Recent trends in inter-laboratory precision at ppb and sub-ppb concentrations in relation to fitness for purpose criteria in proficiency testing

Recently conducted collaborative trials in which the analyte concentration was below 100 ppb provided reproducibility standard deviations that were systematically lower than the predictions of the Horwitz function. This study shows that such statistics are better represented by a model with a constant relative standard deviation. A modified function is suggested as suitable for use (with due caution) as a fitness-for-purpose criterion.