Mass spectrometer and method for calibrating a mass spectrometer

Abstract

The invention relates to a mass spectrometer (1), comprising: a gas inlet (2), which is adapted to supply a sample gas (4) to be ionized to an ionization region (5) of the mass spectrometer (1); a calibration unit (6), which is adapted to supply a calibration gas (7) to be ionized to the ionization region (5); and an ionization unit (8), which is adapted to ionize the sample gas (4) and / or the calibration gas (7) in the ionization region (5). The calibration unit (6) comprises at least one evaporation source (9) for generating the calibration gas (7) by evaporating a source material (10). The invention also relates to a method for calibrating a mass spectrometer (1).

Description

Technical Field

[0001] The present invention relates to a mass spectrometer and a method for calibrating a mass spectrometer. Background Art

[0002] The calibration of the mass scale and signal intensity / sensitivity of a mass spectrometer is generally performed by introducing a calibration gas into the vacuum system of the mass spectrometer via a gas inlet. The calibration gas is typically composed of components with known atomic masses or (equivalently) mass-to-charge ratios (m / z). The known atomic masses of the components of the calibration gas produce one or more calibration peaks in the resulting mass spectrum, which correspond to the mass-to-charge ratios of the (multiple) components of the calibration gas. Since the components of the calibration gas are known, the calibration peaks can be used as a mass scale, by which the mass-to-charge ratios of the peaks corresponding to the unknown components of the sample gas can be determined.

[0003] For quantitative measurements, the signal intensity detected by the mass spectrometer at a specific m / z ratio must be related to the number of ions or to the partial pressure of the corresponding component of the sample gas. For this purpose, the sensitivity of the mass spectrometer for a specific m / z ratio must be determined when calibrating the mass spectrometer. For this purpose, a calibration process described, for example, in the article "Methods for in situ QMS calibration for partial pressure and composition analysis" by Robert E. Ellefson in Vacuum 101 (2014) 423-432 can be performed.

[0004] Introducing calibration gas into a mass spectrometer typically incurs costs for an additional gas inlet system. However, in many UHV (ultra-high vacuum) or XHV (extremely high vacuum) systems, additional gas inlets are generally undesirable. Furthermore, the calibration gas may cause contamination of the mass spectrometer, more specifically, of the mass spectrometer vacuum system.

[0005] The heaviest volatile non-radioactive element in the periodic table is xenon, which has several isotopes with atomic mass range from 124 to 136a.mu. Therefore, the mass scale of mass spectrometer may only be calibrated to the maximum value of 136a.mu when using the calibration gas consisting of volatile (non-radioactive) chemical elements. In addition, the lighter isotope of Xe may cause interference when performing calibration. Due to the presence of several isotopes, Xe also has limited applicability for calibration signal intensity / determination of mass spectrometer sensitivity: on the one hand, the signal intensity of Xe isotopes with different amu must be measured in a timely manner, on the other hand, the fractions of different Xe isotopes in (well defined) calibration gas must be known in advance.

[0006] The calibration of mass scales above 150 a.mu is complicated because molecules or atoms with such large atomic masses are non-volatile or have low volatility. Larger molecules, for example organic molecules or molecules such as SF6, are fragmented during ionization and thus generate ionized fragmentation products with several different smaller atomic masses. Under the corresponding measurement conditions, the ratio of these fragmentation products must be determined or must be known, which complicates the calibration process. Moreover, practically all organic molecules with masses above 150 a.mu, such as dodecane (C 12 H 26 , 170a.mu) etc., are deposited as contaminants on the vacuum components of the mass spectrometer and its environment, and can only be removed at increased temperatures, consuming considerable time. An example of a strong need for good calibration in the range of 200a.mu is the widely used quadrupole mass spectrometer (QMS). These are known for their discrimination of higher mass signals, which is highly dependent on different factors, such as background, environment, and temperature etc.

[0007] US 4,847,493 discloses an apparatus and method for calibrating a mass spectrometer. A calibration gas tank is located inside the same housing containing an ion source assembly and the analytical portion of the mass spectrometer. Each calibration gas and each sample gas is connected to its own associated valve. Two valves control the flow of a selected one of the sample gas and the calibration gas to the ion source assembly.

[0008] US 6,797,947 B2 discloses an apparatus and method for calibrating a mass spectrometer by internally introducing a calibration (lock) mass at a post-source stage of the mass spectrometer. The mass calibration apparatus comprises an ion source for providing analyte ions to a mass analyzer, an ion optical device located between the ion source and the mass analyzer, and a lock mass ion source comprising a lock mass source and a lock mass ionization source adjacent to the ion optical device for creating lock mass ions within the ion optical device.

