Atmospheric pressure MALDI Fourier transform mass spectrometry of labile oligosaccharides.

An atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) source coupled to Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) under UV laser and solid matrix conditions has been demonstrated to analyze a variety of labile oligosaccharides including O-linked and N-linked complex glycans released from glycoproteins. Spectra were acquired by both AP MALDI and vacuum MALDI and directly compared. The results presented here confirm that AP MALDI can generate significantly less energetic ions than vacuum MALDI and is able to produce the intact molecular ions with little or no fragmentation in both positive and negative ion mode analyses. Under certain conditions, noncovalent complexes of sialylated oligosaccharides were observed. The sensitivity attainable by AP MALDI was found to be comparable to conventional MALDI, and tandem mass spectrometry of oligosaccharides ionized by AP MALDI was shown to allow detailed structural analysis. Analysis of N-glycan mixtures derived from human fibrinogen further demonstrated that AP MALDI-FT ICR MS is ideal for the study of complex glycan samples as it provides high-accuracy, high-resolution mass analysis with no difficulty in distinguishing sample constituents from fragment ions.