The suitability of different DHB isomers as matrices for the MALDI-TOF MS analysis of phospholipids: which isomer for what purpose?

Although the analysis of large biomolecules is the prime application of matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS), there is also increasing interest in lipid analysis. Since lipids possess relatively small molecular weights, matrix signals should be as small as possible to avoid overlap with lipid peaks. Although 2,5-dihydroxybenzoic acid (DHB) is an established MALDI matrix, the question whether just this isomer is ideal for lipid analysis was not yet addressed. UV absorptions of all six DHB isomers were determined and their laser desorption spectra recorded. In addition, all isomers were used as matrices to record positive and negative ion mass spectra of selected phospholipids (phosphatidylcholine and -serine): In the order 2,5-, 2,6-, 2,3- and 2,4-DHB, the quality of the positive ion lipid spectra decreases. This correlates well with the decreasing acidity of the applied DHB isomers. The 3,4- and 3,5- isomers give only very weak positive ion signals especially of acidic lipids. In contrast, the most suitable matrices in the negative ion mode are 2,5-, 2,4- and 3,5-DHB. 2,6-DHB does not provide any signal in the negative ion mode due to its marked acidity. Finally, differences in the crystallization behavior of the pure matrix and the matrix/lipid co-crystals were also monitored by atomic force microscopy (AFM): 2,5-DHB gave the smallest crystals and the skinniest layer. It is concluded that basically all DHB isomers can be used as MALDI matrices but the 2,5-isomer represents the most versatile compound.