As recently shown in Fourier transform ion cyclotron resonance mass spectrometry studies (Rapid Commun. Mass Spectrom. 11, 1602, 1997), photochromic systems can be used as molecular probe for the study of matrix-assited laser desorption ionization (MALDI) processes inducing the typical formation of the triply hydrogenated molecules [M 2H H] simultaneously with the color change. In this paper, a novel photochromic system blocked in its open form allows the proposal of a possible mechanism for protonated ions. The results suggest that H-atom transfer from a matrix molecule to an analyte plays an important role in the ionization step. The transferring H-atom may be derived from electronically excited states of matrix and analyte molecules via the triplet states. Among several ionization processes described in the literature, we observe that H-atom transfer following analyte photoionization is a possible ionization mechanism in MALDI. # 1998 John Wiley & Sons, Ltd. Received 15 July 1998; Revised 1 September 1998; Accepted 1 September 1998 Since its development, extensive efforts have been focused on understanding the mechanism of desorption and ionization in MALDI processes. Up to now, several different models have been proposed to explain the mechanism of desorption of large molecules in MALDI.1 However, the process by which ionized analytes are produced is still not fully understood and no adequate quantitative model for the complete process exists, despite extensive studies.
2-6In MALDI, evidence exists that analyte ionization occurs both in the surface and in the expanding plume. Initiated by the laser shot, ions may form by ion-molecule reactions (a collisional mechanism) between matrix molecular ions and the analyte with the participation of radical ions, 7 or may exist after excited state proton transfer (ESPT) between electronically excited matrix molecules and neutral analyte molecules, 8,9 or most probably they may originate from a combination of both processes. Due partly to the experimental conditions and partly to the character of the analyte itself, different ionization mechanisms can be activated. Proteins and peptides ionize almost exclusively by proton transfer, for other classes of compounds such as carbohydrates and synthetic polymers the dominant ionization process is the attachment of sodium, potassium ions. Recently, Allwood et al. 10 proposed a scheme in which thermionic electron emission from excited singlet states provide the main ionization route for species in MALDI. Thus, ionization processes in MALDI appear far more complex than were first believed.In our earlier paper, 11 the MALDI processes were studied by the structural change of photochromic probe incorporated in a matrix. The photochromic probes are constituted by a series of substituted spirooxazines exposed to the UV light. These compounds rapidly change color (transparent to blue in the time scale`10 À9 s) and return to the original color if the irradiation is stopped. The blue color is due to the opening of the spiromoiety t...