Laser desorption/ionization time-of-flight mass spectrometry on sol-gel-derived 2,5-dihydroxybenzoic acid film.

This work presents a novel method for direct desorption/ ionization of analytes from sol-gel-derived film. 2,5-Dihydroxy benzoic acid (DHB), a common MALDI matrix, was incorporated into a sol-gel polymeric structure. The sol-gel-derived DHB thin film can assist the mass analysis of analytes by laser desorption/ionization, with a matrix interference-free background in the mass spectra. The sol-gel-derived film can function as an energy absorber during laser irradiation because it contains DHB molecules. Furthermore, laser irradiation with normal laser power (70-110 microJ) is not likely to generate any background ions from this sol-gel-DHB derived film. The samples were prepared straightforwardly. After a thin film was formed on a Parafilm membrane from the sol-gel-derived DHB solution coating, the sample solution was directly added to the top of the film, for laser desorption/ ionization mass analysis. The analyte signals were homogeneously obtained on the sol-gel-derived DHB film. Experimental results show that the optimum concentrations of DHB incorporated in the sol-gel solution were between 7,500 ppm and 10,000 ppm, providing a matrix interference-free background. Analytes, including small proteins, peptides, amino acids, and small organics, were used to demonstrate the effectiveness of the proposed method. However, a higher laser power (> 110 microJ) than normal was required to desorb small proteins from the sol-gel-derived DHB film. Therefore, a few matrix ions desorbed from the thin film were generated during laser irradiation. The detection limit for both small molecules and proteins, using this sol-gel-assisted laser desorption/ ionization (SGALDI) mass spectrometry (MS), was as low as 81 fmol. However, a mass spectrometer with cutoff-mass selection could detect 8.1 fmol of cytochrome c. The largest analyte observed by the SGALDI-MS in this study was myoglobin.