TOF mass spectra of zircon M257 measured by VUV laser desorption ionization†
A method was developed to measure the time-of-flight (TOF) mass spectra of zircon M257 based on our significantly improved vacuum ultraviolet (VUV) laser desorption ionization (VUVDI) mass spectrometer. A zircon sample embedded in an epoxy slice was inserted in the middle of an extraction electric field. A high voltage pulse was applied to the side surface of the slice to correct the deformation of the electric field arising from the polarization of the epoxy slice. A mass gate was also used in the TOF tube to maintain the sensitivities of the microchannel plate detector towards U+ and Pb+ fragments by depleting the earlier signals in the TOF spectra. For comparison, a TOF-SIMS mass spectrum of M257 was also recorded, using Bi1+ as the primary ion beam. We found that the VUVDI-TOF spectrum with a sampling volume of ∼2 fL (2 × 10−15 L) has similar signal to noise ratios with those of TOF-SIMS spectrum with a sampling volume ∼2000 fL. The Hf+, Pb+, Th+ and U+ and their oxide signals in the VUVDI-TOF spectra were found to feature more intense signals than those in TOF-secondary ion mass (SIMS) spectra. Similar results were also observed for the ion fragments Zr2On+(n = 1–4), ZrSiO2+ and ZrSiO3+.