Ion detection device and method for controlling ion detection device

Abstract

A method of controlling an ion detection apparatus is provided. The method comprises: converting the received ions into the emitted electrons; multiplying the emitted electrons; detecting the multiplied electrons; generating a detection signal in response; and determining the ion strength according to the detection signal.

Description

Technical Field

[0001] The present invention relates to ion detection devices, such as secondary electron multiplier (SEM) detection devices. SEM detection devices can be used to detect ions emerging from a mass analyzer of a mass spectrometer. Background Art

[0002] Figure 1 A simple SEM detection device 10 according to the prior art is shown. The SEM detection device 10 consists of a multiplication section 12 and a detection section 13. The ion beam IB enters the multiplication section guided by an (optional) deflector 11. When the ions hit the first dynode or "conversion dynode" 12A of the multiplication section, the conversion dynode emits secondary electrons. The conversion dynode is at a suitable potential so that the charged ions are accelerated toward the conversion dynode 12A. The subsequent electron emission is called secondary emission.

[0003] Electrons emitted from the impact of ions on the conversion dynode are attracted by the electric field in the direction of the next dynode, where the charge causes the emission of secondary electrons. There is a potential difference between the conversion dynode and the next dynode, so that the next dynode emits secondary electrons for each impact of the charge received from the conversion dynode (the second dynode can have a positive potential difference compared to the first dynode, and the third dynode can have a positive potential difference compared to the second dynode, and so on). With multiple subsequent stages, it should be anticipated that each dynode can have a smaller negative potential (more positive potential), where the secondary electrons emitted from the previous dynode are attracted to the next dynode, which can have a more positive potential. Therefore, it is possible that a single ion entering the front end of the SEM detection device can cause more than one million electrons to leave the multiplication section 12, which are directed to the detection section 13 of the detection device 10, where a pulse detection signal P is generated. In this way and by means of charge multiplication by secondary electrons, it is possible to register the input of even very small numbers of ions or even single ions to the SEM detection device.

[0004] For example, for inductively coupled plasma mass spectrometry (ICP-MS), the detection device should be able to operate over a dynamic range of 9 orders of magnitude, with a goal of operating over a dynamic range of 11 orders of magnitude. This makes the detection device suitable for detecting both major and minor components of a sample.

[0005] US 7,220,970 B2 describes an example of a SEM detector and an example of a mode of operation of the SEM detector. The SEM detector can be used in a counting (or pulse) mode and a current (or analog) mode. In ICP-MS, the detector is usually provided with different sections, the first section being used in analog mode and the second section being used in pulse mode (or being switched off), e.g. Figure 1 To record electrons in current (or analog) mode, an analog signal is acquired from the central dynode or one of the multiplication stages within the multiplication section (e.g. Figure 1 ). The counting (or pulse) mode records the electrons that reach the final stage of the SEM detection apparatus (i.e., the detection section or detector 13). High ion currents can be measured using the analog mode, while the counting mode evaluates relatively small ion currents. The SEM can selectively operate in both analog mode and counting mode to provide a wider dynamic measurement range.

[0006] Another option for increasing the dynamic range of an assay is the Thermo Fisher Scientific iCAP TM Q instruments, where the electrons after the first n dynodes are extracted from the multiplication section of the SEM and transferred to a Faraday cup where the current is measured.

[0007] In counting mode, where all multiplied electrons are fed to the detector, it has been observed that the detector degrades over time due to continued exposure to relatively high electron currents. In addition, in the case of higher counting ranges (about 106cps or higher), the counting mode is affected by the saturation effect of the detector. After each peak (a burst of multiplied secondary electrons caused by ions entering the SEM), the detector needs to be turned off for a specific time because after the pulse, the detector may not be suitable for supporting the charge for the next pulse and / or the detector is susceptible to "ringing" that causes double pulses. In addition, depending on the intensity, each pulse may have a different duration (or "pulse width"). Any additional pulses that fall during this time cannot be detected. In order to mitigate these effects, the counting mode may require a "dead time" during which the detector is "turned off" for a specific period of time after the detection of each peak. The pulse width may be different for different pulses. Therefore, the dead time must be at least as long as the longest pulse width considered. During the period when the detector is "turned off", the electronics can be switched to a mode in which counts are not recorded (although power can still be supplied).

[0008] Compared to counting mode, analog mode has a fast reaction time and is therefore sensitive to changes in ion current. However, because the analog signal depends on the lifetime of the dynodes, drift can occur over time. Therefore, analog mode detection inherently lacks stability and accuracy. In addition, the average amplification factor of each dynode can depend on the work function, so residual gas near the detector can cause further drift.

[0009] In the case of using two modes to increase the dynamic range of the detection device, it is necessary to take into account the drift of the analog mode and the degradation effects associated with the counting mode. This is usually done by performing a cross calibration between the counting signal and the analog signal. Due to unknown drift / degradation effects, the cross calibration needs to be performed regularly. In addition, the cross calibration is also component-dependent and requires calibration reagents.

[0010] Another method of increasing the dynamic range of the detection device is to adjust the current at the first dynode by adjusting the dynode voltage, thereby attenuating the high-level signal in a mode called decay mode. However, the required voltage in this mode can change over time, which may lead to poor stability. In addition, the required voltage depends on the mass of the sample ions. If the amplification voltage is increased during this mode, the detector aging problem may worsen (and the device may show a relatively poor steady state). In addition, the advantages of the counting mode (high precision, linearity, low drift) do not exist in the decay mode. Therefore, this mode is not better than the analog mode. In fact, this mode may lack the rapid response of the analog mode (for example, to protect the detector) and does not provide overlap between modes, where measurements are performed simultaneously with different decays. Instead, it is necessary to perform a pre-measurement, measure the intensity, and then determine the decay of the mode. If the pre-measurement is not performed, there is a risk that the detector is damaged due to high signals. The steady curve also depends on the decay voltage. Therefore, providing a decay mode is not a preferred solution to this problem.

