COLLISION CELL HAVING AN AXIAL FIELD

Abstract

The present invention addresses ways to facilitate the detection and analysis of ion abundance, in particular for analysis of elemental ions, and in particular embodiments for isotope ratio analysis, by use of collision cells that employ an axial drag field, i.e. an axial electric field that exerts a drag force on ions within the cell. By means of the invention, the drag field allows an increase in the transmission in the case of Li from a few % up to almost 100%. The drag field is generated by electric fields and can be switched on and off within microsecond (μs) timescales and thus improves the sensitivity for the lighter elements dramatically. The invention allows use of collision cells for analysis of elemental ions in a simple and fast workflow with high throughput and without compromising transmission.

Claims

1. A method of increasing sensitivity of an elemental mass analysis in a mass spectrometer, the method comprising: i. providing an ion beam comprising at least one elemental ion into a multipole reaction cell; ii. applying an axial electric field gradient in the reaction cell, wherein the axial electric field gradient can be adjusted so that during a mass analysis, a first element is analysed with a first setting of the axial electric field gradient in the reaction cell and a second element is analysed using a second setting of the axial electric field gradient in the reaction cell; and iii. analyzing an ion abundance or isotope ratio of the at least one elemental ion transmitted through the multipole reaction cell using a multicollector, wherein prior to the transmission of ions through the multipole reaction cell, the reaction cell is filled with at least one reaction gas.

2. The method of claim 1, wherein the elemental ions comprise elemental ions that have an atomic mass similar to the molecular mass of the reaction gas.

3. The method of claim 1, wherein the elemental ions comprise elemental ions that have an atomic mass that is the same as or less than the atomic or molecular mass of the reaction gas.

4. The method of claim 1, wherein the at least one elemental ion has an atomic mass of less than 40 amu, preferably less than 30 amu, more preferably less than 20 amu.

5. The method of claim 1, wherein the isotope ratio is determined using a multicollector sector mass analyzer.

6. The method of claim 1, wherein the reaction gas is selected from H.sub.2, O.sub.2, NH.sub.3, and SO.sub.2.

7. The method of claim 1, wherein the reaction gas is provided into the reaction cell at a flow rate of 0.5 to 10 mL/min, preferably 1 to 8 mL/min, more preferably 2 to 6 mL/min.

8. The method of claim 1, wherein an energy spread of the at least one elemental ion after transmission through the reaction cell is reduced compared to an energy spread of the ion generated by an ion source by at least about 50.

9. The method of claim 1, wherein the energy spread of the at least one elemental ion after transmission through the reaction cell is less than 1 eV.

10. A mass spectrometer for mass analysis of elements in a sample, comprising a. at least one ion source, for generating an ion beam from a sample, the ion beam comprising elemental ions and optionally molecular ions that interfere with elemental ions in a mass spectrum; b. at least one reaction cell arranged downstream of the ion source, the reaction cell having an internal volume through which ions travelling in an axial direction from the ion source are transmitted, wherein the elemental ions comprise elemental ions that have an atomic mass similar to, the same as, or less than the atomic or molecular mass of a reaction or reaction gas in the reaction cell; c. at least one sector field mass analyzer, arranged downstream from the reaction cell, d. at least one multicollector detector, for detecting ions that are analyzed in the mass analyzer, wherein the collision cell is configured to provide an axial electric field in the volume, wherein the axial electric field improves the transmission of light elemental ions through the collision cell relative to heavier elemental ions.

11. The mass spectrometer of claim 10, wherein the mass analyzer is a double-focusing sector field mass analyzer.

12. The mass spectrometer of claim 10, wherein the axial field has a gradient in the range of about 0.02 V/cm to about 4 V/cm.

