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Abstract
Photoionisation schemes for mass spectrometry, either by laser or discharge lamps, have been widely examined and deployed. In this work, the ionisation characteristics of a Xenon discharge lamp (Xe-APPI, 9.6/8.4 eV) have been studied and compared to established ionisation schemes, such as atmospheric pressure chemical ionisation, atmospheric pressure photoionisation with a Krypton discharge lamp (Kr-APPI, 10.6/10 eV) and atmospheric pressure laser ionisation (266 nm). Addressing the gas-phase ionisation behaviour has been realised by gas chromatography coupling to high-resolution mass spectrometry without the usage of a dopant. For the multicomponent standard, it has been found that Xe-APPI is able to ionise a broad range of polycyclic aromatic hydrocarbons as well as their heteroatom-containing and alkylated derivatives. However, thiol and ester compounds could not be detected. Moreover, Xe-APPI revealed a high tendency to generate oxygenated artefact, most likely due to a VUV absorption band of oxygen at 148 nm. Beneficially, almost no chemical background, commonly caused by APCI or Kr-APPI due to column blood, plasticisers or impurities, is observed. This advantage is noteworthy for evolved gas analysis without pre-separation or for chromatographic co-elution. For the complex mixtures, Xe-APPI revealed the predominant generation of radical cations via direct photoionisation with a high selectivity towards aromatic core structures with low alkylation. Interestingly, both Xe-APPI and Kr-APPI could sensitively detect sterane cycloalkanes, validated by gas chromatographic retention. The narrowly ionised chemical space could let Xe-APPI find niche applications, e.g., for strongly contaminated samples to reduce the background.
Supplementary materials
Title
Supporting Information
Description
Compound list for the analytical standards (Table S1/S2); Average odd/even ratios for standards grouped into compound classes (Table S3); Intensity- and number-based odd/even ratios for the complex samples (Table S4/S5); Total intensities of selected compounds for all ionisation techniques (Figure S1); Odd/even ratios for selected compounds for APCI (Figure S2); Mass spectrum for 5α-cholestane for Kr-APPI and Xe-APPI (Figure S3); Comparison of background signal of all ionization techniques (Figure S4); Comparison of Kr-APPI and Xe-APPI for complex sample thermal analysis with contamination (Figure S5); Summed mass spectra of the sterane-type compounds observed in diesel (Figure S6); intensity-and number-based odd/even ratio for all compounds as well as divided into the CH-class and heteroatom-containing classes (Figure S7).
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