[0009] Purpose of the Invention

[0010] It is an object of the present invention to provide a mass spectrometer and a method for calibrating a mass spectrometer which allow to simplify the calibration of the mass spectrometer, in particular for large atomic masses. Summary of the invention

[0011] One aspect of the invention relates to a mass spectrometer comprising: a gas inlet adapted to supply a sample gas to be ionized to an ionization region of the mass spectrometer; a calibration unit adapted to supply a calibration gas to be ionized to the (same) ionization region; and an ionization unit adapted to ionize the sample gas and / or the calibration gas in the ionization region, wherein the calibration unit comprises at least one evaporation source for generating the calibration gas by evaporating a source material. Typically, the mass spectrometer also comprises an analysis part. The analysis part comprises: a mass analyzer / mass filter for selecting a specific mass-to-charge ratio of the sample / calibration gas; and a detector for detecting the ionized sample gas and / or calibration gas.

[0012] The calibration unit, more specifically the evaporation source, is typically located in the same housing containing the ionization unit and the analysis part. During calibration, the calibration gas is ionized in the ionization region by the ionization unit, typically by providing an ionization energy input / radiation (e.g., electrons, laser radiation, ...), and at least a portion of the calibration gas is ionized. The ions of the calibration gas are processed in the mass spectrometer in the same way as the sample gas (analyte), i.e. they are detected by the detector of the mass spectrometer after passing through the mass analyzer.

[0013] Since the calibration gas is generated by evaporation of the source material in the evaporation source, no additional gas inlet system is required for supplying the calibration gas to the ionization region. In addition, the source material can be a chemical element with a mass greater than 136 a.mu, such as a metallic material, which is non-volatile under standard conditions. A calibration gas consisting of atoms of such a chemical element does not fragment during ionization, thus simplifying calibration for large atomic masses up to approximately 200 a.mu. When using a form that employs a specific isotope of a chemical element or employs a form that has only one stable isotope (e.g. 27 Al or 197 Calibration can be further simplified when the source material is in the form of a chemical element such as Au. Furthermore, it is possible to use a source material for calibration which has a probability of adhesion close to 1 on the surface of a vacuum component of a mass spectrometer, for example made of stainless steel. In this case, the atoms of the source material deposited on the vacuum component after calibration adhere to the surface of the vacuum component affected by the calibration gas and do not contaminate other components of the mass spectrometer and the vacuum system to which the mass spectrometer is attached.

[0014] In one embodiment, the evaporation source - more precisely, the source material - and the ionization region are arranged along a line of sight, i.e., along a straight line that is generally not blocked by any component of the mass spectrometer. In this way, the calibration gas can be supplied from the evaporation source to the ionization region as a beam that propagates essentially along a straight line. This is advantageous because the calibration gas typically comprises only neutral atoms or molecules and therefore cannot be deflected by ion optics or the like before reaching the ionization region. It is also possible that the calibration unit or the evaporation source is arranged inside the ionization unit, in particular when the ionization unit (at least partially) surrounds the ionization region.

[0015] In a further embodiment, the evaporation source is a thermal evaporation source, preferably a resistive evaporation source, an electron beam evaporation source, or an effusion evaporation source. In a thermal evaporation source, the source material is heated to a temperature close to the melting point or boiling point, thereby converting the source material into a gas phase. In a resistive evaporation source, a high current is passed through a resistive element, such as a filament, a resistive boat, or a crucible, where the source material is placed. In an electron beam evaporation source, a focused beam of high energy electrons is used to directly heat the source material. An effusion evaporation source typically includes a crucible containing the source material in solid form, a heating wire, a cooler, and a thermocouple to control the temperature of the source material.

[0016] In one refinement, the resistive evaporation source comprises a heated filament at least partially coated with a source material. To provide the coating, some pieces of source material, typically in the form of a metal wire (e.g. made of gold or aluminum), are hooked to the filament. When the filament is heated, the metal wire will melt and flow along the filament, like solder on a soldering iron, producing a coating, for example, in the form of droplets of source material. When such a filament is heated, the source material evaporates from the heated filament, the latter being made of, for example, tungsten.

[0017] In a further embodiment, the evaporation source is a pulsed laser deposition (PLD) evaporation source. In pulsed laser deposition, a high power pulsed laser beam is focused within a vacuum chamber to strike a target of source material, which is ablated by the laser beam and evaporated from the target (typically in a plasma jet).

[0018] In a further embodiment, the source material is a metal, preferably selected from the group consisting of: Al, Co, Mn, Bi, Ni, Fe, Cu and noble metals, in particular Au. Au has been shown to be particularly suitable as an atomic source for the calibration gas because it has a high atomic mass and has only a single stable isotope at 197a.mu. However, other metals may also be used as source materials, in particular chemical elements with a single stable isotope, such as Al, etc. As a rule, chemical elements with a saturated vapor pressure higher than that of lead (Pb) should be avoided as source materials (if not used in a vacuum system anyway), because these source materials may contaminate the vacuum system of the mass spectrometer, in particular vacuum components such as vacuum tubes, vacuum housings, etc.

[0019] In a further embodiment, the source material is selected from the group consisting of metal nitrides and metal oxides, in particular nitrides and oxides of tantalum, vanadium, tungsten, rhenium or yttrium. Chemical compositions other than nitrides and oxides can also be used as source material for the evaporation source. As indicated above, chemical elements or compositions which have a high probability of adhering in particular to the surfaces of the vacuum components of the mass spectrometer are preferred source materials.

