ISO 5667-14:1998 pdf download – Water quality – Sampling
5.1 General
Sampling is defined in ISO 5667-2 as the process of removing a portion, intended to be representative, of a body of water (or sludge or sediment) for the purpose of examination for various defined characteristics. Guidance is given below with respect to quality control procedures which can be used to identify and quantify errors associated with sampling. A complete overview of quality assurance applied to sampling is outside the scope of this document. However, it is important to emphasize that the quality control measures discussed below should ideally be applied in the context of a well organized approach to quality control. This would include a review of the whole approach to sampling with respect to its fitness for the intended purpose. Within this, the choice of sampling techniques, sampling locations, numbers and types of sample taken, training of sampling staff, sample transport, preservation and storage should be considered. The chosen approach should be adequately documented and a system of record-keeping established. A suitable quality control programme could contain any or all of the techniques listed below. The effort expended on sampling quality control is dependent on the objectives of the programme, but it is recommended that at least 2 % of analytical efforts should be devoted to quality control for sampling.
As noted earlier, quality control measures in sampling have three main objectives:
a) to provide a way of monitoring and detecting sampling errors and hence a means of rejecting invalid or misleading data;
b) to act as a demonstration that sampling errors have been controlled adequately; and
c) to indicate the variability of sampling and thereby to give a guide to this important aspect of error.
The following quality control techniques are described below.
– the collection of replicate samples as a check on the precision of sampling;
– the use of field blank samples to monitor sources of sample contamination;
– the use of spiked samples as quality controls to assess sample stability during transport and storage.
5.2 Replicate quality control samples This term can be used to cover a range of approaches to quality control which aim to assess the random error associated with different levels of the sampling process:
a) analytical variance: replicate analyses of the same sample prepared in the laboratory can be used to estimate short-term analytical error;
b) analytical + subsampling/transport variance: analyses of replicate samples taken in the field (B 1 and B 2 ) from the bulk sample (B) (the sample obtained by a single application of the sampling procedure). The difference between such data gives an estimate of analytical plus sampling variance (includes storage but excludes the effect from sampling containers).
c) analytical + total sampling variance: analysis of bulk samples obtained by separate application of the sampling procedure. This provides an indication of the variance of the whole process of sampling and analysis (A 1 and A 2 ).
The relationship between the different sampling variances in examples b) and c) is illustrated schematically in figure 1.