Negative ion graphitised carbon nano-liquid chromatography/mass spectrometry increases sensitivity for glycoprotein oligosaccharide analysis

Negative ion nano-liquid chromatography/mass spectrometry (nano-LC/MS) and tandem mass spectrometry (nano-LC/MS²), using graphitised carbon as separating medium, were explored for analysing neutral and acidic O-linked and N-linked oligosaccharide alditols. Compared to the sensitivity of capillary LC/MS (flow rate of 6 μL/min) coupled with a conventional electrospray ionisation source, the nano-LC/MS (flow rate of 0.6 μL/min) with a nanoflow ion source was shown to increase the sensitivity ten-fold with a detection limit in the low-femtomole range. The absolute signals for the [M-nH]^n- ions of the oligosaccharides were increased 100-fold, enabling accumulation of high-quality fragmentation data in MS² mode, in which detection of low abundant sequence ions is necessary for characterisation of highly sialylated N-linked oligosaccharides. Oligosaccharides with high numbers of sialic acid residues gave dominant fragments arising from the loss of sialic acid, and less abundant fragments from cleavage of other glycosidic bonds. Enzymatic offline desialylation of oligosaccharides in the low-femtomole range prior to MS² analysis was shown to increase the quality of the spectra. Automated glycofragment mass fingerprinting using the GlycosidIQ software confirmed the oligosaccharide sequence for both neutral desialylated as well as sialylated structures. Furthermore, the use of graphitised carbon nano-LC/MS enabled the detection of four sialylated O-linked oligosaccharides on membrane proteins from ovarian tissue (5 μg of total amount of protein).