[0011] The advantages arising from the counting mode are mitigated by the introduction of effects already seen in the simulation mode. Furthermore, the attenuation must be pre-set prior to analysis. Thus, this approach may suffer from the disadvantages of both modes. Summary of the invention

[0012] The present invention is directed to an ion detection device, such as a SEM detection device, having an increased dynamic range. The present invention is also directed to an ion detection device having an improved lifetime. The present invention is also directed to an ion detection device having the ability to measure ion currents in the analog range, but with count range accuracy and stability. In addition, the present invention is directed to a method of controlling an ion detection device.

[0013] Therefore, a method for controlling an ion detection device is provided. The method comprises:

[0014] Converts received ions into emitted electrons,

[0015] Multiply the emitted electrons,

[0016] Detecting the multiplied electrons,

[0017] generating a detection signal in response, and

[0018] The ion intensity is determined from the detection signal.

[0019] The method comprises controlling the number of multiplied electrons reaching a detector by intermittent gating depending on the ion density.

[0020] According to the present invention, intermittent gating is provided (e.g., by gating secondary electrons within the multiplication section of the SEM detection device and / or gating the ion beam before the ions hit the conversion element). Advantageously, the proposed method and device can achieve an improved dynamic range while addressing the shortcomings of the counting mode of the previous devices described above. The stability and reproducibility of the SEM detection device can also be improved.

[0021] The proposed method provides a detection device that is capable of providing a large dynamic range using only the counting mode of the detection device without the analog (current) mode. As a result, cross-calibration of the analog mode and the counting mode may not be required. Therefore, a detector calibration solution for cross-calibration may not be required, and the time spent on performing detector cross-calibration may be saved.

[0022] By providing intermittent gating, it is possible to switch the ion flow and / or secondary electron flow on or off (or at least significantly attenuate the flow). By implementing a random algorithm, it is possible to reduce the overall exposure of the detector to electrons, which reduces detector degradation.

[0023] Furthermore, by implementing intermittent gating according to the proposed method, the switching time can allow for intermittent "dead time" during which no electrons are received by the detector. In this way, useful readings can be obtained from the detector while maintaining the overall readiness of the detector. As long as the "off" time of the gating is at least as long as the dead time of the detector, the detector is not overloaded with electrons.

[0024] By implementing the present invention, the detection device does not necessarily require an analog section. Therefore, the technical complexity and cost of the detection device can be reduced. In addition, calibration of the analog mode may not be required, which significantly increases the efficiency of the preparation and operation of the instrument (and can avoid the costs associated with calibration, such as the use of calibrants). Because analog electronics and dedicated analog high voltage power supplies and several feedthroughs can be omitted compared to the dual-mode SEM detection device of the prior art, there are also benefits for the entire instrument (i.e., mass spectrometer).

[0025] If the counting portion of the detection device sees too many counts, it will degrade. As the number of counts on the detector decreases, the detector lifetime increases. Therefore, the lifetime of the detector can be improved by the proposed method by applying intermittent gating to the detection device to improve the range and keep the number of detection events small. The proposed measurement technique can also be used for intensities previously used for counting detection, but fewer events are needed to make an accurate measurement.

[0026] In the high count range (e.g., in the range of more than one million counts per second) and in the higher ranges previously measured by analog mode, the proposed method can determine the ion intensity using fewer detector events. The results can be more accurate and reliable than the methods provided by the prior art.

[0027] As mentioned above, the detection device may not require an analog section. This means that the detection device may require fewer components (which may make the detection device less expensive). In addition, this also means that fewer components are required in the entire instrument because the following components may no longer be required:

[0028] Analog electronics,

[0029] Dedicated analog high voltage and

[0030] Multiple leads.

[0031] According to the proposed method, the way of counting ions is changed from pure counting to a "variable chopping technique". The proposed method provides considerable benefits by providing a variable "chopping" time, and also using this "chopping" time for data evaluation, as described above.

[0032] Determining the ion intensity from the detection signal may comprise determining an ion count value.The number of multiplied electrons reaching a detector of the detection device may be controlled by intermittent gating depending on the count value.

[0033] The conversion element may also be referred to as conversion electrode or conversion dynode.A multiplication unit may comprise a plurality of electrodes, which may also be referred to as dynodes.

[0034] A multiplication unit may be used to multiply the emitted electrons downstream of the conversion element.A detector may be downstream of the multiplication unit.

[0035] The method may further include:

[0036] emits electrons upon receiving ions (by a conversion element),

[0037] multiplying the emitted electrons (by the multiplication unit),

[0038] detecting the pulse of multiplied electrons and generating a detection signal in response (by a detector), and

[0039] A count value is determined (by the detector or a processor in communication with the detector) based on the detection signal.

[0040] The number of electron pulses reaching the detector is controlled by gating the electron pulses reaching the detector.

[0041] The emission of the multiplied electrons may be controlled by intermittent gating based on the determined count value.

[0042] The number of ions received may be controlled by intermittent gating based on the determined count value.

[0043] Intermittent gating can occur after the electrons are emitted. In one example, gating can be performed after the conversion element but before the multiplication unit. In another example, gating can occur after the first few dynodes (e.g., after the first dynode, the second dynode, the third dynode, the fourth dynode, or the fifth dynode).

[0044] The emitted electrons can be attenuated by gating.

[0045] The gate can control the propagation of secondary electrons within the multiplication section of the detection device by attenuating the secondary electrons.

[0046] Controlling the number of multiplied electrons reaching the detector by intermittent gating may include intermittently switching a charged gate to attenuate emitted electrons or to divert ions away from the conversion element.

[0047] Controlling the number of multiplied electrons reaching the detector by intermittent gating may comprise intermittently switching ion optics arranged upstream of the conversion element to divert ions from the conversion element.

[0048] Controlling the number of multiplied electrons reaching the detector by intermittent gating may comprise intermittently switching one of a plurality of dynodes within the multiplication cell so as to attenuate all multiplied electrons within the multiplication cell.

[0049] A potential may be applied to one of the dynodes (preferably one of the first few dynodes downstream within the multiplication section) such that all electrons are attenuated by that dynode, which acts as a gate.

