The chiral herbicide dichlorprop
(2,4-dichlorophenoxy-2-propionic acid), which is sold and applied as the
racemic mixture, was observed to degrade completely
in soil within 31 days, with a half-life of 6.6 d.
Degradation occurred with enantiomeric selectivity,
indicating biologically mediated reactivity as opposed to strictly abiotic degradation. The
S-(−)-isomer
degraded significantly faster (t
1/2 = 4.4 d)
than the
R-(+)-isomer (t
1/2 = 8.7 d);
this is contrary to other
published results that show selective degradation
of the R-(+)-enantiomer, although in other media.
Soil
samples taken from a field plot at increasing time
intervals after application of Foxtril, a commercial
herbicide formulation, were solvent-extracted and
analyzed for total dichlorprop by capillary zone electrophoresis (CZE), using an acetate buffer at pH 4.7.
Heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin, a
chiral
reagent, was then added to the buffer to effect
separation of the (+)- and (−)-isomers of dichlorprop.
Baseline resolution allowed calculation of relative
concentrations (enantiomer ratios) of the two isomers.
CZE is a fast and efficient technique for the
analysis
of ionic organic species (such as the anion of
dichlorprop), including their enantiomers, in pesticide
formulations as well as in environmental samples.
It thus was possible to analyze Foxtril directly
after
dilution with water for ioxynil (2,6-diiodo-4-cyanophenol) as well as for dichlorprop. Ioxynil also
was detected in the soil extract on the day of application. The hydrolysis product [methyl
2-nitro-5-(2,4-dichlorophenoxy) benzoic acid] of bifenox methyl
ester, another herbicide component of Foxtril, was
detected in the soil samples taken at 17 and 31
d.
The photodecomposition of imazamox, a herbicide of the imidazolinone family, was investigated in pure water. The main photoproducts from the photolysis were followed over time by liquid chromatography mass spectrometry and structures were proposed from exact mass determinations obtained by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The method comprised exact mass determination with better than 0.2 ppm mass accuracy and a corresponding structural visualization taking care of respective isotopes with an adapted van Krevelen diagram that enabled a systematic approach to the characterisation of the elementary composition of each photoproduct. By taking advantage of the high resolving power of FT-ICR MS to make precise formula assignments, the derived 2D van Krevelen diagram (O/C; H/C; m/z) enabled one to structurally differentiate the formed photoproducts and to propose a degradation pathway for imazamox.
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