Patent Number: 
Section: 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; andpassing 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 14C+, 14C2+, and 14C3+. 7. The method according to claim 6 wherein the treated particle beam is passed through a mass spectrometer to select 14C−, and receiving the selected portion of the beam at the particle detector configured to detect 14C−. 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 14C2+ 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 CO2 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 14C2+;selecting the 14C2+ 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 14C ions to negative ions by interaction with the gaseous target material and to suppress 13CH and 12CH2 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 14C−; andreceiving the selected portion of the treated particle beam at the particle detector to detect 14C−. 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; anda 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.