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

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.