The results of any analysis can be relied upon only if the technique has been calibrated against a reference material and this calibration is never permanent. It has to be repeated at intervals specified by the instrument manufacturer. The calibration process will decide the degree of accuracy of the reported results. You may question” Why is it necessary to calibrate the instrument from time to time and not only once at time of installation?” The reason is that errors can creep in unexpectedly due to number of factors such as changes in environmental conditions, drifts in power supply, changes in useful life of component parts such as light sources, detectors, columns, etc or even improper handling of equipment. As calibration is carried out against acceptable standards giving known response output can be corrected by a correction factor. It is here that the availability of calibration materials from globally recognized sources plays a major role.
Certified Reference Materials
Certified reference materials or quality control samples are used for calibration of testing instruments, validation of test methods and global acceptance of international trade products. These are characterized for one or more specified properties by means of a certificate that provides value of specified property, its associated uncertainty, and a statement on its traceability to recognized sources such as
- Institute of Reference Materials and Measurements
- Bureau of Analytical Samples Ltd, Middlesborough, UK
- National Research Council, Canada
- Sigma Aldrich
- High Purity Standards
- Choice Analytical
Besides the international sources National laboratories complying to requirements of ISO 17025:2005 also provides standards which are accepted regionally.
A certified reference material has specified quantity of analyte of interest
Certified reference material can be a real sample and not necessarily a pure compound. It is required to have same matrix composition as the actual sample under analysis and can also be termed as a quality control sample.
Certified reference materials are very expensive and a small quantity in the range of milligrams can cost more than a couple of hundred dollars. Fortunately the same purpose is served by making use of working standards which are compounds of high purity and have been standardized against standards through a source such as NIST through a chain of comparisons.
It is important to preserve CRM’s and working standards under specified environmental conditions such as exposure to light, temperature and humidity.
Working standards required to be qualified before expiry.
I hope that I have explained the differences adequately. Please to leave your comments on the basis of your experience on working with such standard materials.