Temperature Effects in Particle Bombardment Mass Spectrometry of Methanol
The spectra produced by methanol when bombarded with a mixed beam of argon ions and atoms with a nominal energy of 8 keV have been obtained at temperatures between −94°C and −173°C. The freezing point of methanol is −98°C, so measurements were made on both liquid and solid methanol. The spectrum of the liquid at −94°C consists primarily of clusters of methanol around the proton, H(CH3OH)n+, where the maximum value of n observed is 18. The spectrum of methanol at t= −104°C (below the freezing point) is very similar to that at t= −94°C. Thus freezing has no apparent effect on the spectrum. The spectrum of the same sample when cooled to −173°C is quite different, for the H(CH3OH)n+ ions lose their prominance, and the high mass spectrum (above m/z 33) is without character, consisting of ions at more or less every mass. This change in the spectrum is reversible, for when the sample is warmed back to −107°C, the spectrum reverts to its original form with prominant H(CH3OH)n+ ions. At an intermediate temperature (−136°C) the spectrum is a mixture of the features observed at −104°C and −173°C. Another way of representing this change in the character of the spectrum with temperature is to plot ΣH(CH3OH)n/TIC (TIC = total ion current)as a function of t, and such a plot shows a sharp decrease between t= −120°C −140°C. We have no explanation for this phenomenon. To investigate the possibility that the phenomenon could be related to surface melting of the solid produced by the ion bombardment, we measured the spectrum as a function of temperature at a bombardment intensity that was 7 times lower than that originally used. The same decrease in the relative concentration of H(CH3OH)n+ clusters as the temperature decreased was observed.