MASS DISCRIMINATION IN MATRIX-ASSISTED LASER-DESORPTION IONIZATION TIME-OF-FLIGHT MASS-SPECTROMETRY - A STUDY USING CROSS-LINKED OLIGOMERIC COMPLEXES

The determination of the mass discrimination effect of matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) over a wide mit range was made by determining the relative amounts of prepurified protein monomer and dimer species present in reconstituted mixtures. Protein complexes were covalently cross-linked with glutaraldehyde and monomeric and dimeric species were separated by gel filtration chromatography. Individual protein species were recombined in specific ratios and analyzed by both MALDI MS and sodium dodecyl sulfate polyacrylamide gel electrophoresis. The contributions of the peak areas of the doubly charged ions and singly-charged dimeric ions generated during the MALDI MS analyses relative to that of the singly-charged [M + H](+) species were measured, The calculation of mass discrimination for two different MALDI time-of-flight instruments showed the loss of a factor of similar to 2.6 in relative peak area as the mit value doubled over the m/z range 30000-145000. An overall correction factor for mass discrimination is given which includes effects associated with sample preparation, sample desorption/ion formation, ion transmission and ion detection.