In the early 1980s, Horwitz et al. reviewed the reported results in the frame of the Association of Official Analytical Chemists (AOAC) PT rounds and derived an empirical relation estimating the coefficient of variation for the reproducibility (CV R) as a function of the mass fraction (C) expressed in g g−1:
Twenty years later, Thompson re-evaluated the results reported in several PT schemes and confirmed the validity of the Horwitz equation at mass fractions ranging from 1.2×10−7 to 0.138 g g−1, while suggesting a constant CV R of 0.22 (or 22 %) for mass fractions below 1.2×10−7 g g−1.
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