IMR-MS device

Abstract

The present invention relates to an apparatus and a method for IMR-MS and/or PTR-MS, comprising a sample gas inlet (202, 206), a first ion source (209), a reaction chamber (203), a mass analyzer (204), wherein the reaction chamber (203) and the mass analyzer (204) are arranged along a central axis (A), characterized by a second ion source (209), wherein the sample gas inlet (202, 206) is arranged to introduce gas essentially along the central axis (A) and is connected to the reaction chamber (203); wherein the first ion source (209) and the second ion source (209) are arranged so as to emit reagent ions essentially perpendicularly to the central axis (A); said apparatus further comprising at least one electrode (302, 303, 304, 305), such that the reagent ions emitted from the first or second ion source (209) can be deflected into the reaction chamber (203) essentially in the downstream direction of the central axis (A).

Claims

1. An apparatus for Ion-Molecule-Reaction Mass Spectrometry and/or Proton-Transfer-Reaction Mass Spectrometry, comprising: a sample gas inlet; an ion source section; a first reagent ion source; a second reagent ion source; a reaction chamber; and a mass analyzer, wherein the reaction chamber and the mass analyzer are arranged along a central axis, wherein the sample gas inlet is arranged to introduce gas essentially along the central axis into the ion source section that is connected to the reaction chamber, wherein the reaction chamber is placed downstream and essentially adjacent to the ion source section, wherein the first reagent ion source and the second reagent ion source are arranged essentially in a plane in front of the reaction chamber so as to emit reagent ions essentially perpendicularly to the central axis into the ion source section, said apparatus further comprising at least one electrode, such that the reagent ions emitted from the first or second reagent ion source into the ion source section can be deflected into the reaction chamber essentially in the downstream direction of the central axis, wherein each reagent ion source comprises at least one electrode with which the injection of reagent ions into the ion source section and/or rejection back into the respective reagent ion source is feasible, wherein said electrodes are connected to a switching device so that emitted reagent ions from one reagent ion source are deflected into the central axis while reagent ions from any other reagent ion source are prevented from entering the reaction chamber.

2. The apparatus according to claim 1, wherein at least one further reagent ion source is arranged so as to emit reagent ions essentially perpendicularly to the central axis.

3. The apparatus according to claim 1, wherein the reagent ion sources are arranged essentially in a plane which is essentially perpendicular to the central axis of the reaction chamber.

4. The apparatus according to claim 2, wherein each one of the three reagent ion sources is capable of producing one type of ions out of the group of H3O+, O2+ and NO+, wherein each one of the three reagent ion sources is configured to produce a different type of ions.

5. The apparatus according to claim 1, wherein the apparatus comprises a sample inlet bypass line, wherein the sample inlet bypass line is arranged essentially perpendicular to the sample gas inlet.

6. The apparatus according to claim 5, wherein the apparatus comprises a gas line, wherein the gas line is arranged essentially perpendicular to the sample gas inlet.

7. The apparatus according to claim 1, wherein at least one electrode is positioned opposite to each reagent ion source and/or at least one electrode is positioned upstream in the vicinity of the sample gas inlet and one electrode is positioned downstream in the vicinity of the reaction chamber.

8. The apparatus according to claim 7, wherein each reagent ion source comprises at least one electrode at its exit.

9. A method to operate an apparatus for Ion-Molecule-Reaction Mass Spectrometry and/or Proton-Transfer-Reaction Mass Spectrometry according to claim 8, comprising the following steps: introducing sample gas into the ion source section via the sample gas inlet in the direction of the central axis of the reaction chamber; continuously generating at least two different reagent ions in the at least two reagent ion sources, wherein in one respective reagent ion source one specific type of reagent ions is produced; applying a voltage to the at least one electrode at the exit of each reagent ion source, wherein a certain value of voltage leads to injection of the respective reagent ions into the ion source section and another certain value of the voltage leads to rejection back into the respective reagent ion source; applying a voltage to the at least one electrode positioned upstream in the vicinity of the sample gas inlet, wherein this electrode causes a repulsive force onto the reagent ions, and applying a voltage to the at least one electrode positioned downstream in the vicinity of the reaction chamber, wherein this electrode causes an attracting force onto the reagent ions, such that the reagent ions are injected into the reaction chamber essentially in the downstream direction of the central axis (A) of the reaction chamber; introducing the reagent ions and the sample gas into the reaction chamber, wherein subsequent to the reaction chamber the ions are analyzed with the mass analyzer.

