Direct analysis of saturated hydrocarbons using glow discharge plasma ionization source for mass spectrometry

Abstract

The ionization source based on glow discharge plasma using ambient air is driven by a pulsed direct-current voltage for soft plasma ionization (SPI). The novelty of this work is that molecular ions [M+13]+ related to the analyte species (M), which may be formed by numerous oxidation, can be dominantly detected as a base peak with little or no fragmentation of them in an air plasma at a pressure of several kPa. The unique ion [M+13]+ was assigned to the oxidation product, [M+O−3H]+, which was confirmed as a deuterated ion [M+O−3D]+ ([M+10]+) by using a deuterated solvent. The ionization reactions were suggested that the product ion [M+O−3H]+ may arise from hydride abstraction reaction of M with O2+•, dehydrogenation reaction of [M−H]+• and subsequently oxidation reaction of [M−3H]+ with O3. n-Alkane mixtures was also measured to evaluate the intermolecular interaction in this system. The limits of detection (LOD) were in the range of 0.126–1.68 ppmv and the relative standard deviation (RSD) for repeatability was approximately 10.0% at the lowest concentration. To our knowledge, this is the first report demonstrating that the spectrum pattern of saturated hydrocarbons could be directly determined without any complicated fragmentation.