Methods and systems of treating a particle beam and performing mass spectroscopy

Abstract

A method of treating a particle beam is disclosed, of interest in particular for mass spectrometry for .sup.14C. A particle beam including positive ions is passed through a charge exchange cell containing a target gas. The target gas is electrically insulating at room temperature and pressure. At least some of the positive ions of the particle beam are converted to negative ions by interaction with the target gas. The particle beam incident at the charge exchange cell includes molecules and/or molecular ions which interact with the target gas to reduce the concentration of molecules as a result of repeated collisions with particles of the target gas. A corresponding mass spectrometry system is also disclosed.

Claims

1. A method of treating a particle beam, the particle beam including positive ions, including the step of passing the particle beam through a charge exchange cell, the charge exchange cell containing a gaseous target material, the gaseous target material being a material that is electrically insulating at room temperature and pressure, at least some of the positive ions of the particle beam being converted to negative ions by interaction with the gaseous target material, the particle beam incident at the charge exchange cell further including molecules and/or molecular ions which interact with the same gaseous target material in the same charge exchange cell to reduce the concentration of molecules as a result of repeated collisions with particles of the gaseous target material thereby to provide a treated particle beam, wherein the negative ions are selected from the treated particle beam for subsequent analysis.

2. The method according to claim 1 wherein the gaseous target material includes a component that is matched in terms of atomic weight to a species in the particle beam to be detected.

3. The method according to claim 1 wherein the gaseous target material used in the charge exchange cell includes at least one of hydrogen, helium, nitrogen, argon, methane, butane, ethane, isobutane and propane, or a mixture thereof.

4. The method according to claim 1 wherein the gaseous target material is energetically-pumped.

5. A method for performing mass spectrometry on an analyte sample including the steps of: generating a particle beam using the analyte sample, the particle beam including positive ions; passing the particle beam through a charge exchange cell, the charge exchange cell containing a gaseous target material, the gaseous target material being a material that is electrically insulating at room temperature and pressure, at least some of the positive ions of the particle beam being converted to negative ions by interaction with the gaseous target material, the particle beam incident at the charge exchange cell further including molecules and/or molecular ions which interact with the same gaseous target material in the same charge exchange cell to reduce the concentration of molecules as a result of repeated collisions with particles of the gaseous target material thereby to provide a treated particle beam; and passing the treated particle beam to a particle detector configured to detect at least some of said negative ions.

6. The method according to claim 5 used for radiocarbon detection, wherein the beam generated from the analyte sample includes at least one of .sup.14C.sup.+, .sup.14C.sup.2+, and .sup.14C.sup.3+.

7. The method according to claim 6 wherein the treated particle beam is passed through a mass spectrometer to select .sup.14C.sup., and receiving the selected portion of the beam at the particle detector configured to detect .sup.14C.sup..

8. The method according to claim 5 wherein the incident particle beam is subjected to selection using a first mass spectrometer before reaching the charge exchange cell.

9. The method according to claim 8 wherein the incident particle beam is subjected to selection so that it consists primarily of .sup.14C.sup.2+ and incidental interferences.

10. The method according to claim 6 wherein the positive ions in the particle beam are generated using an electron cyclotron resonance (ECR) ion source.

11. The method according to claim 10 wherein the plasma in the ECR ion source is manipulated by the addition of a carrier or by addition of excess sample material, in order that the ECR ion source operates to discriminate against the production of ions of some constituents.

12. The method according to claim 11 wherein a helium carrier gas is added to suppress the production of hydrocarbon molecules where the sample is a CO.sub.2 sample.

13. The method according to claim 6 wherein, in the charge exchange cell, the gaseous target material suppresses at least one interfering species by repeated collision with the gaseous target material.

14. The method according to claim 8 wherein, following the charge exchange cell, the treated particle beam is further subjected to selection using a second mass spectrometer.

15. The method according to claim 14 wherein the selected part of the treated particle beam reaches the particle detector configured to detect at least some of said negative ions.

16. A method for performing mass spectrometry on a carbon-based analyte sample including the steps of: generating a particle beam from the analyte sample using an electron cyclotron resonance ion source operated to generate .sup.14C.sup.2+; selecting the .sup.14C.sup.2+ portion, and remaining interferences, using a first mass spectrometer; passing the particle beam through a charge exchange cell containing a gaseous target material selected from a group comprising one or more of hydrogen, helium, nitrogen, argon, methane, butane, ethane, isobutene, propane, and a mixture thereof to convert positive incident .sup.14C ions to negative ions by interaction with the gaseous target material and to suppress .sup.13CH and .sup.12CH.sub.2 interferences as a result of repeated collisions with particles of the gaseous target material in the same charge exchange cell thereby to provide a treated particle beam containing negative ions; passing the treated particle beam through a second mass spectrometer to select .sup.14C.sup.; and receiving the selected portion of the treated particle beam at the particle detector to detect .sup.14C.sup..

