Page 17 - Volume 2, Issue 1, January 2009

GC determination of residues in wheat flour 17
© Benaki Phytopathological Institute
same day). Repeatability with RSD≤ 20% is
considered acceptable (5).
The sensitivity of the method was as-
sessed by the limit of quantification of the
method (LOQm). The LOQm was established
as the lowest concentration tested for which
recovery and precision were satisfactory
(70–120% and <20% RSD, respectively) in
accordance with the criteria established for
analysis of pesticide residues in foods (5).
The limit of quantification of the an-
alytical instrument (LOQi) was calculated
based on the requirement that the signal-
to-noise ratio should be higher than 10.
5. Preparation of fortified samples
Control samples were prepared from or-
ganically produced wheat flour. Aliquots of
10 g of wheat flour were fortified at two lev-
els, the LOQm and the 10×LOQm which are
shown in Table 2. Working standard mixture
solutions for fortification were prepared in
2,2,4-trimethyl pentane/toluene (90/10) at
100×LOQ m. The blank samples were spiked
with 0.1 mL of the 100×LOQm working stan-
dard mixture for the LOQm and 1 mL of the
100×LOQ m working standard mixture for the
10×LOQ m fortification level.
For validating the method a minimum
of 5 replicates is required according to SAN-
CO 2007/3131 (5). In this study 6 replicates in
each level were performed.
6. Gas-chromatographic analysis
The studied analytes were separat-
ed and determined in an Agilent 6890 gas
chromatograph, with two splitless injectors,
a DB-5-MS column (30 m, 0.32 mm i.d. and
0.25 μm film thickness) connected to the
ECD and a DB-17 MS column (30 m, 0.32 mm
i.d. and 0.25 μm film thickness) connected to
the NPD and equipped with a Chemstation
chromatography manager data acquisition
and processing software. The oven temper-
ature program started from 60o C for 1.5 min
increased to 220o C at a rate 14o C/min, held
for 4 min, then increased to 280o C at 20o C
/min and held for 20 min. The helium car-
rier gas flow rate was 1.5 mL/min for both
columns. The temperature of both injectors
was set at 230 o C and splitless injection was
carried out with the purge valve closed for
1 min. Hydrogen (3 mL/min) and air (60 mL/
min) were used as fuel gases for the NPD,
while nitrogen (60 mL/min) and helium (6
mL/min) were used as auxiliary gases for the
ECD. The temperature of both ECD and NPD
detectors was set at 310o C.
7. Confirmation
The confirmation of the analytes was
conducted, as mentioned earlier, from the
retention time of the analyte by using two
different columns and two different detec-
tors. The retention times acquired for each
analyte by using a combination of two dif-
ferent columns and two different detectors
are shown in Table 1. Most pesticides are
sensitive to both detectors. For pesticides
which are determined by only one detector,
such as bifenthrin, endosulfan etc. confirma-
tion is achieved using two different separa-
tion systems (2 different columns).
Results and Discussion
Acetone, dichloromethane and petroleum
ether showed good performance for extrac-
tion of the tested analytes. The method was
evaluated by assessing the basic parameters,
accuracy, precision and sensitivity. The chro-
matograms of the compounds are shown in
Figures 1 and 2. Furthermore the absence
of any interference from matrix compounds
was confirmed by the analysis of matrix
matched blank samples which gave (recov-
ery) values lower than 30% of the residue
level corresponding to the LOQ m (5).
Mean recoveries of the samples fortified
at the LOQ m were between 66.5 – 120% and
at the 10×LOQ m between 86.4 – 120%. These
results indicate satisfactory accuracy of the
method.
The attained LOQi values are shown in
Table 1 along with the MRLs. The lowest
calculated LOQ i value was 0.004 mg /kg for
the analytes lindane and chlorpyriphos and
the highest 0.37 mg/kg for the analyte thi-
abendazole.