Analysis results are always reported against working standards which have been validated against certified reference materials. A certified reference material is very expensive and is procured in small quantity from a standards body having global recognition. Due to the high cost it is not possible to use certified reference materials every time so a working reference standard is prepared bearing reference to the original certified reference material..
Always remember to use a working standard only during its validity as changes do take place in its properties over time. If you have entered the wrong potency of an expired standard in your calculations your analysis results are bound to go wrong. For this reason the working standards are always revalidated at prescribed intervals.
Changes in the characteristic properties of working standards are numerous and could result from:
- Non-maintenance of recommended storage conditions such as temperature and humidity which can lead to faster degradation of the standard
- Some light-sensitive materials undergo degradation due to exposure to light
- Contamination during use of unclean spatula and storage vial.
- Contamination due to return of used standard into the container after weighing.
Revalidation may include analysis of standard for assay, loss on drying, water content, physical description, solubility, heavy metals, sulphated ash, pH, specific optical rotation, melting point, and other characteristics as per requirements of individual standards.. After revalidation the records should be documented and the new standard stored in an amber coloured vials with proper labels giving standard details such as
Working standard number
Date of preparation
Valid up to
The shelf life of the working standard is fixed at one year from date of preparation but for some standards depending upon their characteristics the validity date can be different.
Access to working standards should be limited and a logbook maintained on usage of each working standard
If you take proper care on usage and storage of working standards they will remain stable during the validity and provide reliable results every time.
Please share your views and offer your comments on the article.