17. A mass spectrometry system suitable for performing mass spectrometry on an analyte sample, the system including: a particle beam generator for generating a particle beam using the analyte sample, the particle beam including positive ions; a charge exchange cell, the charge exchange cell configurable to contain a gaseous target material the gaseous target material being a material that is electrically insulating at room temperature and pressure, the charge exchange cell being operable so that at least some of the positive ions of the particle beam are converted to negative ions by interaction with the gaseous target material the charge exchange cell further being operable so that molecules and/or molecular ions present in the particle beam incident at the charge exchange cell interact with the same gaseous target material in the same charge exchange cell to reduce the concentration of molecules as a result of repeated collisions with particles of the gaseous target material, thereby to provide a treated particle beam; and a particle detector configured to detect at least some of said negative ions in said treated particle beam.

18. The mass spectrometry system according to claim 17 including mass flow gas controllers for controlling the gas formulation in the charge exchange cell at room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

(2) FIG. 1 shows a schematic of an embodiment of the present invention, used to measure radiocarbon.

(3) FIG. 2 shows a graph showing the isotope ratios achieved by different sample gas compositions and pressures.

(4) FIG. 3 shows the ratio of negative to positive ions exiting the charge exchange cell for different charge exchange media.

(5) FIG. 4 shows the ratio of negative to positive ions exiting the charge exchange cell for different charge exchange media. The right hand axis shows the variation in background measurements with charge exchange cell gas flow rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT, AND FURTHER OPTIONAL FEATURES OF THE INVENTION

(6) FIG. 1 shows a schematic of radiocarbon measurement according to an embodiment of the invention. Beginning in the electron cyclotron resonance (ECR) ion source, interferences to .sup.14C detection are increasingly suppressed until reliable radiocarbon detection is possible. In FIG. 1, the two mass spectrometers each comprise an electrostatic spherical analyser (ESA) and dipole magnet. Component electrical-biasing is not shown but by manipulating the beam energy the carbon stable isotopes can be quantified with Faraday cup detectors.

(7) The mass spectrometer components shown in FIG. 1 are given by way of example only. They may be differently ordered, added to or subtracted from, and other components such as ion velocity Wien-filters may be substituted.

(8) As is the case of conventional AMS, the .sup.14C is measured in ratio to stable .sup.12C and/or .sup.13C in the common beam from the ion source. The first spectrometer separates the radiocarbon from stable carbon ions which can then be measured as an electric current in a dedicated Faraday cup detector. The stable ions can be made to also pass through the charge-exchange cell and so also be measured free of hydrocarbon interference in dedicated Faraday cups after the second mass spectrometer by temporarily adjusting the ion energy of beam from the ion source so that the stable nuclides achieve the same rigidity as the radiocarbon ions and transmit the first mass spectrometer. The whole system is calibrated by separate measurements of the isotope ratios produced with standard sample materials of known carbon isotope ratios. Accordingly the production of ions in the ion source or in the charge-exchange cell need not be quantitative, but should preferably be consistent. Nevertheless high efficiency in these processes is desirable for expeditious sample measurement or low minimum sample size.

(9) FIG. 2 demonstrates ion source molecule suppression using stable isotopes. Positive carbon ion beams are extracted from a Pantechnik S. A. Nangon 10 GHz ECR plasma ion source newly mounted (at the time of writing) on an ion source deck of the Scottish Universities Environmental Research Centre (SUERC) bi-polar single-stage accelerator mass spectrometer (SSAMS) (Freeman et al (2008) and Freeman et al (2010)). The SUERC SSAMS is intended for routine conventional radiocarbon AMS but can also undertake positive-ion experimentation (Wilcken at al (2008)). This requires the reversal of some electrical and magnetic polarities but otherwise the spectrometer, including ion optical elements, ion detectors, data system and supporting vacuum and cooling systems, is operated similarly in either polarity. Existing sputter ion source control signals are co-opted to run the plasma ion source and the sample gas is delivered by an existing gas-handling system (Xu et al (2007)).

