Conversion of a commercial gas chromatography-mass spectrometer to a liquid chromatography-particle beam/glow discharge mass spectrometer

Abstract

Presented here is the evaluation of working parameters necessary to convert a commercially available HP 5973 MSD gas chromatography electron impact mass spectrometer into a liquid chromatography particle beam glow discharge mass spectrometer (LC-PB/GDMS) system. The initial GC unit was replaced with a commercially available PB interface that allows analysis of liquid samples via electron ionization (EI) mass spectrometry. The original EI source has been removed and replaced with a simple, home-built glow discharge (GD) source, allowing both molecular and elemental information to be obtained simultaneously. Nebulization characteristics were optimized with respect to liquid flow, nebulizer gas, desolvation temperature, and the ability to remove solvent by a two-stage momentum separator. Glow discharge source block temperatures and plasma conditions were optimized relative to the caffeine (test compound) responses. The optimized conditions determined in this study are comparable to work done on a similar type of GDMS in this laboratory. Comparisons of analyte responses for the EI and GD sources with the PB interface reveal caffeine detection limits of 34 and 15 ng mL⁻¹, respectively. The GD and EI mass spectra for caffeine are similar to those of prior work and the NIST mass spectral library. Analytical responses of 100 µL injections varied from 4.7–9.2% RSDs for concentrations of 0.001–250 µg mL⁻¹ of caffeine, creatinine, a tetra-peptide, selenium-methyl-selenocysteine, and Cs and Pb salts. Detection limits were found to be in the ng mL⁻¹ level with absolute masses ranging from 0.24–31.4 ng.