Page 33 - Volume 3, Issue 2, July 2010

© Benaki Phytopathological Institute
GC multiresidue method of multiclass pesticides 63
mixture solutions for measurements were
prepared in matrix extract, previously anal-
ysed for the absence of compounds inter-
fering with the analytes. The blank extract,
in which the solutions were prepared, was
produced as described in section “Proce-
dure ”. 0.5 ml of the blank extract was evap-
orated to dryness by a stream of N 2 and 0.5
ml of a standard solution of the desired con-
centration, prepared in 2,2,4-trimethylpen-
tane/toluene (90/10), was added. The final
solution was placed in an ultrasonic bath for
30 sec.
(b) Procedure
The following extraction method was
used (5): An aliquot of 15 ± 0.15 g of sample
was weighted into a 250 ml PTFE centrifuge
bottle (Nalgene, Rochester, NY); 30 ml of ac-
etone were added and stirred for 1 min in
an ultra-turrax homogenizer at 15,000 rpm;
30 ml of dichloromethane and 30 ml of pe-
troleum ether were added and the mixture
was stirred for 1 min and then centrifuged at
4,000 rpm for 2 min. An aliquot of 25 ml of
the supernatant liquid were evaporated to
dryness on a water stream bath at 65–70oC,
and 1 ml of 2,2,4-trimethyl pentane/toluene
(90/10) was added. An additional quantity of
15 ml of the supernatant liquid was evapo-
rated to dryness on a water steam bath at
65–70o C, and 3 ml of 2,2,4-trimethyl pen-
tane/toluene (90/10) were added. The two
extracts were placed in ultrasonic bath for
30 sec and then they were transferred into
separate vials with a Teflon stopper, in order
to be used for the NPD and ECD chromato-
graphic analysis, respectively. Simultaneous
injections were performed in the injectors
with the aid of two separate autosamplers.
(c) Preparation of fortified samples
Control samples were prepared from or-
ganically produced tomatoes and grapes.
Aliquots of 15 g of the sample were forti-
fied at two levels, the lowest fortification
level (LFL) and the tenfold fortification lev-
el (10*LFL), as shown in Table 2. For validat-
ing the method (3), five replicates were used
for each level.
Results and Discussion
The accuracy of the method was estimated
by calculating the attained recovery. For val-
idating a method, mean recoveries of 70–
120% are considered acceptable, while in
Table 2 (continued)
Analyte R.T. Column Detector Mixture
demeton -S- methyl 8 DB-17 NPD 3
cadusafos 8.6 DB-17 NPD 3
omethoate 9.7 DB-17 NPD 3
diazinon 10.5 DB-17 NPD 3
atrazine 10.8 DB-17 NPD 3
simazine 11.1 DB-17 NPD 3
dimethoate 12.2 DB-17 NPD 3
chlorpyrifos-methyl 13.9 DB-17 NPD 3
prometryn 14.3 DB-17 NPD 3
metalaxyl 15.1 DB-17 NPD 3
tetraconazole 15.9 DB-17 NPD 3
parathion ethyl 16.4 DB-17 NPD 3
fenthion 17.8 DB-17 NPD 3
methidathion 23.3 DB-17 NPD 3
myclobutanil 24.8 DB-17 NPD 3
ethion 26.9 DB-17 NPD 3
benalaxyl 29.9 DB-17 NPD 3
triazophos 32.4 DB-17 NPD 3
phosalone 35 DB-17 NPD 3
pyrazophos 35.6 DB-17 NPD 3
azinphos-methyl 36.5 DB-17 NPD 3
fluquinconazole 37.5 DB-17 NPD 3