© Benaki Phytopathological Institute
Malhat
et al.
58
Results and Discussion
Matrix effects and linearity
The matrix effect of the present meth-
od was investigated by comparing the rel-
ative responses of standards in solvent with
matrix-matched standards for 5 replicates at
0.5 mg L
-1
. The relative responses (response
matrix/response solvent) were 1.01 and 1.04
for tomatoes and soil, respectively. It can be
concluded that the matrix doesn’t signifi-
cantly suppress or enhance the response of
the instrument. The results showed that no
interfering endogenous peak appeared, and
the retention times of the tested analyte at
the spiked sample completely matched
those of the standard samples. Good lin-
earity was obtained over the concentration
ranges (0.005, -1.0 μg mL
-1
) with
R
2
> 0.999,
under this condition.
Method performance
The analytical method was developed
as to provide a rapid accurate and efficient
means of determining methomyl residues
in tomatoes and soil. Table 1 shows the for-
tified results of methomyl in tomato and soil
samples. The mean recoveries in tomato sam-
ples for methomyl were 87.1–94.5%, with RSD
of 6.9-11.2%. Recovery rates and their rela-
tive standard deviation were acceptable. The
LOQs and LODs were found to be 0.007 mg
kg
-1
and 0.005 mg kg
-1
, respectively, ensuring
LOQ values significantly lower than the MRLs
(0.02 mg/kg) established by the European
Union. These results demonstrate the good
performance of the method according to
document SANCO/12571/2013 (SANCO, 2013).
Dissipation behaviour in tomato
Mean residue levels of methomyl during
the sampling period for the recommended
application dose derived from the extraction
and analysis of three tomato sub samples are
shown in Figure 2 and Table 2. The results
showed that methomyl residues were easy
to be decomposed. The original deposits
of methomyl at the recommended dosage
were 1.272 mg/kg. These deposits dissipated
to 0.007 mg/kg 10 days after methomyl ap-
plication, thereby, showing a loss of 99.44 %.
The residues of methomyl in tomato reached
below the LOQ of 0.006 mg/kg in 13 days af-
ter application at the recommended dos-
age. The dissipation curve of methomyl in
tomatoes is presented in Figure 1. Degrada-
tion equation of methomyl in tomato is as
follows:
C
t
=1.412
-0.517t
(methomyl). Estimated
half-life of methomyl was 1.34 days. A sim-
ilar
t
1/2
to that found in our results was ob-
served in a study of the dissipation rate of
methomyl in cabbage and tomato (Othman
et al.,
1987), although studies made in grape
showed a higher
t
1/2
than ours (Kaushik
et al.,
2006). Different species, weather conditions,
the physical and chemical properties of pes-
ticides, method and rate of application may
be responsible for the different dissipation
rates of this compound (Romeh and Mek-
ky, 2009; Malhat
et al.,
2012b, 2014c). In the
field, besides the effect of some physical and
chemical factors such as light, heat, pH and
moisture (Agnihothrudu and Muraleedha-
ran, 1990; Chen
et al.
, 1987; Cosby
et al.,
1972;
Miller and Donaldson, 1994; Malhat, 2012) on
the degradation of pesticides, growth dilu-
tion factor might play a significant role in the
degradation of methomyl residues (Agniho-
thrudu and Muraleedharan, 1990; Bisen and
Ghosh Hajara, 2000; Chen and Wan, 1988;
Khay
et al.,
2008; Cabras
et al.,
1990, Malhat,
2013).
Dissipation behaviour in soil
Figure 2 shows the decline curve of
methomyl in soil. The dissipation dynamics
of methomyl can be described by the follow-
ing first-order kinetics equation
C
t
=1.404
-0.386t
(
R
2
=0.992). The
t
1/2
for methomyl in soil was
Table 1.
Fortification level and recovery per-
centage (±RSD) of methomyl in tomato and
soil samples.
Fortification levels
(mg/kg) (n* = 5)
Methomyl
Tomato
Soil
0.006
87.10 ± 8.4 92.5 ± 5.2
0.03
94.5 ± 11.2 90.9 ± 12.6
0.15
92.1 ± 6.9
88.3 ± 13
* number of replicates
