Matrix-assisted laser desorption ionization (MALDI) mass spectrometry has proved to be an important tool in synthetic- and bio-polymer characterization, but applications to low molecular weight analytes, i.e. less than lO00 Da, have been restricted. An earlier investigation [l] demonstrated the general utility of MALDI, and this has led us to use MALDI as the routine method of choice for involatile, low molecular weight analytes that prove inaccessible by other methods of ionization.
Linear (single-pass) MALDI-time of flight (tof) instruments in general produce spectra in which the only analyte-related ions are [M+H]+ ions or adduct ions such as [M+Na]+ or [M+K]+. Fragmentation is not usually observed. Calix[3]indoles are macrocyclic molecules comprised of three highly substituted indole rings ioined through the 2- and 7- positions of the indole rings via a single carbon bridge:
Linear MALDI-tof analysis of a group of these compounds has provided evidence of fragmentation. Spectra of Calix[3]indoles having amide groups incorporated at each of the bridging carbons show an [M+H]+ ion together with an abundant ion corresponding to the loss of one of these amide groups. The degree of fragmentation has also been shown to depend on the matrix used. Representative examples of the spectra with the matrices 2,5 dihydroxybenzoic acid, alpha-cyano-4-hydroxy-cinnamic acid and sinapinic acid will be presented.
The rapidity with which the MALDI process occurs emphasises the facile nature of this apparent fragmentation. The possible driving force for this fragmentation will be discussed.