GRAPHITE LIQUID MIXED MATRICES FOR LASER DESORPTION/IONIZATION MASS-SPECTROMETRY/

A combination of liquid matrix and graphite particulates (2 mu m) has been proposed as a method suitable for the laser desorption/ionization mass spectrometry of peptides and proteins (Sunner, J.; et al. Anal, Chem, 1995, 67, 4335). Here we demonstrate the potential of this appro ach as a straightforward, and very general, method of achieving the ul traviolet laser desorption/ionization of a broad range of intermediate weight analytes. The desorption/ionization mechanism, the influence o f preparative procedures, and the breadth of application of this metho dology have been investigated. A simple and robust preparative procedu re is presented for the analysis of proteins, oligosaccharides, and sy nthetic polymers. Detection sensitivities are in the femtomole region for lower molecular weight peptides and oligosaccharides. The graphite acts as an energy transfer medium by absorbing the UV radiation, lead ing to thermal desorption of the liquid matrix and analyte. The liquid matrix was observed to fulfill several important roles, In the case o f peptides and proteins, which preferentially form protonated molecula r ions, it acts as a protonating agent. It also enhances the signal in tensities of cationized species (e.g., polysaccharides and polar polym ers) by assisting their desorption. An excess of liquid matrix serves to cool the analyte during the desorption step and minimize decomposit ion. The presence of liquid matrices increases the sample lifetime at a particular desorption spot, minimizing the time-consuming search for ''hot spots''. The addition of cationizing salts has been shown to im prove the quality of mass spectra obtained for polar polymers and exte nd the range of materials that can be investigated to include apolar s ynthetic polymers.