LIQUID SAMPLE INJECTION USING AN ATMOSPHERIC-PRESSURE DIRECT-CURRENT GLOW-DISCHARGE IONIZATION SOURCE

An atmospheric pressure DC glow discharge in helium has been used as an ionization source for organic samples introduced by liquid injection into atmospheric pressure ionization mass spectrometry (API/MS). The glow source operates typically in the range up to 1 mA of current at less than 1 kV, although the source can be operated up to a discharge current of 10 mA. Even at the high current used in this work, the protonated molecule, MH+, is observed with little or no fragmentation for many of the samples studied. The detection limits achieved for API glow discharge detection are typically in the low femtomole region for small organic molecules including small biological neurotransmitters, drugs, pesticides, phenylthiohydantoin-substituted amino acids, and explosives. A detection limit of approximately 2 pg has been achieved for tyramine with linear quantitation over at least 3 orders of magnitude. The sensitivity in these experiments has been further improved by optimization of the skimmer-interface system and the liquid injection/nebulization design.