Abstract
The availability of a result’s uncertainty of measurement in an analytical procedure is an integral part of a laboratory and is one of ISO17025:2017’s requirements. Uncertainty of measurement is one of three indispensable parameters to ensure the quality of measurement results – the others are metrological traceability and method validation.
Although there is numerous information available to estimate the uncertainty of measurement, a step-by-step procedure covering a test method from sample preparation to analysis by WD-XRF could not be located. The Guide to the Expression of Uncertainty in Measurement, also known as the GUM, is the main referring document but cumbersome. Furthermore, most publications in WD-XRF use the top-down approach to quantify a test method’s uncertainty of measurement in contrast with the bottom-up approach, where a test sample’s results’ uncertainty of measurement are estimated.
The study test method was set up to analyse major elements (SiO2, TiO2, Al2O3, total Fe as Fe2O3, MnO, MgO, CaO, Na2O, K2O, P2O5 and Cr2O3) on glass disks by WD-XRF and LOI by gravimetry. The test method analysed a wide range of sample types to cover typical geological samples from South Africa. Method validation data was used to estimate uncertainty, while metrological traceability was established by evaluating the test method’s trueness using certified reference materials.
Bias and precision data of a test method was used by both the top-down and bottom-up approaches. The top-down approach uses the quality control sample’s variance to quantify precision and either certified reference materials or proficiency tests for bias quantification. The bottom-up approach in the study required four functional models combining the uncertainties of the input quantities such as the flux components, regression slope and intercept on the Y-axis to quantify the measurand’s uncertainty of measurement.
EXCEL workbooks were developed for the GUM framework, Kragten and Monte Carlo methods, and the top-down approach. Uncertainties for the different approaches were compared, and significant differences, especially for SiO2, CaO and LOI at high concentration values, exist between the bottom-up and top-down approaches.