Multiphoton ionization study of water and ammonia elimination from dipeptides with pulsed rapid heating and laser desorption for sample vaporization

Abstract

The origins of the peaks at 17 and 18 mass units lower than the molecular ion peak, often found in the multiphoton ionization (MPI) mass spectra of dipeptides, have been investigated. Further experimental results confirm earlier reports that thermal decomposition takes place during laser desorption (LD) as well as in a pulsed rapid heating process in a time-of-flight mass spectrometer with or without the use of a supersonic jet. The M — 18 peak is generated from the MPI of the thermally decomposed product, cyclodipeptide. The other process is the fragmentation of the molecular ion which results in the formation of the M — 17 peak along with the M — 16 peak, corresponding to the elimination of ammonia and amino radical respectively. However, the M — 17 and M — 16 peaks are only observed for the dipeptides having an N-terminal aromatic group. It is noted that these results are different from those obtained using LD-MPI with a Fourier transform mass spectrometer. A possible explanation for the discrepancy is given.