You indicate that your digested matrix varies and is around 10-20% in your samples. Are you matrix matching your standards to your samples? Are you using an internal standard, and if so, what element (and line) are you using? The Si 251.611 emission line is an atomic line. If you are using an internal standard element with an ionic emission line, you can be introducing a bias (positive), because an ionic line is more affected by matrix loading of the plasma (because it requires more energy than an atomic emission line), and so especially if you're not matrix (acid) matching your standards to your samples, you can have a situation wherein your sample matrix reduces the intensity of your internal standard more than it reduces the Si emission line, thereby causing an over-correction. I would try to match your standards and sample acid matrix as closely as possible, and experiment with different internal standards.
City of Fresno Wastewater Laboratory
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). [mailto:[log in to unmask]] On Behalf Of Linda Upp
Sent: Tuesday, May 01, 2018 7:02 AM
To: [log in to unmask]
Subject: high bias on silicon, low niobium
Got a question for the group. Here at AK we frequently will run a carbon or stainless steel by ICP, OES, and XRF - then compare the results from all three instruments to each other. My silicon is pretty much always running higher than that found by the other two methods. I'm using the
251.6 wavelength, and my background / baseline look good. My matrix varies from nitric, perchloric, or aqua regia most often, depending on which of the types of steel I'm doing. I always run NIST or ARMI certified standards at the same time, and will 'correct' to these certified materials if the results do not fall within expected limits.
(meaning if my Nickel is slightly low, I multiply results by the factor necessary to bring it in). I wouldn't anticipate that with these acids my torch would be dissolving into the matrix and contaminating my samples. By the time I run my acid concentration is around 10-20%, and I'm rinsing with water in between. I'm not sure if there's something I'm missing, but I thought I'd poll the community for thoughts.
Likewise, my niobium is frequently low - or missing altogether. I'm running this axially, and have just cleaned the window and aligned the torch. None of the niobium wavelengths are finding the niobium, which was present at about .01% in the recent round of proficiency samples.
I'd appreciate any advise / insights / etc on what I can do to improve these results.
As always, many thanks!
Linda S. Upp
AK Steel, Middletown Research Lab
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