High-Sensitivity Analysis of Neutral Underivatized Oligosaccharides by Nanoelectrospray Mass Spectrometry

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Abstract
Nanoscale electrospray ionization (nano-ESI) overcomes the sensitivity problems found and reported for neutral oligosaccharides in conventional microscale forced-flow ESI. For a series of compounds ranging from trisaccharides to larger polymers with molecular masses up to 6 kDa, sample concentrations of 10-5 M, i.e., 10 pmol total sample load, yielded very intense singly or multiply cationized molecule ions in an ion-trap mass spectrometer. In a dilution series, it is exemplified that, at the 10-8 M level, molecule ion signals can be clearly registered with a S/N ratio of about 7. Only 100 amol of sample has been consumed in this experiment. Investigation of an oligosaccharide−peptide mixture revealed that the oligosaccharide is suppressed in conventional ESI, whereas in nano-ESI both analytes are detected at comparable and high intensities. Mechanistic implications are discussed, emphasizing the influence of surface activity for the two ESI techniques. The very low flow rates inherent to nano-ESI of about 30 nL/min, together with the high signal intensity, make it possible to fully employ the MSn capabilities of an ion-trap mass spectrometer for structural analysis. From less than 1 μL of sample solution, it is possible to make consecutive fragmentation experiments up to MS7 to obtain valuable information about the structure of complex oligosaccharides.