Ionization Characteristics of Glycan Homologues in Various Modes of Electrospray

Fluorescence labeled glycan homologous mixtures were
quantified
using fluorescence and then used to evaluate ionization performances
in electrospray ionization at micro, nano, and femto flow modes. nanoESI
produced higher (2+ and 3+) charged ions adducted with sodium and
calcium. In comparison, femtoESI was found to favor the generation
of [M + H]+ ions against metal adducts, even with nonvolatile
salts up to 1 mM for NaCl and 100 μM for CaCl2. For
labeled glucose homopolymer (GHP) glycans, nanoESI and femtoESI had
0.81 and 3 nM detection limits, respectively. With LC separation and
a much higher flow rate, conventional microflow ESI detected all glycans
with 10-fold lower concentrations. Overall, nanoESI had the optimum
uniformity in the relative ionization efficiency (RIE). When summing
up intensities of analyte ions formed with all charge carriers, the
RIE of the midsized glycans (10 to 16 glucose units) appear to be
uniform (RIE 95%–105%). For the smaller (1–5 glucose
units) glycan components, femtoESI provided better uniformity than
nanoESI and conventional ESI. For the labeled IgG N-glycans, the impact
of chemical structure on the ionization efficiency was revealed by
the strong correlation between their RIE trends in different ionization
modes.