[0020] In a further embodiment, the mass spectrometer comprises at least one sensor, preferably for determining the pressure (or background pressure) of the calibration gas, wherein the sensor is preferably arranged along a line of sight with the ionization zone and / or along a line of sight with the source material. The sensor can be used to control or regulate the evaporation rate of the source material. For this purpose, at least one sensor can communicate signals with a control unit that is typically part of the mass spectrometer. The control unit is a programmable device in the form of suitable hardware and / or software, such as a microprocessor, a programmable controller, a computer or another electronic device. The control unit can be integrated into the calibration unit or can be arranged at another position in the mass spectrometer.

[0021] In a refinement, the sensor is a pressure sensor, preferably an ionization vacuum gauge, more preferably a cold cathode vacuum gauge, in particular a Penning vacuum gauge, or a hot cathode vacuum gauge, in particular a Bayard-Alpert vacuum gauge or an extractor ionization gauge. An ionization vacuum gauge is used to measure the gas pressure by ionization of a residual gas, in the present case a calibration gas or a background gas contained in a mass spectrometer. A hot cathode or a cold cathode is used to generate electrons for ionizing the (residual) gas supplied to such a pressure sensor by electron beam ionization. The use of an ionization vacuum gauge such as a Penning vacuum gauge, a Bayard-Alpert vacuum gauge or an extractor vacuum gauge is particularly advantageous when the ionization unit is adapted to ionize the sample gas and / or the calibration gas by electron impact ionization, i.e. when the ionization unit uses the same type of ionization as the ionization vacuum gauge, in particular with similar ionization cross sections for different atoms. Alternatively, a piezoelectric sensor may be used as a pressure sensor to measure the pressure in the environment of the pressure sensor based on the piezoelectric effect.

[0022] In one refinement, the pressure sensor or the control unit is adapted to determine the flow rate of the calibration gas based on the pressure of the calibration gas. For this purpose, an ionization vacuum gauge such as a Bayard-Alpert ionization vacuum gauge can be used. Such a vacuum gauge (gauge head) is typically used as a beam monitor in molecular beam epitaxy to determine the flow rate of atoms forming a source (such as an effusion evaporation source) (see, for example, www.mbe-komponenten.de / products / pdf / data-sheet-bfm.pdf). Such a beam monitor based on a Bayard-Alpert ionization gauge allows the beam equivalent pressure (BEP) of the atomic or molecular beam to be determined. The beam equivalent pressure is the local pressure of a directed gas beam on a surface measured by a pressure gauge. A Bayard-Alpert ionization meter or another type of ionization vacuum meter thus allows both the determination of the signal intensity of the mass spectrometer during mass spectrometric analysis of a sample gas or gas mixture as described in the above-cited article by Robert E. Ellefson, and the determination / monitoring / calibration of the sensitivity of the mass spectrometer to atoms of (multiple) evaporation sources.

[0023] In a further embodiment, the sensor is a quartz crystal microbalance (QCM), which is preferably used to determine (and possibly control) the flow rate of the calibration gas. The flow of calibration atoms or molecules can be measured or controlled by using such a sensor, which is well known in thin film deposition. Atoms or molecules of the calibration gas with a high probability of sticking on the sensor build up a thin film on the QCM sensor, thus changing the resonant frequency of the QCM sensor at a rate corresponding to the flow rate of the atoms or molecules of the calibration gas. Therefore, the QCM sensor allows the flow rate of the atoms or molecules of the calibration gas to be determined directly, i.e. without determining the pressure of the calibration gas.

[0024] In a further improvement, the mass spectrometer includes a removable cover for blocking the line of sight between the source material and the ionization region and / or the line of sight between the source material and the pressure sensor. The removable cover can typically be moved from a first position and a second position, in which the cover does not block the line of sight between the source material and the ionization region / pressure sensor, and in which the removable cover blocks the corresponding line of sight. The movement of the cover between the two positions can be a translational movement and / or a rotational movement. By blocking the corresponding line of sight, the flow of the calibration gas from the calibration unit to the ionization region / pressure sensor is substantially hindered. In this way, at least one pressure sensor can be used to determine the pressure increase in the vacuum system of the mass spectrometer when the evaporation source is heated without simultaneously measuring the pressure increase due to the flow of the calibration gas. In addition, the removable cover can provide an airtight seal between the calibration unit or the evaporation source and the rest of the mass spectrometer.

[0025] In a further embodiment, the ionization unit comprises (or consists of) an electron ionization source. The electron ionization source ionizes atoms or molecules of the sample gas and / or calibration gas by electron bombardment. The electron ionization source can, for example, be implemented as an electron gun or the like. A person skilled in the art will appreciate that the ionization unit can be adapted to perform ionization in different ways, for example by inductively coupled plasma (ICP), by glow discharge ionization, etc. Depending on the type of ionization unit, the ionization region can be outside the ionization unit, as in the case of using an electron gun, or the ionization region can be part of the ionization unit, i.e. the ionization unit can at least partially surround the ionization region.

