3 Instrumental Analysis
3.1 GC Methods
The most used capillary columns available for pyrethroid analysis are the nonpolar
stationary phase columns {e.g., 5%-phenyl-95% methylpolysiloxane (DB5, HP5%,
CP-Sil 8 BC, or similar)} [15, 19]. However, semipolar stationary phases {e.g., 35%
diphenyl 65% dimethylpolysiloxane (SPB-608) [16] and methyl 50% phenyl
polysiloxane (DB 17 MS, HP-608)} have been also successfully employed
[15, 33]. A more polar stationary phase (methyl 7%, cyanopropyl 7%, phenyl
polysiloxane, DB17-01) was used for the analysis of sediment pore water samples
[22]. Some authors have also proposed the use of short columns in order to reduce
analysis time (DB-5, 10 m  0.18 mm  0.18 lm) [26]. The chromatogram of
synthetic pyrethroids by multiple peaks due to the separation of diastereosimomers
(Fig. 1) [29, 56]. Pyrethroids are classified as type I or type II, depending on the
alcohol substituent. Type I pyrethroids (resmethrin, phenothrin, tetramethrin, permethrin) have two chiral centers on their cyclopropyl ring; thus, they are resolved in
two peaks corresponding to cis- and trans-isomers. However, type II pyrethroids
(cyfluthrin, cypermethrin, deltamethrin, fenvalerate, fluvalinate, fenpropathrin) contain a third asymmetric center, and they are resolved into four peaks. Esfenvalerate is
a type II pyrethroid exception: it does not possess a cyclopril ring and has only two
diastereoisomers. It is not possible to distinguish esfenvalerate and fenvalerate by
GC methods, since esfenvalerate is one of the four isomers found in fenvalerate, and
it is the biologically active component of fenvalerate. Undergoing exposure to polar
solvent [58], heat [59], and light [58, 60], isomerization of pyrethroids can occur,
and additional peaks appear in the chromatogram. This happens, for example, during
the GC analysis of lambda-cyhalothrin and deltamethrin. Tralomethrin can be
transformed into deltamethrin in the injector port of the GC system [61]. Such
pyrethroid transformation can be avoided by using LC-MS instead of GC-MS.
With LC-MS, deltamethrin and the two diastereoisomers of tralomethrin were
separated and identified by Velverde et al. [61]. Another possible solution to
isomerization is reducing the residence time of the sample in the GC inlet where
isomerization occurs [62]. Therefore, injection techniques {e.g., pulsed splitless
injection [63, 64] and programmed temperature vaporization (PTV)} are
recommended to achieve this. Another solution to reduce pyrethroid isomerization
used apolar solvent as hexane in presence of an isomer-stabilizing agent (e.g., acetic
acid) [62]. GC is generally combined with electron capture detector or a mass
spectrometer. Although GC-ECD is robust and highly sensitive for these compounds
having halogenated atoms [37] as known, the selectivity of GC-MS is much better
than that of GC-ECD. During GC-MS analysis, negative chemical ionization mode
(NCI) is preferred to electron ionization (EI) because under EI conditions, pyrethroids give low-mass ions, most of them with the same m/z ratios. Otherwise, NCI
reduces fragmentation, which is mainly due to the labile-ester linkage, generating
negative molecular ions. Bondarenko et al. found that the instrument response of
36
M. L. Feo
3.1 GC Methods
The most used capillary columns available for pyrethroid analysis are the nonpolar
stationary phase columns {e.g., 5%-phenyl-95% methylpolysiloxane (DB5, HP5%,
CP-Sil 8 BC, or similar)} [15, 19]. However, semipolar stationary phases {e.g., 35%
diphenyl 65% dimethylpolysiloxane (SPB-608) [16] and methyl 50% phenyl
polysiloxane (DB 17 MS, HP-608)} have been also successfully employed
[15, 33]. A more polar stationary phase (methyl 7%, cyanopropyl 7%, phenyl
polysiloxane, DB17-01) was used for the analysis of sediment pore water samples
[22]. Some authors have also proposed the use of short columns in order to reduce
analysis time (DB-5, 10 m  0.18 mm  0.18 lm) [26]. The chromatogram of
synthetic pyrethroids by multiple peaks due to the separation of diastereosimomers
(Fig. 1) [29, 56]. Pyrethroids are classified as type I or type II, depending on the
alcohol substituent. Type I pyrethroids (resmethrin, phenothrin, tetramethrin, permethrin) have two chiral centers on their cyclopropyl ring; thus, they are resolved in
two peaks corresponding to cis- and trans-isomers. However, type II pyrethroids
(cyfluthrin, cypermethrin, deltamethrin, fenvalerate, fluvalinate, fenpropathrin) contain a third asymmetric center, and they are resolved into four peaks. Esfenvalerate is
a type II pyrethroid exception: it does not possess a cyclopril ring and has only two
diastereoisomers. It is not possible to distinguish esfenvalerate and fenvalerate by
GC methods, since esfenvalerate is one of the four isomers found in fenvalerate, and
it is the biologically active component of fenvalerate. Undergoing exposure to polar
solvent [58], heat [59], and light [58, 60], isomerization of pyrethroids can occur,
and additional peaks appear in the chromatogram. This happens, for example, during
the GC analysis of lambda-cyhalothrin and deltamethrin. Tralomethrin can be
transformed into deltamethrin in the injector port of the GC system [61]. Such
pyrethroid transformation can be avoided by using LC-MS instead of GC-MS.
With LC-MS, deltamethrin and the two diastereoisomers of tralomethrin were
separated and identified by Velverde et al. [61]. Another possible solution to
isomerization is reducing the residence time of the sample in the GC inlet where
isomerization occurs [62]. Therefore, injection techniques {e.g., pulsed splitless
injection [63, 64] and programmed temperature vaporization (PTV)} are
recommended to achieve this. Another solution to reduce pyrethroid isomerization
used apolar solvent as hexane in presence of an isomer-stabilizing agent (e.g., acetic
acid) [62]. GC is generally combined with electron capture detector or a mass
spectrometer. Although GC-ECD is robust and highly sensitive for these compounds
having halogenated atoms [37] as known, the selectivity of GC-MS is much better
than that of GC-ECD. During GC-MS analysis, negative chemical ionization mode
(NCI) is preferred to electron ionization (EI) because under EI conditions, pyrethroids give low-mass ions, most of them with the same m/z ratios. Otherwise, NCI
reduces fragmentation, which is mainly due to the labile-ester linkage, generating
negative molecular ions. Bondarenko et al. found that the instrument response of
36
M. L. Feo