Abstract
Lithium ion attachment mass spectrometry provides a non-specific, non-fragmenting, sensitive and robust method for the detection of volatile species in the gas phase. The design, manufacture and results of lithium based ion attachment ionisation sources for two different mass spectrometry systems are presented. In this study trace gas analysis is investigated using a modified Chemical Ionization Mass Spectrometer (CIMS) and vapour pressure measurements are made using a modified Knudsen Effusion Mass Spectrometer (KEMS). In the Li+ CIMS, where the Li+ ionization acts a soft and unselective ionization source, limits of detection of 0.2 ppt for formic acid, 15 ppt for nitric acid and 120 ppt for ammonia were achieved, allowing for ambient measurements of such species at atmospherically relevant concentrations. In the first application of Lithium ion attachment in ultra-high vacuum (UHV), vapor pressures of various atmospherically relevant species were measured with the adapted KEMS, giving measured values equivalent to previous results from electron impact KEMS. In the Li+ KEMS vapour pressures <10-3 mbar can be measured without any fragmentation, as is seen with the initial electron impact (EI) set up, allowing the vapor pressure of individual components within mixtures to be determined.
Highlights
Mass spectrometry has provided considerable insight into atmospheric science, notably in the fields of gas kinetics, trace gas analysis and aerosol composition elucidation
Region where sample ionisation takes place with a pressure between 10 and 0.1 mbar. This is linked via a biased pinhole to the collision dissociation chamber (CDC) where there is a DC electric field to pull apart weakly bound cluster ions, the pressure in this region is around 10−2 mbar
Of the filaments tested in the Chemical Ionization Mass Spectrometer (CIMS), the best results were achieved by using a Pt : Ir mesh and the LiCO3 based β-eucryptite synthesis method, giving a factor 500 improvement in primary ion current over the tungsten wire method
Summary
Mass spectrometry has provided considerable insight into atmospheric science, notably in the fields of gas kinetics, trace gas analysis and aerosol composition elucidation. Chemical ionization (CI),[3] for example, has proved popular for trace gas analysis and gas kinetics, providing a source of ions at high pressures (including up to atmospheric pressure) which is highly specific to the target analyte. It requires suitable ionization reactions for the analyte to detect and is not universal. Electron impact (EI) has proved a consistently popular as an ionization method, through the provision of high sensitivity and nonselectivity, albeit with the twin drawbacks of including the requirements for high vacuum (10−6 torr) and the inherent fragmentation of gas analytes that results in complicated mass spectra
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