Quantitative Assessment of a Corona Discharge Ion Source in Atmospheric Pressure Ionization-Mass Spectrometry for Ambient Air Monitoring

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Abstract
In a corona discharge (CD) source for atmospheric pressure ionization-mass spectrometry (API-MS), ion abundances and signal stability for reactant ions in ambient air were governed by gap size, discharge current, and point-to-plane alignment. Reactant ion currents were related directly to discharge current fluctuations and varied 3–20% RSD with a 5 mm gap where ion intensities were greatest. Relative abundances for reactant ions were insensitive to source parameters with 10–30mm gaps, however, absolute ion intensities were decreased more than 90% Product ions were formed from gaseous analyte in air through collisional protontransfer from reactant ions and intensity and variance for product ions paralleled reactant ion trends. Product ion spectra were complicated by masses 91–163 amu from higher hydrated reactant ions with high intensity in the CD source. Response curves for product ions agreed with existing low-field API rate theory and competitive ionizations with analyte mixtures showed selective distribution of available protons. Preferential depletion of reactant ions by product neutrals was not observed and userdirected selectivity through control of ionization with reactant ions naturally abundant in ambient air alone seemed unpromising.