[0050] Intermittent gating may occur prior to multiplying the emitted electrons.

[0051] Intermittent gating may occur prior to emission of electrons.

[0052] The gate may be a grid in front of or within the detection device. For example, the grid may be in front of the conversion element, in front of the multiplication section (immediately downstream of the conversion element) or between the dynodes of the multiplication section.

[0053] Ions can be deflected by gating.

[0054] The gate can control the impact of ions on the conversion dynode by attenuating the ion flow directed to the conversion dynode. Alternatively, the gate can control the impact of electrons on the next dynode after the conversion dynode by attenuating the electron flow between the conversion dynode and the next dynode.

[0055] Intermittent gating can be achieved by a gate that provides an "energy barrier" to block, attenuate, or divert electrons or ions.

[0056] Intermittent gating can be performed in a repeating gating cycle that includes an "off" time during which the number of multiplied electrons reaching the detector is attenuated (or during the "off" time, the emission of the multiplied electrons is stopped), and an "on" time during which the number of multiplied electrons reaching the detector is not attenuated (or during the "on" time, the emission of the multiplied electrons occurs).

[0057] The duration of the "on" time may be controlled depending on the count value during intermittent gating. The duration of the "off" time may be controlled depending on the count value during intermittent gating and / or may be based on the dead time required by the detector.

[0058] The gate may be operated in a cycle, wherein the gate switches between an "on" time, during which the gate allows ions or secondary electrons to pass, and an "off" time, during which the gate attenuates the ions or secondary electrons.

[0059] The multiplied electrons can be detected as pulses of multiplied electrons.The pulses of multiplied electrons can be detected by means of a detector of an ion detection device (also called a "counting detector").

[0060] The "off" time of each gating cycle can be greater than or equal to the detector dead time.

[0061] If the count value of the previous gating cycle was zero, the "on" time of the gating cycle can be increased.

[0062] The "on" time may be set to a duration such that the probability of a count value being greater than zero during the "on" time of at least one gating cycle is between 0.2 and 0.8, preferably between 0.4 and 0.6.

[0063] If the count value of one or more of the previous gating cycles was zero, the "on" time may be increased from the preset duration.

[0064] If the count value of one or more of the previous gating cycles is greater than zero, the "on" time may be kept constant for at least one gating cycle.

[0065] If the probability that the count value is greater than zero is higher than 0.5 (alternatively, higher than 0.6, 0.7, 0.8 or 0.9), the "on" time may be reduced after a predetermined number of gating cycles.

[0066] If the probability of the count value being zero is lower than 0.5 (alternatively, lower than 0.4, 0.3, 0.2 or 0.1), the "on" time may be reduced after a predetermined number of gating cycles.

[0067] If the probability that the count value is greater than zero is lower than 0.5 (alternatively, lower than 0.4, 0.3, 0.2 or 0.1), the "on" time may be increased after a predetermined number of gating cycles.

[0068] If the probability of the count value being zero is higher than 0.5 (alternatively, higher than 0.6, 0.7, 0.8 or 0.9), the "on" time may be increased after a predetermined number of gating cycles.

[0069] The "on" time of the initial gating cycle may be extended until a count value is determined based on a detection signal (also referred to as a "count signal" or a "voltage signal").

[0070] The detection device can remain in an "on" mode (where the gate allows ions or secondary electrons to pass) until a count is detected, and then switch to an "off" mode (where the gate attenuates ions or secondary electrons). The duration of the "on" mode until a count is detected can be considered the initial value of the "on" time of the initial gating cycle.

[0071] The "on" time may be varied between an upper threshold and a lower threshold for a plurality of gating cycles, wherein the probability of a count value being greater than zero is determined based on the determined ion intensities determined during the plurality of cycles.

[0072] The present invention also provides an ion detection device, which includes:

[0073] - a conversion element for converting the received ions into emitted electrons,

[0074] - a multiplication unit for multiplying the emitted electrons,

[0075] - a detector for detecting the multiplied electrons, generating a detection signal in response to detecting the multiplied electrons, and determining a count value based on the detection signal.

[0076] The ion detection apparatus further comprises a controller configured to control the number of multiplied electrons reaching the detector by intermittent switching of the gate in dependence on the ion intensity determined by the detector.

[0077] The multiplication unit may be downstream of the conversion element.

[0078] The detector may be downstream of the multiplication unit.

[0079] Intermittent switching of the gate can be used to control the number of multiplied electrons reaching the detector from the multiplication unit.

[0080] The gate may be located upstream of the conversion element.

[0081] The gate may be located downstream of the conversion element.

[0082] The grid may be a grid that can be charged. The grid may be configured to switch intermittently to attenuate and / or divert charged particles. Specifically, the grid may be configured to switch intermittently to attenuate emitted electrons. Alternatively, the grid may be configured to switch intermittently to divert ions from the conversion element (and / or attenuate ions that reach the conversion element).

[0083] The gate may be an ion optical element arranged upstream of the conversion element.The ion optical element may be arranged to be switched intermittently so as to divert ions away from the conversion element.

[0084] The multiplication unit may be a secondary electron multiplier. One of the plurality of dynodes within the multiplication section may be arranged to be switched intermittently so as to cause all decay of the multiplied electrons within the multiplication section.

[0085] The controller may be configured to perform the method of controlling the ion detection apparatus as described above.

[0086] The present invention also provides a mass spectrometer, which comprises the ion detection device as described above.

[0087] The detection device may include a multiplication section having a plurality of dynodes and a detection section having an electron detector. Each of the plurality of dynodes may be configured to emit electrons if ions and / or electrons hit the dynode. Each dynode may have a less negative potential than the preceding dynode. The detector may include a grid arranged upstream or within the multiplication section, wherein the grid is configured to intermittently attenuate ions or secondary electrons of the ion flow.

[0088] The detection may be a SEM detection device. The gate may be upstream of the conversion dynode of the detection (upstream of the first dynode of the detection). One of the dynodes (preferably one of the first dynodes downstream in the multiplication section) may act as a gate. A grid in front of the detection device or between the dynodes may act as a gate. The grid may include a two-dimensional grid and / or parallel lines. Gating may be provided by an alternating voltage on adjacent wires.

