Electrospray Ionization (ESI)
Unlike MALDI, ionization of carbohydrates by electrospray can lead to the production of several types of ions, depending on the ion source conditions and additives to the solvent. Multiple charging is common, particularly in negative ion mode with polyacidic glycans. The resulting spectra, therefore, do not accurately reflect the glycan profile. Furthermore, signal suppression can be a problem, particularly when compounds are present that ionize in preference to the carbohydrates. Consequently, ESI is generally inferior to MALDI for profiling mixtures of neutral glycans but can be advantageous with sialylated and sulfated glycans because of their lower tendency to fragment. [M+Na]+ ions, as in MALDI, can be obtained by use of relatively high cone voltages , but these ions are usually accompanied by a considerable amount of in-source fragmentation. Sensitivity generally falls as a function of increasing mass such that it is difficult to record singly charged ions from the larger glycans [119, 150, 244-248]. Part of the reason for this is the increasing tendency for the larger glycans to form doubly charged ions of the type [M+cation2]2+ or [M+anion2]2-. Both [M+H]+ and [M+2H]2+ ions can be formed under milder ion source conditions, particularly from derivatized glycans that contain an amine group. In the negative ion mode, carbohydrates form [M-H]- ions or adducts with various anions [238, 249-254]. Anion addition stabilizes the ions and increases sensitivity. Nanoelectrospray appears to be particularly appropriate for glycan analysis as it has been reported not to suffer from loss of signal at high mass to the same extent as classical electrospray .