10. The method according to claim 9, the method comprising applying a voltage to the at least one electrode opposite to the at least two ion sources, said electrode causing a repulsive force onto the reagent ions.

11. The method according to claim 9, characterized in that a controlling device controls the voltages applied to the electrodes, wherein the controlling device controls which kind of the reagent ions generated in the respective ion source will be injected into the ion source section and/or which kind of reagent ions generated in the respective reagent ion source will be rejected back into the respective ion source.

12. The apparatus according to claim 1, wherein the apparatus comprises a controlling device.

13. A method to operate an apparatus for Ion-Molecule-Reaction Mass Spectrometry and/or Proton-Transfer-Reaction Mass Spectrometry according to claim 10 comprising the following steps: introducing sample gas into the ion source section via the sample gas inlet in the direction of the central axis of the reaction chamber; continuously generating at least two different reagent ions in the at least two ion sources, wherein in one respective ion source one specific type of reagent ions is produced; applying a voltage to the at least one electrode at the exit of each ion source, wherein a certain value of voltage leads to injection of the respective reagent ions into the ion source section and another certain value of the voltage leads to rejection back into the respective ion source; applying a voltage to the at least one electrode positioned upstream in the vicinity of the sample gas inlet, wherein this electrode causes a repulsive force onto the reagent ions, and applying a voltage to the at least one electrode positioned downstream in the vicinity of the reaction chamber, wherein this electrode causes an attracting force onto the reagent ions, such that the reagent ions are injected into the reaction chamber essentially in the downstream direction of the central axis of the reaction chamber; introducing the reagent ions and the sample gas into the reaction chamber, wherein subsequent to the reaction chamber the ions are analyzed with the mass analyzer, wherein a controlling device controls the voltages applied to the electrodes, wherein the controlling device controls which kind of the reagent ions generated in the respective ion source will be injected into the ion source section and/or which kind of reagent ions generated in the respective ion source will be rejected back into the respective ion source, wherein the controlling device adapts the parameters of the apparatus, such that the apparatus on demand acts as an Atmospheric Pressure interface Mass Spectrometer, wherein the parameters comprise at least the voltages applied to the electrodes, wherein the controlling device controls that the ions generated in the ion sources are rejected back into the respective ion sources or the ion sources are switched off by the controlling device such that no reagent ions are generated.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a schematic view of the prior art of IMR/PTR-MS instruments.

(2) FIG. 2 shows a schematic view of an exemplary IMR/PTR-MS instrument according to the present invention.

(3) FIG. 3 shows a schematic view of an exemplary reagent ion source arrangement with three ion sources.

(4) FIG. 4 shows a schematic view of the ion source section according to the present invention.

(5) FIG. 5 shows a schematic illustration of an embodiment where one reagent ion source is set to inject reagent ions into the ion source section and one reagent ion source is set to block ions from entering the ion source section.

(6) FIG. 6 shows a schematic view of a prototype built according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) FIG. 1 is a schematic overview of the prior art, with a reagent ion source comprising a first part 101 and a second part 102, a drift tube 103, a mass analyzer 104 and a sample inlet 105. Water vapor originating from a reservoir filled with purified water enters the first part 101 of the ion source. In the first part 101 a hollow cathode discharge converts H.sub.2O molecules into a series of product ions: H.sub.2O.sup.+, H.sup.+, H.sub.2.sup.+, OH.sup.+ and O.sup.+. As eventually highly pure H.sub.3O.sup.+ reagent ions are needed in the second part 102 of the ion source also called “source drift” region the product ions from the first part 101 and the water vapor, which are both transferred into the second part 102 via gas flow and/or electric fields, undergo various ion-molecule reactions which lead to H.sub.3O.sup.+ purity levels of 99% and above. The underlying ion-chemistry has been discussed in detail in literature. Subsequently, the H.sub.3O.sup.+ reagent ions are injected into drift tube 103, where they can interact with the gas containing the analytes (e.g. air with trace compounds) introduced into the drift tube via sample inlet 105.

