Differences between Primary and Secondary reference standards used for analytical work

Certified Reference Materials

Any chemical analysis can be considered valid only if the method of analysis is validated before adoption and results are reported against internationally recognized standard reference materials. Results of such analysis can be relied upon by consumers as well as other laboratories across the world.

Standards are primarily used for the following types of analytical studies:

  • Assay
  • Identification tests
  • Limit tests for related substances
  • Analytical method validation
  • System suitability for analysis techniques and in particular spectroscopic and chromatographic analysis

It becomes necessary to understand the fine differences between different categories of standards used for chemical analysis. The standards are commonly grouped into two groups:

  • Primary standards or certified reference materials
  • Secondary standards or working standards

Primary Standard

A primary standard reference material is an ultra high purity grade compound used in analysis involving assay, identification or purity tests. It can be a single compound or a mixture having the analyte of interest in a specified and certified amount.

The impurities, if any, should be identified and controlled for use in assay studies. The material selected as a primary standard should be highly stable, free from water of hydration and bear traceability to a national or international standards body

In many cases it may not be possible to procure a reference material from such sources because of following reasons :

  • The new molecule is encountered in R&D activity. In such case a reference may not be available from standard bodies. A laboratory can use a reference compound whose purity is established for its routine use.
  • In- house primary reference material can be selected from a particular batch in manufacturing industries. After assigning its batch number its characteristic properties are documented for reference purpose and comparison with future production lots

Secondary or working standard

Primary standards come with a certificate of analysis and bear traceability to a globally recognized standards body. The cost is often too high for even milligram range quantities. Secondary standards or working standards are also high purity grade materials which are quantified in relation to primary standards and put to routine use in laboratories. Such working standards are assigned a limited validity depending on stability of the material and before expiry fresh working standards should be prepared for future use. It is important to realize that if an expired working standard is used in analysis then no credibility can be placed on the reported results.

Handling and storage of standards

Standards play a crucial role in analysis so require to be preserved under specified conditions so that their authenticity is preserved over the prescribed storage periods.

Following recommendations can prove useful:

  • Store in amber coloured glass vials or bottles which are properly sealed or cappedand stored in controlled humidity conditions .
  • Reseal vial or cap the bottle securely after use
  • Temperature sensitive standards should be stored in cooling chambers or dedicated refrigerators between 2-8^0C
  • Ensure that the standard is within its validity before use
  • Do not return a new standard to original container to prevent contamination of the original stock.

The significant contribution of standards in any chemical analysis is emphasized in the article. It is indeed difficult to imagine a laboratory functioning without making use of standards and reference materials.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

Comments

  1. Is there any clause in the Act and Rules to use internationally recognized standard reference materials.

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