Matrix performance in matrix-assisted laser desorption/ionization for molecular weight determination in sialyl and non-sialyl oligosaccharide proteins.

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry relies on the formation of intact molecular ions to determine molecular weight. In biochemical research, conventional methods of protein analysis at picomol to fentomol sensitivity, such as sodium dodecyl sulphate polyacrylamide gel electrophoresis, have been replaced by this new ionization method. Unfortunately, problems caused by the mass accuracy and low resolution restrict the use of this ionization technique, particularly when a high mass accuracy in a high mass range is required. In this paper it is shown that the appropriate choice of parameters which determine the desorption/ionization of glycoproteins can improve the quality of MALDI mass spectra as well as mass reproducibility and resolution. The study of sample-matrix solution composition, pH and instrumental conditions allow the molecular weight determination of highly glycosylated proteins with a high percentage of sialic acid, e.g. erythropoietin. The glycosylation of this molecule which interferes with the production of multiply charged ions in electrospray ionization does not affect the desorption/ionization in MALDI analysis. We report the best operating conditions used to establish the degree of heterogeneity of erythropoietin.