Polymer coated glass fibers were applied as disposable
samplers to measure dissolved concentrations of persistent
and bioaccumulative pollutants (PBPs) in sediment
porewater. The method is called matrix solid-phase
microextraction (matrix-SPME), because it utilizes the
entire sediment matrix as a reservoir for an equilibrium
extraction: a glass fiber with a 15 μm coating of poly(dimethylsiloxane) (PDMS) was placed in a sediment sample
until the PBPs reached their equilibrium distribution
between the PDMS and the sediment matrix (1−30 days).
PBP concentrations in the PDMS were determined by
gas chromatography, and they were divided by PDMS water
partition coefficients to derive at dissolved porewater
concentrations. This approach was applied to measure
porewater concentrations of spiked as well as field sediment,
and several hydrophobic organic substances (log K
OW 5.2−7.5) were measured with high precision in the pg to
ng/L range. Simple equilibrium partitioning is the basis for
the substantial concentration factors that are built into
matrix-SPME and for the low demands in materials and
operation time. Matrix-SPME was in this study directed at
the determination of dissolved porewater concentrations
in sediment, and it is further expected to be applicable to
other environmental media, to field sampling, and to the
sensing of fugacity.
A model is presented for the acute toxicity of organophosphorus (OP) pesticides belonging to the class of
phosphorothionates. The acute toxicity of these pesticides
is governed by the irreversible inhibition of the enzyme
acetylcholinesterase (AChE), after their metabolic activation
to oxon analogues. The model is based on the idea that,
for chemicals exhibiting an irreversible receptor interaction,
mortality is associated with a critical amount of “covalently
occupied” target sites, i.e., the “critical target occupation”
(CTO). For a given compound and species, this CTO is
associated with a critical time-integrated concentration of
the oxon analogue in the target tissue, which can be
modeled by the critical area under the curve (CAUC) that
describes the time−concentration course of the phosphorothionate in the aqueous phase or in the entire aquatic
organism. In contrast to the classical critical body
residue (CBR) model, the CTO model successfully describes
the 1−14-d LC50(t) data of several phosphorothionates in
the pond snail and guppy. Furthermore, the time dependency
of lethal body burdens (LBBs) of phosphorothionates is
explained by the model. Although the CTO model is specifically
derived for OP pesticides, it can be applied to analyze
the acute toxicity and to estimate incipient LC50 values of
organic chemicals that exert an irreversible receptor
interaction in general.
For aquatic toxicants that act by so-called nonpolar narcosis, it is generally acknowledged that the Critical Body Residue (CBR) at death, as a surrogate dose metric for the amount of target that has interacted with the toxicant, is constant. This constancy is not only maintained across exposure times but also across different (narcosis) compounds as well as species. We present here an alternative model, applicable to reactive and receptormediated toxicants, that implies that for these compounds there is no constant CBR. The model also shows that for each single species-compound combination, the Critical Area Under the Curve (CAUC) is constant and independent of exposure time. These findings can have profound consequences for the interpretation of experimental toxicity data (such as 96 h LC 50 values) in risk assessment. Among other things, it shows us that for compounds other than nonpolar narcotics, LC 50 vs time values may decrease significantly even after bioconcentration steady state has been achieved. Consequently, it also shows us that the incipient LC 50 will be severely overestimated (i.e. toxicity underestimated) when using the familiar models based on just bioaccumulation kinetics.
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