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Measurement and Results - Odd aluminum results
I'm attempting to determine aluminum in a vitamin/mineral premix matrix using ICP-OES. I've been analyzing replicates in an attempt to determine the precision of my method in this matrix. I've found some odd behavior in the results.
It seems like if I analyze the sample digest a short time after digestion, it gives results that are 20 percent higher for aluminum. I looked at 3 wavelengths (308, 394, 396), and they all give similar results. Analyzing the digest right after the digestion gives about 600 ppm. If I wait a few days, the results go down to around 500 ppm. I've tried two different instruments and sets of standards. I'm digesting the samples on a hot plate with about 1:6 ratio of concentrated nitric acid to concentrated hydrochloric acid. The resulting digest is clear with a light yellow color. I tried another mineral premix, and it does not have the same issue.
The method I'm using is a multi-element method. It gives the same results for other elements like Co and Cu between the fresh and older digests. It's just aluminum that drops in concentration over time.
I know Se and As have issues with carbon enhancement in ICP analysis, but I haven't heard anything about aluminum having issues. Does anyone know what could be causing freshly digested vitamin/mineral premix samples to give high results for aluminum?
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Luka Bailey
Total posts: 2
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Thanks a lot for the explanation. |
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Josh Messerly
Total posts: 4
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I solved my own mystery. The issue was very fine alumina or aluminum silicates in the sample digest that then settled out overnight. I did an experiment where I took the same autosampler tubes I analyzed the night before and a new tube with freshly poured off solution out of the container. The tube from the night before was now at 500 ppm when it had been 600 ppm the night before. The freshly poured sample digest was at 530 ppm. So I shook the container and poured off another aliquot. This came in at 680 ppm. It was surprising because the digest was clear. But when I took a unshaken container and swirled it with a flashlight, I could see a faint rotating cloud in the sample. It seems there was a very fine dust of alumina or aluminum silicates in the sample that wasn't digested by our acid blend. Since our lab doesn't want to use HF in the digest, I may have to just indicate that we can't analyse samples that contain alumina or aluminum silicates. |