Measurement traceability

All equipment used for tests/calibrations, including those for subsidiary measurements having effect for the validity of the results, shall be calibrated prior to use. The laboratory shall have an established programme and a procedure for equipment calibration.

Traceability of the measurement standards and measuring instruments to the International System of Units (SI) is established by means of an unbroken chain of calibrations or comparisons linking them to relevant primary standards of the SI units of measurements.

When using external calibration services, traceability of the results shall be assured by the use of calibration services from laboratories that can demonstrate competence, measurement capability and traceability.

Calibrations that can not be made strictly in SI units shall provide confidence in measurements by establishing traceability to appropriate measurement standards (certified reference materials, specified methods or standards)

Even though traceability to an SI unit is the desired goal, direct linkage is not easy and is not the usual line of approach in XRF analysis. The SI unit is rather linked indirectly to a measurement procedure. For this purpose, the measurand needs to be defined as a function of the measurement parameters. This provides the working equation that models the experimental measurement and conditions, and ascribe results to the measurand. Subsequently, the correctness of the working equation (method specification) in representing the real situation is assessed through validation, and uncertainties evaluated.