Toxicity measurements in concentrated water samples - Rivm

possible. In case an analytical method may not meet these minimum criteria laid down in the Decision
paper from the European Commission (Quevauviller, 2008), the best available technique should be
carried out not entailing excessive costs. The Guidance describes the methods how to estimate the
minimum criteria in a pragmatic way.
In section 6.2 an explanation is given for the performance criteria and the results of the quality
parameters that could be applied on the XAD-method. The interpretation of the results is evaluated in
section 6.3
6.2 Performance parameters
For the XAD-method including bioassays it is tried to join the QA/QC performance criteria mentioned
in the Decision paper (Quevauviller, 2008). The XAD-method on environmental samples was difficult
to compare with the method for analytical analysis. For analytical analysis it is clear what target has to
be measured (for instance, to meet the EQS for a priority pollutant). For the method based on results of
environmental samples it is hard to estimate those performance criteria. Bioassays do not specifically
target individually substances. The technique gives a single holistic answer, the overall summation of
the toxicity of all substances present in the sample. Prichard (2006) has specifically written a validation
guideline that deals with the requirements, problems and the validation of emerging tools and possible
ways of establishing performance criteria. She mentioned a possible validation approach by validating
the method with a small number (4-5) of agreed relevant individual target substances. However it will
take a lot of effort to measure these criteria with known samples. Besides, every analytical or
environmental sample will react in a different way. This is a common phenomenon for validation
studies on bioassays. So, for several performance criteria a pragmatic approach has been suggested.
In this section the following criteria will be discussed:
- standardization of the method;
- precision of the entire method;
- specificity / selectivity;
- ruggedness;
- limit of detection / limit of quantification;
- measurement uncertainty.
6.2.1 Standardization of the method
Implementation of the method for monitoring, for instance under the WFD, requires a well-defined
description of the entire method. On basis of the experiences with the method during several years of
monitoring, defined protocols from sampling through interpretation of the results have been drawn up.
Wherever possible the methods according available international standards are used. The detailed
protocols are given in Appendix I.
6.2.2 Precision of the entire method
The precision of a method is a statement of the closeness of agreement between independent results
obtained under stipulated conditions (Prichard, 2006). Two types of precision could be estimated:
repeatability and reproducibility. Repeatability refers to the precision of measurements taken with the
same method, in the same laboratory, with the same equipment and within a narrow time period.
Although not many data are available, the studies suggest that the repeatability of the total procedure
lies within acceptable ranges. An initial validation study with effluent samples (Rotteveel and Roex,
2006) indicated that the variation between toxicity measurements on a sample from one extraction run
is only slightly lower than the variation between toxicity measurements on samples from different
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