Sampling: The Ghost in Front of the Laboratory Door

摘要

Sampling can be the most demanding part of an analysis. Anybody in charge of sampling needs a good understanding of the composition of the material to be investigated, its heterogeneity (or homogeneity, in simple cases), and the chemical properties of the analytes. Sampling procedures must be described in detail. Detecting the bias of a sampling procedure can be difficu this fact is trivial, but it must not be forgotten. In many cases, gaseous and aqueous samples are homogeneous, at least at the local scale, but most solid samples are heterogeneous. A typical example is soil, which presents a twofold demanding problem for the analyst: first, the selection of the sites where the samples are taken, and second, the reduction of a sample (for example, 1 kg) to the analysis size (for example, 10 4). The greater the heterogeneity of a material, the deeper a knowledge of sampling statistics is needed. In demanding cases, one should not do the work without the advice of a statistician.