Laser desorption/ionization mass spectrometry on porous silica and alumina for peptide mass fingerprinting.

We investigated a variant of desorption/ionization on porous silicon (DIOS) mass spectrometry utilizing an aqueous suspension of either porous silica gel or porous alumina (pore size of 60 and 90 A, respectively). Laser desorption/ionization (LDI) from samples directly deposited on a stainless steel surface without any inorganic substrates was also achieved. Synthetic peptides designed to cover large sequence diversity constituted our model compounds. Sample preparation, including material conditioning, peptide solubilization, and deposition protocol onto standard matrix-assisted laser desorption/ionization (MALDI) probe, as well as ionization source tuning were optimized to perform sensitive reproducible LDI analyses. The addition of either a cationizing agent or an alkali metal scavenger to the sample suspension allowed modification of the ionization output. Comparing hydrophilic silica gel to hydrophobic reversed-phase silica gel as well as increasing material pore size provided further insights into desorption/ionization processes. Furthermore, mixtures of peptides were analyzed to probe the spectral suppression phenomenon when no interfering organic matrix was present. The results gathered from synthetic peptide cocktails indicated that LDI mass spectrometry on silica gel or alumina constitutes a promising complementary method to MALDI in proteomics for peptide mass fingerprinting.