Analytical results can be relied upon only if they are related to standards and reference materials. The value of any laboratory estimation is expressed in two units, namely, a digital value followed by a unit measurement e.g. concentration is expressed in moles/liter or gm/liter. The result can be accepted only when both the units are calibrated against known universally accepted standards. It is therefore necessary for you to have clarity on different expressions used for laboratory standards and reference materials.
Before proceeding to standards an understanding of the concept of traceability is important. Traceability is a property or result of a measurement which can be related to national or international standard bodies, such as NIST, through an unbroken chain of comparisons within defined uncertainty limits.
What are standards?
Standards basically fall into following categories:
Metrological standards also known as measurement standards and are based on SI units of measurement. Examples are:
- Length expressed in meters or multiples thereof
- Mass in kilograms of multiples thereof
- Time in seconds
- Temperature in degrees Kelvin
- Luminous intensity in Candela
- Concentration in moles per liter
All the above standards are expressed in relation to internationally preserved and accepted standards. For further details on units you may refer to the article Measurement units commonly used in analytical work. A chemical laboratory relies on several standards which are divided into different categories:
Chemical standards are reagents of very high purity and whose composition is well known. Examples are potassium hydrogen phthalate, sodium oxalate and potassium iodate. Such standards are commonly used for preparing secondary or working standards.
Secondary or working standards are based on primary standards. For most laboratories it is often expensive to afford standard materials from international standard bodies so secondary or working standards are preferred for routine use. Such standards have limited stability and should be prepared afresh after expiry of validity.
Broadly speaking analytical standards can be divided into the following classes
- Compounds with established physico-chemical properties such as melting points, boiling points, refractive index, etc are used routinely. Standards having established absorbance behaviour such as Holmium oxide solution or polystyrene film are used for spectroscopic studies.
- Reference materials are specimens for quantitative studies in specific sample matrices.
- Certified reference materials are reference materials whose composition is certified by international standard bodies such as NIST, ISO, EPA, etc. A certificate from the body accompanies the standard.
- Meteorological standards are standards which are calibrated against certified standards available from national or international laboratories. A laboratory should essentially maintain certified standards for mass, temperature and volume.
Documented standards are equally important for laboratories and these include detailed protocols of methods to be followed for analysis of various classes of compounds or products to be analyzed such as AOAC for analysis of foods, National pharmacopoeia’s for medicines, ASTM or BS for petroleum products, building materials, etc and EPA, OSHA for monitoring of environmental contaminants.
- Safe limits for elements and toxic substances in natural resources, foods, medicines, etc are prescribed by international bodies. These limits are revised from time to time depending upon advances in measurement techniques and awareness on safety for human consumption.
- Quality systems standards are prescribed by international bodies for upkeep of high-quality standards by laboratories. Examples are ISO 9000:2000 series of standards which prescribe principles of quality management. ISO17025:2005 contains requirements on management and technical control for laboratories seeking accreditation from national bodies.
- Standard operating procedures are guidelines developed by quality managers of individual laboratories and are based on good laboratory practices which are enshrined in the international guidelines.
- Client standards are industry specific and notified for different materials. Clients send their samples for testing to accredited laboratories for a compliance report on such samples.
Standards help mutual acceptance of results by different laboratories and are important for promotion of international trade but it is equally important to revalidate them at specified intervals and preserve them under specified environmental controlled conditions.