[0089] The mass spectrometer may comprise a detection device. A mass filter (preferably a quadrupole) upstream of the detection device may be used as a grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] Figure 1 A SEM detection device according to the prior art is shown.

[0091] Figure 2 A SEM detection arrangement according to the prior art is shown, wherein a gate is provided at the beginning of a counting or pulse section after the analog part.

[0092] Figure 3 A SEM inspection arrangement according to the prior art is schematically shown.

[0093] Figure 4 An embodiment of a SEM detection arrangement according to the invention is schematically shown.

[0094] Figure 5 Another embodiment of a SEM detection arrangement according to the invention is schematically shown.

[0095] Figure 6 Another embodiment of a SEM detection arrangement according to the invention is schematically shown.

[0096] Figure 7 Another embodiment of a SEM detection arrangement according to the invention is schematically shown.

[0097] Figure 8 Gating signals that can be used in the present invention are shown.

[0098] Fig. 9 A portion of a gating signal that can be used in the present invention is schematically shown. DETAILED DESCRIPTION

[0099] In mass spectrometry, a secondary electron multiplier can be used to help detect ions that have been separated by a mass analyzer. A secondary electron multiplier includes a plurality of electrodes, referred to as dynodes. Each dynode multiplies the incident charge via a process referred to as secondary emission, in which a single charged particle (e.g., an electron) can cause the emission of more electrons (e.g., between 1 and 10 electrons) when incident on a secondary emission material. An electric potential is applied between each electrode and the next electrode in the SEM. The electrons emitted by the first electrode will be accelerated to the next electrode and cause secondary emission of more electrons from the electrode. This can be repeated multiple times (e.g., using 8 to 14 secondary electrode dynodes), resulting in a large amount of multiplied electrons being emitted from the last electrode. The multiplied electrons can be detected as current, or pulses of the multiplied electrons can be capacitively or inductively coupled and counted by a counting electronic circuit.

[0100] Typically, the analog mode (where current is measured) is used in low amplification mode. Conversely, high gain is used in counting mode. By providing dual modes, a linear dynamic range of up to nine orders of magnitude can be achieved, making it possible to measure both major and minor components of a sample in a single run.

[0101] In some dual mode detectors, a gate is provided at the beginning of the counting section after the analog section. Figure 2 The gate is after the analog section and can shut off the electron beam entering the counting section. Shutting off the electron beam entering the counting section can help increase the life of the pulse detector 13.

[0102] Figure 2 A schematic arrangement of a SEM detector device 10 used in some prior art mass spectrometers is shown. The ion detection device 10 comprises a multiplication section 12 consisting of a plurality of dynodes 12A-12L and a detection section 13. Figure 2A schematic diagram of the gating principle is also shown. The ion beam IB is directed toward the conversion dynode 12A of the multiplication section 12. The deflector 11 can be used to guide the ion beam (although this is optional). When the ion beam IB hits the conversion dynode 12A, the conversion dynode emits secondary electrons. The conversion dynode 12A of the detector can be set to a medium voltage, such as 2kV for a typical ICP instrument, or higher for other instruments. The electrons emitted from the conversion dynode are attracted by the electric field to the direction of the next dynode, where they cause the multiplication emission of secondary electrons. There is a potential difference between adjacent dynode electrodes, so that each dynode emits electrons in response to the impact of the charge received from the previous dynode (or in the case of the conversion dynode, from the ion beam). In the counting mode, the electrons emitted by the last dynode 12L are directed to the detection section or detector 13 of the detection device 10, where a detection signal output P is generated. Therefore, the counting mode records the electrons that reach the last stage of the multiplication section. In analog mode, analog signal "A" is measured from the center dynode 12G within the multiplication section. Relatively high ion currents can be measured using analog mode, while relatively small ion currents can be measured using counting mode. The combination of these modes is intended to provide a wider dynamic measurement range.

[0103] Figure 3 A SEM detector arrangement is schematically shown, which may include Figure 2 The SEM detection device 10 is shown. Figure 3 The SEM detector device 1 includes a conversion dynode 12A, an electron multiplier (multiplication section) 12, a pulse detector (or counter) 13, a current detector 14, and a processor 15. The multiplication section 12 includes a gate 16 arranged between an analog section and a pulse section. The analog section can provide electrons to the current detector 14, and the pulse section can provide pulses to the pulse detector 13. The processor 15 provides a gating signal to the gate 16, which can be based on the current I detected by the current detector or the pulse P counted by the pulse detector 13. When pulse counting is not expected, the gating signal can turn off the pulse / counting part for a period of time.

[0104] The need for an analog detection mode is a source of problems in the prior art. Removing analog detection (at least for accurate measurements) would result in an improved system. However, prior art devices rely on an analog mode in combination with a counting mode to increase the dynamic range of the detector to the required level. In order to switch between modes, a gating function can be provided in a dual mode detector. Figure 2 In the detection device of the present invention, the gate signal "G" is applied to the dynode 12I of the multiplication section 12 (downstream of the dynode 12G from which the analog signal "A" is measured). Figure 2 The dynodes 121 in the analog section are arranged after the analog section and are used to shut off the electron beam entering the counting (or pulse) section. This is done by changing the voltage of at least one dynode between a first voltage, in which the electrons pass through the dynode, and a second voltage, in which the electrons do not pass through the dynode. Thus, the gating signal blocks or attenuates the electrons traveling down the multiplication section to prevent large electron pulses from reaching the detector 13.

[0105] As described above, in prior art devices a gating function is used to switch between analog mode and counting mode. In contrast, the present invention proposes applying a voltage oscillation to the gate based on the count value determined by the detector. The voltage oscillation (which may include a voltage and / or a substantially sinusoidal oscillation that repeatedly switches between two states) will have an "on" time (during which electrons pass through the gate normally) and an "off" time (during which electrons are attenuated or blocked). Preferably, the voltage oscillation is a rectangular voltage oscillation, such as Figure 8 However, the change in voltage may not be instantaneous, and the sides of the wave may exhibit a slight slope. Figure 8 In the example shown, the pulses each have a defined “on” time (OT). Figure 8 In the example shown, the average voltage V A is correct.

