Portable XRF instruments are effective tools to provide ‘fit for purpose’ qualitative chemical data that are precise but typically very inaccurate. Under standardized test conditions, the analytical performance of portable XRF instruments varies significantly between individual instruments from the same manufacturer, and between manufacturers using similar instruments. The precision of the instruments to repeat a constant element concentration from a single sample is typically very good (<5 % RSD); however, the accuracy is generally poor to very poor with huge concentration differences in some elements. For example, Al measured from the same NIST standard reference material (NIST 2709a, an agricultural soil) by six Olympus-Innov-X Delta Premium instruments ranged from 2.56 to 7.93 % Al whilst two Thermo-Niton XL3t GOLD+ instruments gave values of 2.14 and 2.33 % Al against the certified value of 7.37 % Al.
After two to five months, repeat analysis by the same instruments under the same conditions shows a measurable deterioration in element concentration, with elements of lower atomic number, Al and Si, the worst affected; for example, the Si response from the Olympus-Innov-X Delta Premium and Thermo-Niton XL3t GOLDD+ instruments deteriorated up to 43 % and 40 %, respectively. This deterioration in the instruments’ performance was observed in all instruments evaluated.
Lithium-ion battery packs, a key to the instruments’ portability, can have a measurable effect on the precision and accuracy of the data with an instant 0.05 % concentration change in Fe (from 3.39 to 3.44 % Fe) during a routine battery pack change. Batteries’ serial numbers should be recorded to monitor these effects and corrections applied.
Each instrument provides uniquely individual data that should not be combined with data from any other units without appropriate post-processing or recalibration. To significantly improve the quality and value of the pXRF data, a calibration procedure appropriate to the material(s) being evaluated should be implemented with regular baseline data collected to ensure instrument and data stability. Over time, matrix calibrations need to be verified with the instrument undergoing a manufacturer’s recalibration when control limits deteriorate significantly.