Brazilian Journal of Analytical Chemistry - BRJAC - Brazilian Journal ...

taken to the laboratory and stored in a freezer until the
analysis was performed. Samples containing pesticides
above the MLR were analyzed by GC-MS to confirm the
results. Furthermore, samples of potatoes in which pesticide
residues were detected were peeled and the pulp
and peel analyzed separately. A portion of the residue
containing samples was also submitted to a cooking
process and then peeled. The peel and pulp were then
analyzed.
All analyses were performed in triplicate.
3. results a n d dIscussIon
3.1. ch r o m a t o g r a p h Ic an a l y s Is
Figure 1 represents the chromatogram of the GC-ECD
analysis of a standard solution of the pesticides in acetonitrile
at the established conditions. The peaks with
t R equal to 4.1, 6.9, 8.1 and 9.5 minutes correspond
to chlorpyrifos, λ-cyhalothrin, cypermethrin and deltamethrin,
respectively. The second peak, with a retention
time of 6.3 minutes corresponds to bifenthrin, the
internal standard. The presence of a second peak for
each of the pyrethroids is related to the conversion of
isomers during injection into the chromatograph [25].
Therefore, the sum of the peak areas of both isomers
was considered for quantification of each pyrethroid.
fi g u rE 1. Ch ro m a t o g ra m o f a 500.0 µg l -1 standard s o l u t i o n o f thE
aCtivE i n g rE d iE n t s in a C E t o n i t r i lE, w h E r E t r = 6.3 m i n: i n tE r n a l standard,
t r = 4.1 m i n: ChlorPyrifos, t r = 6.9 m i n: λ- Cyhalothrin, t r = 8.1 m i n:
CyPErmEthrin a n d t r = 9.5 m i n: dEltamEthrin.
Quantification of pesticides in the extracts was performed
by employing the internal standardization
method. Calibration curves of each pesticide were constructed
for the concentration interval of 6.0 to 750.0
µg L -1 , obtaining correlation coefficients (r) greater than
0.99.
3.2. optImIzatIon o f t h e sle-ltp method
In the optimization process of the SLE-LTP technique,
www.brjac.com.br
Dardengo et al.
analyses were performed of the pesticides chlorpyrifos,
λ-cyhalothrin, cypermethrin and deltamethrin in potatoes.
Some factors which influenced the results of the
technique, such as type of extraction mixture, time of
ultrasound, the ionic strength effect and fortification
time were evaluated using fortified potato samples.
The experimental planning of the mixtures was performed
on the basis of volumetric proportions of water,
acetonitrile and ethyl acetate, determined by the upper
and lower limits of each solvent in the extraction
mixture established in preliminary studies (Figure 2).
Response surface models, obtained with the assistance
of the Statistica 6.0 ® program (StatSoft) as functions of
the pseudocomponents (Figure 2), show the recovery
percentage estimates obtained for each pesticide.
fi g u rE 2. rEsPonsE surfaCE m o d E l s s h o w i n g rECovEry P E rC E n t a g E EstimatEs
f o r ChlorPyrifos (a), λ- Cyhalothrin (B), CyPErmEthrin (C) a n d dEltamEthrin
(d), as f u n C t i o n s o f thE PsEudo-C o m P o n E n t s ( z i ). z h2o = m i x t u rE 1, z mECn
= m i x t u rE 2, z EtaC = m i x t u rE 3, taBlE i.
It is noted for clorpyrifos that the best response was
obtained with pseudocomponent 2, Z MeCN , composed
of 1.0 mL water, 7.5 mL acetonitrile and 1.5 mL ethyl
acetate, with 88% recovery percentage. For the pyrethroids,
the best response was obtained with pseudocomponent
3, composed of 1.0 mL water, 6.5 mL acetonitrile
and 2.5 mL ethyl acetate. In Figures 2b, 2c
and 2d, the recovery percentages obtained when employing
pseudo-component 3 were 86% for λ- cyhalothrin,
76% for cypermethrin and 90% for deltamethrin.
More polar extraction mixtures favored the extraction
of chlorpyrifos and less polar mixtures the extraction
of pyrethroids. However, as the response obtained with
pseudocomponent 3 was more suitable for extraction
of most insecticides, it was selected as the extraction
mixture. After defining the best extraction phase, some
experimental parameters were tested with the objective
of improving extraction efficiency.
The time of ultrasound was evaluated in this study.
The recovery percentage of the pesticides in all
139