In my experience the use of HF and silicon is a necessity. Keep all digested samples the same acid matrix and same weight/volume (ie. .5g/100ml, 1g/1000ml). Use the aqua regia digest (3 parts HCL-1Nitric if I recall) then add 1-3 drops HF after it has digested and cooled. Swirl and let rest 5 minutes before dilution.I would avoid the perchloric altogether. This little technique may also tighten up the Niobium.
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From: Linda Upp <[log in to unmask]>
To: PLASMACHEM-L <[log in to unmask]>
Sent: Tue, May 1, 2018 10:04 am
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
As always, many thanks!
Linda S. Upp
AK Steel, Middletown Research Lab
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