[0106] During the "on" time, the detector may detect the number of pulses (eg, zero, one, or more).

[0107] The number of pulses received at the detector is approximately Poisson distributed. The Poisson distribution represents the probability of the number of pulses (zero, one, two, etc.) occurring during the "on" time. The pulse events occur at an approximately constant average rate, and also approximately independently of the time since the last pulse.

[0108] The detector may count pulses during the "on" time, but may not count pulses during the "off" time (because the gating prevents electrons from reaching the detector or sufficiently attenuates the number).

[0109] The detector can also be configured to adjust the length of the "on" time so that a pulse event occurs at the detector only during a portion of the "on" time. These pulse events are also called "counts" or simply "events". The probability of no event occurring follows Poisson statistics:

[0110] P∶=exp(-I·t 开启 ) (1)

[0112] where I is the intensity in counts per second and where t 开启It’s “on” time.

[0113] This can be rewritten as

[0114]

[0115] Therefore, for a given duration t 开启 For each "on" time (also called an "on" period), we expect a certain probability of there being zero counts during that period and a complementary probability of there being at least one count during that period (which is one minus the probability of there being zero counts). In other words, each "on" period can be classified by one of the following complementary events:

[0116] Zero pulses are detected; and

[0117] One or more pulses detected.

[0118] From the detection results, it is estimated that the “on” time t 开启 The probability that there are zero pulses during this period, and the intensity of the ion beam can be calculated using Equation 2.

[0119] One way the probability can be estimated is by repeating an "on" time of fixed duration over a number of cycles and counting the proportion of cycles during which zero pulses are detected. This proportion is an estimate of the probability and will be more accurate with more cycles.

[0120] With a probability of about 0.5 it is favorable, so the “on” time t 开启 It is preferably set to a value such that approximately half of the "on" period passes without any counts being detected during the "on" period.

[0121] One reason that the probability is preferably about 0.5 is that if the probability is determined to be about 0.5, it is likely to be more accurate with fewer experiments (number of "on" times). If the probability is close to 0, the number of cycles during which we get one or more pulses will be low, and therefore the estimate of the probability may be less accurate.

[0122] The present invention also provides a method of setting the duration of the "on" time. As described above, the "on" time is set to adjust the probability close to 0.5. Typically, it is preferred to set the duration of the "on" time to a suitable value before determining the intensity measurement value (although the "on" time can also be adjusted in a similar manner when determining the measurement value). There are several different ways in which the duration of the "on" time can be set, and these ways are described separately below.

[0123] In a first preferred method, a measurement from the last on-time is used and the duration of the on-time is continuously adjusted.

[0124] Advantageously, the first method is possible whenever the detection signal increases (e.g., from a lower count area region to a higher count area region) or decreases (e.g., from a higher count area region to a lower count area region). Thus, the method is able to react to an ion beam of changing intensity and provide an updated intensity reading.

[0125] In the second method, the analog detector output is used to determine the on-time duration. This method requires a cross-calibration factor. The cross-calibration factor can be continuously updated without user interaction based on the mismatch between the expected probability and the observed probability. The analog output can be used to set the cross-calibration of the "on" time of the counting mode, but the intensity of the detection signal should preferably be measured in the counting mode rather than using the analog mode.

[0126] A third method is to attenuate the ion beam by any lens in the mass spectrometer. For example, a quadrupole focusing lens can be used to attenuate the ion beam reaching the detection device. In this case, the following steps can be performed:

[0127] When the detection signal is above the normal count range, the ion beam is attenuated until the detection signal is within the count range.

[0128] The attenuation is gradually reduced while adjusting the on-time to the desired duration, as described above. Alternatively, the stored dependency between the Q-focus voltage and the attenuation can be used to directly provide the approximate "on" time.

[0129] In other words, the lens of a mass spectrometer (e.g., a quadrupole) upstream of the SEM detection device can be used to attenuate the ion beam in order to reduce the counts of the detector downstream of the multiplication section. This can be done in addition to the switching of the gate. Once the timing of the gate is established to deliver the desired probability of detection within each cycle, the attenuation is reduced and the timing of the gate is readjusted.

[0130] A fourth method is to increase the "on" time until a suitable value is found. This method may be referred to as performing a "scan". The following steps may be performed:

[0131] 1. Start with a short “on” time;

[0132] 2. Perform multiple loops and estimate the probability of zero counts

[0133] (e.g. 100 cycles);

[0134] 3. If the probability of zero counts is not approximately 0.5, increase the "on" time;

[0135] 4. Go to step 2.

[0136] At step 3, a scan may be performed by increasing the "on" time by a predetermined step size. Alternatively, the "on" time may be adjusted continuously.

[0137] Furthermore, if the estimated probability is close to the 0.5 probability target, the scan steps can be performed with a decreasing step size. This can also be reversed, if the estimated probability is far from the 0.5 probability target, the step size is increased.

[0138] In one example of continuously adjusting the "on" time, it may not be necessary to perform a fixed number of cycles to estimate the probability. Instead, the "on" duration may increase for each cycle of zero counts and decrease for each cycle of one or more counts. The "on" duration should eventually stabilize around the "on" time that gives a probability of approximately 0.5.

[0139] A moving average of the intensities can be taken and the probability can be estimated using the formula provided in equation (2). The scan can be stopped when the estimate of the probability is within a predetermined threshold of the target probability (to the desired accuracy). This can be the case assuming shot noise statistics, if

[0140]

[0141] in

[0142] count 加权 : =∑(P·I) (4)

[0144] For example, the desired accuracy may be 1% (0.01).

