Inter-laboratory Testing for Quality Control
Most laboratories will carry out regular and frequent (daily, weekly, shift-based) quality control checks to ensure that their instruments are performing well and to validate the repeatability of the sample preparation methods used. In most cases, these quality control checks will be carried out using in-house check samples, rather than certified standards, because the cost of bulk quantities of certified standards generally makes frequent use prohibitively expensive.
In addition to these regular in-house validation checks, inter-laboratory testing can also be carried out. In general, laboratory participation is optional, and a fee may be charged for taking part in these types of programs.
This type of testing is often organised through research facilities (such as universities or government laboratories) or by larger companies with multiple laboratories doing similar analysis on a regular basis. The organising entity is responsible for the management and administration related to the inter-laboratory testing program. This includes such considerations of the types of samples to be used. Powdered samples, for example, will need to be split to ensure that all participants receive a representative portion of the bulk sample. Some samples (including liquids) may need to be diluted or to be stabilised by the addition of other chemicals. The samples must also be appropriate for shipping, considering that some sample types may not be permitted into certain countries. Consideration must also be given to climate and temperature when the sample is shipped – a sample may melt or freeze if it is being shipped to or from a very hot or very cold location. Finally, any sample storage requirements must be indicated for samples that won’t be analysed immediately.
Inter-laboratory analyses are carried out on a regular but far less frequent basis – once a month or once a quarter, not daily or weekly. Samples are prepared and shipped by the group or company responsible for the interlaboratory testing. Participating laboratories then analyse these samples using their own standard methods and report their results back to the program’s administrator.
Depending on the type of sample being analysed (an ore versus a metal, a solid versus a liquid), certified reference materials may be available for use. Where samples with assigned values are used in an inter-laboratory testing program, reporting back to a participant laboratory is simply a matter of comparing the laboratory’s results to the assigned values and indicating how similar their results are. Results are often indicated in terms of number of standard deviations from the assigned value.
Where an appropriate and sufficiently stable reference material cannot be found, “round robin” testing may be used. With these types of analyses, the results reported by all participating laboratories are used to calculate an assigned value and standard deviation for each analyte of interest.
In some inter-laboratory testing programs, in addition to these regular reports on their own comparative performance, participant laboratories will receive a report that compare each lab anonymously with all others, giving each laboratory an indication of how their analytical performance compares with others.
Participation in a testing program can give a laboratory additional information about their instrument performance:
If a lab consistent reports above the assigned value for a particular analysis, this may indicate a bias in the instrument or method being used
Limitations to instrumentation or methods can be identified if consistent issues are noted with specific analyte levels (for example, when one particular analyte is higher than normal, the lab’s reported result is more likely to be out-of-specification)
The ability to compare with other laboratories’ analyses of the same samples can increase confidence in the validity of the results being reported by each laboratory
Many inter-laboratory testing programs are long term, which also allows laboratories to compare their performance over many years. This can be useful to help demonstrate analytical consistency when there is a change in instrument or method in the laboratory.