QUANTITATIVE MEASUREMENTS OF ELEMENTAL AND MOLECULAR SPECIES USING HIGH MASS RESOLUTION MASS SPECTROMETRY

Abstract

A method for generating a mass spectrum of sample ions using a multi-collector mass spectrometer, the mass spectrometer comprising a spatially dispersive mass analyzer for directing the sample ions into a detector chamber (326), the method comprising: (a) Generating sample ions of a first ion species A with a mass-to-charge ratio (m / z) A , a second ion species B with a mass-to-charge ratio (m / z) B , and a third ion species C with a mass-to-charge ratio (m / z) C , wherein the ions of species A have a different nominal mass than the ions of species B and C, and further wherein the ions of species B have the same nominal mass as the ions of species C; (b) directing the sample ions of species A, B and C to move through the mass analyzer and towards the detectors (330) in the detector chamber (326), the sample ions being deflected during their movement; (c) scanning the ions of species B and C through a main aperture (520) defined in a main mask (518) of a main detector (505), while the ions of species A pass through a guide aperture (514) defined in a guide mask (512) of a guide detector (500); and (d) generating a guide signal (700) representing the ion intensity received at the guide detector (500) from the ions of species A, and generating a main signal (710) representing the ion intensity received at the main detector (505) while the ions of species B and C are scanned via the main aperture (520); wherein ions of species A are detected by the guide detector (500) during the scanning, i) while ions of species B, but not C, ii) subsequently both species B and species C, and subsequently species C but not B are detected by the main detector (505).

Description

Field of the invention

[0001] The invention relates to a method and apparatus for obtaining a high-precision mass spectrum recorded in a multi-collector mass spectrometer. The invention further relates to a method for determining isotope ratios and quantitative information about elemental and molecular species from the high-precision mass spectrum. The method and apparatus can be particularly useful for obtaining a mass spectrum from ions that have the same nominal mass and thus exhibit mass interference. Background of the invention

[0002] The quantitative analysis of elemental and molecular species is of central interest to numerous scientific fields. For example, the precise and quantitative determination of elemental and molecular species is crucial for applications in environmental sciences, materials science, and life sciences.

[0003] A fundamental problem for accurate and precise quantitative mass spectrometry of molecular and elemental species is interference from other species present in the same sample. For example, polyatomic (or molecular) ions in a sample may have the same nominal mass as the atomic (or elemental) isotopes being analyzed, leading to mass interference. In another example, different molecular isotopes may have the same nominal mass. This can make the contribution of each isotopologue to the measured mass spectrum of the sample difficult to resolve.

[0004] Although two species may have the same nominal mass due to the systematic nuclear mass defect, ions will have a true mass slightly adjusted with respect to their nominal mass. For example, the mass defect causes polyatomic molecules comprising atoms with a nominal nuclear mass less than iron to appear heavier than elemental species with the same nominal atomic mass. Alternatively, for polyatomic ions consisting of atoms with a nominal mass greater than iron, the polyatomic ions appear lighter than the elemental species at that atomic mass. Therefore, the mass peak of each isotope can be resolved by achieving a sufficiently high resolution.

[0005] As described in Weyer et al., International Journal of Mass Spectroscopy, 226, (2003), pp. 355-368, a double-focusing, multi-collector inductively coupled plasma (MC-ICP) mass spectrometer can be used to determine isotopic fractions of atomic and polyatomic ions. The detector chamber of the mass spectrometer is equipped with a plurality of Faraday collectors. The Faraday collectors are precisely aligned with respect to the atomic and polyatomic ions with the same nominal mass, which have been separated in a mass analyzer according to their mass-to-charge ratio. An example of the configuration of such a Faraday collector, which is state-of-the-art for measuring 56 Fe and 40 Ar 16 O is used in Fig. 1. The Faraday collector 116 includes an aperture or entrance slit 114 positioned in the beam path of the elemental ions 110 and the molecular ions 112 (whereby the elemental and molecular ions with the same nominal mass and those with the true mass are separated due to the mass defect). The Faraday collector 116 is precisely positioned with respect to the ion beams so that only the elemental ion species 110 enters the entrance slit 114 of the Faraday collector 116. In contrast, the interfering molecular ions 112 are aligned offset from the entrance slit 114 and thus prevented from entering the Faraday collector 116.

[0006] By adjusting the parameters on the mass analyzer of the mass spectrometer, the atomic ions and polyatomic ions can be "scanned" through the entrance slit of the Faraday collector. The signal received by the Faraday collector during the change in the deflection of the ion beams results in a mass spectrum, or mass scan. The mass spectrum represents the intensity or ion current received in the Faraday collector over the extent of the deflection. As an example, Fig. 2A shows a mass spectrum representing Fe isotopes and their corresponding molecular interferences, whereas Fig. Figure 2B shows an enlarged view of a section of the same mass scan. The resulting mass spectrum exhibits a series of flanking and plateau regions. A first flanking region occurs when the atomic ion beam enters the entrance slit, resulting in an increase in the ion current (region A of Fig. 2A). A plateau region occurs at the point where the full atomic ion beam is received through the entrance slit, and thus a maximum intensity of atomic ion species is recorded (region B of Fig. 2A). A second plateau is observed at the point where both the full atom and polyatomic ion beams are collected in the Faraday collector (area C of Fig. 2A). Finally, a third plateau indicates a region where only the polyatomic ion beam is captured through the entrance slit at the Faraday collector (region D of Fig. 2A).

[0007] To determine the isotopic ratios present in the sample, Weyer et al. compare the isotopic ratios at different points on the first plateau of the mass spectrum. Fig. 2B shown diamonds represent 56 Fe / 54Fe isotope ratios (see y-axis on the right) measured at the marked positions in the plateau. It can be seen that the three data points in the central part of the plateau are in good agreement (within a margin of error). However, those data points at the edges of the plateau appear to yield an anomalous result for the isotope ratio.

[0008] Accordingly, the known method for determining isotope ratios described in Weyer et al. requires broad and flat plateaus in the mass spectrum to provide accurate and consistent measurements. Providing a suitable plateau in samples containing ions exhibiting mass interference relies on very high resolution separation of the ion species. Furthermore, these effects can cause peak position drift over the measurement time when temperature fluctuations and mechanical or electrical instabilities occur in the mass spectrometer, thus leading to inaccuracies in the estimates.

[0009] Mass resolution can be improved in sector-field mass spectrometers by reducing the width of the Faraday trap entrance slit. However, this also reduces the overall transmission of ions through the slit, and thus the sensitivity. Increased mass resolution comes at the expense of ion beam transmission, which is why it cannot be increased indefinitely.

[0010] Further limitations of the peak-plateau method described above are evident in the presence of three ion species causing mass interference. Under these circumstances, the ion species with the middle atomic mass does not exhibit a clean plateau where a representative signal can be identified, independent of the high- and low-mass ion species. Further improvement in mass resolution by selecting smaller source slits is technically impossible due to the significant loss in transmission and ion beam intensity at the detector.

[0011] Thus, an improved method for providing high-precision quantitative measurements of elemental and molecular species using high-resolution mass spectrometry is needed.

[0012] In addition, reference is made to the applications DE 11 2011 102 323 B4, WO 2013 / 134 833 A1 and WO 2004 / 047 143 A1, which relate to mass spectrometers with ion detection arrangements. Summary of the invention

[0013] Against this background, a method for generating a mass spectrum of sample ions using a multicollector mass spectrometer according to claim 1, an apparatus according to claim 25, a computer program product according to claim 46, and a method for determining the abundance of ion species in a sample using a multicollector mass spectrometer according to claim 47 are provided to obtain a mass spectrum of ions subject to mass interference in a mass spectrometer comprising multiple detectors. The ions subject to mass interference can be atomic or molecular species with the same nominal mass. The method provides a normalized mass spectrum having particularly high precision and improved clarity of the plateau features resulting from the mass interference.The method also produces a mass spectrum that is less affected by ion beam intensity fluctuations that can occur over the duration of the measurement. The improved mass spectrum can be used to determine the abundances and isotopic ratios of species within a sample and is particularly useful in a deconvolution technique for resolving the peak shape of the ion species causing mass interference.

[0014] To obtain a mass spectrum of sample ions, a mass spectrometer is used, which has a plurality of detectors. The detectors can be of any type known for use in multi-collector mass spectrometers. Such detectors are preferably Faraday detectors (also known as Faraday cups or Faraday traps) or ion counters, including, for example, electron multipliers (e.g., discrete dynode electron multipliers and / or continuous dynode electron multipliers) or other types, or any combination thereof, e.g., a combination of Faraday detectors and electron multipliers. Although the description of the invention generally refers to Faraday detectors as the type of detector used in the invention, it is understood that the Faraday detectors can be replaced by any of the other detector types.Each type of detector used has an entrance slit or entrance aperture.

[0015] The detectors are arranged in a detector chamber of the mass spectrometer. Sample ions are transferred from a spatially dispersed mass analyzer to the detector chamber, and the ions are collected by the detectors. In the mass analyzer, the sample ions are deflected, with the magnitude of the deflection depending on the mass-to-charge ratio of the ions. This results in the ions entering the detector chamber spatially separated from each other according to their mass.

[0016] In the method described here, at least ions of a first species, ions of a second species, and ions of a third species are projected from the mass analyzer into the detector chamber. The ions of the first species have a different nominal mass than the ions of the second and third species, and the ions of the second species have the same nominal mass as the ions of the third species. Given this fact, the ions of the first species are separated from the ions of the second and third species in the mass spectrum, but the ions of the second and third species are subject to mass interference and are separated in the mass spectrum only by the mass defect.

[0017] In the detector chamber, the ions of the first species are received by a lead detector. At the entrance to the lead detector is a lead aperture or entrance slit through which the ions must pass to be detected by the lead detector. The ions of the second and third species are received in a master detector, which has a master aperture or entrance slit through which the ions must pass to be detected by the master detector. The lead entrance slit of the lead detector (which has a first width) may, in some examples, be wider than the master entrance slit of the master detector (which has a second width), although this is not necessary in every embodiment of the invention. In one embodiment, the lead entrance slit and the master entrance slit may have the same width.

[0018] An ion intensity or ion current is measured at the lead and master detectors, while the deflection or direction of the ions of at least the second and third species is changed or "scanned" at the master detector. For example, a change or deviation in the deflection or direction of all sample ions can be induced by establishing or changing the magnetic field in the mass analyzer. In another example, only the second and third ions can be scanned using an ion deflection unit downstream of the mass analyzer (and upstream of the master aperture). In other examples, the electric field could be established in an electrostatic sector of a mass analyzer, or the ion energy of the sample ions could be scanned. Changing any of these parameters on the mass spectrometer results in a change in the deflection of the ions.The described ion deflection methods could be used separately or in combination. In an alternative example, the ions of the second and third species can be scanned at the master detector by moving the entrance slit relative to the ion "beam." In further examples, a combination of the described deflection method can be used together with movement of the master detector (for example, by changing the magnification or dispersion of the instrument using a zoom lens).

[0019] As a result of a change in the deflection of the ions (or the movement of the master entrance slit), the position at which the ions of the second and third species impinge on the master detector is scanned or shifted across the entrance slit. A signal is then generated at the master detector that is proportional to the intensity of the ions of the second and third species entering the master detector during the change in direction. This represents the mass spectrum of the ions subject to mass interference. Similarly, a signal is generated at the lead detector that is proportional to the intensity of the ions of the first species entering the lead detector during the period in which the second and third ion species are being scanned.Ideally, a maximum ion intensity of the first species is recorded at the lead detector throughout the entire scan of the second and third ions (in other words, for the entire period width of interest for the mass spectrum of the ions subject to mass interference).

[0020] The signal received by the master detector can then be normalized using the signal received by the lead detector. This results in a normalized mass spectrum of ions of the second and third species (in other words, a normalized mass spectrum subject to mass interference). Because the different ion species are generated in a common ion source, any signal variation is assumed to be similar for each ion species (or ion current). Recording the intensity of the first ion species for the situation where all ions of the first species (i.e., maximum intensity) are collected by the lead detector while changing the position of the second and third species at the master detector means that the lead detector signal reflects the variations within the ion source.Therefore, the normalized mass spectrum for the second and third species, normalized with reference to the lead detector signal from the first ion species, is essentially independent of signal fluctuations or signal drifts. When the signals are recorded over extended periods of time, i.e., over many scans, where signal fluctuations would otherwise significantly affect the detected peak shapes and intensities, the normalization approach results in significantly improved peak shape accuracy. The benefit of the lead detector is therefore to generate a signal that can be used to normalize the second and third signals. Ideally, the signal measured at the lead detector is completely flat and exhibits no structure due to peak overlaps from different ion species.

[0021] This limitation is completely avoided in cases where the ion beam position of the first ion species is stable (i.e., the ion beam position of the first species is not changed at the lead detector), while the species impinging on the master detector are scanned through the master detector aperture to capture the full structure of the interfered peak. Referencing the master detector signals to the lead detector always guarantees normalization and eliminates drift effects that would otherwise distort the peak shape at the master detector.

[0022] According to the invention, the lead detector is oriented such that the ions of the first species enter through the entrance slit of the lead detector during the entire scan of the second and third ion species. Thus, the degree of variation in the deflection (or displacement) of the ions of the first species at the lead detector is less than the width of the lead entrance slit. In the most preferred embodiments, the position of the beam of ions of the first species at the lead detector is not changed while the second and third ion species are scanned at the master detector. In some alternative embodiments, the lead detector is arranged such that the first ions are deflected during the variation in the deflection (or displacement) of the second and third ions through the entrance slit of the lead detector. Accordingly, the displacement of the second and third ion beams at the master detector is greater than the width of the entrance slit of the master detector.In some other embodiments, the second and third ion species could be scanned at the master aperture and detector by moving the master aperture and optionally the master detector.

[0023] The mass spectrum measured at the lead detector preferably always exhibits maximum intensity throughout the entire, or almost entire, scan or deflection of the second and third ion beams. The maximum lead signal is generated when the full ion beam of the first ion species in the lead detector is recorded through the lead entrance slit during the entire deflection or scanning of the second and third ion species. However, the mass spectrum measured at the master detector exhibits slopes and plateaus corresponding to the change in ion intensity as the mass peaks of the ions of the second and third species move through the entrance slit of the master detector. This means that after normalizing the signal recorded at the master detector with the signal recorded at the lead detector, any small fluctuations in the intensity of the original, unseparated ion beam can be eliminated.Accordingly, the complex peak shape of the signal recorded at the master detectors can be measured with very high precision. Small artifacts can be identified, for example, due to scattering events and small peak tailing effects that occur when the ion beam enters the slit of the master detector.

