Page 36 - Volume 3, Issue 2, July 2010

Vlastaras

et al.

66

fragments for the same compounds are the

following: 260, 270, 290 and 300 for aldrin,

123, 167, 224 and 332 for dichlofluanid, 162,

191, 208 and 252 for pendimethalin and 159,

163, 213 215, 248 and 250 for (162, 191, 208,

252), penconazole. If we compare the ions of

the compounds aldrin and dichlofluanid, the

m/z fragments of 123, 167 and 332 of dichlo-

fluanid are not produced by aldrin. Similarly

if we compare the compounds penconazole

and pendimethalin, the m/z fragments of

213, 215, 248 and 250 of the former are not

produced by the latter. Therefore, and ac-

cording to the principles of mass spectrom-

etry, even if these compounds are co-eluted

we can still identify and confirm which com-

pound is present.

There has been a high uncertainty in the

results of dichlorvos (RSD = 53.55% in to-

matoes and 27.05% in grapes) mostly at the

LFL, which most probably was due to some

losses of the analyte at the stage of evap-

oration, as dichlorvos has a very high va-

pour pressure. In order to estimate the rea-

sons of this uncertainty, we conducted the

following test: first, a standard solution of

dichlorvos at 0.84 mg/ml in 2,2,4-trimeth-

ylpentane/toluene (90/10) was prepared.

Then, a standard solution (A) of 60 ml ace-

tone, 60 ml of dichloromethane and 60 ml

of petroleum ether spiked with 1.6 ml of the

previous solution was prepared. An aliquot

of 25 ml of the solution (A) was transferred

into five 50 ml volumetric flasks and the so-

lution was evaporated to dryness on a wa-

ter stream bath at 65–70

o

C. Finally, 1 ml of

2,2,4-trimethyl pentane/toluene (90/10) was

added. The final solutions were transferred

into five vials with Teflon stoppers. In order

to evaluate the level of uncertainty during

the evaporation and injection stages, we

injected the five solutions five times in the

chromatographic system. Based on the re-

sults of this test, the uncertainty at the stage

of injection was significant (RSD = 12,9%)

but the uncertainty at the stage of evapo-

ration, as a result of dichlorvos high vapor

pressure, was about 5 times higher (RSD =

58%). Lehotay

et al.

(10) reported that di-

chlorvos may be lost during either the evap-

oration step or the sample processing step.

As our results showed, the loss of dichlorvos

was attributed to a large extent to the evap-

oration step.

The most polar analytes, acephate and

omethoate, gave low recovery values out

of the acceptable range (35.5% in tomatoes

and 50.8% in grapes for acephate and 46.2%

in tomatoes and 52% in grapes for ometh-

oate) but consistent (RSD <10%). Therefore,

these values are considered acceptable and

adjustment for recovery in the case of quan-

tification of these compounds is suggested

(3). Nevertheless, the method is still able to

serve as a semiquantitative method for the

detection and confirmation of the presence

of acephate and omethoate in samples.

In the case of metribuzin, the recovery

results at the ECD detector gave low recov-

eries (25-30%). In addition, the presence of

an unknown peak (R.T. = 11.1 min) for both

samples was observed. The area of the un-

known peak is significant smaller than that

of the peak that belongs to metribuzin (R.T.

= 10.2 min). After injection of a standard so-

lution of metribuzin in the mass spectrome-

ter, the ions of the unknown peak were the

same as that of metribuzin. Therefore, the

unknown peak may have been the result of

decomposition of metribuzin in its metab-

olites, mainly desamino-metribuzin. Figure

4(a) shows the chromatograph of metribuz-

Table 5.

Relative retention times (R.R.T.) of the compounds aldrin, dichlofluanid, pendime-

thalin and penconazole (relative to parathion ethyl) in four different columns.

Analyte

R.R.T.

(DB-1)

R.R.T.

(DB-5)

R.R.T.

(DB-17)

R.R.T.

(DB-WAX)

aldrin

1.01

0.99

0.89

0.61

dichlofluanid

0.98

0.96

0.99

pendimethalin

1.05

0.99

penconazole

1.05

1.03

1.07