Two Methods for Certification
Michael was curious about why we use both ICP analysis and gravimetric analysis to validate our standards. Yet, we only quote the ICP analysis as the Certified Value rather than averaging the two results. Paul explains.
According to ISO Guide 34, a number of requirements exist for the certification of CRMs. The use of two independent methods is strongly suggested. The two methods should include one that is a 'primary' method. The second method can be secondary, but both methods must be validated, executed with a known QC standard, and compared using a statistical procedure that would determine if the means agree or disagree (independent method student T test).
In our case, agreement of the two methods is a requirement for release of the product. This agreement demonstrates the accuracy of the standard. The requirement of traceability to the International System of Units (SI) is where the use of one method (rather than the average of two) comes in for the purpose of reporting a certified value. The traceability requirement necessitates that the uncertainty be reported using ISO guidelines (Guide 35). The uncertainty calculation includes all sources of random and systematic error and must be 'provable' with an error budget and clear presentation of errors involved in the preparation of the CRM.
Consequently, a choice is left to the manufacturer as to which method to use for certification. It is common practice to use the method that has the lower uncertainty. The validity of this approach rests upon the accuracy of the uncertainty calculation, the inclusion of all possible sources of error, and a demonstration that the two methods used have Means that are statistically in agreement.
In short, there is no mathematical way of writing an equation that would make scientific sense from a traceability perspective based on an average of uncertainties. Free to call me with any related questions or comments.
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.