(8) It has been shown that this ion source design can also be used for the production of very pure reagent ions apart from H.sub.3O.sup.+, e.g. NO.sup.+, O.sub.2.sup.+, Kr.sup.+, Xe.sup.+, NH.sub.4.sup.+, OH.sup.−, etc. by switching the source gas and adjusting the currents and voltages applied to and the pressure in the ion source.

(9) The invention relates to a front end of an IMR/PTR-MS instrument, denoted 201 in FIG. 2. Sample gas is drawn into the instrument via a sample gas inlet 202, 206 (also termed as sample inlet), which is essentially in direction of the central axis A of the reaction chamber 203 to the mass analyzer 204. A sample inlet bypass line 205 allows for regulating the sample inlet flow while keeping the flow into the reaction chamber 203 constant. That is, a vacuum pump (membrane pump, scroll pump, multi-stage turbomolecular pump, etc.) is connected to the sample inlet bypass line 205.

(10) In a preferred embodiment a mass flow controller, valve or similar device which allows for regulating the air flow is installed between the sample inlet bypass line 205 and the vacuum pump. If this flow regulating device is completely closed so that no suction is created via sample inlet bypass 205 only a minimum amount of gas is sampled (the sample gas flow which enters the reaction chamber 203). If the flow regulating device is opened, virtually any sample inlet flow higher than the minimum can be set. Typically IMR/PTR-MS reaction chambers are operated between 0.1 and 100 hPa, preferably between 1 and 10 hPa. This vacuum is usually maintained by one or more vacuum pumps connected to one or more pumping ports of the reaction chamber 203 and determines the minimum gas flow needed for operating the instrument, i.e. the pressure in 203 results from the gas entering and being pumped away. In line of central axis A is also the second part of the sample gas inlet 206. This part, which is adjacent to the connection point of the sample inlet bypass 205, preferably has a smaller inner diameter than the first part of the sample gas inlet 202, so that the gas flow through 206 is restricted.

(11) Gas line 207 is connected to the second part of the sample gas inlet 206 and to a vacuum pump, which can be the same vacuum pump that is connected to 205 or an additional pump of the same or different type. Preferably a pressure controller, a valve or any gas flow regulating device is interconnected between gas line 207 and the vacuum pump. Via the gas flow/suction through gas line 207 the pressure in the reaction chamber 203 (which itself is evacuated by a vacuum pump) can be regulated.

(12) The sample gas flows into the ion source section 208. This section can be part of the reaction chamber 203 or a separated section. At least two reagent ion sources 209 are mounted in this section essentially perpendicular to axis A, i.e. in plane B. The ion sources 209 can be any IMR/PTR-MS reagent ion sources (e.g. point discharge, plane electrode discharge, microwave discharge, radioactive, etc.). In a preferred embodiment the ion sources 209 are hollow cathode ion sources.

(13) FIG. 3 shows a schematic view of an exemplary embodiment in direction of central axis A. Here, three reagent ion sources 209 are mounted at 0°, 120° and 240° (angular degrees) in plane B, which is essentially perpendicular to axis A. In this exemplary embodiment the ion source section 208 has a circular cross section. Any other cross section is also possible, e.g. triangular, rectangular, polygonal, elliptic, any combination of curved and/or straight forms. Any positions of the ion sources 209 are possible, e.g. opposite of each other or at any angle to each other. The number of ion sources 209 has to be at least two.

(14) FIG. 4 shows a schematic view of an exemplary embodiment of the ion source section 208. In this figure the sample gas is introduced from the left via orifice 301 (connected to sample gas inlet 206, not shown here). A DC voltage can be applied to electrode 302. At position 303 is at least one electrode with an orifice (ion lens) where a DC voltage can be applied. By applying an appropriate voltage to electrode 303 the reagent ions generated in the reagent ion source 209 can be injected into the ion source section 208 or rejected, so that they do not enter the ion source section 208. Electrode 304 can either be the first electrode of the IMR/PTR-MS drift tube or an ion lens for injecting reagent ions into the reaction chamber. 305 is an electrode opposite of the reagent ion source, which can e.g. be a metal plate or the electrically conducting inner housing of the ion source section 208. In some embodiments no electrode 305 is present. In a preferred embodiment the function of electrode 305 is substituted or supplemented by electrode 303 of the at least one additional ion source 209 other than the ion source 209 currently injecting reagent ions. By applying appropriate electric potentials/voltages to 302, 303, 304 and 305 reagent ions are guided into the IMR/PTR-MS reaction chamber as schematically indicated by arrow 306.

