# Intermediate checking on Working Standards

Working standards play a crucial role in chemical analysis. Comparison with a standard is essential for both identification and quantification of unknown components of samples. In other words a standard serves to authenticate an analysis. Ideally a certified reference material bearing traceability to national or international standardization bodies is essential every time an analysis is carried out but this becomes impractical as the cost of certified reference materials is highly prohibitive so working standards offer a practical solution.

#### What is a Working standard?

A working standard is material of highest grade purity which contains the analyte of interest. The composition of the working standard relates to the composition of the certified reference material with which establishes a chain of traceability to the original standards body.

A working standard has a specific validity as it decomposes due to several factors such as external contamination, environmental changes and handling misuse. Before the expiry date another standard is prepared which bears traceability to the standard and so on the chain continues.

Working standards stored in recommended storage conditions are valid for at least three months. If it gets consumed in less than the validity period should be replaced through a written request and any amount remaining after expiry date should not be used any further.

#### Shelf life Studies

Shelf life conditions are :

$$30^O C$$ +/- $$2^OC$$ at 65 % +/-5% RH

Assay and other quantifying parameters are evaluated at initial, 6months and 12 month periods. On the basis of results of shelf life is specified. As a general rule purity of working standard should check at 6 months for expiry of standard having expiry of 1year and 3 months for other working standards

The parameters usually checked are assay, loss on drying and water

Triple analysis is conducted by three different lenses and the acceptance criteria are

 Test parameter Acceptance criteria (RSD) Assay by HPLC +/-0.5% Chemical assay +/- 1.0% Microbiological assay +/-2.0% Water +/- 0.5%

#### Packaging and labelling requirements

Working standards should be stored in amber coloured vials and stored at $$2^O – 8^O$$ C. Records of working standards issued to external parties or to different units of same laboratory should be maintained. A typical working standard label is represented below:

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#### Responses

1. How can we set the acceptance criteria of Intermediate check of CRM ?

1. No standard guideline can be set for all CRM’s. Intermediate checks will depend on nature of the standard and the recommendations of supplier based on its stability and prescribed storage conditions