Differentiation of 'isobaric' peptides and human milk oligosaccharides by exact mass measurements using electrospray ionization orthogonal time-of-flight analysis

Exact mass determination is performed by electrospray ionization orthogonal time-of-flight mass analysis. For peptides in the mass range of 1200-1500 Da a mass error of <5 ppm is achieved with internal calibration within a si ngle mass measurement provided peak intensities are high. Peptides containi ng isobaric amino acids like glutamine and lysine can thus be easily differ entiated by their mass. In cases where more than one of these isobaric amin o acids are present, the position of the amino acid can be revealed by exac tly determining the mass differences between adjacent Y-i" fragment ions in the collision-induced dissociation spectrum. Mass determination accuracy c an be enhanced to 0.5-2 ppm by averaging over 8-10 mass measurements. There by compositional analysis of human milk oligosaccharides in a mixture can b e performed in the mass range up to 3000 Da, even for low-intensity molecul e-ion signals and for isobaric compounds with a mass difference of only 0.0 25 Da. Copyright (C) 1999 John Whey & Sons, Ltd.