(15) If according to the present invention at least two reagent ion sources 209 are installed in the ion source section 208, reagent ions can be selected by simply changing the voltages applied to the electrodes. Preferably the voltages that are changed are the voltages of the ion source exit lenses 303.

(16) In FIG. 5 two reagent ion sources are schematically shown, where the reagent ions of one ion source are rejected (do not enter the ion source section and eventually the IMR/PTR-MS reaction chamber) and the reagent ions produced by another ion source are injected into the ion source section and eventually the IMR/PTR-MS reaction chamber. That is, if e.g. one reagent ion source continuously produces one type of reagent ions and a second reagent ion source continuously produces another type of reagent ions, the reagent ions used for chemical ionization in the IMR/PTR-MS reaction chamber can be very rapidly switched by simply changing electrode voltages. In stark contrast to existing designs, no time consuming source gas switching or pressure adjustments have to be performed. It has been shown that the reagent ion yields entering the reaction chamber are of comparable intensity to reagent ion yields of a common single ion source in-line with the reaction chamber's central axis, i.e. intensity losses due to the perpendicular position are negligible.

(17) In a preferred embodiment at least three reagent ion sources are installed, which continuously produce at least H.sub.3O.sup.+, NO.sup.+ and O.sub.2.sup.+, respectively.

(18) In another embodiment at least four reagent ion sources are installed, which continuously produce at least H.sub.3O.sup.+, NH.sub.4+, NO.sup.+ and O.sub.2.sup.+, respectively.

(19) Obviously, if for a prolonged measurement it is foreseeable that switching of the reagent ions will not be required, i.e. only one type of reagent ions is necessary, all reagent ion sources except for the one producing the required reagent ions can be turned off in order to save source gas and prevent wear effects. However, in this case no rapid switching is possible.

(20) In another embodiment all reagent ion sources are turned off or set so that they do not inject reagent ions into the reaction region. In this mode of operation the novel front end acts as an APi (Atmospheric-Pressure interface) device and atmospheric ions can be analyzed with the IMR/PTR-MS instrument. In this case no chemical ionization reactions between reagent ions and the sample gas take place within the reaction region, but the reaction chamber only transports the atmospheric ions to the mass analyzer. Again, only electric potentials and voltages have to be changed to enable this mode of operation so that the atmospheric ions are guided into the reaction region and subsequently into the mass analyzer.

Exemplary Embodiment

(21) A schematic view of a prototype embodiment, which only acts as an example and should by no means limit the invention to this embodiment, is displayed in FIG. 6.

(22) In an inlet block 401 an opening 402 is drilled in direction of the central axis A. This opening 402 is equipped with a thread to screw in a 1/16 inch sample inlet line (e.g. made of PEEK (PolyEther Ether Ketone), passivated stainless steel, PTFE, etc.) with 1 mm inner diameter. An opening 403 is drilled perpendicular to opening 402 and connected to a membrane vacuum pump via a mass flow controller. By adjusting the mass flow controller the amount of gas containing the analytes sampled by the instrument can be adjusted from the minimum which is needed for operation of the IMR/PTR-MS instrument (between 10 and 100 cm.sup.3/min at standard conditions for the prototype instrument) to the maximum pumping power of the membrane vacuum pump. Adjacent to the drilling 402 there is a drilling 404 with a smaller diameter in direction of the central axis A. Perpendicular to drilling 404 there is a drilling 405 which is connected to the same vacuum pump as drilling 403, but with an interconnected pressure controller instead of a mass flow controller. By adjusting this pressure controller the (resulting) pressure in the IMR/PTR-MS reaction chamber can be adjusted. The second part of the inlet line 404 ends in an electrode/ion lens 406 at the beginning of the ion source section 407.

