Certified Reference Material (CRM), also referred to as a ‘standard’, is an essential part of the daily use of portable XRF instruments. They enable us to evaluate the functionality and accuracy of our instrument readings, to create new custom calibrations and ensure quality control of data.
But what exactly is a CRM?
Certified Reference Material (CRM) are defined as a material that has been analysed with high accuracy measurement techniques and has a certificate that provides the certified values of the measured properties, the associated uncertainties and a statement of metrological traceability, including method precision, method trueness, homogeneity and stability.
Two important aspects of a CRM are for it to be homogenous and chemically stable. These attributes enable the certified values of the CRM to be accurately duplicated and can be used for quality control and quality assurance across short and long time periods.
The certified values for various elements in a CRM are determined by a round-robin laboratory analysis method. This involves each CRM being sent to several laboratories, usually more than 15, for analysis using various methods (XRF, 4-Acid Digest and Aqua Regia) to obtain well-constrained values for the composition of the material, with a 95% confidence interval and low standard deviations.
For everyday uses, what does this really mean?
(1) Validation of methodology
A CRM is used for validation purposes to check the adequacy of instrument performance. For pXRF instruments, it is important to validate the data that is being collected by the instrument. By analysing a CRM, we can remove unwanted bias as there is a set range of values that the instrument should be reading. Anything out of this range can indicate that an instrument may not be performing properly.
(2) Performance and proficiency testing
The most important aspect of a CRM is that it has a certificate that states the composition of the material and the uncertainties associated with these values. When checking the performance and proficiency of an instrument it is effective to use a selection of CRMs with different concentrations ranging from light to heavy elements that relate to the project parameters. Having a wider range of concentrations allows the user to be able to plot the instrument’s performance, be it over the period of a day, or even months into a project.
(3) Process control and quality assurance checks
The replicability of CRM values enables them to be used for QAQC throughout a project. Having CRMs of known values analysed with the collection of samples of unknown values enable quality assurance and control processes to be applied to the data. When comparing the measured values for a CRM to the concentration on the certificate, the measured value should consistently be within a specific range of the certificate. If there is an unexpected spike or drop in the measured CRM composition and the value is no longer within an acceptable range from the certificate value, then there is a quality control problem. This could be due to contamination of the instrument or the calibration is no longer performing correctly. The quick identification and resolution of this error means the quality of the data is continuously checked and assured. Implementing a procedure that includes the use of CRM as part of regular QAQC will ensure the collection of good data.
Want to be able to implement your own QAQC but don’t know where to start? Check out our intermediate course here.
If you would like more information on implementing pXRF QAQC protocols, click here.
(4) Creating calibrations
CRMs are also used in the creation of custom calibrations, as the well-constrained element values of the CRMs enable them to be a good source of high-quality chemical data that is reliable. These calibrations are created to aid better data collection and accuracy for specific elements or matrices that are particularly challenging for currently available calibrations.
If you would like more information on how custom calibrations are created, click here.