Issues with Low Bias on Quality Control Samples of Sn and Sb

Aoife writes...
We are having some issues with low bias on our quality control samples by ICP-MS. We have quality control samples at 10 and 90ug/L. Our calibration ranges from 01ug/L to 1000ug/L and we have 26 elements on the one run. Sn and Sb are being particularly problematic. The results for the 10ug/L tend to read a bit high and the 90ug/L results are consistently low - Sb in particular is very low e.g. 60ug/L. The Sn and Sb also seem to be carrying over i.e. if we run a blank straight after the 1000ug/L standard the Sb reads at 30ug/L. if we run a 90ug/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 117.9016 for Sn and 120.9038 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!

Dear Aoife,

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.

Serving you in chemistry,

Paul R. Gaines, Ph.D.
CEO of Inorganic Ventures & Fellow Chemist

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