(23) Three hollow cathode reagent ion sources 408, 409 and 410 (the latter two are only indicated in the schematic view) are mounted perpendicular to central axis A in plane B at 120° offset angle, respectively (compare FIG. 3). All three reagent ion sources are of the same design: A first ionization chamber 411 and a second ionization chamber 412. Both ionization chambers are essentially made of a conductive material. Preferably the conductive material is stainless steel, such as stainless steel type EN 1.4301, 1.4405 or 1.4407. One or more source gases are introduced via mass flow controllers into the first chamber 411, where a hollow cathode discharge ionizes the source gas. Typical source gases are H.sub.2O vapor for H.sub.3O.sup.+, O.sub.2 for O.sub.2.sup.+, a mixture of N.sub.2 and O.sub.2 for NO.sup.+ and a mixture of N.sub.2 and H.sub.2O vapor for NH.sub.4.sup.+ production. Ions and neutrals subsequently enter a second ionization chamber 412 where they react via ion-molecule reactions and highly pure reagent ions of one particular type are formed. Ionization chamber 412 is connected to a vacuum pump via an electronically controlled (proportional) valve so that the pressure can be regulated. Electric fields can be applied to ionization chambers 411 and 412 in order to control the hollow cathode discharge, transport the ions and control the ion-molecule reactions. At least one ion lens at the exit of chamber 412 into the ion source section 407 enables blocking (rejecting) or transmitting the ions.

(24) Each of the three reagent ion sources 408, 409 and 410 continuously produces one particular type of reagent ions. However, only the voltage applied to the exit ion lens of one ion source is set so that these reagent ions can enter the ion source section 407. The remaining two reagent ion types are hindered from entering the ion source section 407 by the voltage applied to the exit ion lenses. After entering the ion source section 407 the reagent ions are drawn in direction of central axis A by electric fields. These fields are e.g. created by electrode 406 and the ring electrodes of the reaction chamber 413.

(25) Additionally, the electrodes at the exits of chambers 412 can create electric fields which penetrate to some extent into the ion source section 407. That is, at the exit electrodes of those reagent ion sources which are set so that they do not inject reagent ions into the ion source section 407 a repulsive electric potential is applied to. This repulsive potential hinders reagent ions from exiting the respective ion sources.

(26) On the other hand, it pushes reagent ions originating from another reagent ion source towards central axis A. In other words the reagent ions which are injected into the ion source section 407 from one “active” reagent ion source 408 are repelled by electrode 406 and the exit electrodes of the other two “inactive” reagent ion sources and drawn into the reaction chamber 413. For example, if the reagent ion source at 0° (angular degrees) produces H.sub.3O.sup.+ reagent ions and injects these reagent ions via acceleration due to at least one electrode at the exit of chamber 412 into the ion source section 407, the exit electrodes of the remaining two reagent ion sources at 120° and 240° are set so that they do not inject reagent ions into the ion source section 407. The repulsive electric field created by these two electrodes helps to keep the H.sub.3O.sup.+ reagent ions originating from the reagent ion source at 0° close to central axis A, i.e. force them on a flight path close to the one indicated by arrow 306 in FIG. 4.

(27) The reaction chamber 413 in this exemplary embodiment comprises a series of ring electrodes with constant orifice diameters 414 and an adjacent series of ring electrodes with decreasing orifice diameters 415. DC voltages are applied across electrodes 414 and 415 so they act as an IMR/PTR-MS drift tube. Additionally applied RF voltages allow for focusing the ions and thus prevent ion losses. The reaction chamber is evacuated to between 1 and 10 hPa by a vacuum pump connected to a pumping port in 413. Eventually, the ions are transferred into region 416, which represents a differential pumping region, a TOF mass analyzer and a microchannel plate detector.

(28) With the present invention sample gas can be introduced in-line with the central axis of the reaction chamber of an IMR/PTR-MS instrument. Although the inlet gas flow as well as the pressure in the reaction chamber can be fully controlled, the sample gas does not pass any valve, mass flow controller or similar device on its way into the reaction chamber. Moreover, there are no bendings or kinks the sample gas has to pass, but it can directly enter the reaction chamber. This greatly improves the instrument's response and decay time because wall-effects, such as sample-wall interactions, are suppressed. In combination with the possibility of extremely rapid reagent ion switching, because of at least two reagent ion sources simultaneously producing reagent ions, an IMR/PTR-MS instrument according to the present invention is much faster, more selective and sensitive compared to existing designs.

(29) By switching off all reagent ion sources or setting them so that none of them injects reagent ions into the reaction region, the front end according to the present innovation acts as an APi. This is only possible because the sample gas inlet design suppresses contact between the sample gas and walls, which would inevitably lead to the loss of (atmospheric) ions. Thus, an instrument equipped with this front end is extremely cost efficient as two types of instrumentation are combined in one: APi-MS and IMR/PTR-MS.