Effects of the physico-chemical properties of amino acids and chemically functionalized surfaces on DIOS-MS analysis

Desorption ionization on silicon mass spectrometry (DIOS-MS) allows for the detection of low molecular weight species from fluid samples. However, this method remains scarcely used for clinical diagnosis likely because of a lack of knowledge about the desorption/ionization mechanism as well as about the interplay between the surface and analyte properties which are effective in desorption/ionization, impeding the optimization of the DIOS-MS analysis. Herein, the normalized intensity of the DIOS-MS peaks at [M+H]⁺ of seven amino acids on four different porous silicon modified surfaces are investigated. These amino acids (arginine, phenylalanine, methionine, glutamine, leucine, cysteine and valine) have different isoelectric points, proton affinities, and octanol-water partition coefficients. The four selected surfaces were oxidized porous silicon (SiO₂), the same porous silicon modified with a propyl dimethyl ethoxy silane, octadecyl dimethyl ethoxy silane or 3 amino propyl dimethyl ethoxy silane (CH₃-short, CH₃-long and NH₃⁺, respectively). These surfaces present different electrical charges, alkyl chain lengths, and hydrophilic/hydrophobic properties. For each surface, the intensities of the protonated molecules ([M+H]⁺) are discussed with respect to the electrical charge and proton affinity of the amino acids, their z-distributions inside the pores (determined by time of flight secondary ion mass spectrometry profiling), their surface interaction energies (calculated by molecular dynamics simulations), the interfacial water content and the proton availability for each surface.