We recently encountered a DA processing error for many of our batches when running for Fe, Al, Zn and Cd. For some background, where are a nanotechnology/materials lab synthesizing core-shell nanoparticle structures, and use ICP to determine composition and concentration after processing.
I found that often, the ISTD wasn't defined in the batch DA, stating <none> instead of mass 89 for Yttrium. Mass 89 was acquired and properly defined as the ISTD in the prior DA tab, but not selected in the fully quant tab. What exactly is the ISTD doing, in the context of post acquisition DA? How and why are the results changed when the ISTD is defined in the fully quant tab?
When I selected mass 89 for the ISTD and re-processed the batch, many of the calculated sample ppb values increased by a few to 10% or more. In some cases they decreased slightly, but mostly increased. Turns out this has been a problem with more than a few of our batches from these elements (EPA multi element standard) since we typically "Create from existing"... At least we caught it. Going forward, we will be aware of this setting and implement some control solutions for these lower throughput analytes for us. We run tons of gold and silver and have good control over those.
I understand the basics of IS correction for matrix, SIS or plasma effects in ICP-MS, but the corrections with this field weren't proportional to the ISTD recovery.... In some cases, samples with IS recovery within a couple percent would be hugely different after re-processing DA when the ISTD box was changed from <none> to 89 in the Fully Quant tab. In other cases, not as much.
Can anyone explain this to me???
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