Why do some secondary standards have a shelf life?

Document ID

Document ID TE11846

Published Date

Published Date 31/10/2019
Question
Why do some secondary standards have a shelf life?
Summary
Explanation of the shelf life for secondary standards.
Answer
Primary standards are solutions with a known concentration of the analyte of interest and are commonly used to perform accuracy checks or calibrations. The downside of these solutions is that they can often have short shelf lives due to chemical stability, they often will need to be prepared (for example via dilution to the testing range or put through the full test procedure which can include digestions), and results on the standards can be impacted by handling issues such as storage or preparation.

Secondary standards are physical standards that replicate samples that have been prepared for measurement and can often be used as alternatives to primary standards. The benefits of secondary standards is that they have little to no issues with chemical stability, do not need to be prepared in any way, and there are far fewer issues with handling. However, the trade offs are that these standards check only the instrument. They do not verify that the procedure is being followed correctly.  They do not interact with any reagents and therefore cannot be used to verify reagents or check for sample interferences like primary standards can through matrix spikes.

Because there are little to no issues with chemical stability with secondary standards the question can often come up as to why they expire. The shelf life for most certified secondary standards is two years (this includes specchecks and the glass rod secondary standard for TU5). This shelf life isn't actually in reference to the physical standard itself, but is instead in reference to the validity of the lot specific certificate of analysis. It's more correct to think of the certificate as the thing that is expiring, not the physical standard.

All secondary standards that expire have a lot specific certificate of analysis that provides a manufacture set tolerance for that secondary standard. As an ISO9001 certified company, all validation tools (including secondary standards) are required to have an expiration date, or date for needed re-certification. From experience with similar validation tools, with an average rate of use and average rate of wear, the standards would be expected to stay within the manufacture set tolerance for two years.

Within this frame work, the shelf life does not mean that the standard will no longer be within the manufacture set tolerance outside of expiration. It also does not mean that until the expiration is reached the secondary standard is guaranteed to be within the manufacture set tollerance. Customers who continue to use expired secondary standards do so at their own risk, and customers who use secondary standards to verifiy instrumentation that is used for compliance reporting may not be allowed by their regulators to continue to use secondary standards that have expired.

The exception is for the Gelex secondary standards for turbidity. Because of the intended use of these standards, the values and tollerance for the standards are user set and not manufacture set and provided on a certificate of analysis. Therefore they do not have a shelf life. (see also: How are Gelex secondary standards used? , What is the shelf life of Gelex standards?)))  )

This applies specifically to secondary standards that are certified. These include:
Glass Rod Secondary Turbidity Standard <0.1 NTU for TU5200, TU5300 sc, and TU5400 sc Laser Turbidimeter (Product # LZY901)
Test Filter Set for DR2800, DR3800, DR3900, DR5000 and DR6000 Spectrophotometers (Product # LZV537)
Test Filter Set for DR 2700 Spectrophotometer (Product # LZV770)
Set of test filter for Lico (Product # LZM339)
SpecCheck Secondary Gel Standards Set, DPD Chlorine - LR (Product # 2635300)
SpecCheck Secondary Gel Standards Set, DPD Chlorine - MR (Product # 2980500)
SpecCheck Secondary Gel Standards Set, DPD Chlorine - HR (Product # 2893300)
SpecCheck Secondary Gel Standards Set for Monochloramine & Free Ammonia. (Product # 2507500)
SpecCheck Gel Secondary Standard Set, Fluoride (Product # 2712500)
SpecCheck Gel Secondary Standard Set, Ozone, Mid-Range (Product # 2708000)