[0026] In a further embodiment, the mass spectrometer comprises an ion trap for storing ions of the sample gas and / or calibration gas, wherein the ionization region is formed inside the ion trap (in the storage region for the ions). The mass spectrometer may in particular be a Fourier transform (ion cyclotron resonance) mass spectrometer. In an FT ion trap mass spectrometer, in addition to storing ions in the storage region of the ion trap, the ions are also excited (mass selection) in the storage region and detected in the FT ion trap, more specifically at the electrodes of the ion trap. The ionization region is preferably located in the center of the ion trap. In this case, the line of sight between the source material and the ionization region typically leads from the source material to the center of the ion trap.

[0027] A further aspect of the invention relates to a method for calibrating a mass spectrometer, comprising: generating a calibration gas by evaporating a source material in at least one evaporation source of the mass spectrometer; supplying the calibration gas to an ionization region and ionizing the calibration gas in the ionization region; detecting the ionized calibration gas in a detector of the mass spectrometer; and calibrating the mass spectrometer based on the detected ionized calibration gas. As indicated above, the ions of the calibration gas are supplied to the ionization region and are processed in the mass spectrometer in the same manner as the sample gas (analyte), i.e. they are detected by the detector of the mass spectrometer after passing through a mass analyzer which may be embodied as an excitation device, typically when the mass spectrometer includes an ion trap.

[0028] As indicated above, the calibration gas can allow the mass scale of the mass spectrometer to be determined. For example, when the mass spectrometer includes a quadrupole analyzer, the correlation between the applied quadrupole voltage and the mass-to-charge ratio can be calibrated / determined, allowing the identification of specific chemical elements by their mass-to-charge ratio in the corresponding mass spectrum. Similar calibrations can be performed in time-of-flight mass analyzers in which the conversion between ion drift times and mass-to-charge ratios can be calibrated.

[0029] In one variant, the step of calibrating the mass spectrometer comprises determining the sensitivity of the mass spectrometer based on the signal intensity when detecting the ionized calibration gas and based on the pressure detected by the pressure sensor when the calibration gas is supplied to the ionization region. In addition to the mass scale, it is also advantageous to also determine / calibrate the sensitivity of the mass spectrometer, for example for both small and large atomic masses. The calibration of the sensitivity of the mass spectrometer can be performed as indicated in the article by Robert E. Ellefson which has been cited above and is incorporated by reference in its entirety into the present application.

[0030] For example, to determine the sensitivity of a mass spectrometer, in a first step, before supplying a calibration gas to the ionization region, the background pressure p0 and the signal intensity B at the mass-to-charge ratio(s) of interest k (i.e. at the mass-to-charge ratio(s) of the source material) are determined. kIn a second step, the source material is evaporated to produce a calibration gas, and the signal intensity S at the mass-to-charge ratio(s) k of the source material is determined a second time. k and the pressure p1 of the calibration gas in addition to the background pressure. Sensitivity K k It can be obtained by calculating the difference S between the signal intensities in the first and second steps at the mass-to-charge ratio (s) k of interest: k -B k Determined from the ratio of the difference p1-p0 between the pressure values ​​in the second step and the first step:

[0031] K k =(S k -B k ) / (p1-p0) (1)

[0032] In this way, the sensitivity K for the corresponding mass-to-charge ratio k (for example for k=27 (Al) or for k=197 (Au)) can be determined / calibrated. k .

[0033] Different evaporation sources with different source materials can be used to calibrate the mass spectrometer at different mass-to-charge ratios. Those skilled in the art will appreciate that the equation (1) given above is used to explain the basic principle of calibrating the sensitivity of the mass spectrometer. In practice, further steps may be required during calibration in order to take into account different effects, which are typically for the evaporation of metals. One of such effects is the gettering effect of the surface coated with a fresh film, which is composed of atoms of the calibration gas, especially Al, Ti, Ta and other gettering metals, causing a pressure reduction of other gases. On the other hand, when the evaporation source is heated, a pressure increase occurs due to the enhanced desorption of molecules from the surrounding parts at an increased temperature.

[0034] In a variant, the method further comprises: before and / or after supplying the calibration gas to the ionization region: coating the surfaces of the vacuum components in the mass spectrometer with a getter material for the source material. Suitable getter materials for the source materials typically used in this application are, for example, Al or Ti. In order to avoid that the source material is peeled off from the surfaces of the vacuum components in which the deposit of the active material is formed, these surfaces can be coated with the getter material. The coating can be applied to the (multiple) affected surfaces by supplying the getter material to the (multiple) affected surfaces using a further evaporation source for evaporation of the getter material. The coating with the getter material can be applied before a subsequent calibration, or it can be applied after the calibration in preparation for a subsequent calibration.

