Detailed comparison of Xenon APPI (9.6/8.4 eV), Krypton APPI (10.6/10.0 eV), APCI, and APLI (266 nm) for gas chromatography high resolution mass spectrometry of standards and complex mixtures

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.