Ion-to-neutral ratio of 2,5-dihydroxybenzoic acid in matrix-assisted laser desorption/ionization

RATIONALE In most previous studies, the ratios of desorbed ions and neutrals from matrix-assisted laser desorption/ionization (MALDI) were measured outside the common MALDI conditions. In this work, we measured the ratios under common MALDI conditions. METHODS Ions were detected using a time-of-flight mass spectrometer in combination with a time-gated ion imaging detector. Mass-resolved desorbed neutral molecules at different angles and velocities were measured using a modified crossed molecular beam apparatus. RESULTS The upper limit of the ion-to-neutral ratio from pure 2,5-dihydroxybenzoic acid (25DHB) is 4x109 at laser fluence 40J/m2, it increases to 3x107 at laser fluence 250J/m2. The ratios of matrix from the mixture of 25DHB and analyte remain in the same order of magnitude as pure 25DHB. However, the ratio of analyte depends strongly on the analyte. Values as large as 103104 for bradykinin and as small as <108 for glycine were observed at laser fluence similar to 100J/m2. CONCLUSION The ion-to-neutral ratios of 25DHB matrix measured in this work are much smaller than some of the values reported in previous work using different methods and/or under different MALDI conditions. Copyright (c) 2013 John Wiley & Sons, Ltd.