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 C using time-of-flight mass spectrometry. A steep rise in ion production occurs at 90 C, achieving a maximum between 120 C - 130 C, then sharply decreasing to a minimum at 130 C - 140 C, and returning to a second maximum value at 150 C. Above 150 C - 160 C useful information could not be obtained because of rapid volatilization of the sample into the vacuum. The overall trend in ion production is well described by a comprehensive 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 showed an increase in thermal desorption of 2,5-DHB from 110 C - 150 C, maximizing at approximately 130 C - 140 C, as well as an increase of water vapor in the vacuum from 120 C - 150 C, also having a maximum at 130 C - 140 C. This excess water vapor coupled with 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 matrix molecules. Thermal dehydration, condensation, and decarboxylation increase the volume of gas released at high temperatures. Lastly, to confirm the mass spectrometry results, the thermal desorption and decomposition under vacuum was measured using a quartz-crystal microbalance. The maximum desorption rate was found at 130 C - 140 C.
Citation: International Journal of Mass Spectrometry
Issue: No. 1
Pub Type: Journals
crystal growth, desorption, ionization, laser desorption ionization, mass spectrometry, matrix assisted laser desorption, organic acid