Ionization Characteristics\nof Glycan Homologues in\nVarious Modes of Electrospray

Fluorescence labeled glycan homologous mixtures were\nquantified\nusing fluorescence and then used to evaluate ionization performances\nin electrospray ionization at micro, nano, and femto flow modes. nanoESI\nproduced higher (2+ and 3+) charged ions adducted with sodium and\ncalcium. In comparison, femtoESI was found to favor the generation\nof [M + H]<sup>+</sup> ions against metal adducts, even with nonvolatile\nsalts up to 1 mM for NaCl and 100 μM for CaCl<sub>2</sub>. For\nlabeled glucose homopolymer (GHP) glycans, nanoESI and femtoESI had\n0.81 and 3 nM detection limits, respectively. With LC separation and\na much higher flow rate, conventional microflow ESI detected all glycans\nwith 10-fold lower concentrations. Overall, nanoESI had the optimum\nuniformity in the relative ionization efficiency (RIE). When summing\nup intensities of analyte ions formed with all charge carriers, the\nRIE of the midsized glycans (10 to 16 glucose units) appear to be\nuniform (RIE 95%–105%). For the smaller (1–5 glucose\nunits) glycan components, femtoESI provided better uniformity than\nnanoESI and conventional ESI. For the labeled IgG N-glycans, the impact\nof chemical structure on the ionization efficiency was revealed by\nthe strong correlation between their RIE trends in different ionization\nmodes.