2,5-dihydroxybenzoic acid: laser desorption/ionisation as a function of elevated temperature

The temperature dependence of laser desorption/ionization (LDI) ion yields has been measured for 2,5-dihydroxybenzoic acid (2,5-DHB) single crystals from room temperature to 160 degrees C using time-of-flight (TOF) mass spectrometry. A steep rise in ion production occurs at 90 degrees C, achieving a maximum at 120 degrees C, then decreases sharply to a minimum at 140 degrees C, and returns to a second maximum at 150 degrees C. Above 160 degrees C, useful information could not be obtained because of rapid volatilization of the sample into the vacuum. The overall trend in ion production, but not some of the details, is well described by a recent two-step theory of the laser desorption/ionization process, which takes into account the temperature-dependent effects of plume expansion. Measuring the background vacuum composition with a quadrupole mass spectrometer residual gas analyzer (RGA) showed an increase in thermal desorption of 2,5-DHB starting at 90 degrees C and maximizing at 130 degrees C. The increased neutral production by thermal desorption is believed to be the cause of the decrease in LDI ion production due to reduced pooling probabilities for laser-excited 2,5-DHB molecules. Thermal dehydration, condensation, and decarboxylation increase the volume of gas released at high temperatures which also serve to decrease LDI ion production at elevated temperatures. Lastly, to confirm the mass spectrometry results, the thermal desorption of 2,5-DHB single crystals under vacuum was measured using a quartz-crystal microbalance (QCM). The onset of desorption was found to occur at 90 degrees C and the maximum desorption rate was found at 135 degrees C. Published by Elsevier B.V.