Deuterated solvents and DIOS surfaces derivatized with different functional groups are used to investigate impacts of local chemical environment on analyte ionization. Both solvent molecules and surface functional groups are found to directly participate in analyte protonation in the condensed phase. The corresponding protonation effectiveness is quantitatively estimated based on the relative MS peak intensities of [M ϩ 2] ϩ /[M ϩ 1] ϩ . A direct correlation between ionization of triethylamine and the relative acidities of the surface and the solvent is evident. In addition, the proton donating effectiveness of a solvent is found to be related to its vapor pressure. Improved MS detection of small molecules via proper surface treatment and solvent selection is demonstrated. (J Am Soc Mass Spectrom 2008, 19, 8 -13) © 2008 American Society for Mass Spectrometry E xtensive research in metabolite profiling in recent years has revived interest in desorption ionization on porous silicon mass spectrometry (DIOS-MS). Elimination of matrix molecules in DIOS-MS reduces background noise in the low-mass range and enables direct detection of biologically significant small molecules [1][2][3][4]. It also overcomes matrix-induced analyte redistribution on a sample surface, which allows two-dimensional imaging of tissue samples with good spatial accuracy [5]. Recent studies have shown that the local chemical properties of a DIOS substrate, including the residual solvent on the surface and the surface functional groups, are critical in analyte ionization [6 -9]. Quantitative investigation of these factors, however, is lacking. In this report, we semiquantitatively assessed contributions of various proton sources in DIOS-MS and the feasibility of improving MS detection of lipids and peptides by proper sample treatment.
Experimental
DIOS Substrate PreparationDIOS substrates were prepared by dipping the wafer into a solution of 5% HF/EtOH for 1 min to remove the oxidized layer, followed by a 1.5-min anodic etching in 25% HF/EtOH at 5 mA/cm 2 [10,11]. Before MS experiments, the DIOS substrates were dipped in 5% HF/ EtOH to regenerate the H-terminated surface. For OHterminated surfaces, the DIOS substrates were dipped in a solution of 15% H 2 O 2 /MeOH (or 15% D 2 O 2 / MeOH-d 4 ) for 30 min. For RNH 2 -terminated surfaces, the same OH-terminated surfaces were refluxed in a solution of 10% APTMS/toluene under N 2 for 4 h. To remove residual solvent molecules, the DIOS or chemically modified DIOS substrates were dried under high vacuum overnight, unless specified otherwise.