The ABCs of CRMs...
Reference materials have many uses. Instrument calibration, type-standardization, calibration curve drift correction, “check standards,” and statistical process control (SPC) requires reference materials with specific attributes. Following is a discussion on the specifics for the uses of each type of reference material.
Certified Reference Materials (CRMs), with Confidence Intervals, should be used for establishing calibration curves. The quality of a CRM allows for estimating the accuracy of the analytical results obtained from the curve. The Confidence Intervals, of the CRM, and quality of the curve fit will act as a guide for determining accuracy.
Type Standards are exactly the same CRMs as used for calibration. Type Standards are used when the utmost in accuracy is required. When a CRM is run just before similar alloy unknowns (using the Type Standard mode of operation), the accuracy of analysis is the same as the accuracy of the CRM’s Confidence Interval. Also when a CRM is used as a Type Standard, drift correction is automatically incorporated.
Set-Up-Standards (SUSs) are used to “drift correct” the calibration curves. The process of drift correcting the curves has many names: normalization, standardization, and re-calibration. Regardless of the name given the process, the curves are adjusted back to their state at the time of the original calibration.
It is not important to have accurate chemistries for SUSs; chemistries at +/-10 percent are sufficient. Corrections to the calibration curves are made on intensities, not concentrations. It is important to have 1) homogeneous material, and 2) material that does not vary within a specific lot. It is perfectly fine to have chemistries vary from one lot to another lot of SUSs; it is a very fast and easy change over when one lot of the SUSs are exhausted.
Timing for implementing drift correction is critical. Some laboratories periodically run check or SPC standards to determine if the instrument is within the allowed tolerances. The stability and duty cycle of the instrument determine the period for checking drift. It should at least be checked every hour or before a batch of samples are run to assure quality of results.
If drift detection tolerances are not set up, it is imperative that drift correction, at a minimum, is accomplished every shift for laboratory instruments and hourly for mobile instruments. If the analyst is unsure about the state of the instrument, drift correction should immediately precede the analysis of any samples, for optimum accuracy.
The best way to assure routine analytical performance is to periodically run a CRM. The permitted tolerances can be established by running precision on the check standards right after calibrating. How often you run the check standards will depend on the stability of the spectrometer.
SPC reference materials can be the same as Check Standards. SPC is a long established practice for appraising product quality. SPC for the analytical measurement process is analysis of good quality reference materials at regular time intervals, storing that data for various statistical applications (e.g., control charts with upper and lower control limits), and using the statistical summary to decide if corrective action is necessary. Also, the data record can be used to prove that the instrument was “in control” over a certain time period or during a specific sample analysis.
If this brief discussion on various uses of reference materials raises questions or comments, it can be expanded or clarified by calling or faxing
ARMI. We look forward to everyone understanding and using reference materials appropriately and effectively.