ZnS is an excellent standard for elemental sulfur measurements. I also utilize a ultra pure pyrite FeS2 which is also excellent (we have ICP-MS on the traces so we can assume stoichiometry on the Fe and S).
Obviously due to the peak shift of sulfur with oxidation, an sulfate standard would be better for sulfate measurements. In fact, in typical basaltic glasses which often have a mixture of sulfide and sulfate at the 1000 PPM level or so, a good "rule of thumb" is to peak on the pyrite/sphalerite standard, then detune the spectrometer by 1/3 towards the sulfur peak on anhydrite. On a Cameca instrument that is about a 10 points lower sin theta (*10^5) spectrometer position. The perform your basaltic glass measurements at that spectrometer position.
Remember, for high accuracy trace elements, the standard choice is relatively unimportant compared to the accuracy of the background determination. In fact, looking at the detection limit expression from Scott and Love 1983, we can see that the higher the standard intensity (normalized to the pure element concentration), which is in the denominator of the expression, the lower the calculated detection limit for the unknown. In other words, the higher the standard intensity per concentration (which is highest in the pure element due to less absorption of the primary emitted X-ray), the better the trace element statistics.
For ultra high accuracy trace element measurements the best methods utilize the appropriate background measurement type (off-peak vs. MAN vs. multi-point bgd) and a matrix matched blank standard, which is run as an unknown sample. That is, a standard with a similar matrix to the unknown in question, which contains a *zero* concentration of the trace element.
As Mike Jercinovic says "If you can't measure something, trying measuring nothing, because if you can't measure nothing, you can't measure anything".
A nice summary of considerations of background measurement choices is this topic here:
https://probesoftware.com/smf/index.php?topic=1378.0See also:
https://iopscience.iop.org/article/10.1088/1757-899X/32/1/012012/metaand
https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/abs/multipoint-background-analysis-gaining-precision-and-accuracy-in-microprobe-trace-element-analysis/55B6A8EC500550EEDCFBF5BE50D81555An extended discussion of these trace element considerations with Mike Jercinovic can be found here:
https://probesoftware.com/smf/index.php?topic=928.0