[0035] Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Exemplary embodiments are shown in the schematic drawings and are explained in the following description. The following are shown:

[0037] Figure 1 is a schematic illustration of an example of a mass spectrometer having a calibration unit with an evaporation source for generating a calibration gas by evaporation of a source material,

[0038] Figure 2 Is with Figure 1 The calibration unit shown in FIG. 1 is similar to a schematic illustration of a calibration unit for an ion trap mass spectrometer. Figure 3a to Figure 3c is a schematic illustration of a resistive evaporation source and a filament partially coated with an active material. DETAILED DESCRIPTION

[0039] Figure 1 A mass spectrometer 1 is schematically shown having a gas inlet 2 (more precisely, a gas inlet system) for supplying a sample gas 4 from a process chamber outside a (vacuum) housing 3 of the mass spectrometer 1 to an ionization region 5 inside the housing 3 of the mass spectrometer 1. The mass spectrometer 1 has a calibration unit 6 adapted to supply a calibration gas 7 to the ionization region 5 of the mass spectrometer 1. The calibration unit 6 is arranged inside (i.e. in situ) the housing 3 of the mass spectrometer 1. An ionization unit 8 is also provided in the housing 3 and is adapted to ionize both the sample gas 4 (analyte) and the calibration gas 7 in the ionization region 5.

[0040] In the present example, the ionization unit 8 is an electron ionization source in the form of an electron gun, and generates an electron beam 8a directed to the ionization region 5 for ionizing the respective gases 4, 7 by electron impact ionization. The sample gas 4 and the calibration gas 7 are provided to the ionization region 5, i.e., the sample gas 4 and the calibration gas 7 may be provided to the ionization region 5 simultaneously, but typically not at the same time. The sample gas 4 having typically unknown components and / or unknown amounts of components is provided to the ionization region 5 for mass spectrometry analysis thereof. The calibration gas 7 is provided to the ionization region 5 for calibration of the mass spectrometer 1.

[0041] After (partially) ionization in the ionization region 5, both the sample gas 4 and the calibration gas 7 are provided to the analysis part of the mass spectrometer 1. The analysis part has an analyzer 11, in the form of a quadrupole mass filter in this example, for selecting a suitable range of mass-to-charge ratios of components of the sample gas 4 or calibration gas 7. The analysis part also has a detector 12 for performing mass spectrometry measurements of the ionized gases 4, 7. It will be appreciated that other types of analyzers, such as time-of-flight analyzers, sector field analyzers, etc., may be used in the mass spectrometer 1. The detector 12 may include a plurality of detector elements, such as a Faraday cup, etc.

[0042] For the purpose of selectively supplying the sample gas 4 or the calibration gas 7 to the ionization region 5, a control unit 13 is provided in the mass spectrometer 1. The control unit 13 may be adapted to control the gas inlet 2, for example a controllable valve or the like, to supply the sample gas 4 to the ionization region 5 or to prevent the sample gas 4 from flowing to the ionization region 5. Those skilled in the art will appreciate that the gas inlet 2 does not necessarily have a controllable valve. In this case, the sample gas 4 may be supplied to the ionization region 5 in a continuous manner. Those skilled in the art will also appreciate that the housing 3 may be omitted. The control unit 13 is also adapted to control the calibration unit 6 to supply the calibration gas 7 to the ionization region 5 or to avoid generating the calibration gas 7. In the present example, the calibration unit 6 has a single evaporation source 9 for generating the calibration gas 7 by evaporating the source material 10. In Figure 1 In the example shown in FIG. 1 , the evaporation source 9 is a thermal evaporation source in the form of a resistive evaporation source, where an electric current is passed through a resistive element, such as a filament, at which the active material 10 is placed, as will be described in detail below. The calibration unit 6 may also include other types of thermal evaporation sources, such as an electron beam evaporation source, an effusion evaporation source, etc.

[0043] If you can Figure 1 As inferred, the source material 10 and the ionization region 5 (or ionization volume) are arranged along a line of sight 14a. More precisely, the line of sight 14a extends from the source material 10 along a straight line corresponding to the main flow direction of the calibration gas 7 and intersects the electron beam 8a generated by the ionization unit 8 in the ionization region 5.

[0044] The source material 10 is typically a non-volatile material, in particular a metal. Suitable metals are noble metals, in particular gold (Au), but other metals may also be used as the source material 10, such as Al, Co, Mn, Bi, Ni, Fe, Cu, etc.

[0045] By evaporating a source material 10 in the form of a metal, a calibration gas 7 comprising atoms of the source material 10 is provided. The calibration gas 7 in the form of atoms of a metal vapor is not split during ionization, which simplifies the calibration process. However, the selection of the source material 10 is not limited to metals. For example, chemical compositions such as metal nitrides or metal oxides (for example nitrides or oxides of vanadium, rhenium or tantalum, tungsten or yttrium) can also be provided as source materials. In addition, the calibration unit 6 can have more than one evaporation source 9 for evaporating different source materials 10. The calibration gas 7 associated with these evaporation sources 9 can be provided to the ionization region 5 simultaneously with the sample gas 4 (possibly together).

