Toxicity measurements in concentrated water samples - Rivm

Drying of XAD
XAD must be dried to get rid of any excess water that will affect distillation. In general,
compounds that dissolve well in acetone will come off the XAD after drying overnight. In
spite of drying, however, some water will remain in the XAD pores. Acetone mixes well with
water, so the remaining water can be eluted from the XAD with acetone. The acetone eluate
will thus always contain a certain amount of water.
Some of the more polar or hydrophilic substances, particularly substances that do not
dissolve well in acetone, may be eluted with the remaining water in the XAD pores. The drier
the XAD, the poorer the recovery of this kind of component. On the other hand, too much
water in an acetone eluate may lead to incomplete distillation (section 3.3). Because acetone
can lead to toxicity, drying time was optimized for distillation efficiency.
The amount of water can be checked by weighing the XAD before and after drying. Dried
XAD should not weigh more than 0.3 g per ml. When using 2.5 ml XAD for a sample of
10 litres, a dried XAD sample should not weigh more than 0.75 g.
Acetone elution
The volume of acetone used to desorb the chemicals from the XAD should be large enough
to elute them to a high extent, and small enough to keep the time needed to evaporate or distil
the acetone from the sample to a minimum. The acetone volume has therefore been
standardized at about 1.5 times the XAD wet volume.
Elution column shape
The elution column should be narrow, to ensure contact between XAD and acetone is
efficient. The height to diameter ratio of the XAD in the elution column should be no less
than 3:1.
Acetone flow rate
In order to stimulate desorption of the chemicals, the flow rate should be kept low during
acetone elution, at approximately 1 ml/min.
3.3.3 Kuderna-Danish distillation
For toxicity testing, a sample needs to be free of any substances related to the treatment of the sample
that can cause unintended toxic effects. An acetone eluate is not suitable for toxicity testing, so the
acetone must be removed. Kuderna-Danish distillation (KD-distillation) is employed to remove the
majority of the acetone from a sample. This type of distillation is widely used for concentration of
chemicals by evaporating the solvent.
Care must be taken that the XAD does not contain too much residual water, as boiling of the acetone
eluate will cease before the residue volume is sufficiently reduced.
An acetone eluate that has been dried overnight usually contains little water. On the other hand, when
the amount of water is too small, the sample may boil dry during distillation. This will result in
complete loss due to evaporation and precipitation of chemicals. A controlled amount of water (0.5 ml)
is therefore added to the acetone eluate at the start of the distillation to prevent it from boiling dry. The
distillation residue (0.2 ml) generally consists of approx. 40 % water, the remainder is acetone. If a
distillation residue volume is too large, the water concentrate will contain too much acetone and will
cause background toxicity.
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