Insights into Desorption Ionization on Silicon (DIOS)

Desorption ionization on silicon (DIOS), a variant of surface-assisted laser desorption ionization (SALDI) which uses porous silicon (pSi) as a substrate, is a well-established and effective soft-ionization technique in mass spectrometry. In this work DIOS experiments found that the caffeine analyte could be detected by mass spectrometry as both a radical cation and a protonated species with relative intensities that depend critically on the position of the incident laser focus relative to substrate surface. In both cases analyte desorption appears to be driven by a thermal mechanism. Radical cation formation is attributed to electron transfer reactions between the desorbed neutral analyte and pSi due to the large electron affinity of the substrate. Preliminary experiments where different incident laser wavelengths were used suggest that the ionization mechanism leading to the detection of protonated peptide Dalargin involves in part electrons and holes formed when photoexciting pSi above its electronic band gap.