Working standards play a critical role in any laboratory. The results of any analysis are meaningful only when they are related to a reference standard material. Under ideal circumstances a certified reference material should be used every time an analysis is performed but this is not a practical proposition as the cost of certified reference materials is very high so the only option available is to make use of working standards.
What is a working standard?
A working standard is a material of highest purity grade available whose composition is known in relation to a certified reference material having established traceability to a recognized global standard body. As a working standard is to be used over a prolonged time period it is necessary to evaluate its composition periodically to verify that it does not vary beyond acceptable limits. This also necessitates care in handling and preservation under prescribed storage conditions. Before its expiry date another working standard is prepared and its composition is compared to the existing standard .In this manner a record of traceability is maintained with the original certified reference material through an unbroken chain of traceability.
Stages of Procurement, handling and intermediate checking of Working Standards
A working standard can be prepared in-house or it can be procured from a reputed supplier or another laboratory. A certificate of analysis should accompany in case the material is procured from external sources.
On receipt proper arrangements must be made for its storage, ie, temperature, humidity and exposure to light. Care needs to be taken after every use to properly close and seal the container and ensure that same storage conditions are maintained. It is also helpful if a log record is maintained on its consumption.
The material should be analyzed on receipt or in – house preparation. The standard is analyzed by at least 3 validated analysts for its assay, and other parameters such as loss on drying, moisture content, melting point, sulphated ash, etc. The average of results obtained is stored for future reference. Thereafter the vial or bottle is affixed a label assigning unique number, date of analysis, storage conditions and the use by date.
Working Standard validity
The working standard should be used only till the mentioned date on the label. Before this date a new standard should be made available. In case a standard gets consumed before its expiry a fresh replacement should be made available in advance. The list of working standards should be kept updated every time a new standard is introduced.
Shelf life studies
The validity of use is based on shelf life studies. Normally the validity of stable products is set for 3,6 or 12 months but for unstable materials shelf life studies are conducted generally at 30 +/- 2 deg C and 65+/- 5% RH. On the basis of shelf life studies the utility period can be decided.
As a general rule the assay of a working standard including loss on drying and moisture content should be checked after 6 months for standards having expiry of 1 year or 3 months for other working standards. In case any unexpected variations are observed or the appearance changes the standard should be discarded even before its use by date. As a useful guideline the table below provide the maximum deviation acceptance criteria for different tests on working standards.
|Test Parameter||Acceptance Criteria|
|Assay by HPLC||+/- 0.5 %|
|Chemical Assay||+/- 1.0 %|
|Microbiological Assay||+/- 2.0%|