| Desorption/Ionization On porous Silicon (DIOS) is a recent addition to the arsenal of analytical chemists interested in identification of low molecular weight compounds in complex mixtures. This technique (usually coupled to time-of-flight mass analyzers) makes use of the unique properties of porous silicon (such as high absorptivity in the UV, porous structure and high surface area) to provide ‘soft’ desorption/ionization of intact molecular ions. The porous silicon substrate performs a similar function to the organic matrix of Matrix-Assisted Laser Desorption/Ionization (MALDI) techniques, namely separation of analyte molecules (to minimize clustering) and absorption and transfer of laser energy to analyte molecules. While MALDI is typically employed for analysis of high molecular weight compounds, DIOS is generally applied to analysis of thermally or photochemically labile low molecular weight compounds for which acquisition of singly-charged molecular ion is problematic.; Recent research on this technique has been mostly applications driven with few systematic investigations into factors that influence the DIOS process. This study attempts to remedy some of these shortcomings with an examination of the physical and chemical characteristics of effective DIOS surfaces, determination of applicability of analytes with various functional groups, influence of storage and sample solvent and DIOS behavior of samples with high salt concentrations. Three distinct porous silicon production techniques are presented and useful DIOS lifetimes of these surfaces are determined. The origin of ‘background’ ions (a difficulty due to the high surface area of the substrate) and the origin of protons for ionization (primary ions obtained in DIOS analysis are of the form [M+H]+) are investigated. Various silicon substrate cleaning protocols are examined and an efficient means of minimizing background ion signal is introduced. In addition, upper molecular weight limitations of DIOS surfaces are examined and limits of detection for several compounds are discussed. |
