Glow discharge ionization source for liquid chromatography/particle beam mass spectrometry

A detailed evaluation of the analytical characteristics of a liquid chromatography/particle beam-glow discharge mass spectrometry (LC/PB-GDMS) system is described for applications in the area of inorganic (free metals in solution) and organic compound analysis. A highly efficient particle beam interface is used to introduce analyte species into a glow discharge source for subsequent vaporization and ionization. The GD operating current and gas pressure were optimized, with the general responses found to be similar to those obtained previously for particulate matter analysis by this PB-GDMS system. The mass spectra obtained for inorganic species are comprised exclusively of the respective elements' isotopic patterns, with no evidence of appreciable oxide species formation. Organic species introduced into the discharge through the particle beam interface yield mass spectra that are virtually identical to those from standard electron impact (70 eV) ionization. An analytical response curve for caffeine, using 200-microL (H2O/MeOH) injection volumes, showed less than 5% RSD for replicate injections over a concentration range of 10-500 ppm, with a detection limit of 13 ppb (2.7 ng) obtained for the caffeine molecular ion. Similarly, detection limits for Fe, Ag, and Cs ranged from 5.8 to 6.1 ppb (approximately 1 ng, each) in the injected volume. As an example of the feasibility of the PB-GDMS system as a detector for liquid chromatography, the separation and identification of the organic constituents in diet soda was performed.