A method is described to measure freely dissolved compounds, using solid-phase microextraction (SPME) as sampling technique. Currently applied techniques, like solvent extraction, have the disadvantage of extracting the total amount of a compound that is present in a sample. For many applications, though, the freely dissolved fraction of a compound is of more interest than the overall amount. The procedure described in this work is based on the principle that only the freely dissolved compound is available for partitioning to the SPME fiber. By extracting only a small amount of the freely dissolved fraction, the equilibrium between the fraction of a compound bound to a matrix and the fraction dissolved in the aqueous phase is not perturbed. The procedure was applied to study the binding of four polar compounds, i.e., aniline, nitrobenzene, 4-chloro-3-methylphenol, and 4-npentylphenol, to bovine serum albumin to illustrate the possibilities of the method.
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.
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