[0145] In another example, the step size may be based on the estimated probability of the previous "on" time. Thus, if the estimated probability p(zero) = 0.9, a large step size may be applied (e.g., increasing the "on" time by a factor of ln(0.5) / ln(0.9) = 6.56), and if the estimated probability p(zero) = 0.6, a small step size may be applied (e.g., increasing the "on" time by a factor of ln(0.5) / ln(0.6) = 1.36). In general,

[0146]

[0147] Since the number of cycles used to determine the probability is small, the estimate may be inaccurate. The new "on" time can then be used for the increased number of cycles to provide an updated probability and an updated t 开启 ,etc.

[0148] The sweep can also be performed in reverse (starting with a long "on" time and decreasing). However, starting with a shorter "on" time and increasing will likely result in reaching the desired "on" time faster than starting with a longer "on" time and decreasing.

[0149] The number of "on" time events can also vary over time. In other words, the number of cycles can be quite low to start with when the "on" time may need significant adjustment, and as the "on" time approaches the desired value (when the probability approaches 0.5) the number of cycles is increased to obtain a more accurate estimate of the probability.

[0150] Once the "on" time is selected, it can be measured by counting the number of cycles where zero or one count is detected. The result of the count can be used to adjust the "on" time setting for the next series of cycles.

[0151] In a fourth method, a scan is performed within an "on" time range. The "on" time is varied (arbitrarily, scanned) between upper and lower thresholds for a number of cycles, and the probability of detection of an electron is determined from the detection results over a number of cycles.

[0152] In a fifth method, if no electrons are counted by the counting detector during at least one switching cycle of the gate, the on-time is increased from a preset duration. If electrons have been counted by the counting detector during the previous switching cycle of the gate, the "on" time is kept constant for at least one cycle. If the probability of detecting electrons is lower than 0.5, the "on" time is increased after a predetermined number of switching cycles of the gate.

[0153] In a sixth method, the probability is determined for a fixed number of cycles (e.g., 100), and if the probability is below 0.5, the "on" time is increased. The detection probability is calculated for multiple cycles with a constant "on" time, where if the detection probability is below 0.5, the "on" time is increased.

[0154] A seventh method is to mix pulses of different lengths and evaluate the results.

[0155] For example, the "on" period can be varied stepwise from an upper limit to a lower limit (one period per step). A preferred duration can be estimated based on the results. This can be performed as a sweep through the possible "on" times. Alternatively, a statistical mixture of "on" times can be used (in other words, larger steps between less likely values ​​and smaller steps between more likely values).

[0156] Between each repetition, sufficient "off" time is required. The "off" time is preferably as short as possible to reduce the time spent. In one example, the "off" time can be set to the dead time. Alternatively, the "off" time can be shorter than the dead time (and as short as possible) unless at least one count is seen (in which case the "off" time should be the dead time). This approach can reduce the total time spent. This applies to any of the methods 1 to 7 mentioned above, and can also be applied when taking measurements.

[0157] In the eighth method, the gate is opened and the time until the first pulse arrives is measured. This is repeated several times (for example, 10 times). In this case, there is a risk that several pulses may pass through the detector before the gate is closed. This is because the running time effect in the detection device can cause a delay between the electrons passing through the gate and the detection signal being detected. In addition, it may take some time to close the gate after the detection signal is detected. Therefore, the second pulse may pass through the gate before the gate is closed.

[0158] In the eighth method, the gate is kept open until an electron is detected. The "on" time is determined by measuring the time until the detector detects the first secondary electron, while keeping the gate in a state that allows ions or secondary electrons to pass.

[0159] Another option is to end the "on" time immediately after a count has been registered. In this case, the running time of the pulse through the detector should also be considered.

[0160] Working examples for setting the "on" time (according to the fourth, fifth and sixth methods) are provided below.

[0161] First, assume an intensity of 2,000,00 zero counts per second (cps). Start with an "on" time of 1 nanosecond (ns) and an "off" time of 1 ns. According to Equation 1, the probability of zero counts during the "on" period is 1.00 (or more precisely 0.998). After 10 "on" periods with no events (20 ns total), it is determined that the "on" time should be increased.

[0162] The "on" time is then increased to 10ns. If there are no events, the "off" time is set to 1ns, and if there are one or more events, the "off" time is set to 20ns. The probability of zero events occurring during the "on" time is 0.98. Therefore, after 10 "on" cycles, we expect to have registered 10 "on" cycles during which no counts were made. This takes 110ns.

[0163] The "on" time is then increased to 100ns. The probability of zero events occurring during the "on" period is 0.82. The expected number of cycles during which an event occurs is 1.8. For this example, assume that there are two "on" periods during which at least one event occurs. The total duration of the "on" time and the "off" time is 100ns x 10 ("on" time) + 20ns x2 ("off" time after an event) + 1ns x 8 ("off" time after no event) = 1.048 microseconds (μs). Based on the detection results, the probability is estimated to be 0.8. Therefore, the intensity determined from the estimated probability is 2,230,000cps (according to Equation 2). The calculated "on" time required to make the probability 0.5 (based on the detection results) is provided by the following equation:

[0164]

[0165] This would give a true probability of 0 for 0.537 events in this example (according to equation 1). This may depend on the requirements set in the method being sufficiently precise. In a preferred example, a probability between 0.4 and 0.6 is acceptable. Therefore, the t 开启 The value of provides a probability with an acceptable range.

[0166] Overall, in this example, determination of the correct "on" time takes 1.168 μs.

[0167] In another approach, instead of measuring a fixed time, the measurement can be made until the shot noise limit is at a predetermined value. If 1% accuracy is desired, this would mean ten thousand counts. This is because the noise is proportional to the square root of the ions if it is shot noise limited. Therefore, to get 1% RMS, 1 / Square(0.01) = 10,000 ions are required. At 2,000,000 cps, this would require less than 0.03 seconds. During this measurement, the "on" time can be fine-tuned.

[0168] Theoretically, the proposed method can provide a detector with unlimited dynamic range. As long as the ions arrive at the second dynode as individual events, the dynamic range is limited only by the electronics and its ability to set very short and precise "on" and "off" times. This can be calibrated out when the electronics are at their limits, for example when the pulse shape deviates from a rectangular shape.

