Absolute and/or relative detection limits in laser-based analysis: the end justifies the means.

The absolute limit of detection usually expresses the minimum amount of analyte detectable, while the relative limit of detection refers to the minimum concentration of analyte detectable. These concepts and their differences are obviously familiar to all analytical spectroscopists. Nevertheless, the two definitions are used liberally in the literature. For example, it is not uncommon to refer to exceptional sub-femtograms detection limits for a technique used to analyse ultratrace levels of an element in water and to a modest part per million detection limit of another technique used to characterise the microdistribution of an element in a sample mass of about one microgram. In this paper, an attempt is made to point out that the terms "ultratrace analysis" and "microanalysis" must refer to two conceptually different approaches and that there are cases in which one definition is more appropriate than the other. It is argued that, while there is no objection in reporting both detection limits when a single technique is evaluated, one has to be careful in choosing the most appropriate definition when different analytical techniques are compared.