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Certified Concentration of a Standard

Jaime purchased an Arsenic standard for ICP and had a question regarding the certificate of analysis. Jaime noticed that we report two independent assay methods, but we report the certified concentration of the lower of the two. Jaime asked Paul why we didn't average of the two together.


Dear Jaime,

First, thank you for your question. I have only gotten this question, which I believe is very important, once before.

In response, we used to take the average of both methods. When we applied for our ISO Guide 34 accreditation, we posed this question to A2LA. Their response requested us to do the following:

  1. Validate all methods and only use validated methods for assay purposes.
  2. Compare the two methods for agreement statistically.
  3. Then, if there is agreement, to use the method giving the lowest uncertainty.

The key is the validation and statistical comparison that allows for certification using the most precise data. Otherwise it is just "putting your best foot forward" with no scientific basis.

Please feel free to contact me with any other questions. I hope that you find our product and service to be excellent in every way.

Serving you in chemistry,

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

DISCLAIMER: Advice offered by the chemists at Inorganic Ventures is intended for the individual posing the question. Feel free to contact us to verify whether these suggestions apply to your unique circumstances.