(10) The graph of FIG. 2 is of the .sup.13C.sup.+/.sup.12C.sup.+ ratio obtained from the first mass spectrometer (see FIG. 1) where .sup.12CH interferes with .sup.13C. It is evident that the measured .sup.13C/.sup.12C ratio can be reduced by increasing CO.sub.2 sample gas in the ion source or else by adding He carrier to increasingly remove .sup.12CH from the ion beam until the expected .sup.13C/.sup.12C ratio is reached. The same effect is employed for .sup.14C measurement in the preferred embodiment of the present invention.

(11) The preferred embodiment of the invention for sample radiocarbon measurement suppresses interference to .sup.14C detection in steps:

(12) Step 1: Partial hydrocarbon molecule suppression in an ECR ion source producing positive carbon ions in a variety of charge states from CO.sub.2 sample, optionally in the presence of He carrier gas.

(13) Step 2: Partial hydrocarbon molecule suppression by the selection of the .sup.14C.sup.2+ with a first mass spectrometer.

(14) Step 3: Suitable additional hydrocarbon molecule suppression and .sup.14N atomic isobar suppression with a thick non-metallic gas charge-exchange cell.

(15) Step 4: Resulting .sup.14C.sup. separation from molecular-fragments and remaining positive ions in the treated particle beam (exiting the charge-exchange cell) using a second mass spectrometer.

(16) Step 5: .sup.14C.sup. ion detection and counting with a final particle detector.

(17) The inventors observe that selecting the 2+ charge state partially suppresses molecular interference. It is considered that using this charge state for measuring natural-abundance .sup.14C has not been disclosed previously. 1+ selection produces super-natural .sup.14C measurement background at SUERC, whereas the selection of less-copious 3+ or even more highly charged positive ions is unnecessary.

(18) FIG. 3 shows why thick non-metal charge-exchange gas is employed to both remove remaining molecules and suppress .sup.14N by ion charge inversion. FIG. 3 shows the ratio of C to C+ ions exiting the SUERC SSAMS charge-exchange cell with various non-metallic gases measured with the instrument second mass spectrometer, using incident C2+ ions of the stable isotope noted. The SiN [7] data is from Wilcken et al. (2013) and the other dashed curves [1]-[6] from the references cited therein for comparison.

(19) Tenuous metal vapours are known as efficient means of charge-exchanging positive ions negative at low ion energy. However, molecule suppression requires sufficiently thick gas and therefore incident ion energies of 10 s keV or more to traverse the gas and be quantifiable with a mass spectrometer. At these energies non-metallic gases are considered to be similarly efficient. Also, such gases can be readily manipulated with conventional gas-handling equipment (mass-flow controllers, etc.), whereas metal-vapour control is more cumbersome and imprecise, and electrically-insulating gas cannot compromise the electric fields employed in mass spectrometry in a way that leaking metal vapour can. Moreover, a gas or gas blend can be chosen to provide the optimal combination of molecule suppression without excessive beam scattering and negative-ionisation.

(20) The gas requirements for good molecule suppression are the same as conventional AMS utilising thick stripper. Accordingly we can employ the same N.sub.2 gas metered into the same differentially-pumped open-ended tube between the mass spectrometers of the SSAMS as when the instrument is functioning conventionally. In that case this serves as the stripper-canal, whereas in the positive-ion method this serves as an electron-adder. Gases other than pure N.sub.2 are conjectured to be the optimum, for example propane or isobutane. More electropositive gases such as isobutane are more efficient at donating electrons as shown in FIG. 3. The amount of gas employed is found empirically by adjusting gas flow whilst monitoring the measured .sup.14C and stable carbon isotopes and their ratio. Gas thickness is an acceptable compromise of that best for molecule-removing and for charge-exchanging, and in a further improved embodiment can be adjusted automatically in feedback depending on the abundance of individual sample .sup.14C and interferences.

(21) The beam energy is determined by the electrical biasing of the ion source and the charge-exchange cell deck. By the method of the present invention, and with radiocarbon-dead CO.sub.2 sample, radiocarbon measurement background of about 2% Modern (after correction for PIPS detector dark count) with 280 keV 14C ions has been achieved, chosen to match the ion energy employed when the SSAMS is operating conventionally, and good results also achieved at 140 keV, half this ion energy. This indicates that accelerator-free analysis is also possible in some embodiments in which ion source bias alone is sufficient.