13. The mass spectrometer of claim 10, wherein the mass analyzer is a single-focusing sector field mass analyzer.

14. The mass spectrometer of claim 10, wherein the reaction cell comprises at least one multipole ion guide.

15. The mass spectrometer of claim 14, wherein the multipole comprises a plurality of rod electrodes configured to be supplied with RF voltage, wherein the rods are arranged according to at least one of the following arrangements to provide an axial electric field gradient: (i) at least some of the rods are slanted along the axial direction, (ii) the rods are each provided as a plurality of segments spaced along the axial direction wherein stepped voltages, are applied to the segments, (iii) the rods have a resistive coating or comprises a resistive material, (iv) at least one of the rods are tapered along the axial direction.

16. The mass spectrometer of claim 10, wherein the reaction cell comprises at least one auxiliary electrode disposed to create an axial field within the volume of the reaction cell.

17. The mass spectrometer of claim 10, wherein the reaction cell is configured to provide an axial field along all of, or a portion of, the internal volume.

18. The mass spectrometer of claim 10, wherein the ion source is selected from: an inductively coupled plasma (ICP) ion source and secondary ion mass spectrometry (SIMS) ion source.

19. The mass spectrometer of claim 10, further comprising at least one mass filter, arranged upstream from the reaction cell and downstream from the ion source.

20. The mass spectrometer of claim 10, further comprising at least one electrostatic lens, for selectively and alternately transmitting or reflecting the ion beam, wherein the electrostatic lens is preferably arranged between the mass filter and the reaction cell.

21. The mass spectrometer of claim 10, wherein the axial electric field has a monotonically progressive electric field gradient in the reaction cell.

22. The mass spectrometer of claim 10, wherein the reaction cell comprises a plurality of auxiliary electrodes that are arranged between adjacent rods in the multipole ion guide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0088] The skilled person will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way.

[0089] FIG. 1 shows a schematic overview of a mass spectrometer employing a collision cell configured to generate a drag field, in accordance with the invention.

[0090] FIG. 2 shows an example of increased sensitivity of Li detection, by employing a drag cell during elemental analysis of Li.

DESCRIPTION OF VARIOUS EMBODIMENTS

[0091] In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples are provided to provide further understanding of the invention, without limiting its scope.

[0092] In the following description, a series of steps are described. The skilled person will appreciate that unless required by the context, the order of steps is not critical for the resulting configuration and its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps.

[0093] It should be appreciated that the invention is applicable for elemental and isotope analysis of solid liquid or gaseous samples in general by mass spectrometry techniques. In general, therefore, the sample that is being analyzed in the system will be variable. Further, the system and method according to the invention is illustrated in the embodiments that follow with a preferred embodiment of a mass spectrometer for determining isotope ratio.

[0094] Referring to FIG. 1, there is schematically shown an example of a mass spectrometer that includes a collision cell according to the invention. Here, a double focusing multicollector inductively coupled plasma mass spectrometer (ICP-MS) is shown. The instrument has an ICP source 10 and a deflection lens 20 arranged upstream from a quadrupole mass filter 30 and a dual-mode reflection lens 40 that is optional, but when present is preferably arranged between the mass filter and the collision cell 50. The optional reflection lens can be used for transiently diverting the incoming ion beam to a supplementary analysis section of the instrument, which can for example be an electron multiplier detector. By inclusion of the lens and such a detector, the first mass filter can be used to obtain a full mass spectrum of the ion beam, and during such analysis the lens is set to reflect the ion beam that is transmitted by the mass filter into the detector.

[0095] An advantage of the reflection lens is the placement of the detector 45 for detecting ions that are reflected in the lens 40, adjacent to the quadrupole. The pressure in this region of the instrument is significantly lower (i.e., higher vacuum) than within the chamber surrounding the downstream collision cell, or near the upstream ICP source. As a consequence, noise at the off-axis detector 45 can be kept to a minimum, leading to improved signal-to-noise when the mass filter is set to obtain a full mass spectrum, and the reflection lens configured to reflect transmitted ions into the detector.

[0096] The collision cell 50 comprises a quadrupole assembly that is configured to generate a drag field. For example, the collision cell can include auxiliary electrodes that generate an axial field in the cell. The quadrupole collision cell can also include modified rods for generating axial fields, as described herein.