[0046] The preferred source material 10 for the calibration unit 6 has a high probability of adhering to the surfaces of the vacuum components of the mass spectrometer 1 which come into contact with the calibration gas 7, for example to the surfaces 3a at the interior of the vacuum housing 3 of the mass spectrometer 1, which is typically made of stainless steel. In this way, the deposition of the source material 10 on the corresponding surface 3a adheres to this surface 3a and does not contaminate the mass spectrometer 1. In order to avoid that the source material 10 is peeled off from the affected surfaces 3a, these surfaces 3a can be coated with a getter material 17 for the source material 10, such as Al or Ti, before or after the calibration gas 7 is supplied to the ionization region 5.

[0047] exist Figure 1 In the example shown in , the mass spectrometer 1 comprises two sensors 15a, 15b, which are not required but facilitate the operation of the mass spectrometer 1 with the calibration unit 6. The first sensor 15a is a pressure sensor arranged along the line of sight 14a with the ionization region 5 and with the source material 10. The above-mentioned pressure values ​​p1 and p0 can be determined using the first sensor 15a. Additionally, if the first sensor 15a is arranged at a suitable position in the gas flow of the sample gas 4, the pressure of the sample gas 4 can also be measured using the first sensor 15a.

[0048] The second sensor 15b is arranged along the (further) line of sight 14b to the source material 10. The second sensor 15b allows for direct control / measurement of the flow rate Q of the calibration gas 7 c For this purpose, the second sensor 15b is a quartz crystal microbalance. Alternatively, a pressure sensor such as a Bayard-Alpert vacuum gauge can also be used for this purpose. Other types of vacuum gauges, for example cold cathode vacuum gauges such as Penning vacuum gauges or extractor ionization gauges, can also be used as first / second sensors 15a, 15b.

[0049] The pressure p of the calibration gas 7 determined by the first pressure sensor 15a c can be used in the control unit 13 to determine the flow rate Q of the calibration gas 7 c (Assuming the flow rate of calibration gas 7 is Q c is not directly determined by the quartz crystal microbalance 15b). Generally speaking, during the calibration process for quantitative mass spectrometry, the flow rate Q of the calibration gas 7 is c The control unit 13 may be adapted to control or regulate (in closed-loop control) the flow rate Q of the calibration gas 7 c . The flow rate Q of the calibration gas 7 c or the pressure p of the calibration gas 7 c can be used in the calibration of the mass spectrometer 1 , as will be explained in further detail below.

[0050] In the calibration process, a calibration of the mass scale of the mass spectrometer 1 is performed. In the present example, the calibration involves the correlation between the quadrupole voltage applied to the quadrupole analyzer and the mass-to-charge ratios of the known atomic masses (of the components of the calibration gas 7 detected by the detector 12. The known masses and, accordingly, the mass-to-charge ratios of the peaks of the component (s) of the calibration gas 7 in the mass spectrum of the calibration gas 7 are used as a mass scale by which the peaks of the (unknown) components of the sample gas 4 present in the mass spectrum of the sample gas 4 can be assigned to their correct mass-to-charge ratios.

[0051] In addition to identifying specific components of the sample gas 4, for quantitative measurements the sensitivity / signal strength of the mass spectrometer 1 should also be calibrated.

[0052] For this purpose, in a first step, for a given mass-to-charge ratio k of a source material 10, in this example gold ( 197 Au, k = 197), the background pressure p0 in the mass spectrometer 1 is determined using the first pressure sensor 15a and / or the second pressure sensor 15b (i.e. in the absence of the calibration gas 7 or the sample gas 4). In addition to the background pressure p0, the background signal intensity B measured by the detector 12 at the mass-to-charge ratio k = 197 is determined. k In a subsequent step, calibration gas 7 is introduced into the ionization region 5 and the pressure p1 (or equivalently, p c ) is measured by the pressure sensors 15a, 15b. The signal intensity S of the ionized calibration gas 7 at a mass-to-charge ratio or amu of k=197 is determined by the detector 12. k .

[0053] In the subsequent step, the difference S between the signal intensities in the first step and the second step at the mass-to-charge ratio k is calculated. k -B k The sensitivity K of the mass spectrometer 1 for a mass-to-charge ratio k=197 is determined by the ratio of the difference p1-p0 between the pressure values ​​in the second step and the first step. k (See also the article by Robert E. Ellefson cited above):

[0054] K k =(S k -B k ) / (p1-p0) (1)

[0055] In this way, the sensitivity K for a mass-to-charge ratio k=197 (ie for Au) is determined kAdvantageously, the mass spectrometer 1 is calibrated for at least one further amu value (or equivalently, m / z ratio) that is relatively small, for example for k=27 (i.e. Al). The sensitivity of the mass spectrometer 1 for k=27 can be determined in the manner indicated above by using a further evaporation unit for evaporating Al as source material 10.

[0056] In order to determine the pressure increase in the ionization region 5 or in the mass spectrometer 1 when the evaporation source 9 of the calibration unit 6 is heated, Figure 1 The mass spectrometer 1 of FIG. 5 has a movable cover 16. The movable cover 16 is arranged close to the calibration unit 6 and can be moved from a first position in which the cover 16 does not block the line of sight 14a between the source material 10 and the ionization region 5 and to the pressure sensors 15a, 15b, and a second position in which the movable cover 16 blocks the line of sight 14a. In this example, the movable cover 16 can be moved between the two positions in a translational movement, as indicated by Figure 1 By blocking the respective sight lines 14a, 14b, at least one pressure sensor 15a, 15b can be used to determine the pressure increase in the vacuum system of the mass spectrometer 1 when the evaporation source 9 is heated, without simultaneously measuring the pressure increase due to the calibration gas 7. The pressure increase due to the temperature increase of the evaporation source 9 can be taken into account for the calibration of the mass spectrometer 1.

