Page 38 - Volume 3, Issue 2, July 2010

Vlastaras

et al.

68

R.T. of metribuzin, as in our case. The Kovats

RI of the other metabolite is much small-

er than the Kovats RI of metribuzin, which

makes metribuzin-diketo (M.W.=184) and

metribuzin-desamino-diketo (M.W.=169) as

smaller compounds to have earlier R.T. At

the time of the study, no reference materi-

al was available in our laboratory to confirm

the presence of any metabolite. Therefore,

the identification of any degradation prod-

uct or metabolite was not possible and this

investigation was used to partly explain the

low recovery of the certain compound. The

spectra presented in Figure 4(b) are in full

scan mode and in accordance with the com-

mercial available library of Dr Ehrenstofer,

which is a special collection of pesticides

and pollutants.

In conclusion, the peak with R.T.=13.17

min in the GC-MS system, shown in Figure

4(b), may have been a metabolite, a degra-

dation product or a pollution of the system

or matrix. However, in the framework of the

present study, it was difficult to determine

its identity. Further studies dedicated to the

identification of this compound are required.

Conclusions

Themethodof analysis proposedby thepres-

ent study is fast, easy and effective. By using

44 representative analytes, the method ap-

plied for the determination of more that 180

pesticides by gas chromatography with elec-

tron capture (ECD) and nitrogen phosphorus

(NPD) detectors was validated. Neverthe-

less, several difficulties occurred with certain

problematic molecules (dichlorvos, metibuz-

in) or specific chemical subgroups (polar or-

ganophosphorus pesticides). In addition,

the matrix effect played an important role in

the sensitivity of the analytes. For example,

in the case of demeton-s-methyl, propicon-

azole and penconazole, the signal to noise

ratio was higher in tomatoes than in grapes.

Although the validation of the method by

choosing representative analytes is of low

cost, effective and easy technique for testing

if a method is suitable for the determination

of a wider range of pesticides, it is not ade-

quate. According to the European

Commis-

sion (3),

for checking the methods perfor-

mance for all the analytes in its scope, each

laboratory should have a rolling programme

to include all other analytes at least every 12

months, but preferably every 6 months

.

Literature cited

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Pesticide Science,

Vol. 17

(5): 473-476.

Received: 4 January 2010; Accepted: 17 June 2010