(22) FIG. 4 shows the variation in C.sup./C.sup.+ ratio for multiple gas flow rates. It shows that negative ionisation efficiency is constant once there is gas flow sufficient for charge state equilibrium. The level of ionisation efficiency is dependent on the charge exchange gas used, as well as the ion energy. Radiocarbon background measurements with isobutane gas are also shown in FIG. 4. The background measurements were observed to be lowest where the gas flow was sufficient to destroy molecules without significantly scattering ions into the detector.

(23) Accordingly the described embodiment of the present invention is capable of reproducing the .sup.14C abundance measurement range of the conventional AMS technique. This is done with an ion source superior to the sputter negative-ion sources normally used. By virtue of leveraged higher initial ion charge in the ion source biasing electric field, the new method is also a better route to accelerator-less .sup.14C mass spectrometry than conventional AMS with potential considerable equipment cost savings.

(24) Additional details and explanations of the preferred embodiment and modifications of the preferred embodiment will now be set out.

(25) Particle Beam Source

(26) The positively charged particle beam is generated in an ion source such as electron cyclotron resonance (ECR), inductively couple plasma (ICP) or a capacitively coupled plasmas (CCP) ion source. An ECR ion source is the presently preferred ion source. It has the advantage over ICP and CCP in that it can readily make higher charge states than the 1+ and so is better at eliminating molecular interferences.

(27) Different charge states of the particle beam can be utilised from the ion source. Higher charge states, such as 3+ and above, have the advantage of being molecular free however they are more difficult to produce and therefore result in smaller beams (i.e. beams with fewer particles) and make less efficient use of the sample being measured.

(28) Going down in charge state to the 2+ and then 1+, the molecular interfering content increases but bigger and more efficiently produced beams are possible. In any charge state it is also possible to optimise the source conditions to reduce molecules, such as using an additional carrier gas such as He in the source (see FIG. 2). As explained above, the preferred embodiment uses the partial molecular suppression provided by the 2+ charge state which provides sufficient beam for accurate measurements.

(29) Sample Input

(30) Samples can be inputted into the ion source in solid, liquid or gas form. Sample loading can be automated. Samples can be pre-treated and prepared separately from the system or they can be taken directly from another system, such as in the example of carbon, CO.sub.2 can be combusted automatically from an organic source or generated in an elemental analyser and feed directly into the ion source. This has the advantage over conventional Cs sputter ion sources that typically only use samples prepared separately from the machine increasing labour and costs. In the case of carbon, the sample can comprise CO.sub.2 prepared separately.

(31) Ion Beam Analysis

(32) The system of the preferred embodiment is a high-resolution mass spectrometer. It utilises the different bending radius for charged particles with different momentum to identify the mass of the particles. An electrostatic analyser (ESA) and magnet work together to select mass, the magnet selects a momentum (i.e. species with the same mass*velocity combination) and the ESA selects the same energy regardless of mass. These steps are standard in mass spectroscopy.

(33) Interferences in this system are from particles with the same mass such as molecules or isobars. There is already at least partial molecular suppression in the ion source. The positive particle beam is then passed through the target gas in the charge exchange cell where the particles collide with the particles in the gas breaking apart the molecules. Ideally the target gas particles have a similar mass to the particle beam, i.e. heavy enough to create a strong collision and break the molecules apart without scattering the beam and destroying beam quality. The mass of the target gas is preferable to be similar to that of the ion beam for best performance, but it will work with other gases, but at potentially reduced performance. This removes the remaining molecular interferences.

(34) As the particle beam passes through and collides with the gas, it exchanges electrons with the gas, such that some of the particles in the beam will pick up additional electrons and become negatively charged.

(35) The charge exchange process works more efficiently when the target gas has low electronegativity. Metal vapours have low electronegativity, but are disadvantageous for the reasons already discussed. Of greater importance in the present invention is that the target gas is (or components of the target gas are) simple to flow in to the system. A metal vapour gas is difficult to maintain and it must be kept at a high temperature at all times to stop it condensing back into a liquid or solid. If metal gas vapour moves or migrates out of the charge exchange cell it can condense on insulators in the apparatus causing them to conduct and leading to potential electrical discharges. Using a gas which will not condense in use keeps the system cleaner and makes the system considerably simpler and cheaper to build. It is preferable that the gas has as low an electronegativity as possible but a high electronegativity may be acceptable provided that the loss in efficiency is acceptable.

(36) In some cases, the isobar of the particle of interest cannot create a negative beam. Some such cases are:

(37) .sup.14N will not produce a negative beam to interfere with .sup.14C, to measure its content in bulk carbon.

