An Atmospheric-Pressure Glow Discharge Ionization Source

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Abstract
In this work, a novel atmospheric-pressure glow discharge source is explored. This discharge operates in the 1 kV range at typically 50–250 μA current in atmospheric-pressure helium. The characteristic I vs. V curve for a glow discharge is observed, and the discharge can operate in the positive or negative modes. The background ions observed in the positive mode from the atmospheric-pressure ionization (API) cell in the mass spectrometer are characteristic of water clusters from background water in the cell, and almost no helium background ions are detected. This atmospheric-pressure glow discharge source has been used to ionize various small biologicals, drugs, peptides, and explosives analogs that have been introduced into the API cell via a direct insertion probe. The resulting mass spectra exhibit generally the MH+ with minimal or no fragmentation in the positive mode, indicating that ionization of the analyte occurs softly through proton transfer from water clusters. Detection limits in the low-pg range have been obtained in both the positive and negative modes.