[0024] According to a first aspect of the invention, there is provided a method of generating a mass spectrum of sample ions using a multicollector mass spectrometer, the mass spectrometer comprising a spatially disperse mass analyzer for directing the sample ions into a detector chamber, the method comprising: (a) Generating sample ions of a first ion species A with a mass-to-charge ratio (m / z) A , a second ion species B with a mass-to-charge ratio (m / z) B, and a third ion species C with a mass-to-charge ratio (m / z) C , wherein the ions of species A have a different nominal mass than the ions of species B and C, and further wherein the ions of species B have the same nominal mass as the ions of species C; (b) directing the sample ions of species A, B and C to move through the mass analyzer and towards the detectors in the detector chamber, whereby the sample ions are deflected during their movement; (c) scanning the ions of species B and C across a master aperture defined in a master mask of a master detector, while the ions of species A pass through a lead aperture defined in a lead mask of a lead detector; and (d) generating a lead signal representing the ion intensity received at the lead detector from the ions of species A and generating a master signal representing the ion intensity received at the master detector while the ions of species B and C are scanned across the master aperture; where during scanning, ions of species A are detected by the lead detector, while ions of species B but not C, then both species B and species C, and then species C but not B are detected by the master detector. Preferably, the method further comprises: (e) Normalizing the master signal from ions B and ions C using the lead signal to determine a normalized mass spectrum of ions B and ions C. In this embodiment, the master signal is calibrated by referencing the lead signal (which reflects signal fluctuations) to determine the normalized mass spectrum of ions B and ions C. When the master signal is recorded over a long period of time or across many scans, signal fluctuations are thus taken into account and a more accurate peak shape is obtained. This, in turn, allows for better peak deconvolution and thus a better determination of the ion abundances.

[0025] The mass spectrometer may be a double-focusing mass spectrometer, although the method can also be applied in a single-focusing mass spectrometer. The mass spectrometer comprises a plurality of detectors in the detector chamber. The detector chamber may comprise Faraday detectors or other detector types, such as ion-counting detectors, for example, electron multipliers, or conductivity detectors (such as a controlled drift detector). In a particular example, the lead detector is a lead Faraday detector, which is one among many movable Faraday detectors, and the master detector is a master Faraday detector, which is a fixed, central Faraday detector.

[0026] The mask or entrance plate for each detector can be an integral part of the detector or a separate part. For example, the mask can be mounted at a distance from the rest of the detector, or it can form a wall plate or wall of the detector. In either case, the mask is positioned in the beam path of the ion beam, between the entrance to the detector chamber and the detector's detection area. Ions received in the detector strike the mask and enter through the aperture or entrance slit formed in the mask.

[0027] The aperture can be a slit or opening defined in a mask at the entrance to the detectors. The size of the aperture describes the dimension of the slit or opening in the direction in which the ions are scanned or deflected, for example, the slit width. Normalizing the master signal to the lead signal to obtain a normalized mass spectrum can involve adjusting the scale of the master signal to a scale common to the lead signal. For example, the scale of the master signal can be adjusted with respect to the maximum and minimum values ​​of the lead signal. Adjusting or normalizing the master signal by the lead signal can be achieved by dividing each data point on the master signal at a given time by the lead signal acquired at the same time.In a particular example, the normalization may refer to a standard point in the lead signal, so that each data point of the master signal is scaled relative to the standard point.

[0028] Advantageously, the resulting normalized mass spectrum provides a very accurate peak shape. The described method is particularly well suited to reducing the effects of intensity fluctuations or drifts in the ion beam intensity over time (e.g., due to electrical or mechanical drift or temperature changes).

[0029] The nominal mass of ions of species A, ions of species B, and ions of species C is the mass that would be expected based on the number of protons and neutrons present in the nuclei of an ion of a given species. For elemental or atomic ions, the nominal mass should be representative of the true atomic mass. For molecular or polyatomic ions, however, the true molecular mass compared to the nominal mass is adjusted by the mass defect. The mass defect results from the equivalence of mass and energy and describes the mass difference representative of the nuclear binding energy.

[0030] Adjusting the scanning of ion species B and C may involve adjusting the mass analyzer parameters to change the deflection of the ions passing through. For example, the mass analyzer's magnetic field can be scanned or set up. This results in a change in the deflection of the ions of species A, the ions of species B, and the ions of species C. Each ion species is deflected during its flight through the mass analyzer at an angle determined by its mass-to-charge ratio. This results in a spatial separation of the ions into "ion beams" of each ion species. The degree of deflection of the ions can be adjusted by changing the magnetic field applied in the mass analyzer. Since the same magnetic field is applied simultaneously to all ions passing through the mass analyzer, any adjustment to the mass analyzer parameters will result in a simultaneous change in the deflection or direction of all ion species.

[0031] Other methods and techniques may be used to scan ion species B and C. These are described in detail below. In one example, the mass analyzer may comprise an electrostatic sector in a dual-focusing mass spectrometer, and the deflection or change in direction of the ion beams may be proportional to the electric field applied to the electrostatic sector. Therefore, the ions may be scanned by adjusting or building up the electric field at the electrostatic analyzer, preferably while keeping the magnetic field constant. In another example, the deflection of the sample ions is adjusted by scanning or building up the energy of the sample ions. In another embodiment, the ions may be scanned by adjusting the electric field at the electrostatic analyzer and the energy of the ions in parallel.In a preferred embodiment, the parallel adjustment of the electric field and the ion energy is performed in a fixed ratio. Alternatively, the deflection of particular ion species (or ion beams) can be adjusted using a pre-aperture deflection unit downstream of the mass analyzer. For example, a master pre-aperture deflection unit can be used upstream of the master aperture to specifically change the deflection of ion species B and C, causing ion species B and C to be scanned across the master aperture while maintaining the position of ion species A in the lead detector constant.

[0032] Optionally, in one embodiment, the method comprises scanning ion species A across at least a portion of the lead aperture of the lead mask of the lead detector, while scanning ion species B and C across the master aperture. In other words, the ions of species A can be scanned simultaneously with the ions of species B and C. For example, parameters of the mass analyzer can be adjusted (to change the magnetic or electric field) and thus cause a simultaneous change in the deflection of all sample ions.In an example where the deflection of ions of species A is adjusted, the configuration of the lead aperture, master aperture, and degree of deflection of ion species A, B, and C is such that the ions of species A pass through the lead aperture and are collected by the lead detector during the period in which the master detector detects ions of species B but not C, then both species B and species C, and then species C but not B. Advantageously, this results in a maximum current at the lead detector during the deflection period, which can then be used to normalize the mass spectrum measured at the master detector.

[0033] Conveniently, the lead aperture is wider than the master aperture. This configuration can be particularly advantageous when scanning both species A ions and species B and C ions. Providing a lead aperture that is relatively larger or wider than the master aperture can be useful to provide maximum ion intensity of species A ions at the lead detector during the scanning or deflection period of species B and C ions. This is because the species A ions can pass through the lead aperture during the scan or deflection and are not blocked by the lead mask. However, in some examples, the lead aperture and master aperture may be the same size or width, or the lead aperture may be smaller than the master aperture. Where, for example, For example, if the ions of species B and C are scanned, but the ions of species A are not scanned, the lead aperture can be narrower than the master aperture.This scanning, or adjusting the deflection of ions of species B and C, but not species A, can be achieved, for example, by using a pre-aperture deflector installed downstream of the mass analyzer and upstream of the mass detector, as described in more detail below.

[0034] Preferably, scanning the ions of species B and C comprises adjusting the deflection of the ions of species B and C as they move through the mass analyzer and toward the detectors in the detector chamber. In other words, scanning the ions of species B and C (and in some examples, also scanning the ions of species A) can be achieved by changing the deflection applied to the ions as they move through the mass analyzer and toward the detectors at the detector chamber. For example, this may comprise changing the deflection applied in the mass analyzer, or it could comprise applying an additional deflection prior to entry into the detector (for example, by using a pre-aperture deflection unit in front of the aperture at the entrance to the detector).

[0035] Optionally, scanning the ions may involve moving the ion beam relative to an entrance aperture at the entrance to a detector. For example, scanning species B and C ions may involve moving the ion beam relative to the master aperture. Moving the ion beam can be achieved either by changing the deflection of the ion beam to scan across the aperture or by moving the aperture relative to the stationary ion beam. In either case, the appropriate movement causes the ions to pass through the aperture, so that the ion beam is first blocked by the aperture, then passes through the aperture, and is subsequently blocked by the aperture.

[0036] Where scanning is optionally employed by adjusting the deflection through the mass analyzer, scanning involves setting up the magnetic field at the mass analyzer. Applying a magnetic field to the sample ions at the mass analyzer causes ion species with different mass-to-charge ratios to disperse spatially. Changing or setting up the applied magnetic field results in an adjustment of the degree of deflection (or changing the angle of the ion beams). Accordingly, the ion beam moves across the aperture at an entrance plate to a detector (such as the lead mask or master mask) while the degree of deflection changes. Applying a magnetic field affects all sample ions passing through the magnetic analyzer. Therefore, the deflection of both ion species A and ion species B and C would be adjusted as a result of setting up the magnetic field at the mass analyzer.Advantageously, the structure of the magnetic field offers an easily controllable method for adjusting the deflection of the ions.

[0037] Where scanning is optionally employed by adjusting the deflection of the mass analyzer, and a dual-focusing mass analyzer is used, scanning involves scanning the electric field at the electrostatic sector of the mass analyzer. The magnetic field at the mass analyzer is preferably kept constant. In a similar way to applying a magnetic field in a mass analyzer, establishing an electric field at the electrostatic sector of a mass analyzer causes an adjustment of the degree of deflection (or changing the angle of the ion beams). The establishment of an electric field will affect all sample ions, and thus the deflection of each of the ion species A, B, and C will be adjusted. Adjusting the deflection results in the movement of the ion beam incident on the entrance plate relative to the aperture.The electric field setup provides a straightforward method for controlling the deflection of sample ions at an electrostatic sector of the mass analyzer.

[0038] For example, where optional scanning or deflection adjustment of all ion beams (for species A, B, and C) is used, deflection adjustment involves scanning an energy of the sample ions. The degree of deflection of the sample ions in the mass analyzer depends on the energy of the ions. By increasing or building up the energy of the sample ions (for example, by accelerating the ions in the mass spectrometer), the extent to which the ions are deflected in the mass analyzer is adjusted and the ion beam is scanned relative to an aperture on the detector. By scanning or changing the energy of the sample ions, the deflection of ion species A, B, and C can be adjusted simultaneously.

[0039] Preferably, scanning involves constructing or adjusting a deflection provided by an ion deflection unit downstream of the mass analyzer. In other words, an ion deflection unit can be placed in the beam path of the ions of a particular species, after the ions have exited the mass analyzer, and before an aperture or entrance slit to a detector. The ion deflection unit (or pre-aperture ion deflection unit) can then be used to deflect only some of the sample ion species. Further information on a pre-aperture ion deflection unit of this type can be found in this applicant's patent publication WO 2012 / 007559 A2, which is incorporated herein by reference in its entirety.In a specific example, ion species B and C can be scanned across the master aperture by altering the deflection caused by a master pre-aperture deflection unit installed downstream of the mass analyzer and upstream of the master aperture. In this case, the pre-aperture ion deflection unit can be installed downstream of the mass analyzer and upstream of the master aperture in the beam path of the ions of species B and C. The ion deflection unit can then be used to further deflect ions of species B and C (in other words, to adjust their overall deflection). In this way, ions of species B and C can be scanned across the master aperture while the deflection of the ions of species A remains unchanged.Conveniently, since species A ions are not scanned or further deflected, they can be precisely directed to pass through the lead aperture during the entire deflection period of ions B and C. Therefore, a narrower lead aperture can be used, and appropriate configuration of the lead aperture, master aperture, and change in deflection of species B and C ions can be more easily achieved.

[0040] This use of the pre-aperture deflection unit for the master detector offers another significant technical advantage. An important aspect of using the pre-aperture deflection unit in front of the master detector (and not adjusting the ion beam deflection at the lead detector) is that the lead detector signal therefore corresponds to a fixed ion beam position and ion mass, so it is not necessary for the lead detector signal to be free of interference. The reason for the lead detector is to generate a signal that can be used to normalize the B and C master detector signals. This requires that the lead detector signal from ion species A always reflect the fluctuations in the ion source, and this signal should therefore ideally not be complicated by a peak structure containing interference effects from different ion species in the lead detector signal due to scanning of the species at the lead detector.Ideally, the signal measured at the lead detector is completely flat and exhibits no structure due to peak overlaps of different ion species. This limitation is completely avoided in embodiments where the ion beam position of ion species A is stable, while species B and C impinging on the master detector are scanned through the master detector aperture to capture the complete structure of the interfered B and C peaks. By referencing the master detector signals to the lead detector signals, normalization and elimination of signal drift effects that would otherwise distort the peak shape at the master detector are always guaranteed.

[0041] In an alternative embodiment, a lead pre-aperture deflection unit, installed downstream of the mass analyzer and upstream of the lead aperture, deflects the ion species A such that the ion species A completely passes through the lead aperture, while the ions of species B and C are scanned via the master aperture. For example, ion species B and C can be scanned across the master aperture and detector by adjusting the deflection caused by the mass analyzer (according to one of the methods described above) or by adjusting the beam energy, and the beam of ion species A can be kept substantially stationary at the lead aperture by compensating for the deflection of ion species A caused by the mass analyzer or the beam energy change, the compensation being performed by a pre-aperture deflector mounted in front of the lead aperture and detector.Alternatively, compensation for the deflection of ion species A caused by the mass analyzer can be achieved by changing the position of the lead aperture / detector such that ion species A can be kept substantially stationary relative to the lead aperture.

[0042] In another embodiment, ion species B and C can be scanned across the master aperture by moving the master aperture and optionally the master detector while maintaining the deflection by the mass analyzer. In this case, ion species A can be kept essentially stationary relative to the lead aperture.

