Mass Spectrometry of Glycans
Aspects of Ionization for Mass Spectrometry Specific to the Analysis of Glycans
Electron Impact (EI)
Electron impact ionization is used mainly for small glycans and is used in GC/ MS studies of permethylated monosaccharides as discussed in Section З.188.8.131.52. The method relies on creating volatile derivatives of the carbohydrates and is generally restricted to carbohydrates with a maximum of about 11 residues. Larger carbohydrates need temperatures in excess of 400 ° C [185-190] for analysis. Such temperatures cause extensive decomposition of the compounds, and, in addition, few GC/MS instruments are capable of operation at these temperatures. Thus, N-glycans must be ionized by other techniques.
Fast Atom Bombardment (FAB)
FAB, introduced in 1981 , was used extensively for ionization of N-glycans until the advent of MALDI and ESI techniques but is now mainly of historical interest. Glycans needed to be derivatized, usually by permethylation, in order to form a hydrophobic layer over the surface of the matrix that was necessary for efficient ionization. Thioglycerol was widely used as the preferred matrix for N-glycans, but the spectra suffered from a high background that limited sensitivity and the spectra contained extensive fragmentation. Nevertheless, much excellent work was performed with the technique, particularly by the Imperial College group (e.g., Refs [192-195].