Control samples

Control samples are normally samples of a matrix-matched RM or CRM, made and priced so that they can be used to check the accuracy and precision of runs of analytical results for samples being analysed by a specific analytical method. They are inserted into each batch of samples.   The control samples assay results are checked on control charts or by using statistical techniques.

The manufacturer will have tested and documented the values expected for different tests and different methods.  These “Recommended Concentrations and Limits” are summarised on a “Certificate of Analysis”, which is useful for selecting a specific control material.

To behave the same during analysis control sample characteristics should be the same or similar to the sampled material.  However, there is a catch.  Because the Control Sample property values are based on a measurement campaign using a number of independent laboratories (round robin), the recommended concentrations and limits will reflect the average results and cumulative precision produced by the participating laboratories.  These “consensus values and limits” should not generally be used for setting the control limits in a single lab because they include a between-laboratory component of variation that can make the manufacturers control limits too wide for single lab use; depending on the commutability of the CRM with respect to the samples being tested (see Limits).

Control samples that have not been tested by an interlaboratory process may be used to test lab precision, but thats only half the job.  Properly characterised (by a sufficiently large round robin) RM’s or CRM’s are still needed to test laboratory bias (accuracy).