[0043] Preferably, the ions of species A pass through the lead aperture and are collected by the lead detector at all times that at least one of the ion species B and C is collected by the master detector. In other words, the lead detector can be positioned so that the entire beam of ions of species A passes through the aperture during the scanning period. In contrast, the beam of ions of species B and species C can be positioned so that the ion beams are blocked in a first position by the master mask on one side of the master aperture. Both ion beams can then be deflected or scanned across the master aperture until they reach a second position on the side of the master aperture opposite the first side. At this second position, the ions of species B and species C are blocked from entering the master detector by the master mask.As the ions are scanned between the first and second positions, the ions of the second species B first enter the master detector alone through the master aperture, then both the second and third species enter, and finally only the third species enters the master detector. This conveniently provides a mass spectrum for the ions of species B and species C to be obtained at the master detector, while simultaneously receiving a signal of maximum intensity at the lead detector. The size of the scan or translation of the ions, the alignment of the lead and master detectors, and the width of the lead and master apertures must be appropriately selected to provide a master and lead signal that exhibit the desired characteristics for normalization.These variables will depend on the sample isotopes to be measured, as well as the characteristics of the mass spectrometer and the method used to scan or adjust the deflection of the ions.

[0044] To obtain the lead signal and master signal described above, the distance between the first and second positions of the ions of species A on the lead mask is preferably smaller than the width of the lead aperture in embodiments in which the deflection of all ion species is varied. Furthermore, both distances—the distance between the first and second positions of the ions of species B and the distance between the first and second positions of the ions of species C on the master mask—are larger than the width of the master aperture.

[0045] Optionally, the ions of species A comprise a first species of an atomic isotope, the ions of species B comprise a second species of an atomic isotope, and the ions of species C comprise a species of a molecular isotope. For example, the ions of species A and species B may be different isotopes of the same element, and the ions of species C may comprise an isotope of a molecule. As an illustrative example, the ions of species A 56 Fe + the ions of species B can 57 Fe + and the ions of species C can 40 Ar 16 OH + be.

[0046] Alternatively, the ions of species A comprise a first species of a molecular isotope, the ions of species B comprise a second species of a molecular isotope, and the ions of species C comprise a third species of a molecular isotope. For example, the ions of species A, ions of species B, and ions of species C may be different isotopes of the same molecule, typically where ionic species A is the most abundant molecular isotope. As an illustrative example, the molecule may be methane and the ions of species A may 12 CH4 + the ions of species B can 13 CH4 + and the ions of species C can 12 CH3D + be.

[0047] It is understood that any number of additional ion species may also be present and detected in the ion beam. For example, one or more ion species may be present that cause mass interference with the second ion species B and the third ion species C (e.g., a fourth species D, etc.). In this way, three, four, five, or more interfering ion species can be detected by the master detector (or master Faraday detector).

[0048] Advantageously, the method further comprises positioning the lead detector within the detector chamber for receiving the ions of species A. The lead detector may be a movable detector within the detector chamber. The position of the detector may be adjusted relative to the ion beam of the ion species of interest to provide the correct alignment for successful application of the method (i.e., so that the ions of species A impinge on the lead detector). The movable detector may be motorized, with the positioning of the detector controlled via computer or manually to allow adjustment of the detector from outside the detector chamber. Alternatively, the lead detector could be a fixed detector.

[0049] Optionally, the method further comprises positioning the master detector within the detector chamber for receiving the species B ions and the species C ions. The lead detector may be a movable detector within the detector chamber. The position of the detector may be adjusted relative to the ion beam of the ion species of interest to provide the correct alignment for successful application of the method (i.e., so that the species B and C ions impinge on the master detector). The movable detector may be motorized, with the positioning of the detector controlled via computer or manually to allow readjustment of the detector from outside the detector chamber.

[0050] Alternatively, the master detector can be a fixed detector or a fixed Faraday cup. For example, the master detector can be the fixed, central detector within a plurality of detectors arranged in a plane within the detector chamber for receiving the sample ions.

[0051] In certain embodiments, both the lead detector and the master detector may be positioned as described, ie, may be movable within the detector chamber.

[0052] Optionally, the mass spectrometer comprises a plurality of detectors in the detector chamber, each detector comprising a mask defining an aperture. Preferably, the aperture size is not the same for each of the plurality of detectors. The lead detector and / or the master detector can be determined from the plurality of detectors to select the size or width of said lead and / or master aperture. For example, the lead and / or master detector can be selected with regard to their match with the ion species of interest and with regard to the size of the aperture relative to the displacement of the ion beam during the scan of the ions. Advantageously, this allows greater flexibility in the configuration and arrangement of the mass spectrometer and a wider range of variables available for successful implementation of the method according to the invention.For example, it allows the mass spectrometer to be configured to measure a specific sample ion species.

[0053] Alternatively, the optics (or specifically the zoom optics) can be configured to align the ion species (or ion beam) of interest with the entrance aperture of the master or lead detectors. This allows for proper alignment of the ion species to the appropriate detector for successful implementation of the method. The use of zoom optics can ensure proper alignment without requiring any movement of the detectors relative to each other (or so that only a minor movement is required).

[0054] Optionally, determining a normalized mass spectrum of ions B and C involves dividing the master signal at a given time point by the lead signal acquired at the same time point. This type of "point-by-point" normalization can be particularly useful when only one master signal trace is acquired, as fluctuations, particularly within the ion beam at the time point of the trace, can be effectively eliminated.

[0055] Preferably, the normalized mass spectrum of the ions of species B and the ions of species C is a first normalized mass spectrum. Subsequently, a second or further mass spectrum can be obtained using the same method as described above. This allows an average of the first and second normalized mass spectra to be calculated to determine an average normalized mass spectrum. For example, the average of the ion intensity of the first and second normalized mass spectra can be calculated at each value of the mass deviation (or scan of the ions). Conveniently, this average provides a normalized mass spectrum with increased precision and is particularly useful for reducing the effects of anomalous data points in the measured mass spectrum and for improving statistics.

[0056] Advantageously, the average normalized mass spectrum described above can be referred to as the previous average mass spectrum. A further or new normalized mass spectrum can then be obtained by repeating the above steps. A new average normalized mass spectrum can then be determined from an average of the new normalized mass spectrum and the previous, previous average normalized mass spectrum. This process can be repeated many times, each time "overlaying" the average determined based on previous measurements with a newly measured normalized mass spectrum.

[0057] The scan direction or scan speed can be changed between multiple acquisitions, and the various scans to be added or averaged can be aligned and stretched to achieve optimal results. This alignment and stretching method can utilize methods familiar from gas and liquid chromatography.

[0058] In some cases, data points in the newly measured mass spectrum may be discarded if they are found to be outside a predefined range or set of limits (e.g., outside an error margin), e.g., by referring to the mean value established based on previous measurements. Using this method, small drifts in mass peak positions and small instability spikes in ion intensity (e.g., due to high-voltage spikes) can be effectively eliminated.

[0059] Alternatively, an average of a set of measured normalized mass spectra can be obtained. As a further alternative, a best-fit normalized mass spectrum can be obtained from a large number of measured normalized mass spectra by applying statistical methods such as least squares regression.

[0060] In one embodiment, multiple normalized mass spectra (i.e., peak scans) can be obtained. Before averaging the mass spectra, each new spectrum is compared to the mean of the previous mass spectrum. The new mass spectrum is then fitted to the mean spectra using known statistical methods to compensate for mass drift due to magnetic hysteresis or other possible system instabilities, so that the residual overlap is reduced to the smallest value. In this way, the new spectra best match the mean spectra to effectively reduce mass drift effects on peak averaging. The best-fit spectra can then be averaged.

[0061] Ideally, the method further comprises varying the adjustment rate of the deflection, or other scanning means, to scan the ions of species A and / or B and C at a plurality of different scan rates while scanning them across the respective lead and / or master apertures. Consequently, the number of data points recorded per unit of scan or deflection of the ions in a first segment of the lead signal and the master signal may be different from the number of data points recorded per unit of scan or deflection of the ions in a second segment of the lead signal and the master signal. In other words, the density of data points is greater for a first portion of the lead and / or master signal than for a second portion of the lead and / or master signal.In one example, the parameters on the mass analyzer can be changed at different rates for different segments of a scan, so that more data points are recorded during a first portion of the ion scan than during a second portion of the ion scan. Alternatively, the scan rate, or rate of change of deflection, used by an ion deflection unit can be adjusted during different portions of the deflection setup. This method is referred to below as the segmented scan method. Alternatively, the frequency of sampling data points during a segment of the scan could be changed.

[0062] Advantageously, this segmented scan method allows a larger number of data points to be recorded in regions of the signal of greater interest. For example, a larger number of data points can be recorded where the shape of the mass spectrum changes rapidly (e.g., in flanking regions). Fewer data points can be recorded in the flat plateau regions of the mass spectrum, as these can be assumed to be flat. Consequently, the mass spectrum measurement can be made more efficient, as the overall measurement time can be reduced while still providing a sufficient number of data points in the regions of interest for an accurate representation of the mass spectrum shape.

[0063] In a second aspect, an apparatus is provided for generating a mass spectrum from sample ions using a multicollector mass spectrometer, the mass spectrometer comprising a spatially disperse mass analyzer, the sample ions being directed to move through the mass analyzer and towards detectors in a detector chamber, the sample ions being deflected during their movement, the mass spectrometer comprising a lead detector arranged in the detector chamber for detecting sample ions of a first ion species A having a mass-to-charge ratio (m / z) A and wherein the mass spectrometer comprises a master detector arranged in the detector chamber to detect sample ions of a second ion species B having a mass-to-charge ratio (m / z) B and a third ion species C with a mass-to-charge ratio (m / z) Cand wherein the ions of species A have a different nominal mass than the ions of species B and C, and further wherein the ions of species B have the same nominal mass as the ions of species C, the device comprising: a control module configured to scan ion species B and C relative to the master aperture, preferably by deflecting the ion species B and C to scan the ions of species B and C across a master aperture defined in a master mask of a master detector, while the ions of species A pass through a lead aperture defined in a lead mask of a lead detector; and an analysis module configured to receive a lead signal generated by the lead detector, the lead signal representing the ion intensity of the species A ions received at the lead detector while the species B and C ions are scanned via the master aperture; and receive a master signal generated by the master detector, the master signal representing the ion intensity received at the master detector while the ions of species B and C are scanned via the master aperture; wherein the control module is configured such that during scanning, ions of species A are detected by the lead detector, while ions of species B but not C, subsequently both species B and C, and subsequently species C but not B are detected by the master detector.

[0064] The lead or master detector can be any detector type used in mass spectrometry, or any combination of detector types. In a preferred embodiment, the detectors are Faraday detectors.

[0065] The analysis module and the control module may be part of a computing processor. Although described as separate modules, the control and analysis modules may not be separate from each other. The analysis and control modules may be controlled by computer software, which may include a graphical user interface for accepting user input and outputting data. A processor comprising the control module and the analysis module, or the computer software for interacting with the control module and analysis module, may be an integral part of the mass spectrometer or separate from it.

[0066] Preferably, the analysis module is further configured to normalize the master signal from ions B and C using the lead signal to determine a normalized mass spectrum of ions B and ions C. Advantageously, the device can be used to obtain a particularly precise mass spectrum in which mass interference is particularly visible, if any. Advantageously, the resulting normalized mass spectrum can reduce the effects of intensity fluctuations or drifts in the ion beam intensity over the measurement time. Consequently, the analysis methods applied to the mass spectrum can provide more accurate estimates of the isotopic ratios within the original sample. Furthermore, the more precise mass spectrum can be suitable for analysis using different, more accurate methods.

[0067] Optionally, the control module is further configured so that, while the ions of species B and C are scanned via the master aperture, the control module is also configured to scan the ion species A via at least a portion of the lead aperture of the lead mask of the lead detector. For example, the deflection of the ions of species A can be adjusted simultaneously with the adjustment of the deflection of the ions of species B and C.

[0068] Optionally, the control module is further configured to vary the deflection of ion species A at a lead pre-aperture deflection unit mounted downstream of the mass analyzer and upstream of the lead aperture, wherein the control module is configured to deflect ion species A such that ion species A completely passes through the lead aperture while the ions of species B and C are scanned via the master aperture. In other words, while ion species B and C are being scanned, a deflection can be applied to ion species A to compensate for any deflection applied to all ions to keep the ions of species A aligned toward the lead aperture.

[0069] Preferably, the control module is configured to scan ion species B and C across the master aperture by adjusting the deflection of the ions of species B and C as they move through the mass analyzer and toward the detectors in the detector chamber. In other words, scanning of ions B and C (and, in some examples, also scanning of ions of species A) can be achieved by varying the deflection applied to the ions as they move through the mass analyzer and toward the detectors at the detector chamber. For example, this may involve varying the deflection applied in the mass analyzer, or it could involve applying an additional deflection prior to entry into the detector (for example, by using a pre-aperture deflection unit in front of the aperture at the entrance to the detector).

[0070] Optionally, scanning the ions may involve moving the ion beam relative to an entrance aperture at the entrance to a detector. For example, scanning species B and C ions may involve moving the ion beam relative to the master aperture. Moving the ion beam can be achieved either by changing the deflection of the ion beam to scan across the aperture or by moving the aperture relative to the stationary ion beam. In either case, the appropriate movement causes the ions to pass through the aperture, so that the ion beam is first blocked by the aperture, then passes through the aperture, and is subsequently blocked by the aperture.

[0071] Where scanning by adjusting the deflection through the mass analyzer is employed, the control module can be configured to scan ions of species B and C by adjusting the deflection through the establishment of the electric field. Alternatively, in cases where the mass analyzer comprises an electrostatic analyzer, the control module is configured to scan the ions by adjusting the deflection by changing or establishing the electric field at the electrostatic sector of the mass analyzer. Advantageously, changing the magnetic or electric field causes the ion beams to move across or scan the surface of the detector (via the mask containing the entrance slit). Therefore, adjusting the mass analyzer parameters provides a more straightforward means of scanning ion beams relative to the aperture on the detector than by adjusting the position of the detector itself.The setting of the mass analyzer parameters can be controlled by the control module depending on the user's inputs via the computer software.

[0072] Optionally, the control module is configured to scan the ions or adjust the deflection by scanning the energy of the sample ions. For example, the control module can be configured to change the acceleration of the sample ions in the mass spectrometer. Since the deflection of the ions in the mass analyzer depends on the energy of the ions, adjusting or scanning the energy of the ions causes an adjustment in the deflection of the sample ions (for example, ions of species A, B, and C) passing through the mass analyzer.

