2.5. Structural determination of oligosaccharides by negative-ion ESI -CID-MS/MS
Negative-ion ESI–MS and ESI–CID–MS/MS were carried out on a Q-TOF mass spectrometer (Micromass Q-TOF Ultima Global, Waters, UK). Nitrogen was used as desolvation and nebuliser gas at a flow rate of 250 and 150 L h 1, respectively. Source temperature was 4 °C and the desolvation temperature was 150 °C. A cone voltage of 35 V was used for ESI–CID–MS/MS. The capillary voltage was maintained at 3 kV. Product-ion spectra were obtained from CID using argon as the collision gas at a pressure of 0.17 MPa. The collision energy was adjusted to 11–50 V for optimal fragmentation. A scan rate of 1.5 s per scan was used for both ESI–MS and ESI–CID–MS/MS experiments, and the acquired spectra were summed for presentation. Oligosaccharides were dissolved in acetonitrile/water (1:1, v/v), typically at a concentration of (100 lg mL 1), of which 10 lL was loop-injected. Solvent (acetonitrile/1 mM ammonium bicarbonate, 1:1, v/v) was delivered by a Harvard syringe pump (Harvard Apparatus, Holliston, MA, USA) at a flow rate of 10 lL min 1.