Is the incorporation of analytes into matrix crystals a prerequisite for matrix-assisted laser desorption/ionization mass spectrometry?: A study of five positional isomers of dihydroxybenzoic acid

The 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-positional isomers of dihydroxybenzoic (DHB) acid were investigated with respect to their function as matrix-assisted laser desorption/ionization (MALDI) matrices. Optical absorption spectra of solid matrix samples, recorded in diffuse reflection from samples revealed peak broadening and a red shift of the peaks relative to the solution spectra. Single crystals of all isomers were grown from solution and analyzed by x-ray crystallography. Single crystals, as well as standard dried droplet and thin layer preparations with added cytochrome c, were analyzed by UV- (266, 308, 337, 355 nm) and IR- (2.94 mu m) MALDI time-of-flight mass spectrometry (MALDI-TOF-MS), A spectrophotometric measurement of the heme absorption around 400 nm of redissolved single crystals showed a quantitative incorporation of the protein into crystals of 2,5-DHB and a partial protein incorporation with large statistical fluctuations into single crystals of 2,4-DHB. No protein incorporation above the detection limit of 10(-5) molar analyte-to-matrix ratio was found for the other isomers. Best MALDI spectra from standard preparations were recorded for dried droplet preparations of 2,5-DHB at 337 and 355 nm and for 2,4-DHB at 266 and 308 nm, Both matrices performed well in the IR, too. 2,6-DHB yielded spectra of comparable quality at 337 nm and 2.94 mu m, but only when prepared in a thin layer from an acetone solution. The results suggest that protein incorporation into the crystals of solid MALDI matrices is helpful, but not a prerequisite for MALDI. A large surface-to-volume ratio, typical for microcrystalline thin layer preparations supports protein desorption if no measurable incorporation occurs. Undesirable matrix adduct formation to the protein ions was seen for all DHB isomers at the wavelengths of 308 and 266 nm. (Int J Mass Spectrom 185/186/187 (1999) 859-870) (C) 1999 Elsevier Science B.V.