Q UALITY OF S AMPLES The purpose of sampling is to provide for a specific aim (determine one or some of

5.2.1 Q UALITY OF S AMPLES The purpose of sampling is to provide for a specific aim (determine one or some of

the characteristic properties) a part of the object that is representative and suitable for analysis. The part of the object taken for further examination is the sample which is

usually a very small portion (10 5 to 10 6 ) of the sampled object (e.g., 1 –2 kg of apples taken from an orchard of 2 ha yielding 50,000 –60,000 kg fruits, or taking

20 soil cores from 5 ha field). The sample may be a single unit or an increment, or it may contain a number of primary samples* defined by the sample size in case of a

* One or more units taken from one position in a lot.

Quality Assurance 133 composite bulk sample, from which the laboratory sample may be prepared. The test

portion (usually 2 –50 g) is a representative part of the laboratory sample, which is extracted.

To prepare such a small fraction of the sampled object providing unbiased information with quantifiable uncertainty requires well-defined procedures per- formed by very responsible and technically highly qualified staff. The samples and the test portions analyzed should satisfy some basic quality requirements:

Represent the properties of the object under investigation (composition, particle size distribution)

Be of the size that can be handled by the sampler and the analyst; keep the properties the object had at the time of sampling; be suitable to give the required information (e.g., mean composition, composition as a func- tion of time or place); and keep its identity throughout the whole procedure

of transport and analysis 21 To develop a quality sampling plan, the following actions should be taken and points

may be considered:

Purpose of the study (different sampling procedure would be required if we want to obtain information on the average residue in a commodity or the distribution of residues in crop units, within one field (or lot) or between

fields)

Clear definition of the object, which can usually be properly defined by the lot=batch number, the space coordinates and the time

Collection of information of the properties of the objects before sampling (it may be necessary to inspect the site to determine the conditions and equipment required)

Selection of suitable sampling method and tools; testing the suitability of containers to be used to collect, pack, and ship the samples, taking also into account the health, safety, and security precautions

Determination of the time required for reaching the sampling site and handling the samples

Provisions for prevention of contamination and deterioration of samples at all stages, including size reduction of bulk sample

Arrangement for sealing, labeling, delivering the samples and the sampling record to the laboratory in unchanged conditions, and assuring integrity of the whole operation

Preparation of preprinted sampling record sheet which guides the operator to collect and record all essential information including deviations from the sampling protocol

Training of sampling personnel to assure that they are aware of the purpose of the operation and the provisions to be taken for obtaining reliable samples (e.g., permitted flexibility to adapt the sampling method for the particular conditions, recording requirements, legal actions, etc.)

134 Analysis of Pesticides in Food and Environmental Samples