[0073] The control module can be configured to scan ions B and C by varying the deflection provided by a master pre-aperture ion deflector downstream of the mass analyzer and upstream of the master aperture. In other words, a deflector can be placed in the beam path of a particular ion species or multiple ion species downstream of the mass analyzer and upstream of the entrance aperture of a detector. The deflector can then be used to adjust the deflection of only some of the ion species. In one particular example, the deflector is placed downstream of the mass analyzer in the beam path of ion species B and C. The deflector is then used to adjust the deflection of ions B and C to scan ions B and C across the master aperture.Conveniently, this method of adjusting the deflection of ion species B and C avoids the need to also adjust the deflection of ions of species A. Therefore, the ions of species A can be very precisely aligned to the lead aperture.

[0074] Preferably, at all times that at least one of the ion species B and C is collected by the master detector, the ions of species A pass through the lead aperture and are collected by the lead detector. Advantageously, the distance between the first position of the ions of species A at the lead aperture at the beginning of the scan and the second position of the ions of species A at the lead aperture at the end of the scan is smaller than the width of the lead aperture. Furthermore, the corresponding first position of the ions of species B and the ions of species C at the master mask at the beginning of the scan and the corresponding second position of the ions of species B and the ions of species C at the master mask at the end of the scan are preferably separated by a distance greater than the width of the master aperture.Conveniently, with correct alignment, this can allow a mass spectrum to be obtained for the ions of species B and C in the master detector while the ion beams are scanned or deflected across the aperture as they move between the first and second positions. At the same time, the full ion beam of species A ions can be acquired at the lead detector throughout the entire scan or deflection, with the recorded lead signal consistently measured at maximum intensity.

[0075] Optionally, the ions of species A comprise a first species of an atomic isotope, the ions of species B comprise a second species of an atomic isotope, and the ions of species C comprise a species of a molecular isotope. Alternatively, the ions of species A comprise a first species of a molecular isotope, the ions of species B comprise a second species of a molecular isotope, and the ions of species C comprise a third species of a molecular isotope. For example, the ions of species A and B may be different isotopes of the same element, and the ions of species C may comprise an isotope of a molecule.

[0076] Advantageously, the lead detector is movable, and the control module is further configured to position the lead detector within the detector chamber to receive ions of species A. For example, the position of the detector relative to the ion beam of the ion species of interest can be adjusted to provide the correct alignment for successful application of the method (i.e., so that the ions of species A impinge on the lead detector). The lead detector can have a motor, so that the control module controls a motor to move the lead detector within the detector chamber without requiring direct access to the interior of the detector chamber.

[0077] The master detector may be movable, and the control module may be further configured to position the master detector within the detector chamber to receive the species B ions and the species C ions. The position of the detector may be adjusted relative to the ion beam of the ion species of interest to provide the correct alignment for successful application of the method (i.e., so that the species B and C ions impinge on the master detector). The master detector may have a motor drive so that the control module controls a motor to move the master detector within the detector chamber without requiring direct access to the interior of the detector chamber. Alternatively, the master detector may be stationary, while the lead detector is movable with respect to the master detector.

[0078] Optionally, the mass spectrometer comprises a plurality of detectors in the detector chamber, each detector comprising a mask having an aperture, and the control module is further configured to select the lead detector and / or the master detector from the plurality of detectors to select a respective lead and / or master aperture size. This advantageously allows greater flexibility in the configuration and arrangement of the mass spectrometer that can be used to apply the method. The aperture size can be the dimension of the aperture in the direction of the scan, for example, the width of the aperture or the entrance slit.

[0079] Optionally, the analysis module is configured to determine a normalized mass spectrum of ions B and C by dividing the master signal at a given time by the lead signal acquired at the same time. This can be particularly useful for eliminating fluctuations in the electron beam in cases where only a trace of the master signal is acquired.

[0080] Advantageously, the analysis module is further configured to store the normalized mass spectrum of the species B ions and the species C ions as a first normalized mass spectrum, obtain a second normalized mass spectrum in the same manner as the first normalized mass spectrum was measured, and determine an average normalized mass spectrum from an average of the first and second normalized mass spectra. This advantageously provides a normalized mass spectrum with increased precision and is particularly useful for reducing the effects of anomalous data points in the measured mass spectrum.

[0081] Preferably, the analysis module is further configured to store the averaged normalized mass spectrum as a first or previous average normalized mass spectrum, obtain a further or new normalized mass spectrum, and determine a mean normalized mass spectrum from an average of the first and previous average normalized mass spectrum and the further or new normalized mass spectrum. This method can be repeated many times to overlay each subsequent new normalized mass spectrum with the previous average mass spectrum to provide a new average normalized mass spectrum. Using this method allows small drifts in peak positions and small instability peaks (for example, due to high-voltage spikes) to be effectively eliminated.

[0082] The new normalized mass spectrum can be fitted to the mean normalized mass spectra using known statistical methods (e.g., fitting the new spectra to reduce the residual overlap to the lowest value). Conveniently, the new mass spectra best fit the averaged normalized mass spectra to effectively reduce mass drift effects on peak averaging. The averaged normalized mass spectrum can then be recalculated to incorporate the best-fit new normalized mass spectra. In this way, newly measured normalized mass spectra are "superimposed" on the previous mean normalized mass spectrum.

[0083] Advantageously, the control module is configured to vary the scan or deflection adjustment rate such that the ions of species A, B, and / or C are scanned at a plurality of different scan rates while being scanned across the corresponding lead and master apertures. In other words, where the sampling frequency of the measurement points remains unadjusted, the change in the scan rate (or change in the variation rate of the scan or deflection of the ions) results in the collection of a different number of data points in a first segment of the lead or master signal than in a second segment of the lead and / or master signal. Alternatively, the sampling rate for measuring the data points can be adjusted for different segments of the measurement.

[0084] Advantageously, this method allows for the recording of a larger number or density of data points in regions of the lead and / or master signal where the shape of the mass spectrum changes rapidly (for example, in the flanking regions). Fewer data points can be recorded in the flat plateau regions of the mass spectrum, as these can be assumed to be flat. Consequently, the mass spectrum measurement is more efficient, and overall measurement times can be reduced.

[0085] In a third aspect, there is provided a multi-collector mass spectrometer comprising: a spatially disperse mass analyzer; a detector chamber, wherein the sample ions are directed to move through the mass analyzer and towards detectors in a detector chamber, wherein the sample ions are deflected during their movement, wherein the detector chamber comprises a lead detector arranged to detect sample ions of a first ion species A having a mass-to-charge ratio (m / z) A are aligned with a lead aperture defined in a lead mask on the lead detector, wherein the detector chamber further comprises a master detector arranged to detect sample ions of a second species B having a mass-to-charge ratio (m / z) B and ions of a third species C with a mass-to-charge ratio (m / z) C are aligned with a master aperture defined in a master mask on the master detector; and a device comprising: a control module configured to adjust the position of the ion species B and C relative to the master aperture such that the ions of species B and C are scanned through the master aperture defined in the master mask of the master detector, while the ions of species A pass through the lead aperture defined in the lead mask of the lead detector; and an analysis module configured to receive a lead signal generated by the lead detector, the lead signal representing the ion intensity of the species A ions received at the lead detector while the species B and C ions are scanned via the master aperture; receive a master signal generated by the master detector, the master signal representing the ion intensity received at the master detector while the ions of species B and C are scanned via the master aperture; and to normalize the master signal from ions B and C using the lead signal to determine a normalized mass spectrum of ions B and ions C.

[0086] The mass spectrometer may further comprise the apparatus, including the additional features described earlier in this document, wherein the lead aperture, the master aperture, and the control unit are configured such that, during scanning, ions of species A are detected by the lead detector, while ions of species B but not C, subsequently both species B and C, and subsequently species C but not B are detected by the master detector. For example, the control unit may be configured to apply a deflection such that, during scanning, ions of species A are detected by the lead detector, while ions of species B but not C, subsequently both species B and C, and subsequently species C but not B are detected by the master detector, the degree of deflection being relative to the lead and master apertures.

[0087] Ideally, the mass spectrometer is a dual-focusing mass spectrometer, featuring an electrostatic analyzer and a magnetic sector. Optionally, the mass analyzer is a magnetic mass analyzer, which deflects and separates the ion species by applying a magnetic field. The spatially dispersed mass analyzer causes ions of different species (and different mass-to-charge ratios) to be deflected at different angles or to different degrees as they pass through the mass analyzer, thus separating the ions at a distance.

[0088] Preferably, the mass spectrometer may further comprise an ion deflection unit downstream of the mass analyzer. The ion deflection unit may be positioned upstream of the master aperture so that the ions of species B and C pass through the deflection unit. During use, the deflection unit can adjust the deflection of the ions of species B and C.

[0089] In a fourth aspect, a computer program is provided that is configured such that, when executed by a processor, the computer program controls an elemental mass spectrometer for operation according to the method described above. For example, the computer program may include logic that, when executed on a computer, enables control of the mass spectrometer and the apparatus described above to perform the method.

[0090] In a fifth aspect, a method is provided for determining the abundance of ion species in a sample using a multicollector mass spectrometer, the mass spectrometer comprising a spatially disperse mass analyzer for directing the sample ions into a detector chamber, the sample comprising sample ions of a first ion species A having a mass-to-charge ratio (m / z) A , ions of a second ion species B with a mass-to-charge ratio (m / z) B and ions of a third ion species C with a mass-to-charge ratio (m / z) C, wherein the ions of species A have a different nominal mass than the ions of species B and the ions of species C, and ions of species A do not cause mass interference or do not cause significant mass interference with any other ion species, and further wherein the ions of species B have the same nominal mass as the ions of species C, the method comprising: (a) performing the method of any one of claims 1 to 12 to provide a normalized mass spectrum of the ions of species B and the ions of species C; (b) deconvolving the mass peak of each of the ions of species B and species C from the normalized mass spectrum, assuming that the shape of the mass peak of the ions of species B is identical to the shape of the mass peak of the ions of species C; and (c) Determining an abundance of the ions of species A, the ions of species B and / or the ions of species C in the sample.

[0091] The determined abundances can be used to determine one or more abundance ratios, e.g., isotope ratios. The ratio can be determined according to the relative abundances (amplitudes of the mass peaks) of each of the ions of species A, the ions of species B, and the ions of species C.

[0092] The ions of species A preferably do not cause mass interference or significant mass interference with any other ion species, so that an accurate peak shape can be measured for species A. In other words, the mass of species A differs sufficiently from the mass of ions B and C such that the mass peak of species A lies at a distance from the mass peaks of species B and species C.

[0093] The method for obtaining a normalized mass spectrum for ions of species B and C is described in detail above. Advantageously, a plurality of normalized mass spectra are measured for ions of species B and C to obtain a mean or best-fit normalized mass spectrum. For example, the mean normalized mass spectrum can be obtained by "overlaying" a mean normalized mass spectrum previously obtained (according to the method described above) with newly measured mass spectra.

[0094] Deconvolution of a mass peak for each of the species B and C ions can be achieved by applying standard signal processing methods to the normalized mass spectrum. Deconvolution assumes that the shape of each mass peak for the species B ions and the species C ions will be identical. This assumption can be considered valid due to the common ion source and common acceleration phases for each ion species, which result in a similar angular momentum and energy distribution for ions of each species. Although this assumption is best applied to mass spectra measured in a double-focusing mass spectrum, the assumption can also be usefully applied to mass spectra obtained in a mass spectrometer with a single-focusing magnetic sector.Any suitable algorithm can be applied (for example, in an analysis module of a computer processor) to deconvolute or resolve the mass peaks from the mass spectrum subject to mass interference.

[0095] The use of the master and lead detectors for normalizing the mass peaks, optionally together with the spectrum averaging and segmented scan methods, improves the quality of the mass spectrum measurement and thus allows an effective deconvolution of the interfering mass peaks and thus an improved determination of the ion abundances.

[0096] Once the mass peaks of the ions of species B and species C have been obtained according to the above procedure, methods standard in mass spectrometric analysis can be applied to obtain the isotopic ratio of each ion species.

[0097] Preferably, the method comprises measuring a full mass spectrum of the ions of species A or the ions of another species that does not cause mass interference or does not cause significant mass interference with another ion species, on a detector arranged in a detector chamber, to determine a mass peak having a mass peak shape for the ions of species A or for the ions of the other species that do not cause mass interference. The step of deconvolving the mass peak of each of the ions of species B and species C from the normalized mass spectrum is based on the assumption that the shape of the mass peak of the ions of species B is the same as the shape of the mass peak of the ions of species C and is the same as the shape of the mass peak measured for the ions of species A or the other species that do not cause mass interference.

[0098] The mass peak of the ions of species A or of the ions of another species that does not cause mass interference or does not cause significant mass interference with any other ion species can be obtained by measuring a mass spectrum at the detector, e.g., the lead detector or master detector, but preferably the master detector. The ion should be chosen so that the mass spectrum is not subject to mass interference.The mass spectrum for ions of species A, or the ions of another species that do not cause mass interference or do not cause significant mass interference with any other ion species, is completely scanned or deflected across the entrance slit to the detector (for example, from an initial position where the ion beam is blocked by the mask on one side of the aperture to a position where the ion beam is blocked by the mask on the opposite side of the mask). The shape of the mass peak can then be deduced from the shape of the measured mass spectrum. In particular, the shape of the flanking regions of the mass spectrum, as the ions begin to enter or exit the detector aperture, reflects the shape of the mass peak.It may be advantageous to measure the mass spectrum of ions of species A or other ion species not subject to mass interference using a method (as described above) that provides a greater density of measurement points in the flanking segments of the mass spectrum.

[0099] Preferably, measuring a full mass spectrum of the ions of species A or of ions of another species that does not cause mass interference or does not cause significant mass interference with any other ion species comprises: Measuring a full high-resolution mass spectrum of a first ion species at a master detector while measuring a full high-resolution mass spectrum of a second ion species at a lead detector, wherein the first and second ion species are different ion species and are either ions of species A or ions of another species that does not cause mass interference or does not cause significant mass interference with any other ion species; and Normalize the signal measured at the master detector to the signal measured at the lead detector to determine a normalized full mass spectrum for the ions of species A or the other non-mass-interfering species. This conveniently provides a very high-precision peak shape for the non-mass-interfering ion, which can then be used to convolve the peak shape of the ions of species B and species C. A more accurate peak shape for the non-mass-interfering ion leads to better quantification of the contributing species in the measured, normalized non-mass-interfering mass spectrum. This is due to more accurate deconvolution of the mass peaks.Using the lead and master detectors to capture the normalized peak shape of an ion that does not cause mass interference allows for the derivation of a normalized peak shape that is independent of small fluctuations at the sample ion source or elsewhere in the mass spectrometer. Thus, this method is useful for correcting for ion beam drift.

