Toxicity measurements in concentrated water samples - Rivm

Table 3-5. Average measured concentration as percentage of actual initial concentrations of the narcotic
mixture for 2 datasets.
Dataset 1 Dataset 2
Average %
(n=10)
S.D.
Average %
(n=3)
S.D.
1,4-dichlorobenzene 98 2 100 2
Hexachloroethane 90 3 99 1
1,3,5-trichlorobenzene 92 3 97 2
3,4-dichlorotoluene 100 3 99 2
1,2,3-trichlorobenzene 93 2 102 1
3-chloronitrobenzene 86 4 95 1
2,4-dichloroaniline 84 3 76 1
1,2,3,4-tetrachlorobenzene 96 3 101 2
3,4-dichloro-nitrobenzene 86 5 95 1
2,4,6-trichloroaniline 96 6 97 1
Pentachlorobenzene 96 4 99 1
Average 92 5 96 7
However, not much is known about the actual initial concentrations of the other chemicals that were
tested. Artefacts in chemical analysis may not be noticed, but also interpretation of chemical recoveries
can easily be different when the actual concentration differs from the nominal concentration. Therefore
it is not possible to draw ‘hard’ conclusions on the effectiveness of the method for the pesticides, the
organotin substances and the detergents, especially when the behaviour of the test substance is not
known.
When data are lacking on initial concentrations in spiked water, an indication on extraction efficiencies
can be obtained from chemical analysis in acetone after elution. Although this may be a legitimate way
of thinking, misinterpretation of results may occur when mistakes were made, e.g. accidental elevated
or lowered initial concentrations due to erroneous spiking of water.
Without data on initial concentrations in spiked water and concentrations in waste water, it is not
possible to produce a mass balance. Mass balances are helpful in recognising or excluding mistakes or
artefacts due to chemical analysis. When the aim is to validate or to improve the method, it is advised
to chemically check all steps in the procedure when possible.
Optimization
The narcotic mixture comprises of hydrophobic substances that were expected to give good extraction
recoveries. It should be noted that the method development was based mainly on the narcotic mixture.
The choices for XAD/ water volume ratio, the extraction time, the elution volume and the KDprocedure
were all based on experiments only with this mixture.
For the narcotic test mixture it is therefore known, also from former experiments during method
development, that the XAD-concentration method works well, as long as the chemicals are not volatile,
and less satisfactory for the relatively volatile substances (Struijs and Van de Kamp, 2001; Collombon,
2007). However, it is not known to what extent the method can be optimized for chemicals that are less
hydrophobic.
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