[0169] exist Fig. 9 An illustrative example is provided in which a graph of (a portion of) the gate voltage over time is shown. On the same axis the pulses received at the detector are shown. During the "on" time OT two events occur, namely the detector pulses.

[0170] In the event that the start / initiation of an electronic pulse occurs during the "on" time, the pulse will continue to be detected after the gate is closed again. Closing the gate may prevent further pulses from reaching the detector, but pulses initiated during the "on" time may not be blocked.

[0171] At least for this reason, it is preferred to set the "off" time to be at least as long as the detector dead time. In the case where the start of a pulse coincides with the end of an "on" period, an event may be detected during the "off" period (when the discriminator level is crossed). In this case, the duration of the "off" period may need to be slightly longer than the detector dead time, at least the duration of the long pulse.

[0172] There may also be a delay required for the pulse to propagate through the detector. Fig. 9 This delay is not shown. Those skilled in the art will understand the modifications required to take this delay into account.

[0173] The following alternative method can also be used to calculate ionic strength:

[0174] 1. Set the door to the "open" position;

[0175] 2. Measure the time until the first ion event is recorded;

[0176] 3. Set the door to the "closed" position during the dead time;

[0177] 4. Repeat the above steps and calculate the average time between ion events.

[0178] Intermittent gating can be performed at a number of different locations within the SEM detection apparatus. In a first embodiment, gating can be performed using existing gates, i.e., at existing locations in the multiplication section. However, in contrast to prior art devices (which use gates to switch between analog mode and counting mode), the proposed method applies intermittent gating signals to the gates based on the determined count value. The first embodiment Figure 4 Shown in.

[0179] Figure 4 The SEM detection device 1 also includes a conversion dynode 12A, a multiplication section 12, a pulse detector or counter 13 and a processor 15, such as Figure 3 The multiplication section 12 further comprises a gate 16 arranged between what in the prior art device are called the analog part and the pulse part or the multiplication section (in the detector arrangement according to the invention, only pulses can be detected without current, and thus the current detector can be omitted). According to the invention, the processor 15 provides an intermittent gating signal to the gate 16, which is based on the pulses P counted by the pulse detector 13. The intermittent gating signal periodically switches off the downstream part of the multiplication section.

[0180] like Figure 4 As schematically shown, gating can be performed at a different dynode of the detection device (such as a conversion dynode or one of the first few dynodes) instead of using gating at the location present in the dual mode detector (at the dynode after the analog part AS). Figure 5 A second embodiment of the present invention is shown. Figure 5 As shown, compared with the prior art ( Figure 3 ), the gate is moved from a position after the conventional analog portion to a position toward the start of the electron multiplier 12. This position may correspond to, for example, the second dynode, the third dynode, or the fourth dynode. Figure 3 14) is no longer needed and is removed. The gating signal that previously switched between analog mode and counting mode is replaced by an intermittent gating signal based on the determined count value. By using intermittent gating as described in this document, the dynamic range of the counting mode can be increased so that the analog mode is not needed (at least for measuring ion intensity, although the analog mode can be used to set t in some of the above methods). 开启 ).

[0181] Alternatively or additionally, a grid may be provided to switch the electron beam or ion beam on / off. Thus, the function of the grid may be provided by a separate grid rather than by the dynodes. The grid may be provided before the electron multiplier (e.g. Figure 6 In the third embodiment shown in FIG. 1 ) or before the ion converter (as shown in FIG. Figure 7 4. As shown in the fourth embodiment shown).

[0182] exist Figure 6 In an embodiment of the invention, the multiplication section 12 has no gate. Alternatively, the gate 16 (composed of a grid) is arranged here in front of the multiplication section 12, immediately downstream of the conversion dynode 12A. In this embodiment, the intermittent gating signal generated by the processor 15 is used to intermittently prevent the electrons generated by the conversion dynode 12A from reaching the multiplication section 12. As a result, the multiplication section 12 intermittently generates electrons and the detector 13 intermittently detects the electrons.

[0183] exist Figure 7 In the embodiment of the invention, the multiplication section 12 also has no gate. Instead, a gate 16 (composed of a grid) is here arranged in front of the conversion dynode 12A. In this embodiment, an intermittent gating signal generated by the processor 15 is used to intermittently prevent ions from reaching the conversion dynode. As a result, the multiplication section 12 intermittently produces electrons.

[0184] The principles above regarding this method can be applied anywhere gating is performed.

[0185] Gating is preferably performed earlier in the electron multiplier than in the case of a gate present in prior art devices. This is because earlier gating can reduce the power consumption of the device since electron multiplication is not performed when the gate is off. Furthermore, gating should preferably be performed when the pulse is as short as possible. If gating occurs at the ion, then before the SEM, the pulse will be a Dirac function with a pulse width of zero.

[0186] In some cases, it may be advantageous to maintain the position of the gate after the analog section, rather than moving the gate upstream. In this way, existing equipment may be modified to perform the invention with fewer changes than if the position of the gate were moved.

[0187] It may be advantageous to provide multiple gates at different stages of the device and drive all gates using the same gating signal. In the case where the gates are only able to attenuate charged particles rather than completely block them, this can further reduce the total number of multiplied electrons reaching the detector in the off mode.

[0188] As described above, there are alternative embodiments in which:

[0189] The dynodes after the analog output provide a gating function (first implementation);

[0190] One of the first dynodes is used to provide a gating function (preferably one of the first five dynodes; second embodiment);

[0191] The grid is disposed within the SEM (third embodiment); and

[0192] Another type of energy filter is provided (this may be provided by a combination of grids, deflection plates, ion choppers, special grids, etc.).

[0193] In a second embodiment, one of the dynodes before the analog output (wherein the analog output is retained) can be used to provide a gating function. In another example, the gating function can be provided by a conversion dynode, a second dynode, a third dynode, a fourth dynode, or a fifth dynode.