[0100] Preferably, deconvolving a mass peak for each of the ions of species B and species C (in step (b) above) further comprises: Fitting a mass peak of ion species B and a mass peak of ion species C to the normalized mass spectrum, wherein the shape of the mass peak of ion species B and the shape of the mass peak of ion species C are the same as the shape of the mass peak of the ions of species A or the other non-mass-interfering species measured with the master detector; wherein the mass peak for ion species B and for ion species C is fitted by adjusting the amplitude of each mass peak and / or the mass at which each mass peak occurs. In other words, the mass peaks of ion species B and C are assumed to have an identical shape to the mass peak measured for the non-mass-interfering ion. Preferably, the peak shape for the non-mass-interfering ion is measured with very high precision using a normalized signal measured at the lead and master detectors. Using the high-precision peak shape measurements, the mass peaks for ion species B and C can be deconvolved using a fitting method to fit the identical peak shapes to the normalized mass spectrum.Since the peak shape is known, the mass peaks for ion species B and C can be found by fitting at least one fitting parameter to the measured normalized mass spectrum of the mass-interfering ion species B and C. The fitting parameter will primarily be a scaling factor for the amplitude of each mass peak, although where the exact mass of ion species B and C is unknown, the mass can also be used as a fitting parameter.

[0101] As a result of the effective deconvolution method described, the calculation of the abundance of sample ions B and C is not based on the identification of plateaus in the full mass spectrum, unlike previously known methods. The measurements used here to determine the abundance of ion species in a sample can accordingly require measurements with lower resolution (e.g., using a higher transfer of electrons to the ion detector) without compromising precision. Consequently, the abundance of species separated only by very small mass defects can be better distinguished. Furthermore, the method is particularly effective in cases where a large number of different ion species exhibit mass interference.

[0102] Although mass can be used as a fitting parameter during the deconvolution of the mass peaks for ion species B and C, the exact mass (the exact atomic or molecular mass) of each ion species present in the sample will generally be known. For example, the mass of the ions of an atomic or molecular species will be known from previously documented measurements. The exact mass will be the mass generally accepted as the true atomic or molecular mass for a given atom or molecule, respectively.

[0103] Optionally, the determined abundances can be used to determine one or more abundance ratios, wherein the ratios are determined according to the relative amplitudes of the deconvolved mass peaks of at least two of the ions of species A, the ions of species B and the ions of species C.

[0104] Preferably, the sample ions may further comprise ions of one or more further ion species, wherein the ions of each of the one or more further ion species have the same nominal mass as the ions of species B and species C; and the method further comprises deconvolving a mass peak of each of the ions of species B, species C, and each of the one or more further ion species from the normalized mass spectrum, assuming that the shape of the mass peak of the ions of species B, species C, and each of the one or more further ion species is the same as the shape of the mass peak measured for the ions of species A or for the ions of the other non-mass-interfering ion species. Each of the one or more further ion species may each have a mass-to-charge ratio (m / z), wherein the mass-to-charge ratio is different for each of the further ion species. Brief description of the drawings

[0105] A method and apparatus for obtaining a mass spectrum of sample ions according to one aspect of the present disclosure will be described, by way of example only, with reference to the following drawings, in which: Fig. Figure 1 is a schematic view of the alignment of Faraday detectors according to a method described in the prior art; Fig. Figure 2A is a graph of the mass spectrum obtained by methods described in the prior art, showing all Fe isotopes and their corresponding molecular interferences; Fig. 2B is an enlarged view of a section of the mass spectrum from Fig. 2A, which was obtained by methods described in the prior art; Fig. Figure 3 is a schematic view of a double focusing multicollector mass spectrometer; Fig. 4A is a schematic view of the arrangement of the plurality of Faraday detectors and CDD detectors in a plane within the detector chamber of a mass spectrometer; Fig. 4B is a plan view of the arrangement in the detector chamber of the mass spectrometer of the Fig. 4A is the Faraday detectors and CDD detectors; Fig. Figure 5A is a schematic view of the configuration of the lead Faraday detector with respect to the ions of the first species (A); Fig. Figure 5B is a schematic view of the configuration of the master Faraday detector with respect to the ions of the second and third species (B and C, respectively); Fig. Figure 6A is a graph of the lead signal at the Faraday detector with insets showing the position of the ion peak of the first species (A) relative to the aperture; Fig. Figure 6B is a graph of the master signal at the master Faraday detector with insets showing the position of the ion peaks of the second and third species (B and C) relative to the aperture; Fig. 7 is a graph showing an example of the lead signal and master signal measured by the method; Fig. Figure 8 is a flowchart illustrating the method for obtaining a mass spectrum of sample ions; Fig. Figure 9 is a graph showing segments of the mass spectrum in which a higher density of data points is measured; and Fig. Figure 10 is a flowchart illustrating the method for determining the ratio of ion species in a sample using a multicollector mass spectrometer.

[0106] Where applicable, like reference numerals refer to like elements throughout the figures. The figures are not to scale. Detailed description of specific embodiments of the invention

[0107] With regard to Fig. Figure 3 shows a schematic representation of a dual-focusing mass spectrometer 300. The ions are generated at the ion source 314, which is powered by a power supply 310 connected via terminals 311, 312. The ions are accelerated and focused at an entrance slit and then pass through an electrostatic analyzer (ESA) 316, which helps focus the ion beam and select ions with the required energy. The ions next enter a focusing quadrupole 318 to further focus the ion beam. Upon exiting the focusing quadrupole, the ion beam passes through an exit aperture defined in a mask 320 and then through a magnetic field applied to the electromagnetic sector 322.

[0108] The exit aperture on mask 320 has different possible widths that determine the resolution of the ion beam. Since the aperture only allows a portion of the focused ion beam to pass through, selecting an aperture with a larger area or wider gap allows a larger portion of the ion beam (in other words, a larger number of ions) to pass into the magnetic field, thus enabling a more sensitive measurement. However, a smaller area or narrower aperture may be useful for reducing ion optical aberrations and thus providing improved resolution for the measurement. Using a higher resolution aperture may be preferable for implementing the method described here, as this results in ions that have a narrower mass peak with a relatively smaller overlap. Therefore, the high resolution is useful for resolving ion species that have mass peaks subject to mass interference.

[0109] Within the magnetic mass analyzer at the electromagnetic sector 322, the applied magnetic field causes a change in the direction or deflection of the ions. Ions with a larger mass are deflected less than ions with a smaller mass, resulting in a spatial separation of the ions according to their mass-to-charge ratios. The separated ions exit the magnetic mass analyzer 322 and enter the detector chamber 326. A plurality of detectors 330, including Faraday detectors and conventional differential detectors (CCD), are arranged in the detector chamber (see also Fig. 4B). The detectors 330 are arranged in a plane to collect each species of spatially separated ions simultaneously. The operation of the mass spectrometer and the acquisition of data can be controlled by a computer 328, which includes a control module and analysis module.

[0110] A Faraday detector (also known as a Faraday cup or Faraday collector) is a conductive pan designed to collect ions within its chamber or cavity. As the charged particles are collected in the Faraday detector, the buildup of charges on the walls of the conductive pan causes a current whose magnitude is proportional to the number or intensity of the collected ions.

[0111] Fig. Figure 4A shows the arrangement of Faraday detectors 410, 412, 414 and CDD detectors 416 in the detector chamber 326. A number of movable Faraday detectors 412, 414 (labeled L1 through L4 and H1 through H4) are arranged on either side of a fixed, central Faraday detector 410 (labeled C). Three CDD detectors 416 are also mounted on one side of the central Faraday detector 410. Each Faraday detector includes an aperture 420 defined by a mask 422. The masks are arranged along the entrance to the Faraday detector so that ions must pass through the aperture 420 to enter the Faraday detector. Across the range of Faraday detectors 410, 412, and 414, the apertures have a range of slit widths. Slit L3 has the widest slit width, e.g., 1.8 millimeters in one particular example.Compared to slit L3, the other detectors can have a relative width of, for example, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, and / or 0.1 of the width of L3. This would result in a slit 0.7 of the width of L3 being approximately 1.3 millimeters wide, and a slit 0.3 of the width of L3 being approximately 0.5 millimeters wide.

[0112] Fig. Figure 4B shows the Faraday detectors arranged in a plane 418 within the detector chamber 326, viewed from above. The position of the movable Faraday detectors 412, 414 can be adjusted within the plane to align them with each of the incident separate ion species (or incident ion beams). Adjusting the position of the movable Faraday detectors allows the selection of a specific Faraday detector with a specific aperture width for a given ion species. In some cases, the central Faraday detector (C) 410 is fixed. Therefore, the direction (or deflection) of the ion beams can be adjusted so that ions of one species of interest are aligned with the central Faraday detector (C) 410, and the movable Faraday detectors 412, 414 can be mounted around the central Faraday detector 410 to align with the ions of the other species in the sample.

[0113] An ion deflection unit 424 is mounted downstream of the mass analyzer and in front of the aperture at the entrance to a detector (in this case, detector H2). The ion deflection unit 424 can be used to apply a deflection to a particular ion species aligned to a specific aperture and detector, without applying the same deflection to all sample ions.

[0114] A sample to be analyzed can contain a number of different ion species. As explained above, ion species of different masses are separated in the mass analyzer according to their mass-to-charge ratio (m / z). In cases where the nominal mass of the different ion species is not the same, the ion beams are separated to be relatively well resolved. However, in cases where ions with the same nominal mass are present in the sample, the ion beams cannot be well resolved (so their mass peaks overlap). If the ion beams are not well resolved, mass interference is observed.

[0115] As discussed above, different ions of nominally the same mass have slightly different true masses due to the mass defect. In this case, ion species subject to mass interference can only be resolved by obtaining a very high-precision mass spectrum. This high-precision mass spectrum can be obtained according to the method and apparatus described below with reference to Fig. 5 to 9. Furthermore, the following are described with reference to Fig. 10 describes a method and apparatus for estimating isotope ratios in the presence of mass interference. Obtaining a normalized mass spectrum

[0116] Fig. 5A and Fig. 5B represent Faraday detectors. Fig. Figure 5A shows a lead Faraday detector 500 with a lead mask 512 at the entrance to the detector's cup section 510. A lead aperture 514 is formed in the mask 512 and has a first width d1. Fig. 5B shows a master Faraday detector 505 with a master mask 518 extending across the entrance to the cup portion 516 of the Faraday detector. The mask includes a master aperture 520 having a second width d2. The first width d1 of the lead aperture 514 of the lead Faraday detector 500 is greater (or wider) than the second width d2 of the master aperture 520 of the master Faraday detector 505 (in other words, d 1> >d2). In this example, the master Faraday detector 500 corresponds to the one in Fig. 4A and Fig. 4B, and the lead Faraday detector 505 is selected from among the movable Faraday detectors 412, 414 that have a wider aperture. Thus, the lead Faraday detector 500 can be selected to provide a desired lead aperture width. For example only, the lead Faraday detector 505 can be detector L3 in Fig. 4A, which has the widest entrance slit to the detector.

[0117] As in Fig. As shown in Figure 5A, the lead Faraday detector 500 is oriented to receive or collect an ion species (represented at a first position by an ion beam 522, shown as a solid line). This is a first ion species (which could be referred to as ions of species A) that has a different nominal mass than any other ion species present in the sample being analyzed. Consequently, the ions of the first species are well resolved in the mass spectrum from the other ions present in the sample, and the ion beam is separated from the ion beam of any other species. Consequently, only a single ion species impinges on the lead Faraday detector.

[0118] As in Fig. 5B, the master Faraday detector 505 is arranged to collect a second and third ion species (where the second species is represented by an ion beam 526 (solid line) at a first position, and the third ion species is represented by a second ion beam 530 (solid line) at the first position). The second and third species may be referred to as species B ions and species C ions, respectively. The second and third ion species have the same nominal mass and are separated in the mass spectrum only by the mass defect. Thus, the second and third ion species represent a mass-interfered ion beam. In one particular example, the second ion species is an atomic ion species, and the third ion species is a polyatomic or molecular ion species. In an alternative example, the second and third ions are each molecular species.In further examples, a fourth or further ion species may also cause mass interference with the second and third ion species.

[0119] At a first time, the lead Faraday detector 500 is arranged so that the ions of a first species enter the detector section 510 of the Faraday detector 500 through an aperture 514 of the mask 512. As in Fig. 5A, the ion beam 522 of the first ion species is ideally aligned at a first time to a first position within a first edge of the lead aperture 514.

[0120] At the same first time, the master Faraday detector 505 is positioned such that the ions of the second and third species are prevented by the master mask 518 from entering the cup portion 516 of the Faraday detector 505. The ions of the second and third species are incident on the master Faraday detector 505, but intersect with the Faraday detector 505 at the mask 518 outside and adjacent to a first edge of the master aperture 520. The arrangement is such that the ion beams of the second and third ion species do not enter the cup portion 516 of the master Faraday detector 505 at the first time and at the first position.

[0121] The ion beams of the first, second, and third species are simultaneously scanned or deflected across the surface of the mask 512, 518 of the master Faraday detector 510, or lead Faraday detector 505, respectively. In this example, the movement of the ion beams is the result of a change in the magnetic field applied to the magnetic mass analyzer 322. The gradual buildup or scanning of the magnetic field leads to a gradual change in the direction or deflection of the ion beams in the mass analyzer 322. Consequently, the position of each ion beam at the mask 512, 518 of each Faraday detector 510, 505 moves relative to the aperture 514, 520 of each Faraday detector 500, 505. An example of the direction of the displacement of the ion beams during the scan is indicated by the dotted arrows in the Fig. 5A and Fig. 5B.

[0122] As a result of the scan or the change in deflection, the ion beams of the first, second, and third species shift. At the maximum degree of change in the deflection of the ion beams, the shift of the ion beams at the corresponding masks 512, 518 of the lead Faraday detector 500 and master Faraday detector 505 reaches a maximum. At the lead Faraday detector 500, the ion beam of the first ion species is shifted by a distance D1 until it reaches a second position. The second position of the ion beam of the first species is aligned adjacent to the second, opposite inner edge of the lead aperture (the ion beam in the second position 524 is shown as a cross-hatched line in Fig. 5A). In other words, when the ions of the first species are in the second position 524, they can still enter the cup section 510 of the lead Faraday detector through the lead aperture 514. Therefore, during the period in which the deflection or scan of the first ion species is taking place, the full ion beam enters the cup section 510 of the lead Faraday detector 500.