[0097] Elemental ions are generated in the ICP source from a sample, including for example low mass ions such as Li.sup.+ ions. The ions are transmitted through the mass filter, which is set to transmit ions in a selected mass range that includes the elements that are being analysed. The mass filter can preferably be set to transmit ions in a mass range that does not include the mass of plasma Ar.sup.+ isotopes, where argon is used to generate the plasma. This is expected to lead to reduced interference in the collision cell and hence improved sensitivity. The ions are transmitted through the electrostatic lens 40, when present, and into the collision cell 50, which is flooded with a collision gas such as He. The axial field in the collision cell increases the speed of transmissions of the incoming ions, in particular such that lighter elemental ions such as Li.sup.+ that would otherwise significantly lose momentum in the cell are able to travel through the collision cell with reduced energy loss. From the collision cell, the ions are transmitted into the downstream dual sector multicollector instrument, where their isotope composition is determined, such as .sup.6Li.sup.+ and .sup.7Li.sup.+ in the case of lithium, and .sup.10B.sup.+ and .sup.11B.sup.+ in the case of boron.

[0098] Downstream from the collision cell is a mass analyser that comprises an electric sector 60 and a magnetic sector 70, followed by a multicollector detector assembly 80.

[0099] Collision cells in current mass spectrometers are used to remove or attenuate interferences by kinetic energy discrimination, fragmentation, resonant charge transfer or mass shifting by ion-molecule reactions. Ideally one would like to work with one instrumental configuration for all elements of the periodic table. One problem that arises when analysing lighter elements like Li and B is that just a few collisions with the collision gas, which in most cases is He, can result in a complete energy loss of the elemental ions. As a result, these lighter ions essentially come to rest, and are trapped inside the collision cell and therefore not detected. This is why the sensitivity for light element detection is severely hampered by collision cells when they are filled with gas.

[0100] In current instruments, this can only be circumvented when the gas in the collision cell is pumped away, for light element analysis. However, this is time consuming and reduces throughput of the instrument. The present invention provides a solution to this problem, since the loss of sensitivity for lighter elements can be significantly reduced by the use of a collision cell with an axial drag field.

[0101] Exemplary data that illustrate the advantage of using a collision cell with a drag field in the analysis of Li is shown in FIG. 2. For this analysis, a drag field high energy collision dissociation (HCD) cell, as used on the Q Exactive mass spectrometer (Thermo Scientific), was installed in an instrument configured as shown in FIG. 1. He is used as the collision gas.

[0102] The light dots (bottom curve) show the decrease in Li ion transmission through the collision cell without any axial drag field as a function of He flow rate. With increasing He flow rate the transmission drops significantly, and is as low as 10% at 9 ml/min He flow. This flow rate is about an average He flow rate that is needed to achieve collisional focusing for mid mass and high mass elements. Applying a drag field in the cell significantly improved Li sensitivity, and the increase in sensitivity is proportional to the applied field. Thus, as an increased field is applied, there is a peak in the transmission curve that is obtained with increased He flow rate, and the curve peak shifts to the right, towards a higher He flow rate, as the drag field is increased. One can thus see that when applying drag voltages in the range of 100 V at 9 ml/min He flow the transmission of Li is greater than 60%. This means that the sensitivity of Li detection, compared with no drag field, is improved by more than a factor of 6. Comparable improvement in detection can be obtained for other lighter elements, such as boron.

[0103] Although the improved transmission and detection for elemental analysis has been illustrated by the particular instrument configuration and for Li analysis as shown by the data in FIG. 2, it should be appreciated that the advantage of using a drag collision cell for elemental analysis can be realized in a variety of instrument setups. Thus, the concept of using an accelerating drag field within the collision cell for improving transmission and detection of elemental ions, in particular light element ions, can be implemented in mass spectrometers that are suitable for such analysis.