[0194] As used herein, including in the claims, unless the context indicates otherwise, the singular form of the terms herein shall be interpreted as including the plural form, and vice versa. For example, unless the context indicates otherwise, singular references herein (including the claims), such as "a" or "an" (such as an analog / digital converter) mean "one or more" (e.g., one or more analog / digital converters). In the specification and claims of the present disclosure, the words "include", "comprising", "having" and "containing" and variations of these words, for example, "including" or similar words mean "including but not limited to", and are not intended to (and do not) exclude other components.

[0195] Although embodiments according to the present disclosure have been described with reference to specific types of devices and applications (particularly those deployed in ICP-MS or iCAP TM SEM detectors in devices), and in such cases, as discussed herein, embodiments have particular advantages, but methods according to the present disclosure may be applied to other types of devices and / or applications. Although specific structural details of the instrument have advantages (especially in view of known system constraints and performance), they may be significantly varied to obtain devices with similar or identical operations. Unless otherwise stated, each feature disclosed in this specification may be replaced with an alternative feature for the same, equivalent or similar purpose. Therefore, unless otherwise stated, each feature disclosed is merely an example of a series of equivalent or similar properties.

[0196] Unless otherwise stated, the use of any and all examples or exemplary language ("for example," "such as" and similar language) described herein is intended only to better illustrate the present invention and is not intended to limit the scope of the present invention. No language in the specification should be construed as indicating that any unclaimed element is essential to the practice of the present invention.

[0197] Unless otherwise specified or the context requires otherwise, any steps described in this specification may be performed in any order or simultaneously.

[0198] All aspects and / or features disclosed in this specification may be combined in any combination, except for the mutually exclusive combination of at least some features and / or steps in such features and / or steps. As described herein, there may be a specific combination of aspects with further benefits, such as determining a compensation parameter set and applying the compensation parameter set to the aspects of measurement. Specifically, the preferred features of the present invention are applicable to all aspects of the present invention and, and may be used in any combination. Similarly, the features described in non-essential combinations may be used individually (not in combination).

Claims

1. A method for controlling an ion detection device, the method include: - converting received ions into emitted electrons, - multiply the emitted electrons, - detect the multiplied electrons, - generating a detection signal in response, and - determining the ion intensity based on the detection signal, Features: - Depending on the ion intensity, the number of multiplied electrons reaching the detector is controlled by intermittent gating.

2. The method according to claim 1, in, Determining the ion intensity from the detection signal includes determining an ion count value, wherein the number of multiplied electrons reaching the detector is controlled by intermittent gating depending on the count value.

3. The method according to claim 1 or claim 2, in, The intermittent gating occurs: prior to emitting said electrons; after emitting said electrons; and / or After emitting the electrons and before multiplying the emitted electrons.

4. A method according to any preceding claim, in, The emitted electrons are attenuated by the gating; and / or Therein, the ions are deflected by the gating.

5. A method according to any preceding claim, in, Controlling the number of multiplied electrons reaching the detector by intermittent gating includes one or more of the following: intermittently switching the charged grid to attenuate the emitted electrons or to divert the ions away from the conversion element; intermittently switching an ion optical device disposed upstream of the conversion element to divert the ions from the conversion element; as well as One of the plurality of dynodes in the multiplication unit is switched intermittently so as to completely decay the multiplied electrons in the multiplication unit.

6. The method according to any one of the preceding claims, in, The intermittent gating is performed in a repeated gating cycle, wherein the repeated gating cycle comprises: - an "off" time, in which the number of multiplied electrons reaching the detector is attenuated by the intermittent gating; and - an "on" time, in which the number of multiplied electrons reaching the detector is not attenuated by the intermittent gating.

7. The method according to claim 6, in: The duration of the "on" time and the duration of the "off" time are controlled during the intermittent gating depending on the ionic strength; and / or The "off" time of each gating cycle is greater than or equal to the dead time of the detector.

8. The method according to claim 6 or claim 7, in, The "on" time is set based on one or more of the following: If the ionic strength of the previous gating cycle was zero, increase the "on" time of the gating cycle; The "on" time is set to a duration such that the probability that the ion intensity is greater than zero during the "on" time of at least one gating cycle is between 0.2 and 0.8, preferably between 0.4 and 0.6; increasing the "on" time from a preset duration if the ion intensity of one or more of the previous gating cycles was zero; If the ionic strength of one or more of the previous gating cycles is greater than zero, maintaining the "on" time constant within at least one gating cycle; If the probability that the ionic strength is greater than zero is higher than 0.5, reducing the "on" time after a predetermined number of gating cycles; extending the "on" time of the initial gating cycle until the ion strength is determined based on the detection signal; and The “on” time varies between an upper threshold and a lower threshold for a plurality of gating cycles, wherein the probability that the ion intensity is greater than zero is determined based on the determined ion intensity determined during the plurality of cycles.

9. An ion detection device, the ion detection device include: - a conversion element for converting the received ions into emitted electrons, - a multiplication unit for multiplying the emitted electrons, - Detectors for: - detect the multiplied electrons, - generating a detection signal in response, and - determining the ion intensity based on the detection signal, Features: The ion detection apparatus further comprises a controller configured to control the number of multiplied electrons reaching the detector by intermittently switching a gate, depending on the ion intensity determined by the detector.

10. The ion detection device according to claim 9, in: The gate is located upstream of the conversion element; or The gate is located downstream of the conversion element.

11. The ion detection device according to claim 9 or claim 10, in, The gate is a charged grid, and wherein the grid is arranged to be switched intermittently in order to attenuate emitted electrons or to divert the ions away from the conversion element.

12. The ion detection device according to claim 9, in, The gate is an ion optical device arranged upstream of the conversion element, wherein the ion optical device is arranged to be switched intermittently in order to divert the ions away from the conversion element.

13. The ion detection device according to claim 9, in, The multiplication unit is a secondary electron multiplier, and one of a plurality of dynodes in the multiplication unit is intermittently switched so as to cause all the multiplied electrons in the multiplication unit to decay.

14. The ion detection device according to any one of claims 9 to 13, in, The controller is configured to perform the method according to any one of claims 1 to 8.

15. A mass spectrometer comprising the ion detection device according to any one of claims 9 to 14.

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