Sample Preparation

Almost without exception, food is a complex inhomo-

geneous mixture of a staggering range of chemical

substances. The isolation and measurement of indivi-

dual chemical compounds in food usually represent a

difficult task. Even with powerful modern techniques

of separation and identification, such as HPLC, rarely

is it possible to load a syringe directly with a food

matrix and inject to obtain a sensible result. Perhaps

surprisingly, it is not unusual to find analytical

methods published with precision data reflecting

repeated direct assays of standard solutions. This tells

the reader little about the practicality of the meth-

ods for real world samples. Procedures for prepara-

tion of the sample should be developed, evaluated,

and published as an integral part of any analytical

method. There are three steps involved in sample preparation

for chemical analysis of foods: (a) sampling, obtaining

a sample for the laboratory; (b) homogenization of the

laboratory sample to allow the taking of test portions;

and (c) sample preparation, physical and chemical

manipulation of the test portion prior to analytical

measurement. It should be appreciated that elements

of these steps may occasionally occur in the reverse

order or as combined operations. The fourth and final

step of the analysis is the actual determinative assay

procedure. Paradoxically, although the purpose of

each of the three steps is to increase the accuracy and

precision of the analysis so the test portion reflects the

composition of the bulk; each step also introduces

inherent errors. The error contributions of these steps

for a typical food analysis scheme are shown in Fig. 1.