Investigations of theoretical principles for MALDI-MS derived from solvent-free sample preparation: Part I. Preorganization

Abstract

The success of the recently developed solvent-free MALDI-MS method suggests that the common understanding of a successful MALDI process may not be justified. Analyte incorporation into a matrix crystal is apparently not necessary for the MALDI process and it further appears that crystallinity is obstructive. The analyte cytochrome c (Cyt c) and various matrices were employed in these studies. Light microscopy revealed microscopic defects on the surface of a single crystal of 2,5-dihydroxy benzoic acid (2,5-DHB) with incorporated Cyt c. MALDI analysis of this intact crystal required considerably higher laser power for obtaining mass spectra and showed much more fluctuation in the threshold laser power than was observed for the analysis of a fine powder of the same crystal. These results show that the required threshold laser power is in the order: intact crystal with perfect surface > intact crystal with defect surface > pulverized crystal. The smaller the remaining crystals are, the milder the MALDI process. Further investigations using the solvent-free MALDI method showed that the analysis of Cyt c is possible directly from the individual powders of Cyt c and 2,4-, 2,5- and 2,6-DHB, a result which was not obtained using the solvent-based MALDI method, and revealed, the stronger the absorption of a matrix at the applied laser wavelength, the milder is the MALDI process. Analyte incorporation into a matrix crystal apparently is not necessary, a concept that is strengthened by the successful characterization of Cyt c using incompatible matrices such as dithranol and anthracene. An optimized matrix-assistance in MALDI-MS is achieved by intimate contact between analyte and matrix, the smallest possible remaining crystallinity, and a sufficient absorption of the applied matrix at the laser wavelength.