Quantitative aspects of the matrix-assisted laser desorption mass spectrometry of complex oligosaccharides



Comparative studies of the matrix-assisted laser desorption ionization of complex oligosaccharides were made using a range of mono, di- and tri-substituted benzoic acids, substituted coumarins and cinnamic acids. The best results were obtained with 2,5-dihydroxybenzoic acid (2,5-DHB). Detection limits using this matrix were in the range of 100 fmol for complex and high mannose sugars, some 2-fold better than those obtained with α-cyano-4- hydroxycinnamic acid and 10-fold better than with an earlier oligosaccharide matrix, 3-amino-4-hydroxybenzoic acid. The strongest signals were achieved with a matrix-to-sample ratio of about 5000:1. Little correlation was found between the measured peak height and sample loading when 3-amino-4-hydroxybenzoic acid was used as the matrix and the signal appeared to saturate at a sample level of around 30 pmol. However, good correlation was found between the amount of sample loaded onto the target and the response when an analogous compound was used as an internal standard and peak ratios were measured. In contrast to this, the use of 2,5-dihydroxy-benzoic acid as the matrix enabled the oligosaccharide to be measured over the range 100fmol to at least 100pmol with an excellent linear correlation between sample quantity and response and without the need of a standard. No evidence of saturation was found with this matrix. With 2,5-DHB (other matrices were not tested), the oligosaccharide signal strength was found to fall with increasing molecular weight in a linear manner and similar to that found previously with proteins. Signal strength from individual sugars also fell when these were measured in mixtures. Derivatization (peracetylation) generally caused an increase in signal strength.