Secondary ion mass spectrometer analyses for trace elements in glass standards using variably charged silicon ions for normalization

Trace element analyses of silicate materials by secondary ion mass spectrometry (SIMS) typically normalize the secondary ion count rate for the isotopes of interest to the count rate for one of the silicon isotopes. While the great majority of SIMS analyses use the signal from Si+, some laboratories have used a multiply charged ion (Si2+ or Si3+). We collected data and constructed calibration curves for lithium, beryllium, and boron using these different normalizing species on synthetic basaltic glass and soda-lime silicate glass standards. The calibrations showed little effect of changing matrix when Si+ was used, but larger effects (up to a factor of similar to 2) when using Si2+ or Si3+ are a warning that care must be taken to avoid inaccurate analyses. The smallest matrix effects were observed at maximum transmission compared to detecting ions with a few tens of eV of initial kinetic energy ( conventional energy filtering ). Normalizing the light element ion intensities to Al3+ showed a smaller matrix effect than multiply-charged Si ions. When normalized to O-16(+) (which includes oxygen from the sample and from the primary beam), the two matrices showed distinct calibration curves, suggesting that changing sputter yields (atoms ejected per primary atom impact) may play a role in the probability of producing multiply charged silicon ions.

Automated summary

Trace element analyses of silicate materials using SIMS normalize the ion count rate based on silicon isotopes, with Si+ being the most commonly used. However, using Si2+ or Si3+ as normalizing species may lead to larger matrix effects and inaccurate analyses. Calibrations using different normalizing species on synthetic basaltic glass and soda-lime silicate glass standards showed minimal matrix effects when using Si+, while larger effects were observed with Si2+ or Si3+. Normalizing the ion intensities to Al3+ showed smaller matrix effects than multiply-charged Si ions, and normalizing to O-16(+) resulted in distinct calibration curves, indicating a potential role of sputter yields in producing multiply charged silicon ions.