Sulfur on 7500 ICPMS

I am having an issue with sulfur readings on 7500 ICPMS. The standard curve is fine (r=.998), instrument is in tune, and recently tuned for EM and PA (within last 14 days); however, I’m getting zero readings when their should be some and my S standards are off when ran as samples (10ppm=6ppm, 1ppm=0.5ppm, 0.01 ppm not even readable). Has anyone encountered this issue or know anything that could be causing this?

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