Spirooxazines as a molecular probe for the study of matrix-assisted laser desorption/ionization processes. Part I: Study of the interaction effect between the molecular probe and the matrix

Abstract

In a search for a better understanding of the desorption/ionization process in matrix-assisted laser desorption/ionization (MALDI), photochromic systems such as spirooxazines have been studied by Fourier transform ion cyclotron resonance mass spectrometry coupled with a laser interface. These compounds become intensely blue upon exposure to ultraviolet light and return to the original color if the radiation is stopped. The blue color is due to the opening of the carbon–oxygen bond, creating a delocalized π-electron photomerocyanine structure which absorbs light in the 500–650 nm range. The spirooxazines appear to be an excellent probe for the study of MALDI processes because the opening of the spiro structure induces the typical formation of the triply protonated molecules [M + 3H]+ simultaneously with the color change. The source of hydrogen in the matrix has been deduced by using nicotinic acid, methyl nicotinate and deuterated nicotinic acid as matrices. The differences between the direct laser desorption (LD) (detection of molecular ion, M+.) and the MALDI process are discussed. A detailed mechanism of spiro-compound fragmentation is proposed. © 1997 John Wiley & Sons, Ltd.