Toxicity measurements in concentrated water samples - Rivm

(VIM, 1996). In analytical chemistry it typically encompasses contributions from sample processing
and treatment (e.g. extraction of the analyte) as well as from instrumental readings.
A target uncertainty means the measurement uncertainty that is formulated as a goal and that is decided
on the basis of a specified intended use of measurement results (e.g. like EQS for priority pollutants).
The minimum required relative target uncertainty for the priority pollutants is set on 50 %.
Knowledge of the magnitude of the contributing errors from each step or process in an analytical
method can be helpful to identify the significant errors to address if action is desirable to reduce the
overall measurement uncertainty (Lepom et al., 2008). For the method presented here (sampling,
sample treatment and bioassays), such an approach is not feasible because the composition of the
samples by definition is not known.
An alternative approach of estimating the measurement uncertainty is to use data from analysis of
certified reference material, routine control samples or inter laboratory trials. For the XAD-method a
lot is already known about different mixtures of substances (chapter 3) and blanks. The inter laboratory
trial with a field sample gives an indication of the variation in test results. Important for this criteria is
the uncertainty at the level of the concentration factor of the critical value (EC f 50 = 10). To measure the
uncertainty at this level further intra and inter laboratory surveys should be performed with a known
mixture of substances with a response at the critical level. However every mixture of known substances
will react in a different way as a field sample. The response in the bioassays of the field samples
analysed in the inter laboratory trial were approximately at the level of C f = 100. The measurement
uncertainty (CV) at this concentration varied between 5 – 35 %.
6.3 Interpretation of the results
6.3.1 Ruggedness of pT
Aldenberg et al. (2000) tabulated upper, median and lower estimates for the Fraction Affected for
standardized concentrations in relation to toxicity data sample size. The interval between the lower and
upper estimate (5 % and 95 % confidence respectively) decreases slightly with increasing number of
toxicity tests (sample size, n). The standardized concentration (-ks), calculated as:
f
log(
C ) − mean
− ks =
standard deviation
Equation 5
has a stronger relation with the confidence interval. This means that the difference between the original
water sample (C f = 1) and the mean of the SSD-curve determine the confidence interval together with
the standard deviation of the samples. Increasing the number of toxicity tests in the sample to estimate
an SSD has therefore only little effect. Only very large sample sizes decrease the confidence interval
substantially.
Standardized concentrations are calculated for monitoring data from 2000-2005 (Figure 6-3). Also for
this data, based on calculations for a single fit distribution with Excel, the small effect of increasing the
number of toxicity tests (or sample size = n) on the confidence interval is obvious.
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