(38) Magnesium will not produce a negative beam to interfere with .sup.26Al, to measure its content in bulk aluminium.

(39) Xenon will not produce a negative beam to interfere with .sup.129I, to measure its content in bulk iodine.

(40) Manganese will not produce a negative beam to interfere with .sup.55Fe, to measure its content in bulk iron.

(41) The target gas can be excited or pumped to improve performance. In the simplest case a DC bias can be applied longitudinally to the gas, this will act to accelerate electron which are liberated in a collision between the particle beam and the gas, the accelerated electrons will then interact further with the gas and, if the energy is sufficient, liberate more electrons and/or velocity match with particle beam and promote recombination and negative ion formation. Where the DC voltage and gas pressure is sufficiently high then a cascade effect of the secondary ions will produce a plasma DC discharge. Additional methods of creating a full plasma is to pump the gas with an alternating electro-magnetic field such as RF in a CCP or ICP or microwaves in other plasmas such as the ECR ion source. In this case the low mass electrons are accelerated quickly in the alternating field whereas the ion is too heavy to respond and will remain relatively stationary (this is the typical description of an AC plasma). As the particle beam passes through the plasma these fast moving oscillating electrons energetically collide multiple times with the particle beam causing improved ionisation and molecular dissociation and, in the case of plasma, donate electrons to the ion beam producing the negative ions where the plasma cools or de-excites again.

(42) System Description

(43) FIG. 1 is now described in more detail. This refers to carbon measurement, but the system can be adapted to apply to the other isotopes discussed above.

(44) CO.sub.2 gas 1 is added to the ECR ion source 3 where it is ionised, molecules are at least in part broken up and a particle beam 5 is accelerated out of the ion source.

(45) A dipole magnet 7 is used to select, for example, the 2+ carbon atoms for further analysis. The abundant isotopes, .sup.12C and .sup.13C, are measured in off-axis Faraday cups 10 (the axis of the rare isotope being on-axis), whereas the rare isotope, .sup.14C, is selected for further processing to remove the interferences of molecules such as .sup.13CH.sup.2+, and its isobar .sup.14N.sup.2+.

(46) A fast switching DC bias can be applied to the first magnet vacuum manifold to alter the energy and therefore momentum of the abundant isotope to allow it to be switched on-axis, in this instance the off-axis cups to measure the abundant isotope is situated after the second magnet.

(47) A gas cell 12, consisting of a tube 14 where a small amount of gas is flowed in through a mass flow controller 16 or other needle valve, flows down the tube and removed by differential pumping at either end. The on-axis isotope beam 18 passes through the tube where it interacts with the gas, significantly destroying the remaining molecules and charge exchanging so that the beam exiting the gas cell 20 has negligible molecules and a range of charge states for example, 20% in 1, 50% neutral and 30% in 1+. All nitrogen is neutral or positively charged.

(48) An ESA and dipole magnet 22 (in any order) are then used to select the .sup.14C.sup.1 particles, which are now free from any molecules or isobars, and send them to a single particle detector 24.

(49) Another variation on the system is to remove the first selection magnet and pass everything through the clean-up stage in the gas cell, in which case the .sup.12C, .sup.13C and .sup.14C are all measured in the 1 charge state after the magnet.

(50) While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

(51) All references referred to above and in the lists below are hereby incorporated by reference.

LIST OF REFERENCES APPEARING IN FIG. 3

(52) The reference numbers in square brackets below are references for the data points used in FIG. 3 and are distinct from other reference numbers not in square brackets used elsewhere in the application. [1] J. H. Ormrod, W. L. Michel, Can. J. Phys. 49 (1971) 606-620 [2] J. Heinemeier, P. Hvelplund, Nucl. Instr. Meth. 148 (1978) 425-429 [3] J. Heinemeier, P. Hvelplund, Nucl. Instr. Meth. 148 (1978) 65-75 [4] B. Christensen et al, Phys. Rev. A 18 (1978) 2042-2046 [5] W. N. Lennard et al, Nucl. Instr. Meth. 179 (1981) 413-419 [6] W. N. Lennard et al, Rhys. Rev. A 24 (1981) 2809-2813 [7] K. M. Wilcken et al, Nucl. Instr. Meth. B 294 (2013) 353-355 [8] S. P. H. T. Freeman et al, Nucl. Instr. Meth. B (2015) http://dx.doi.org/10.1016/j.nimb.2015.04.034

LIST OF NON-PATENT DOCUMENT REFERENCES

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