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Measurement and Results - Low bias for Sn and Sb quality control samples

 
We are having some issues with low bias on our quality control samples by ICP-MS. We have quality control samples at 10 and 90 ug/L. Our calibration ranges from 0.1 ug/L to 1000 ug/L and we have 26 elements on the one run. Sn and Sb are being particularly problematic. The results for the 10 ug/L tend to read a bit high and the 90 ug/L results are consistently low - Sb in particular is very low, e.g. 60 ug/L. The Sn and Sb also seem to be carrying over, i.e., if we run a blank straight after the 1000 ug/L standard the Sb reads at 30 ug/L. if we run a 90 ug/L QC then the 90 will read ok for Sb because effectively it is reading 60+30. The subsequent QC's then start dropping lower and lower. We use mass 118 for Sn and 121 for Sb. Looking back at old data from the previous two years the problem seems to have started near end of 2008 and it has gotten progressively worse since then. We have not made any changes to the method for standards. Any advice on this would be appreciated!
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admin Total posts: 529
Sn and Sb are the two most easily hydrolyzed elements in the Periodic Table. It is very likely hydrolytic instability that you are experiencing, i.e., unstable solutions. Check your undiluted standard as well as any intermediate dilutions using ICP-OES (rather than MS - independent method to confirm and eliminate other possible causes). If the checks confirm lower concentrations then look at the dilution chemistry. Sn and Sb need relatively more acid than other elements or need to be fluorinated, i.e., addition of HF. The fact that you are having more difficulty at the higher concentrations strongly suggests hydrolytic instability.
Posted: 09/05/18 21:34:59

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