[0123] At the master Faraday detector 505, the second and third ion beams are moved across the surface of the mask 518 and across the aperture 520 during the period of the scan or change in deflection. The ions of the second and third species are shifted by a distance D2 until they reach a second position (the second ion species in the second position is shown as the cross-hatched ion beam 528 in Fig. 5B, and the third ion species in the second position is shown as a hollow ion beam 532 in Fig. 5B). In the second position, both the second 528 and third 532 ion beams are blocked by the mask 518 and placed adjacent to the opposite outer edge of the master aperture 520. Thus, during the scanning period of the second and third ion beams, the ions of the second species can first enter the cup section 516 of the master Faraday detector 505 through the master aperture 520 on the mask 518, while the ions of the third species are still blocked by the mask. Subsequently, both the ions of the second and third species can enter the cup section 516 of the master Faraday detector 505, and then only the ions of the third species can enter the cup section 516 of the master Faraday detector 505. Finally, in the second position of the ion beams, at maximum displacement D2, both ion beams are again blocked by the mask 518.

[0124] During the entire scan period, the signal is measured at the lead Faraday detector 500 and the master Faraday detector 505. Fig. 6A and Fig. 6B shows the signals received by the lead Faraday detector and the master Faraday detector, respectively. Fig. Figure 6A is a graph of the lead signal, representing the ion current (or intensity) measured at the cup section 510 of the lead Faraday detector 500 during the scan or change in deflection on the mass analyzer. The x-axis shows the mass deviation, which is proportional to the change in deflection of the first species ion as the field in the mass analyzer is changed or established. Fig. Figure 6B is a graph of the master signal representing the ion current (or intensity) measured at the cup portion 516 of the master Faraday detector 505 during the scan or change of deflection. In Fig. 6A and Fig. 6B shows the signal for a deflection of the ion beams that extends beyond the first and second positions discussed above, although the first and second positions are labeled on the x-axis. The insets in Fig. 6A and Fig. 6B shows a representation of the position of the mass peak or ion beams in the mask aperture at the lead and master Faraday detectors.

[0125] In relation to Fig. 6A, the mass peak of the first ion species is located in the first position discussed above (shown by inset b) on the left side, but within the lead aperture. In the center of the scan, the mass peak for the first ion species is located in the center of the lead aperture (in Fig. 6a (marked as c). In the second position of the ion beam (marked as d), the mass peak of the first species lies on the right side, but within the aperture of the lead Faraday detector. During the entire shift of the ion beam from the first position to the second position, the full mass peak lies within the aperture of the lead Faraday detector. Consequently, the ion current between the first and second positions is relatively stable. The lead signal exhibits a flat plateau at the maximum intensity throughout the entire period of the scan or deflection of the first ion beam. Areas a) and e) of Fig. 6A shows the lead signal when the shift of the ion beams is increased to such an extent that the ions of the first species can no longer enter the lead Faraday detector and are blocked by the lead mask. Here, the ion intensity drops to zero.

[0126] When considering Fig. 6B, in region a), the ion beams of both the second and third species are aligned outside the aperture of the mask at the master Faraday detector. As the ion scan progresses, the mass peak for the second ion species moves into the aperture, but the mass peak for the third ion species remains blocked by the mask (marked as region b). In the middle of the scan, or change in deflection (marked as region c), the mass peak of both the second and third ion species passes through the master aperture and is recorded in the master Faraday detector. With a further shift of the ion beams, the mass peak of the second ion species moves out of the master aperture and is blocked by the master mask, although the ion beam for the third ion species remains in the aperture, allowing it to enter the master Faraday detector (marked as region d).Finally, once the ion beams are at the highest degree of scan or deflection and have reached the maximum displacement, both the mass peaks of the second and third ion beams have moved outside the aperture and are blocked by the master mask (marked as region e).

[0127] The Fig. The ion intensity graph shown in Figure 6B represents the mass spectrum of the second and third ion beams measured at the master Faraday detector.

[0128] A plateau in the ion intensity in region c) represents the maximum intensity when both ion beams can enter the cup section of the Faraday detector. Plateaus of lower ion intensity in regions b) and d) of Fig. 6B can be observed, where only one – the second or third ion beam – passes the aperture to be collected by the master Faraday detector.

[0129] The lead signal represents the maximum ion intensity of the ions of the first species throughout the entire ion beam scan. In an ideal system, this would be constant. However, in real experimental data, small changes are observed, resulting from fluctuations and instability of the sample ion beam intensity over time. As a result of the measurement described above, these background fluctuations can be removed from the master signal by normalizing to the lead signal. The result is a more precise, normalized mass spectrum for the second and third ions subject to mass interference. The plateau features in the normalized mass spectrum may be more pronounced, allowing the observation of a narrower plateau related to a very small mass defect between the ion species.Such a precise mass spectrum is extremely advantageous for further analysis and for estimating the isotopic ratios of the ions present in the sample.

[0130] Fig. Figure 7 shows a specific example of a measured lead signal 700 (dotted line) and master signal 710 (solid line) before normalization. The lead signal 700 has been shifted along the mass x-axis to allow the master and lead signals to be aligned for comparison. However, since the master signal 710 and lead signal 700 were acquired simultaneously, no shift has occurred along the time x-axis.

[0131] In Fig. 7 was the first ion species methane measured at the Lead Faraday detector (Signal 700) 12 CH4 +, which has a molecular mass of 16, representing the main ion beam. Three ion species were recorded in the master Faraday detector. The mass-interfering ions (signal 710) measured at the master Faraday detector were 13 CH4 + , 12 CH3D + and 12 CH5 + , each of which has a nominal molecular mass of 17. It can be seen that there are three plateaus in the mass spectrum of the master signal 710, indicating the three ions subject to mass interference.

[0132] It can be seen that small fluctuations are observed in the plateau of both the lead signal 700 and the master signal 710. These fluctuations, which occur simultaneously in both signals, are artifacts of the experimental instability affecting all sample ions (e.g., fluctuations in beam intensity). Normalizing the master signal 710 can be used to effectively eliminate these fluctuations in the resulting mass spectrum for the ions of the second and third species.

[0133] The normalization of the master signal 710 according to the lead signal 700 can be performed by dividing the master signal at a given time by the lead signal acquired at the same time. This type of point-wise normalization is particularly effective for eliminating signal fluctuations when only one track is acquired. As shown in Fig. As shown in Figure 7, each data point at a given time of the master signal Mi through the lead signal data point L i which is simultaneously acquired by the lead detector at the same time. Therefore, each individual data point P i of the normalized mass spectrum of the ions of the second and third species can be calculated as Pi=Wed / Li

[0134] In another example, the master signal can be normalized relative to a standard point in the lead trace. In this case, each data point P i of the normalized mass spectrum of the ions of the second and third species can be calculated as PI=Mi(L0 / Li) Obtaining an averaged mass spectrum

[0135] Further improvements can be made to the precision of the normalized mass spectrum by using additional analytical methods. Fig. Figure 8 describes a method that utilizes the superposition of the measured and normalized mass spectra of the ions subject to mass interference to yield an averaged normalized mass spectrum. A first normalized mass spectrum M1 is obtained using the method described above (step 802). By repeating the method, a second normalized mass spectrum M2 is obtained (step 804). An average mass spectrum M A is subsequently obtained by For example, the weighted mean of the ion intensity of the first and second normalized mass spectra can be calculated at each value of the mass deviation (for example due to the ion beam shift).

[0136] A third normalized mass spectrum M3 can be obtained by repeating the procedure described above (step 808). If the previously calculated mean mass spectrum is a previous mean mass spectrum, ie, M A = M FA (Step 810), a new mean mass spectrum M A be determined by taking the mean value of the previous mass spectrum M FA and the third normalized mass spectrum M3 is calculated (step 812).

[0137] The procedure can be repeated several times. For example, an Nth mass spectrum M N obtained according to the method described above (step 814). The Nth mean mass spectrum is then obtained by calculating the mean of the Nth mass spectrum and the mean mass spectrum obtained after the N-1st repetition of the method (step 818). In other words, the mean mass spectrum M Athe mean value of each new mass spectrum M N and the previous mean mass spectrum M FA .

[0138] A particular advantage of this method is that small drifts in peak position or small fluctuations in the mass spectrum (for example, due to instability in the ion beam resulting from a small high-voltage spike) can be effectively excluded. This is because this averaging method is less affected by an anomalous result than, for example, simply taking the mean of all measured mass spectra for a given mass deviation. Furthermore, this method allows the complete exclusion of data points from the data set if they deviate from the calculated mean mass spectrum by more than specified limits. This makes it easier to exclude anomalous data points.

[0139] Alternatively, the mean mass could be simply calculated by taking an average ion intensity for each mass deviation across N mass spectra. In other embodiments, the best-fit mass scans for N mass spectra could be obtained using least-squares regression. Using a segmented mass scan method

[0140] A further improvement in the precision of the measured mass spectrum can be achieved by using a segmented scan method. Ideally, a large number of data points are recorded at the Faraday detectors during the scan or deflection of the ion beams to generate a mass spectrum. This helps to ensure that the shape of the normalized mass spectrum is correctly preserved. However, measuring a large number of data points may require a very slow change in the scan rate or the deflection adjustment rate (for example, the rate of change of the mass analyzer parameters), and thus very long measurement times may be necessary. Long measurement times are not only inconvenient for the researcher; they can also introduce errors into the measured mass spectrum due to the drift of the ion beam intensity or the position of the mass peaks over time.

[0141] A segmented scan method allows for the collection of an increased density of data points in regions of the mass spectrum that are of particular interest and exhibit the greatest change. For example, peak shape changes rapidly in the flanking regions of the mass spectrum. Therefore, it is advantageous to obtain a larger number of data points (per unit mass deviation) in the region of the scan where the peak shape exhibits a flank to obtain an accurate representation of the peak shape. In the regions of the scan that exhibit a plateau, a reduced number of data points can be recorded, as this region of the mass spectrum can be assumed to be approximately flat. Conveniently, this method allows for the measurement of a highly accurate mass spectrum in an efficient manner, and the overall measurement time can be reduced.

[0142] Fig. Figure 9 shows an example mass spectrum. In this particular example, the mass spectrum has a number of plateaus indicative of a mass-interfered spectrum of the type obtained at the master detector according to the method described above with reference to Fig. 5 to 7. The mass spectrum in Fig. 9 is divided into segments or regions. The plateau regions 910 can be assumed to be approximately flat. The flank regions 912 exhibit rapid changes in ion intensity. In the flank regions 912, a larger number (or greater density) of data points per unit mass deviation is desired to provide an accurate picture of the mass spectrum shape.

[0143] To change the density of data points recorded in different segments of the data scan, the rate or frequency at which data points are sampled at the detectors or analysis module can be kept constant, but the rate of scanning or changing the deflection can be varied. For example, in one example, the slew rate of the field at the mass analyzer can be modified. In this case, the magnetic field in the mass analyzer can be built up more slowly in the flanking segments of the mass spectrum than the slew rate in the flat plateau regions of the mass spectrum. This will result in a larger number of data points per unit mass deviation in the flanking segments of the scan. Alternatively, the rate of scanning or adjusting the deflection can be modified in other ways (for example, by changing the rate at which an ion deflection unit applies a change in deflection).

[0144] Alternatively, the frequency or rate of sampling data points at the Faraday detectors (or at an analysis module connected to the Faraday detectors) can be varied. For example, the sampling rate can be increased in the flanking sections of the mass spectrum compared to the flat plateau segments of the mass spectrum. Obtaining accurate isotope ratios of a mass spectrum subject to mass interference

[0145] The above with reference to Fig. The method described in sections 5 to 9 is particularly advantageous for obtaining a precise mass spectrum for a set of interfered ions, especially for eliminating the effects of intensity fluctuations or drifts in the ion beam intensity over time. When used in conjunction with a number of other measurement and analysis methods (particularly peak deconvolution), the described method can provide an accurate estimate of the isotopic ratios present in the sample.

[0146] The fundamental difference between isotope ratio analysis measurements and trace analysis measurements is that the exact mass of the species of interest and their peak shapes are typically known. Therefore, the most important analytical task is to quantify the species, rather than identifying the species by mass and then quantifying it. In this respect, an isotope ratio analysis evaluation can be simpler, and a peak deconvolution strategy can be used to yield more precise mass peaks for the ions subject to mass interference.

[0147] Peak deconvolution can solve the problem of balancing the requirements for high mass resolution and ion beam transmission. Where peak shape is precisely captured as described below (using a master detector and lead detector to measure a non-mass-interfered ion peak shape), the method described here provides a powerful deconvolution algorithm that does not specifically require peak plateaus or complete separation of species in the mass spectrum. The deconvolution concept relies on capturing a non-mass-interfered peak shape with very high precision, as well as a high-precision mass spectrum for the mass-interfered ions.Using the constraints described here, quantitative information can be obtained from the precise peak shapes for the mass-interfering ions determined from the deconvolution of the normalized mass spectrum for the mass-interfering ions. Accordingly, the better the initial peak shape is captured, the better the quantification of the contributing mass-interfering ion species. Instead of fitting plateaus, we now fit overlays of identical peak shapes to identify the intensity of the mass-interfering ions. This method allows measurements at higher transmission and reduced mass resolution while still providing high precision.

[0148] The procedure described here for high-resolution isotope ratio measurements comprises four steps, which are Fig. 10 and are listed below: 1. Determine the mass peak shape of an ion species that is not subject to mass interference; 2. Obtaining a highly precise measurement of the mass spectrum of the ion species subject to mass interference; 3. Determine the peak shape of each mass peak in the mass spectrum subject to mass interference; and 4. Statistical evaluation of each peak in the mass spectrum to allow an assessment of the relative abundance of the ion species subject to mass interference and the isotopic ratios.

[0149] Each of these steps will now be described in more detail. 1. Determine the mass peak shape of an ion species that is not subject to mass interference (step 1012 of FIGURE 10)

[0150] A preliminary measurement of the mass spectrum of a non-interfered ion species can be taken at a detector in the detector chamber. For example, a first non-mass-interfered ion species (in other words, with a mass different from the other isotopes in the sample) can be measured at the lead detector. Simultaneously, a second non-mass-interfered ion species can be measured at the master detector. Following the procedure detailed above, the scan or deflection of the second non-mass-interfered ion species can be adjusted so that the ions are scanned across the master aperture.In other words, the scan of the second ions can be adjusted so that the second ions do not enter the master detector at the beginning of the scan or deflection, then the second ions can enter the master detector through the master aperture, and finally the second ions are again prevented from entering through the master aperture. In this way, a full mass spectrum of the second ions is measured.

