Handling undetected and low-level components in purity determination

Impurities expected to be present at low levels in a high-purity material may go undetected or be measured with near-zero values. This complicates assessment and reporting of uncertainty, and in particular can lead to underestimation of uncertainty, anomalies in estimating degrees of freedom, and coverage intervals, which extend outside the feasible range for the measurand. These issues are considered and appropriate remedies are suggested. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher.