Laboratory analysis involves measurement and quantification of several common quantities, such as, mass, volume, temperature, pH, specific gravity, refractive index, light absorbance, etc for which the commonly used standards are:
Mass – Certified reference weights
Volume – Volumetric flasks, burettes or pipettes
Temperature – Platinum resistance thermometer
pH – Standard buffers
Specific gravity – water at specified temperature or any other reference liquid or solid
Refractive index – same as for specific gravity
Light absorbance – standard absorbance filters or solutions
Besides the standards mentioned above identification and quantification of both inorganic and organic materials requires standards existing as high purity compounds in either solid or liquid state. Such standards fall into either of the two categories – primary standard or secondary standard. The differences between the two categories have been highlighted in an earlier article (link).
Traceability of Standards
A reference standard material without a trail of its traceability is of little value so defining of traceability of laboratory standards is an important requirement of ISO/IEC 17025:2005 .
Before moving ahead you should have clarity on concept of traceability. It is the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually, national or international standards (expressed in SI units), through an unbroken chain of calibration comparisons all having defined uncertainties. In other words a standard is universally accepted if other laboratories agree to adopt it as a reference for stipulated tests and measurements.
Establishing the traceability requires procurement of certified reference material at least once from globally recognized standard bodies. Procurement of primary standards is also permitted from national metrology laboratories which in turn establish the traceability of such standards with standards procured from international bodies. National laboratories also maintain ideal stipulated environmental conditions for preservation of such standards.
In case it is not possible to establish the chain of traceability of standards within a laboratory then the standards can be sent for calibration at defined intervals to national metrological laboratories or other accredited calibration laboratories and calibration results along with the specified uncertainty values should be obtained along with the report specifying traceability to internationally acceptable standards.
It goes without saying that an accredited calibration and testing laboratory can confidently report its results and ensure that the results are comparable to any other accredited laboratory if traceable standards are used. It is important to bear in mind at all times that credibility of results can have far reaching consequences in terms of public health, safety and in legal judgments based on laboratory reports.