What would cause sealed Stablcal® standards to read outside the tolerance on the Certificate of Analysis?

Document ID

Document ID TE4934

Published Date

Published Date 27/05/2017
Question
What would cause sealed Stablcal® standards to read outside the tolerance on the Certificate of Analysis?
Summary
Stablcal® standards reading outside of tolerance.
Answer
This can sometimes happen if the instrument is calibrated with one lot of Stablcal® standards and then a new lot of standards are read once received and compared the certificate of analysis for the new lot.

The turbidimeters used to check the quality of the Stablcal® standards and create the certificate of analysis are calibrated using formazin standards prepared following Standard Methods for the Examination of Water and Wastewater.

The specification for our Stablcal®standards is that they read within ±5% from Standard Methods prepared formazin.

Normal formazin dilutions are only stable for several minutes to a couple hours. The benefit of using sealed Stablcal® standards is that they can be reused instead of needing to prepare fresh dilutions for every calibration. These suspentions are more stable than formazin dilutions, but will still have the NTU value decrease over time as the vial ages. In production they shoot for a higher percentage but still within +5% so that as the value decreases over time, it will still be within -5% throughout the printed shelf life on the vial.

What's important to note is that the specification is within ±5% from Standard Methods prepared formazin, not from lot to lot. So if the instrument is calibrated with an aged lot near the end of its shelf life that may be at a lower percentage of Standard Methods prepared formazin is then used to measure a new lot which is at a higher percentage of Standard Methods prepared formazin, then the standard would appear to read higher and can provide results that are outside the upper limit of the specification listed on the certificate of analysis.

If there is a concern on the quality of new lots of sealed Stablcal® standards, make sure that the instrument used to check the standards are calibrated using Standard Methods prepared formazin, not with a different lot of sealed Stablcal® standards.