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

Abstract

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 extraction) 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 carbohydrates containing sialic acids as the result of loss of the sialic acid moieties. The relative abundance of these metastable ions was found to decrease with ion-source pulse delay whereas the mass difference between the focused and unfocused ions increased. The mass difference was also found to vary with the instrument used to record the spectra. An equation has been derived that links the masses of the precursor ion with the two fragment ions (focused and unfocused). It contains a term reflecting the geometry of the mass 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 predicted from the masses of the two fragment ion peaks. The metastable ions can also be used to confirm fragmentation pathways in the manner similar to that used for spectra recorded with magnetic sector instruments. Metastable ions were also found in spectra recorded with an orthogonal-TOF mass spectrometer using field ionization. The formulae relating the masses of the fragment and precursor ions were also found to apply in this case, further demonstrating their instrument independence.