[0104] Another advantage of using the drag cell is that due to multiple collisions in the cell the energy spread of the ions is reduced much more effectively than without the axial drag field. The smaller energy spread because of more collisions is expected to result in improved abundance sensitivity in the mass spectrum, in particular in the mass range of the actinides. In nuclear applications for instance the accurate detection of the .sup.236U peak is critical as it serves as an indicator of whether the nuclear material has been processed in a reactor or not. However, during conventional isotope analysis, the peak tail of the major .sup.238U isotope interferes to some extent on the .sup.236U peak. The long peak tail of the .sup.238U peak is caused by the scattering of the .sup.238U ion beam with apertures or residual gas particles in the sector analyzer that results in a small energy loss and change in direction. To detect minor traces of .sup.236U the ion beam is therefore usually passed through an RPQ energy filter lens in order to discriminate against scattered .sup.238U ions. The RPQ energy filter lens sits behind the axial detector slit of the multicollector detector and rejects all ions below a certain energy level and therefore acts a high pass filter to discriminate against scattered .sup.238U ions which have suffered some energy loss. Applying an axial drag field in the collision cell is expected to eliminate the need for such analysis, resulting in simplified instrument configuration and increased sensitivity during isotope analysis.

[0105] Without cooling, the energy distribution of the ions generated in the ICP source using a balanced coil or a shielded torch is in the range of a few eV. With collisional cooling and in particular with using the drag cell which allows for even more collisions, the energy distribution of the ions can be significantly reduced and thus the energy distribution of the ions becomes much sharper and the filter action of the energy filter can be much more specific. For thermal ionization ion sources the energy spread of the generated ions is less than 1 eV and the abundance sensitivity achieved with the energy filter lens is about 10 times better compared to ICP instruments. With improved collisional cooling using a drag cell, the abundance sensitivity can be improved for an ICP source as well.

[0106] The drag cell allows for more collisions inside the collision cell. More collisions means also more interactions with the gas inside the reaction cell. Therefore the drag cell allows the collision cell to be operated with significantly larger gas pressures as can be appreciated from the data shown in FIG. 2. For reactions that have a rather low reaction rate, overall reaction yield can be significantly increased by using a drag cell because of larger gas pressure and as a consequence more interactions between incoming ions and reaction gas. In general, if the gas pressure is increased, a concomitant increase in the reaction yield is expected. As a consequence, the drag cell can also be used for chemistry within the collision cell, for example for mass shift reactions within the cell, with improved reaction yields.

[0107] In summary, the present invention provides numerous advantages, including: [0108] a. improved sensitivity for lighter elements in collision mode; [0109] b. universal operation mode for all elements without compromises in sensitivity reduction for the lighter elements; this is important for quadrupole mass analyzer instruments; [0110] c. reduced energy spread of ions and thus improved abundance sensitivity for large dynamic range isotope ratio analysis; [0111] d. improved energy spread to enhance higher mass resolution; [0112] e. operation at higher gas pressures to allow for higher yield on gas phase reactions inside the collision cell.

[0113] As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.

[0114] Throughout the description and claims, the terms “comprise”, “including”, “having”, and “contain” and their variations should be understood as meaning “including but not limited to”, and are not intended to exclude other components.

[0115] The present invention also covers the exact terms, features, values and ranges etc. in case these terms, features, values and ranges etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., “about 3” shall also cover exactly 3 or “substantially constant” shall also cover exactly constant).

[0116] The term “at least one” should be understood as meaning “one or more”, and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with “at least one” have the same meaning, both when the feature is referred to as “the” and “the at least one”.

[0117] It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention can be made while still falling within scope of the invention. Features disclosed in the specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.

[0118] Use of exemplary language, such as “for instance”, “such as”, “for example” and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless so claimed. Any steps described in the specification may be performed in any order or simultaneously, unless the context clearly indicates otherwise.

[0119] All of the features and/or steps disclosed in the specification can be combined in any combination, except for combinations where at least some of the features and/or steps are mutually exclusive. In particular, preferred features of the invention are applicable to all aspects of the invention and may be used in any combination.