Quantities and units in analytical chemistry

Analytical chemistry is largely concerned with the determination of the composition of mixtures. The result of the analysis of a component in a mixture should comprise the product of a 'numerical value' and a 'unit' in order to express the value of the 'quantity' being measured (and an associated statement of uncertainty). The quantities and units which can be used to express these results are subtly different and can often be confused and misused. This article clarifies their meaning, presents a novel method of demonstrating the relationship between them, and discusses the advantages and drawbacks of their usage in analytical chemistry, particularly with respect to environmental analysis. Suggestions for best practice for use in analytical chemistry are also made.