[0057] Above about Figure 1 The described calibration can also be performed with Figure 2 The electrical Fourier transform of the ion trap 18 shown is performed in the mass spectrometer 1 . Figure 2 The mass spectrometer 1 has an inlet (not shown) for supplying the sample gas 4 to the ionization region 5 via the line of sight 14b. The ionization region 5 corresponds substantially to the center of the ion trap 18. Figure 2 The mass spectrometer 1 is shown in a state in which the calibration unit 6, more precisely its evaporation unit, is activated for evaporating the source material 10, which in the present example is gold. Figure 2 In FIG. 5 , the calibration gas 7 is shown in the ionization region 5 together with a line of sight 14a leading from the calibration cell 6 to the ionization region 5. Figure 2 In the example of FIG. 1 , the evaporation source 9 is a pulsed layer deposition PLD source. However, it is also possible to use a pulsed layer deposition PLD source instead of a PLD source. Figure 1 The thermal evaporation source is shown in FIG. Figure 1 In the example of FIG. 1 , instead of using the evaporation source 9 , a PLD source or another type of ionization source may be used.

[0058] exist Figure 2In the FT ion trap 18, ions 7a and 7b of the calibration gas 7 are trapped between the ring electrode 19 and the first cap electrode 20a and the second cap electrode 20b. In order to store the ions 7a and 7b in the ion trap 18, the RF signal generation unit 21 generates a radio frequency signal V provided to the ring electrode 19. RF Each of the two excitation units 22a, 22b generates an excitation signal S1, S2, which is provided to the corresponding cap electrode 20a, 20b to excite the ions 7a, 7b to achieve oscillation. The oscillation frequency of the ions 7a, 7b in the ion trap 18 depends on the mass-to-charge ratio of the ions 7a, 7b. The two measurement amplifiers 23a, 23b amplify the corresponding measurement current caused by the oscillation. The ion signal u is generated from the difference between the two measurement currents ion (t). The detector 12 including an FFT ("Fast Fourier Transform") spectrometer is used to perform the ion signal u ion (t) and used to determine mass spectral data in the form of a mass spectrum 25. The mass spectrum 25 indicates the number of excited ions 7a, 7b depending on their mass-to-charge ratio m / z. In other words, the mass spectrum 25, and accordingly the mass spectral data 25, indicates the mass-to-charge distribution of the ions 7a, 7b in the calibration gas 7.

[0059] exist Figure 2 In the example of FIG. 5 , the calibration gas 7 is introduced into the ion trap 18 in an electrically neutral state. The mass spectrometer 1 has an ionization unit 8 to ionize at least a portion of the neutral calibration gas 7 introduced into the ion trap 18 in the ionization region 5. In the present example, the ionization unit 8 includes an electron gun (e.g., 70 eV or another suitable ionization energy) for electron beam ionization of the neutral calibration gas 7 introduced into the ion trap 18. Figure 1 As in the example of FIG. 1 , the electron beam 8a intersects the line of sight 14a leading from the calibration unit 6 to the ionization region 5. It will be appreciated that other types of ionization units 8 may also be used, for example using inductively coupled plasma, glow discharge ionization, etc. Figure 1 and Figure 2 of the mass spectrometer 1.

[0060] Prior to detection, the ions 7a, 7b may be selectively excited at least once according to their mass-to-charge ratio m / z, for example by means of SWIFT (Stored Waveform Inverse Fourier Transform) excitation. SWIFT excitation may be used in particular to eliminate ions 7a, 7b having a specific mass-to-charge ratio from the ion trap 18. In particular, ions 7a, 7b of a buffer gas or background gas may be eliminated from the ion trap 18, thus allowing detection of minute traces of ions 7a, 7b of gaseous species of the calibration gas 7. Figure 2The mass spectrometer 1 shown in FIG. 1 further comprises a control unit 13 which controls the mass spectrometer 1 , in particular controls the calibration process, as described above with reference to FIG. Figure 1 The mass spectrometer 1 is explained.

[0061] Figure 3a Shown in more detail Figure 1 The evaporation source 9 has a filament 26 and a voltage source 27. The voltage source generates an (adjustable) voltage for passing an electric current through the filament 26 to heat the filament 26 to a temperature of 1000° C. or higher. In this example, the filament 26 is made of tungsten (W) and has a diameter of about 0.3 mm to 0.5 mm. Figure 3b , Figure 3c As can be inferred, the gold wire 28 is hooked to the filament 26. By heating the filament 26 to a temperature above the melting point of the gold wire 28, the gold wire is melted and flows along the filament 26, so that, for example, Figure 3c The coating is provided in the form of droplets of source material 10 shown in . It will be appreciated that wires made of other (metal) materials such as copper etc. may be used instead of gold to provide source material 10 which may be evaporated when current is passed through the filament 26 .