[0151] During the second ion scan, the first ions are measured at the lead detector. Ideally, the first ions can enter the lead detector during the entire period of the second ion scan or deflection, providing a lead signal of maximum ion intensity. The lead signal and the master signal can then be normalized. This provides a normalized mass spectrum for the second ions, from which a mass peak shape can be determined for an ion not subject to mass interference. Determining a very precise mass peak shape in this way is important because it captures the peak shape independently of small fluctuations that are always present in ion beams.

[0152] In the example of methane described above, the 12 CH4 + -ion ​​with mass 16 is measured at the master detector to determine the mass peak shape. 12 CH4 +The ion is chosen because no other methane isotopes have the same nominal mass. A peak shape for the interference-free ion species can be resolved from the determined mass spectrum.

[0153] For best results, the mass spectrum of the non-interfered ion species should be measured using the segmented mass scan method discussed above. The averaged mass spectra method described above could also be used. 2. Taking a high-precision measurement of the mass spectrum of the ion species subject to mass interference (step 1014 of FIGURE 10)

[0154] The mass-interfered spectrum is measured according to the method described above. Therefore, the interfered mass spectrum is measured at the master Faraday detector and then normalized against a signal obtained simultaneously at the lead Faraday detector for a non-mass-interfered ion. To improve the precision of the obtained mass spectrum, the segmented mass scan method and the averaged mass spectrum method can also be applied. In this way, a highly precise mass spectrum can be efficiently obtained, with reduced influences from intensity fluctuations and instability of the sample ion beams.

[0155] In the particular example of methane, shown in Fig. 7, the lead Faraday detector is aligned to detect the 12 CH4 +-ions of mass 16, and the master Faraday detector is arranged to detect the methane ions with a nominal mass of 17 ( 13 CH4 + , 12 CH3D + and 12 CH5 + ). Although the ion collected in the lead Faraday detector in this example is the same ion as the one for which a peak shape was measured in step 2, this is not strictly necessary. 3. Determine the peak shape of each mass peak in the interfered mass spectrum (step 1016 of FIGURE 10)

[0156] Here, we assume that the mass peak shape of each mass-interfered ion in the sample is identical to the peak shape measured in step 1 for an ion that is not mass-interfered. The peak shape is a measure of the energy distribution of the ions in the ion beam and their angular momentum. In a double-focusing mass spectrometer (as described above with reference to Fig. 3), all image aberrations caused by the ion energy distribution and ion angular momentum are well controlled, and thus, it can be reasonably assumed that the peak shape is identical for each ion species. A similar assumption can be made for mass spectrometers that have a single focusing magnetic sector or for other mass spectrometer types. However, for these latter types of instruments, the assumption may be less accurate but still valid.

[0157] The peak shape of each mass-interfering ion species in the mass spectrum measured in step 2 can be resolved using deconvolution methods. For example, the mass peaks of each mass-interfering ion can be fitted to the normalized mass spectrum. In this example, the fitting is based on the assumption that the shape of each mass peak for a mass-interfering ion is the same as the shape of the mass peak of the non-mass-interfering ions measured in step 1. Specifically, the mass peaks for the interfering ions are fitted by adjusting the amplitude of each mass peak and / or the mass at which each mass peak occurs. In the particular example of methane, shown in Fig. 7, three mass peaks can be deconvolved from the mass spectrum, one for each of the ions with a nominal mass of 17 ( 13 CH4 + , 12 CH3D+ and 12 CH5 + ). 4. Statistical evaluation of each peak in the mass spectrum to enable an assessment of the isotopic ratios (step 1018 of FIGURE 10)

[0158] The isotopic ratio of the ion species present in the sample can be calculated from the mass peaks obtained for each ion according to the procedure described above. In particular, the relative height of the mass peaks can be used to calculate the relative abundance of the ion species and their isotopic ratios, according to methods known in mass spectrometry data processing.

[0159] In a preferred example, the exact mass of the interfering ion species in the sample is known before starting the measurement (step 1010 of Fig. 10). It is assumed that the exact mass is known from previous measurements, since the ion species in a sample is known (but not their abundance). For a given example, the ion species present and their nominal masses, along with their exact mass, may be known from a previous investigation. For example, in the case of (which is considered as an example with respect to the above Fig. 7 related) methane is known to 12 CH4 + with a mass of 16, 13 CH4 + , 12 CH3D + and 12 CH5 + with a mass of 17, as well as four isotopes ( 13 CH3D + , 13 CH5 + , 12 CH2D2 + and 12 CH4D + ) with a mass of 18 are among the common isotopes.

[0160] Conveniently, where the exact mass is known, only the amplitude of each mass peak is used as a fitting parameter for deconvolution of the normalized mass spectrum, as described in step 3. This makes the fitting of the mass peaks of the non-interfered ions less computationally intensive and more accurate.

[0161] Although in the method described above, the mass peak shape of an ion species not subject to mass interference is determined in a first step, in another example, the mass peak shape of an ion species not subject to mass interference is not measured at all. Instead, the mass peaks subject to mass interference are fitted to the normalized mass spectrum (as in step 3) using the peak shape of another fitting parameter. In this case, the mass peaks for the mass-interfering ions can be fitted by assuming that the peak shapes are identical for each mass-interfering ion. Nevertheless, such deconvolution of the mass peaks of the mass-interfering ions from the normalized mass spectrum is likely less straightforward than in cases where the peak shape is known.

[0162] Numerous combinations, modifications, or alterations of the features of the above embodiments will be apparent to those skilled in the art and are intended to be part of the invention. Any of the features specifically described in connection with one embodiment or example may be used in another embodiment by making appropriate changes.

[0163] For example, although the double focusing mass spectrometer in Fig. 3 with an electrostatic analyzer as the first focusing sector and a magnetic mass analyzer as the second focusing sector, the mass spectrometer could be arranged in a reversed configuration—with the electrostatic and magnetic sectors in reverse order. Furthermore, a mass spectrometer with a single focusing magnetic sector could also be used.

[0164] Although in the embodiments described here, the detectors used to implement the described method are Faraday detectors, those skilled in the art will understand that any type of detector could be used in the detector chamber. The method described here could equally be applied to other types of detectors, for example, electron multipliers or conductivity detectors. Furthermore, a combination of different types of detectors could be used as the lead and master detectors.

[0165] Furthermore, although the lead Faraday detector is described as a movable Faraday detector and the master Faraday detector is described as a fixed, central Faraday detector, any pair of Faraday detectors located within the detector chamber of the mass spectrometer could be used. The Faraday detectors could be selected from the plurality of Faraday detectors to provide a suitable orientation and slit width to provide the scan or deflection of the ion beams as described.

[0166] Although the embodiment of Fig. 5A and Fig. While Figure 5B describes a method for adjusting the deflection of the first, second, and third ion species (or the ions of species A, B, and C) by changing the parameters of the mass analyzer (specifically, by applying a magnetic field to the mass analyzer), the ion scan can be performed in many different ways. In particular, the deflection of the ions in the sample can be changed in several ways. For example, the electric field in an electrostatic sector of a mass analyzer can be applied, resulting in an adjustment of the deflection of all passing sample ions.

[0167] In this way, the ions of the first species (species A) are deflected relative to the lead aperture, and the ions of the second and third species (species B and C) are scanned completely across the width of the master aperture from one side to the other in a manner analogous to the method described above with reference to Fig. 5A and Fig. 5B. In a further alternative, the energy of the sample ions can be increased to cause an adjustment of the deflection of the ions. Consequently, the ions of the first species (species A) can be scanned relative to the lead aperture, and the ions of the second and third species (species B and C) can be scanned relative to the master aperture, as described above for Fig. 5A and Fig. 5B.

[0168] In a particularly useful example, an ion deflection unit (also known as a pre-slit deflection unit or a pre-aperture deflection unit) can be used to adjust the deflection of at least some sample ions. The ion deflection unit provides highly controllable deflection only for those ions that pass through the unit. In one example, an ion deflection unit can be placed in the beam path through the mass spectrometer for the ions of the second and third species (species B and C). The ion deflection unit is located downstream of the mass analyzer, in front of the master aperture at the entrance to the master detector. The ion deflection unit can cause deflection of the ions of the second and third species. This deflection is in addition to any deflection applied to all sample ions passing through the mass analyzer. Accordingly, the total deflection of the ions of the second and third species is adjusted.

[0169] The use of the ion deflection unit allows the ions of the second and third species to be scanned via the master aperture according to the claimed invention. However, the deflection of none of the other sample ions is adjusted by the ion deflection unit. Accordingly, the deflection of the ions of the first species is not adjusted, and the "ion beam" of ions of the first species is not scanned at the lead aperture. Instead, the ions of the first species remain precisely aligned to enter for detection by the lead detector throughout the entire period of deflection of the ions of the second and third species through the lead aperture.

[0170] In some cases, the pre-slit ion deflection unit will be used for ions of the first species, usually for alignment with the lead aperture. However, this pre-slit deflection unit would not necessarily be used to adjust the deflection of the ions of the first species simultaneously with the ions of the second and third species. Such use of ion deflection units allows for a relatively coarse adjustment of the position of the lead and master detectors. The ion beams can then be precisely positioned relative to each detector and aperture using the pre-slit ion deflection units to ensure optimal peak overlap. In another example, the pre-slit deflection unit can be used to deflect ions of the first species in such a way that a deflection applied to all ions simultaneously (for example, by changing the magnetic field) is compensated.Therefore, the pre-slit deflection unit can be used to keep the beam of first species ions in a relatively stable position compared to the lead aperture.

[0171] Advantageously, the use of ion deflection units allows scanning of the interfered ion species while keeping the non-mass-interfered ions stable in position. This approach eliminates any requirement for the lead aperture to be larger than the master aperture, and the slit widths of the lead and master apertures can be completely independent of each other. This approach also eliminates any need to scan the magnetic or electric field on the mass analyzer, or to scan the ion beam energy.

[0172] The use of a pre-slit ion deflection unit in front of the master aperture will provide significant technical advantages. For example, the lead signal is stationary, reducing the need for the lead detector signal to be free of interference. The lead signal is used to normalize the master signal, which requires that the lead signal always reflect the fluctuations within the ion source. The lead signal should not be complicated by peak structure due to mass interference between different ion species. Ideally, the measured lead signal is completely flat and exhibits no structure due to peak overlaps of different ion species at the master detector.This is achieved by maintaining the ion beam position of the non-mass-interfered species in a stable position while the mass-interfered ion species are scanned at the master detector via the master aperture to capture the full structure of the mass-interfered spectrum. By referencing the master signal to the lead signal, normalization is always ensured to eliminate drift effects that would otherwise distort the peak shapes measured at the master detector.

[0173] Scanning the deflection using pre-slit ion deflection units also enables microscans that can be performed quickly and are potentially useful for configuration purposes. Furthermore, microscans using pre-slit ion deflection units can also utilize a segmented scanning approach. In this way, microscans provide a higher density of measurement points where mass interference overlaps on the rising and falling edges of the mass spectrum, while fewer points are recorded in the plateau region of the mass spectrum.

[0174] It should be noted that when using the ion deflection unit to adjust the deflection of the ions of species B and C, the measurement is otherwise carried out according to the method and apparatus described with reference to Fig. 5A and Fig. 5B.

[0175] In another example, the scan of the ions can be achieved by changing the aperture position and, preferably, the detector. For example, the scan of the ions of the second and third species at the master detector can result from the movement of the master aperture and the master detector relative to the ion beam containing ions of the second and third species. In this way, the ion beam can remain stationary, but the aperture can be moved such that ions of the second but not the third species, then both the second and third species, and then the third but not the second species are captured by the master aperture in the master detector. The position and movement of the aperture and detectors can be adjusted for high resolution by the mass spectrometer control unit.

[0176] In yet another example, a combination of ion deflection techniques can be used, and this can also be implemented in combination with movement of the master and / or lead detectors. For example, these ion scanning techniques can be used in conjunction with changing the instrument's magnification using a zoom lens. In this way, the beam striking the lead detector can be "held in place" while the other ion beams are scanned across one or more master detectors.

[0177] Although the text only discusses the analysis of two mass-interfering species at the master detector, many mass-interfering ions can be present in a single sample. For example, there can easily be five or more mass-interfering species in a mass spectrum, complicating the mathematical problem of deconvolving the mass peaks. Nevertheless, the above procedure can still be applied as described. The number of interferences in a peak can easily amount to as many as thirty species in cases of complex samples. The procedure described here is particularly advantageous for such complex mass spectra.

[0178] Furthermore, in some cases, multiple mass-interferenced spectra can be measured simultaneously by defining one or more master detectors. For example, a first mass-interferenced ion beam can be measured at a first master detector, and another, spatially separated mass-interferenced ion beam can be measured at a second master detector. The signal recorded at each master detector can then be normalized with respect to the simultaneously recorded lead signal.

