Concept for evaluating results close to the participants’ limits of quantification in proficiency tests for elements in food

Abstract

Proficiency testing (PT) in chemical analysis ideally provides participants with feedback on all aspects of laboratory performance. In the field of analysis of elements in food, analyte levels in test items are often close to the legal limits. Performance criteria such as those in Commission Regulation (EC) No. 333/2007 also refer to maximum values for limits of detection (LOD) and limits of quantification (LOQ), as well as to selectivity and control of contamination. Therefore, PTs with analyte levels in the range of LOD and LOQ are also very beneficial, as at these low levels, insufficient selectivity or contamination during sample preparation are more likely to affect the analytical results.

Since 2017, our unit has provided 7 PTs, each with at least one parameter (e.g. an element prone to contamination such as arsenic or mercury) with very low mass fractions. In these cases, the datasets showed a significant proportion of results reported as “< LOQ”, together with quantitative results as well.

If the “< LOQ”-values are predominant, calculation according to ISO 13528 is not feasible. Due to the wide range of the individual LOQs, the numerical use of the LOQs or a percentage of them is not appropriate. Therefore, we decided to sort the quantitative results and the “< LOQ” results numerically. We then determined the median and evaluated all quantitative results above the median as “false positive”, while all other results were evaluated as “true statement”. In this way, laboratories affected by contamination, for example, received appropriate feedback.