Claims

1. A mass spectrometer (1), comprising: a gas inlet (2) adapted to supply a sample gas (4) to be ionized to an ionization region (5) of the mass spectrometer (1), a calibration unit (6) adapted to supply a calibration gas (7) to be ionized to the ionization region (5), an ionization unit (8) adapted to ionize a sample gas (4) and / or a calibration gas (7) in the ionization region (5), wherein the calibration unit (6) comprises at least one evaporation source (9) for generating the calibration gas (7) by evaporating a source material (10), wherein the evaporation source material (10) and the ionization region (5) are arranged along a straight line that is not blocked by any component of the mass spectrometer (1), wherein the source material (10) is a metal selected from the group consisting of Al, Co, Mn, Bi, Ni, Fe, Cu and Au, or Wherein the source material (10) is selected from the group consisting of: metal nitrides and metal oxides.

2. The mass spectrometer according to claim 1, wherein the evaporation source is a thermal evaporation source (9).

3. The mass spectrometer according to claim 2, wherein the thermal evaporation source (9) is a resistance evaporation source, an electron beam evaporation source or an effusion evaporation source.

4. The mass spectrometer according to claim 2, wherein the resistive evaporation source (9) comprises a heating filament (26) which is at least partially coated with the active material (10).

5. The mass spectrometer according to claim 1, wherein the evaporation source (9) is a pulsed laser deposition (PLD) evaporation source.

6. The mass spectrometer of claim 1, wherein the source material (10) is selected from the group consisting of nitrides and oxides of tantalum, vanadium, tungsten, rhenium and yttrium.

7. The mass spectrometer according to claim 1, further comprising: at least one sensor for determining the pressure (p c ), wherein the sensor is arranged along a line of sight to the ionization region (5) and / or along a line of sight to the source material (10).

8. The mass spectrometer of claim 7, wherein the sensor is a pressure sensor.

9. The mass spectrometer of claim 8, wherein the sensor is an ionization vacuum gauge.

10. The mass spectrometer of claim 9, wherein the sensor is a cold cathode vacuum gauge or a hot cathode vacuum gauge.

11. The mass spectrometer of claim 10, wherein the cold cathode vacuum gauge is a Penning vacuum gauge.

12. The mass spectrometer of claim 11, wherein the hot cathode vacuum gauge is a Bayard-Alpert vacuum gauge or an extractor ionization gauge.

13. The mass spectrometer according to claim 8, wherein the pressure sensor or the control unit (13) of the mass spectrometer (1) is adapted to determine the pressure (p c ) to determine the flow rate (Q) of the calibration gas (7) c ).

14. A mass spectrometer according to claim 7, wherein the sensor is a quartz crystal microbalance for determining the flow rate (Q c ).

15. The mass spectrometer according to claim 7, further comprising: A removable cover (16) is provided for blocking the line of sight between the source material (10) and the ionization region (5) and / or the line of sight between the source material (10) and the pressure sensor.

16. The mass spectrometer according to claim 1, wherein the ionization unit (8) is an electron ionization source.

17. The mass spectrometer of claim 1, further comprising: An ion trap (18) for storing ions (7a, 7b) of a sample gas (4) and / or a calibration gas (7), wherein an ionization region (5) is formed inside the ion trap (18).

18. A method for calibrating a mass spectrometer (1), comprising: generating a calibration gas (7) by evaporating a source material (10) in at least one evaporation source (9) of a mass spectrometer (1), supplying a calibration gas (7) to the ionization region (5) and ionizing the calibration gas (7) in the ionization region (5), detecting the ionized calibration gas (7) in a detector (12) of a mass spectrometer (1), calibrating the mass spectrometer (1) based on the detected ionized calibration gas (7); and wherein the evaporation source material (10) and the ionization region (5) are arranged along a straight line that is not blocked by any component of the mass spectrometer (1), wherein the source material (10) is a metal selected from the group consisting of Al, Co, Mn, Bi, Ni, Fe, Cu and Au, or Wherein the source material (10) is selected from the group consisting of: metal nitrides and metal oxides.

19. The method according to claim 18, wherein the step of calibrating the mass spectrometer (1) comprises: Based on the signal intensity (S) of the detector (12) when detecting the ionized calibration gas (7) k ) and based on a pressure (p) detected by at least one pressure sensor when the calibration gas (7) is supplied to the ionization region (5) c ) to determine the sensitivity of the mass spectrometer (1) (K k ).

20. The method according to claim 18 or 19, further comprising: Before and / or after supplying the calibration gas (7) to the ionization region (5): A surface (3a) of a vacuum component (3) in a mass spectrometer (1) is coated with a getter material (17) for a source material (10).

21. The method of claim 18, wherein the source material (10) is selected from the group consisting of nitrides and oxides of tantalum, vanadium, tungsten, rhenium, and yttrium.

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