Relationship between in-source and post-source fragment ions in the matrix-assisted laser desorption (ionization) mass spectra of carbohydrates recorded with reflectron time-of-flight mass spectrometers

The reflectron matrix-assisted laser desorption (ionization) [MALD(I)] mass spectra of many complex carbohydrates (and other compounds) recorded with time-of-flight (TOF) instruments fitted with time-lag focusing (delayed ext raction) contain focused fragment ions resulting from fragmentation within the ion source, together with unfocused (metastable) fragments derived from post-source decay. The latter ions are particularly abundant from carbohyd rates containing sialic acids as the result of loss of the sialic acid moie ties. The relative abundance of these metastable ions was found to decrease with ion-source pulse delay whereas the mass difference between the focuse d and unfocused ions increased. The mass difference was also found to vary with the instrument used to record the spectra. An equation has been derive d that links the masses of the precursor ion with the two fragment ions (fo cused and unfocused). It contains a term reflecting the geometry of the mas s spectrometer and can, thus, be used with any TOF instrument. The formula can be rearranged such that the mass of an absent molecular ion can be pred icted from the masses of the two fragment ion peaks. The metastable ions ca n also be used to confirm fragmentation pathways in the manner similar to t hat used for spectra recorded with magnetic sector instruments. Metastable ions were also found in spectra recorded with an orthogonal-TOF mass spectr ometer using field ionization. The formulae relating the masses of the frag ment and precursor ions were also found to apply in this case, further demo nstrating their instrument independence. (Int J Mass Spectrom 188 (1999) 13 1-146) (C) 1999 Elsevier Science B.V.