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

27 February 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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.


mass spectrometry
atmospheric pressure ionisation
polycyclic aromatic hydrocarbons
gas chromatography

Supplementary materials

Supporting Information
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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