Answer to Question #5368 Submitted to "Ask the Experts"

Q

I am interested in performing a comparison of gamma spectroscopy data for quality assurance/quality control (QA/QC) purposes (that is data on the same samples from two different laboratories), but I can't seem to find a way to directly compare gamma spectroscopy data that was counted for two different periods of time (one is almost twice the other). Is there a way to compare sample activities and MDA's (minimum detectable activities) under these circumstances?

A

Thank you for your question pertaining to the QA/QC of gamma spectroscopy data. The radionuclide concentrations and uncertainties, even if generated from different counting geometries, should be comparable. For example, consider the many radiochemistry laboratories that participate in performance evaluation (PE) programs where they analyze a blind sample and report their results. (See the Mixed Analyte Performance Evaluation Program on the MAPEP website.) It is inconceivable that every laboratory that performs gamma analysis would use the same geometry and count time, yet the results among these participating labs are often comparable.

For the MDAs, geometry and count time would affect the result. If the laboratories are trying to meet a requested MDA, the MDA values should be comparable. However, it is true that the MDAs will be different if the count times are different, and all other parameters are the same.

One final comment on QA/QC. QC is typically performed on blanks, laboratory control samples, and PE samples. These analyses are the true indicators of QC. Comparing the sample results from two different laboratories analyzing the same sample (not split samples) can be a valuable tool in detecting a potential discrepancy in results. You can compare results, but this is generally not considered a QC function.

Eric W. Abelquist, CHP