Soft X-ray atmospheric pressure photoionization (soft X-ray APPI) as an ionization method in liquid chromatography-mass spectrometry (LC-MS) is presented. The ionization mechanism was examined with selected test compounds in the negative ion mode, using soft X-ray APPI source emitting 4.9 keV photons. Test compounds with an acidic group were ionized by a proton transfer reaction, producing deprotonated molecules ([M - H]−), whereas compounds having positive electron affinity were ionized by a charge exchange reaction, producing negative molecular ions (M-•). Soft X-ray APPI does not require a dopant to achieve high ionization efficiency, which is an advantage compared with vacuum ultraviolet APPI with 10 eV photons, in which a dopant is needed to improve ionization efficiency. The energy of the soft X-ray photons is in the keV range, which is high enough to displace a valence electron and often also inner shell electrons from LC eluents and atmospheric gases, initiating an efficient ionization process in the negative ion mode.
CITATION STYLE
Hieta, J. P., Vesander, R., Sipilä, M., Sarnela, N., & Kostiainen, R. (2021). Soft X-ray Atmospheric Pressure Photoionization in Liquid Chromatography-Mass Spectrometry. Analytical Chemistry, 93(27), 9309–9313. https://doi.org/10.1021/acs.analchem.1c01127
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