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Study Notes: Practical Use of Standards

This Study Notes follows on from Study Notes: Reference Standards and concerns the use of reference standards at the laboratory level.

Reference standards are used in the laboratory for many different functions, for example:

  • the calibration of equipment or methods
  • as a QC check sample to determine that a particular measurement ‘run’ on a piece of analytical equipment has measured the check sample to an appropriate level of accuracy
  • for proficiency testing programs
  • for legal or regulatory reasons
  • for marketing purposes
  • for the training and evaluation of staff.

Knowledge of reference standards, their uses and limitations is vital for laboratory function. Remember that reference standards or reference materials may be defined as a material or substance with one or more property values that are sufficiently homogeneous, stable and well established to be used for calibration of an apparatus, the assessment of a measurement method or for assigning values to materials.

The example of a reference standard that we will consider here is a ‘check sample’ for pH measurement. This is a stable buffer with a known pH that has been purchased from a supplier.

The supplier certifies that the pH level is a certain value and assigns a range for the acceptable value obtained when measuring this check sample. For example, if the mean pH value is 5.90 the manufacturer will have calculated the standard deviation of a large number of replicate measurements and will provide an acceptable range which is usually mean +/- 2 or 3 standard deviations:

5.90 +/- 0.10 pH units
Range = 5.80 – 6.00 pH units.

Thus any value of pH that falls within the 5.80 – 6.00 range will be acceptable.

Why is a range given? All measurements involve deviation (error) and the range gives an acceptable window of acceptable values. For further information see Study Notes: Variability, Accuracy and Precision.

For any reference standard to function correctly it must be stored and handled properly. For instance, rules for storing reference standards may include:

  • store at correct temperature
  • avoid fluctuations in temperature
  • keep tightly stoppered
  • store out of sunlight
  • avoid contaminating the reference standard with other materials
  • do not use after the use-by-date
  • use only for specified equipment/samples/methods
  • monitor for microbial growth and discard if present
  • monitor for precipitation, sediment or crystals and discard if present
  • re-calibrate reference standard against another standard at specified time interval.

Reference Standards are linked back to SI Units as follows:

SI Units
International Standards, e.g. kilogram, litre, second
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International Standards
Accepted to conform to SI Units with an acceptable level of variation
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Australian Primary Reference Material
Developed from International Standards by CSIRO and agents
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Australian Secondary Reference Material
Developed from Primary Reference Material by CSIRO and agents
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State Reference Standards
Developed from Secondary Reference Material by CSIRO and verifying authorities
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Working Reference Material
Developed from Reference Standards by verifying authorities
and NATA-registered laboratories

Used at the level of the laboratory

It is important to remember that reference standards and materials are important tools in the laboratory that ultimately are traceable back to SI units. They should be treated and used with great respect because their use often entails important aspects of the laboratory QC and QA programs and the reliability of results generated by the laboratory.

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