Page 30 - Volume 5, Issue 2, July 2012

Dithiocarbamates and inorganic bromide residues in plant products

59

the calibration curve at 9 concentration lev-

els (0.05-0.1-0.2-0.5-1-2-5-10 and 20μg/L).

Bromide ion residues were also quantified

by calibration curve using 5 solutions equiv-

alent to 5, 25, 50, 100 and 150μg Br.

Thiram was used for the fortification

of the blank samples during the validation

of dithiocarbamates, as it is the only com-

pound of the group that can be diluted in

organic solvents.

Preparation of fortified samples

Control samples were prepared from or-

ganically produced apples, leek, potatoes,

strawberries, wheat, tomatoes, lettuce and

rice and analysed in order to certify that

no interfering peaks with the analytes ap-

peared in chromatograms. Aliquots of each

product blank sample were fortified at two

or three levels as presented in tables 1 and

2, ranging from 0.1 to 1mg/kg for the lowest

level and 1 to 50mg/kg for the correspond-

ing highest. The highest level of fortification

was that of the MRL set for dithiocarbam-

ates or bromide ion by the European Union;

while the limit of determination was set as

the lowest validated level. The validation

procedures were in accordance to the SAN-

CO Document10684/2009.

Sample preparation

The extraction procedures for the deter-

mination of inorganic bromide and CS

2

are

described below. As far as

inorganic bro-

mide

is concerned, 5g portion of homoge-

nated sample (or 1g for dry products) was

weighted into a 100mL Erlenmeyer flask and

3 (or 8mL respectively) of water were added.

5mL of propylene oxide solution and 1mL of

sulphuric acid were added for the derivati-

zation of inorganic bromide to 1-bromopro-

panol-2 and 2-bromopropanol-1. The flask

was closed and shacked briefly and the mix-

ture stand at room temperature for 60 min.

50mL ethyl acetate and 4g of ammonium

sulphate were added to the suspension and

the flask was shaken by hand first vigorous-

ly for 1 min and then occasionally for 20 min.

Τhe upper organic phase was decanted and

dried through anhydrous sodium sulphate,

1 mL of the organic extract was transferred

to an autosampler vial and 100μL of inter-

nal standard solution (10μg/mL) was added.

The calibration solutions followed the com-

plete method procedure (derivatization,

partitioning etc.)

As fοr the

dithiocarbamates

, their resi-

dues are typically located superficially. Thus,

sample comminuting (e.g. cutting, milling,

grinding) is only to be performed where this

is necessary to obtain acceptable sub-sam-

plingvariability, which is alsoa functionof the

analytical portion size. The bigger the por-

tion, the smaller the sub-sampling variability

becomes. In most cases opposite segments

of each unit are cut out. A defined portion

of 50g was taken for analysis into a cleavage

vessel and 25mL isooctane was added. Then

150mL of the hydrolysis reagent tin (II)-chlo-

ride in hydrochloric acid was added and the

vessel was immediately closed with a screw-

cap with septum. The samples were put into

a shaking water bath for 2 hours at 80

o

C. The

reaction mixture was cooled down and 1mL

of the isooctane-phase was pipetted into a

GC vial for analysis.

Gas- chromatographic analysis

Determination of carbon disulfide was

performed by the use of a Shimadzu GC-

2010 chromatographic system with a split-

less injector and a DB-5 column (5%- phenyl-

methylpolysiloxane, 50m, 0.32mm i.d. and

1μm film thickness) connected to a flame

photometric detector (FPD) with sulphur fil-

ter. The oven temperature programme start-

ed from 45

o

C (1 min), increased to 250

o

C at a

rate of 20

o

C/min, and held there for 5min.

The injection volume was set to 1μL.

Determination of inorganic bromide

was based to a 6890N Agilent gas chromato-

graphic system with an electron capture de-

tector (ECD), a splitless injector and a DB-Wax

52 CB column (chemically bonded polyethyl-

ene glycol, 30m, 0.25mm i.d. and 0.25μm film

thickness). The oven temperature programme

started from 50

o

C (1 min), increased to 150

o

C

at a rate of 2.5

o

C/min, increased to 200

o

C at

a rate of 10

o

C/min and held there for 10min.

The injection volume was also set to 1μL.