Claims

[1] A method for generating a mass spectrum of sample ions using a multi-collector mass spectrometer, the mass spectrometer comprising a spatially dispersive mass analyzer for directing the sample ions into a detector chamber (326), the method comprising: (a) Generating sample ions of a first ion species A with a mass-to-charge ratio (m / z) A , a second ion species B with a mass-to-charge ratio (m / z) B , and a third ion species C with a mass-to-charge ratio (m / z) C , wherein the ions of species A have a different nominal mass than the ions of species B and C, and further wherein the ions of species B have the same nominal mass as the ions of species C; (b) directing the sample ions of species A, B and C to move through the mass analyzer and towards the detectors (330) in the detector chamber (326), the sample ions being deflected during their movement; (c) scanning the ions of species B and C through a main aperture (520) defined in a main mask (518) of a main detector (505), while the ions of species A pass through a guide aperture (514) defined in a guide mask (512) of a guide detector (500); and (d) generating a guide signal (700) representing the ion intensity received at the guide detector (500) from the ions of species A, and generating a main signal (710) representing the ion intensity received at the main detector (505) while the ions of species B and C are scanned via the main aperture (520); wherein ions of species A are detected by the guide detector (500) during the scanning, i) while ions of species B, but not C, ii) subsequently both species B and species C, and subsequently species C but not B are detected by the main detector (505). [2] The method of claim 1, further comprising: (e) normalizing the main signal (710) from the ions B and the ions C using the guide signal (700) to determine a normalized mass spectrum of the ions B and the ions C. [3] A method according to claim 1 or claim 2, wherein one or both detectors - the guide detector (500) and / or the main detector (505) is / are an ion counting detector. [4] A method according to claim 1 or claim 2, wherein one or both detectors - the guide detector (500) and / or the main detector (505) is / are a Faraday detector. [5] The method of any one of claims 1 to 4, further comprising scanning the ions of species A across at least a portion of the guide aperture of the guide mask (512) of the guide detector (500) while scanning the ion species B and C across the main aperture. [6] Method according to one of claims 1 to 5, wherein the guide opening is wider than the main opening. [7] Method according to one of claims 1 to 5, wherein the guide opening and the main opening have the same width. [8] A method according to any preceding claim, wherein a guide pre-aperture deflection unit mounted downstream of the mass analyzer and in front of the guide aperture deflects the ion species A such that the ion species A completely passes through the guide aperture while the ions of species B and C are scanned via the main aperture. [9] A method according to any preceding claim, wherein scanning the ions of species B and C comprises adjusting the deflection of the ions of species B and C as they move through the mass analyzer and toward the detectors (330) in the detector chamber (326). [10] A method according to any preceding claim, wherein the ion species B and C are scanned across the main aperture by varying a deflection caused by a main pre-aperture deflection unit installed downstream of the mass analyzer and upstream of the main aperture. [11] A method according to claim 10 when dependent on claim 1 or 2, claim 5 or claim 8, wherein a deflection of ion species A is not changed while the deflection of ion species B and C is changed using the main pre-aperture deflection unit. [12] A method according to any preceding claim, wherein the ion species B and C are scanned across the main aperture by establishing a magnetic field at the mass analyzer. [13] A method according to any preceding claim, wherein the ion species B and C are scanned across the main aperture by adjusting an electric field at an electrostatic sector of the mass analyzer. [14] A method according to any preceding claim, wherein the ion species B and C are scanned across the main aperture by changing a deflection by adjusting an energy of the sample ions. [15] A method according to any one of the preceding claims, wherein the ion species B and C are scanned via the main aperture by i) the main opening is moved, or ii) the main aperture and the main detector (505) are moved. [16] A method according to any preceding claim, wherein the ions of species A comprise a first species of an atomic isotope, the ions of species B comprise a second species of an atomic isotope, and the ions of species C comprise a species of a molecular isotope. [17] A method according to any one of claims 1 to 15, wherein the ions of species A comprise a first species of a molecular isotope, the ions of species B comprise a second species of a molecular isotope, and the ions of species C comprise a third species of a molecular isotope. [18] A method according to any preceding claim, wherein the guide detector is movable within the detector chamber (326), and the method further comprises positioning the movable guide detector (500) within the detector chamber (326) to receive the ions of species A. [19] A method according to any preceding claim, wherein the main detector (505) is movable within the detector chamber (326), and the method further comprises positioning the main detector (505) within the detector chamber (326) to receive the species B ions and the species C ions. [20] A method according to any preceding claim, wherein the mass spectrometer comprises a plurality of detectors (330) in the detector chamber (326), each detector comprising a mask defining an aperture, the aperture size not being the same for each of the plurality of detectors, and the method further comprising: Selecting the guide detector (500) and / or the main detector (505) from the plurality of detectors to select the width of said guide and / or main openings, respectively. [21] A method according to claim 2 or any one of claims 3 to 20 when dependent on claim 2, wherein determining a normalized mass spectrum of the B ions and the C ions comprises dividing the main signal (710) at a given time by the guide signal (700) acquired at the same time. [22] A method according to claim 2 or any one of claims 3 to 21 when appended to claim 2, wherein the normalized mass spectrum of the ions of species B and the ions of species C is a first normalized mass spectrum, the method further comprising: Repeating the method of claim 2 or any one of claims 3 to 21 when dependent on claim 2 to produce a second normalized mass spectrum; and Determining a mean normalized mass spectrum from an average of the first and second normalized mass spectra. [23] The method of claim 22, wherein the mean normalized mass spectrum is a previous mean mass spectrum, further comprising: Repeating (804) the method according to claim 2 or any one of claims 3 to 21 when dependent on claim 2 to produce a further normalized mass spectrum; and Determining (806) a mean normalized mass spectrum from an average of the further normalized mass spectrum and the earlier normalized mass spectrum. [24] A method according to any preceding claim, further comprising varying a rate of change of the deflection to scan the ions of species A and / or B and C at a plurality of different scan rates as they are scanned across the respective guide and / or master apertures. [25] Apparatus for generating a mass spectrum from sample ions using a multi-collector mass spectrometer, the mass spectrometer comprising a spatially dispersive mass analyzer, the sample ions being directed to move through the mass analyzer and towards detectors in a detector chamber (326), the sample ions being deflected during their movement, the mass spectrometer comprising a guide detector (500) arranged in the detector chamber (326) for detecting sample ions of a first ion species A having a mass-to-charge ratio (m / z) Aand wherein the mass spectrometer comprises a main detector (505) arranged in the detector chamber (326) to receive sample ions of a second ion species B having a mass-to-charge ratio (m / z)s and a third ion species C having a mass-to-charge ratio (m / z)c, and wherein the ions of species A have a different nominal mass than the ions of species B and C, and further wherein the ions of species B have the same nominal mass as the ions of species C, the device comprising: a control module configured to scan the ions of species B and C through a main aperture (520) defined in a main mask (518) of the main detector (505), while the ions of species A pass through a guide aperture (514) defined in a guide mask (512) of a guide detector (500); and an analysis module configured to receive a guide signal (700) generated by the guide detector (500), the guide signal (700) representing the ion intensity of the species A ions received at the guide detector (500) while the species B and C ions are scanned via the main aperture (520); and to receive a main signal (710) generated by the main detector (505), the main signal (710) representing the ion intensity received at the main detector (505) while the species B and C ions are scanned via the main aperture (520); wherein the control module is configured such that during scanning, ions of species A are detected by the lead detector (500), while ions of species B but not C, then both species B and C, and then species C but not B are detected by the main detector (505). [26] The apparatus of claim 25, wherein the analysis module is further configured to normalize the main signal (710) from the ions B and C using the guide signal (700) to determine a normalized mass spectrum of the ions B and the ions C. [27] Apparatus according to claim 25 or claim 26, wherein one or both of the guide detector (500) and / or the main detector (505) is / are a Faraday detector. [28] The apparatus of any one of claims 25 to 27, wherein the control module is further configured to scan the ions of species A across at least a portion of the guide aperture of the guide mask of the guide detector (500) while scanning the ion species B and C across the main aperture. [29] Device according to one of claims 25 to 28, wherein the guide opening is wider than the main opening. [30] The apparatus of any one of claims 25 to 29, wherein the control module is further configured to vary the deflection of the ion species A at a guide pre-aperture deflection unit mounted downstream of the mass analyzer and upstream of the guide aperture, the control module being configured to deflect the ion species A such that the ion species A completely passes through the guide aperture while the ions of species B and C are scanned via the main aperture. [31] Apparatus according to any one of claims 25 to 30, wherein at all times that at least one of the ion species B and C is collected by the main detector (505), ions of species A pass through the guide aperture and are collected by the guide detector (500). [32] The apparatus of any one of claims 25 to 31, wherein the control module configured to scan ion species B and C via the main aperture comprises the control module being configured to adjust the deflection of ions of species B and C as they move through the mass analyzer and toward the detectors (330) in the detector chamber (326). [33] The apparatus of any one of claims 25 to 32, wherein the control module is configured to scan the ions of species B and C across the main aperture by varying a deflection caused by a main pre-aperture deflection unit located downstream of the mass analyzer and upstream of the main aperture. [34] Apparatus according to claim 33 when dependent on claim 25, 26 or 27, claim 29 or claims 31 to 33, wherein the deflection of ion species A is not changed while the deflection of ion species B and C is changed using the main pre-aperture deflection unit. [35] The apparatus of any one of claims 25 to 34, wherein the control module is configured to scan the ion species B and C across the main aperture by establishing a magnetic field at the mass analyzer. [36] The apparatus of any one of claims 25 to 35, wherein the control module is configured to scan the ions of species B and C across the main aperture by adjusting an electric field on the mass analyzer. [37] The apparatus of any one of claims 25 to 36, wherein the control module is configured to scan the ions of species B and C via the main aperture by adjusting an energy of the sample ions. [38] The apparatus of any one of claims 25 to 37, wherein the control module is configured to scan the ions of species B and C across the main aperture by moving the main aperture and optionally the main detector (505). [39] The apparatus of any one of claims 25 to 38, wherein the guide detector is movable, and the control module is further configured to position the guide detector (500) in the detector chamber (326) for receiving the ions of species A. [40] The apparatus of any one of claims 25 to 39, wherein the main detector is movable, and the control module is further configured to position the main detector (505) in the detector chamber (326) for receiving the species B ions and the species C ions. [41] Apparatus according to any one of claims 25 to 40, wherein the mass spectrometer comprises a plurality of detectors (330) in the detector chamber, each detector (330) comprising a mask defining an aperture, the aperture size not being the same for each of the plurality of detectors, and the control module is further configured to select the guide detector (500) and / or the main detector (505) from the plurality of detectors (330) to select a size of said guide and / or main aperture, respectively. [42] Apparatus according to claim 26 or any one of claims 27 to 41 when dependent on claim 26, wherein the analysis module is configured to determine a normalised mass spectrum of the B ions and the C ions by dividing the main signal at a given time by the guide signal acquired at the same time. [43] Apparatus according to claim 26 or any one of claims 27 to 42 when dependent on claim 26, wherein the analysis module is further configured to: to store the normalised mass spectrum of the ions of species B and the ions of species C as a first normalised mass spectrum; to obtain a second normalized mass spectrum; to determine a mean normalized mass spectrum from an average of the first and second normalized mass spectra. [44] The apparatus of claim 43, wherein the analysis module is further configured to: to save the averaged normalized mass spectrum as a first average normalized mass spectrum: to obtain another normalized mass spectrum; to determine a mean normalized mass spectrum from an average of the first mean normalized mass spectrum and the previous normalized mass spectrum. [45] The apparatus of any one of claims 25 to 44, wherein the control module is further configured to vary the adjustment rate of the deflection to scan the ions of species A and / or B and C at a plurality of different scan rates while being scanned across the respective guide and / or master apertures. [46] A computer program product comprising instructions configured, when executed by a processor, to control a multi-collector mass spectrometer for operation according to the method of any one of claims 1 to 24. [47] A method for determining the abundance of ion species in a sample using a multicollector mass spectrometer, the mass spectrometer comprising a spatially dispersive mass analyzer for directing the sample ions into a detector chamber (326), the sample comprising sample ions of a first ion species A having a mass-to-charge ratio (m / z) A, ions of a second ion species B having a mass-to-charge ratio (m / z)s and ions of a third ion species C having a mass-to-charge ratio (m / z)c, wherein the ions of species A have a different nominal mass than the ions of species B and the ions of species C, and ions of species A do not cause mass interference or do not cause significant mass interference with any other ion species, and further wherein the ions of species B have the same nominal mass as the ions of species C, the method comprising: (a) performing the method according to claim 2 or any one of claims 3 to 24 when dependent on claim 2 to provide a normalized mass spectrum of the ions of species B and the ions of species C; (b) deconvolving the mass peak of each of the ions of species B and species C from the normalized mass spectrum, assuming that the shape of the mass peak of the ions of species B is identical to the shape of the mass peak of the ions of species C; and (c) Determining an abundance of the ions of species A, the ions of species B and / or the ions of species C in the sample from the mass peaks. [48] ​​The method of claim 47, further comprising measuring a full mass spectrum of the ions of species A or the ions of another species that does not cause mass interference or does not cause significant mass interference with any other ion species at a detector (330) disposed in a detector chamber (326) to determine a mass peak having a mass peak shape for the ions of species A or for the ions of the other non-mass-interfering species; wherein the step of deconvolving the mass peak of each of the ions of species B and the ions of species C from the normalized mass spectrum is based on the assumption that the shape of the mass peak of the ions of species B is the same as the shape of the mass peak of the ions of species C and is the same as the shape of the mass peak measured for the ions of species A or the other non-mass-interfering species. [49] The method of claim 48, wherein measuring a full mass spectrum of the ions of species A or ions of another species that does not cause mass interference or does not cause significant mass interference with any other ion species further comprises: Measuring a full mass spectrum of one ion species at a main detector (505) while measuring a full mass spectrum of another ion species at a guide detector (500), wherein the ion species are either ions of species A or ions of another species that does not cause mass interference or does not cause significant mass interference with any other ion species; and Normalizing the signal measured at the main detector (505) to the signal measured at the guide detector (500) to determine a normalized full mass spectrum for the ions of species A or the other species that does not cause mass interference. [50] A method according to any one of claims 47 to 49, wherein deconvolving a mass peak for each of the ions of species B and species C according to (b) further comprises: Fitting the mass peak of ion species B and a mass peak of ion species C to the normalized mass spectrum, wherein the shape of the mass peak of ion species B and the shape of the mass peak of ion species C are the same as the shape of the mass peak of the ions of species A or the other non-mass-interfering species; where the mass peak for ion species B and for ion species C is adjusted by adjusting the amplitude of each mass peak and / or the mass at which each mass peak occurs. [51] A method according to any one of claims 47 to 50, further comprising using the determined abundances to determine one or more abundance ratios, the ratios being determined according to the relative amplitudes of the deconvolved mass peaks of at least two of the ions of species A, the ions of species B and the ions of species C. [52] A method according to any one of claims 47 to 51, wherein the sample ions further comprise ions of one or more further ionic species, the ions of each of the one or more further ionic species having the same nominal mass as the ions of species B and species C; and further comprising deconvolving a mass peak of each of the ions of species B, species C and each of the one or more further ionic species from the normalised mass spectrum, in accordance with the assumption that the shape of the mass peak of the ions of species B, species C and each of the one or more further ionic species is the same. [53] A method according to claim 52 when dependent on claims 49 or 50, wherein deconvolving a mass peak of each of the ions of species B, species C and each of the one or more further ionic species from the normalised mass spectrum is further based on the assumption that the shape of the mass peak of the ions of species B, species C and each of the one or more further ionic species is the same as the shape of the mass peak measured for the ions of species A or for the ions of the other non-mass interfering ionic species.

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