00G3019-DO columns with hexane/1,2-dichloroethane/ethanol (500:10:0.05, v/v/v)
as eluent [62, 69]. Xu et al. worked on enantiomeric separation of lambdacyhalothrin by HPLC using the columns of Chiralpak AD (amylase tris
[3,5-dimethylphenyl carbamate]), Chiralpak AS (amylase tris[(S)-1-phenyl carbamate]), Chiralcel OD (cellulose tris[3,5-dimethylphenyl carbamate]), and Chiralcel
OJ (cellulose tris[4-methyl benzoate]) with different chiral stationary phases
[70]. The enantiomers of lambda-cyhalothrin were separated completely on all the
columns tested and detected by circular dichroism at 236 nm. In GC, Corcellas et al.
developed a method for simultaneous determination of the different enantiomers of
six pyrethroids (bifenthrin, cyhalothrin, cyfluthrin, cypermethrin, permethrin, and
tetramethrin) [71] using BGB-172 of 30 m  0.25 mm and a column with 0.25 μm of
film thickness. Previously, the same column was used by Chamberlain et al. for
separation of the enantiomers of cypermethrin, cyfluthrin, cis-bifenthrin, and permethrin and also by Liu and Gan who showed that this chromatographic column was
the best one for the enantiomeric separation of pyrethroids [69, 72]. The chromatography method proposed by Corcellas et al. allowed the separation of all cisenantiomers (two pairs, four peaks), but for trans-isomers, the enantiomeric separation was not possible, obtaining two peaks corresponding each one to each pair [71]
(Fig. 2).
5 Quantitative Methods
A complication in analyzing pyrethroids is that the concentration of each isomer of
an individual pyrethroid in the standard mixture is unknown. Generally, the technical standard mixtures of pyrethroids, which are generally used for quantification,
directly provide the sum of the concentrations of the individual isomers for each
pyrethroid. Thus, the concentration of each pyrethroid is determined by summing the
areas of the observed individual isomers. Moreover, pyrethroid-labeled standards are
scarce. Commercially available standards are trans-permethrin-d6 [4, 29] and transcypermethrin-d6 [29] which are generally used as internal standards for an isotope
dilution quantification. Other standards used for pyrethroid quantification are
PCB-166, PCB-195 [37], and caffeine [36]. Dibromooctafluorobiphenyl has been
used as surrogate for aqueous samples and dibutylchlorendate for sediment and biota
samples [15].
6 Conclusion
Sample preparation and cleanup methods for pyrethroids are well established for
environmental and food samples. Recoveries are high, reproducibility is good, and
method limit of detection is adequate for the determination of levels of pyrethroids at
environmentally relevant concentrations. Recently, low-solvent consumption and
Analytical Methods for Determining Pyrethroid Insecticides in Environmental and. . .
39
as eluent [62, 69]. Xu et al. worked on enantiomeric separation of lambdacyhalothrin by HPLC using the columns of Chiralpak AD (amylase tris
[3,5-dimethylphenyl carbamate]), Chiralpak AS (amylase tris[(S)-1-phenyl carbamate]), Chiralcel OD (cellulose tris[3,5-dimethylphenyl carbamate]), and Chiralcel
OJ (cellulose tris[4-methyl benzoate]) with different chiral stationary phases
[70]. The enantiomers of lambda-cyhalothrin were separated completely on all the
columns tested and detected by circular dichroism at 236 nm. In GC, Corcellas et al.
developed a method for simultaneous determination of the different enantiomers of
six pyrethroids (bifenthrin, cyhalothrin, cyfluthrin, cypermethrin, permethrin, and
tetramethrin) [71] using BGB-172 of 30 m  0.25 mm and a column with 0.25 μm of
film thickness. Previously, the same column was used by Chamberlain et al. for
separation of the enantiomers of cypermethrin, cyfluthrin, cis-bifenthrin, and permethrin and also by Liu and Gan who showed that this chromatographic column was
the best one for the enantiomeric separation of pyrethroids [69, 72]. The chromatography method proposed by Corcellas et al. allowed the separation of all cisenantiomers (two pairs, four peaks), but for trans-isomers, the enantiomeric separation was not possible, obtaining two peaks corresponding each one to each pair [71]
(Fig. 2).
5 Quantitative Methods
A complication in analyzing pyrethroids is that the concentration of each isomer of
an individual pyrethroid in the standard mixture is unknown. Generally, the technical standard mixtures of pyrethroids, which are generally used for quantification,
directly provide the sum of the concentrations of the individual isomers for each
pyrethroid. Thus, the concentration of each pyrethroid is determined by summing the
areas of the observed individual isomers. Moreover, pyrethroid-labeled standards are
scarce. Commercially available standards are trans-permethrin-d6 [4, 29] and transcypermethrin-d6 [29] which are generally used as internal standards for an isotope
dilution quantification. Other standards used for pyrethroid quantification are
PCB-166, PCB-195 [37], and caffeine [36]. Dibromooctafluorobiphenyl has been
used as surrogate for aqueous samples and dibutylchlorendate for sediment and biota
samples [15].
6 Conclusion
Sample preparation and cleanup methods for pyrethroids are well established for
environmental and food samples. Recoveries are high, reproducibility is good, and
method limit of detection is adequate for the determination of levels of pyrethroids at
environmentally relevant concentrations. Recently, low-solvent consumption and
Analytical Methods for Determining Pyrethroid Insecticides in Environmental and. . .
39