Evaporation and ion damage in molecular SIMS

Glycerol is often employed in organic SIMS and FABMS because it enhances emission of secondary ions characteristic of analytes dissolved in it. Secondly, the duration of intense secondary ion emission is extended when a glycerol matrix is employed. The behavior or pure glycerol under primary particle bombardment is apparently different from its solutions. Under operating conditions employed here (5 - 10μA/cm2, 5 keV Ar+) an exponential decrease is observed in the characteristic m/z 93 (glycerol + H+) secondary ion intensity as shown in Fig. 1. When other matrices such as H2SO4 or polyphosphoric acid are employed, analogous behavior is observed. The authors attribute this behavior principally to evaporation: the decay rates of characteristic protonated molecular ions from each of the three matrices vary with the vapor pressure of each matrix. It is not apparent from the data that sputtering has any significant effect on the decay rates of the protonated matrix molecular ions. Either the sputtering rate is much less than the normal evaporation rate or the sputtering rates for each of the three matrices is proportioned to the matrix vapor pressure. The authors believe the former to be the case: the rate of glycerol evaporation is calculated to 100 monolayers/sec. For a primary ion flux of 6.2 x 1014 bombarding particle/sec (10-5 A/cm2 Ar+), assuming 100 A2 of the surface is destroyed per incoming ion, the surface erosion of glycerol due to irradiation is ≅.6 monolayers/sec. For all but the most intense radiation, the loss rate of glycerol is principally due to evaporation