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Methods for dithiocarbamates rely on the generation of CS2, which can be

measured by GLC or by colorimetry.

The methods used in the survey of US food items by Slesinski (1990) for ethylenebis(dithiocarbamate) residues in crops, processed commodities, meat, and milk were described by Westberg (1989a-c). The methods rely on the formation of CS2 from dithiocarbamate residues during reaction with

hydrochloric acid + stannous chloride at 100°C in a sealed reaction flask. CS2 is then measured by GLC headspace analysis (flame-photometric

detector). Calibration relies on an ethylenebis(dithiocarbamate) standard similarly prepared and injected.

The laboratory sample (wet and dry crops, meat) was chopped or ground while frozen with dry ice. Frozen milk was quick-thawed to a slush using a cold water bath. The analytical portion (4 g for crops, 10 g for meat, 20 g for milk) was placed in the reaction flask for CS2 generation. Samples had

to be kept frozen at all times until the addition of the reagent, including during weighing (samples kept on dry ice before and after weighing). The detection limit for dithiocarbamates (as CS2) was 0.01 mg/kg in crops and

0.001 mg/kg in meat and milk.

Rogers et al., (1989a-c) described methods used in the Slesinski (1990) survey for ethylenethiourea in crops, meat and milk. ETU was extracted from the sample with water (pH adjusted to 11-12 with ammonia) + ethanol or methanol, the extract was cleaned up on an alumina column, and the ETU was determined by HPLC.

Samples of crops or meat were ground while frozen with dry ice. Milk was thawed to a slush for weighing. Samples must be kept frozen until the extraction solvent is added. All glassware that comes into contact with extracts or ETU solutions must be silanized. Determination was by HPLC with electrochemical detection.

Loftus (1990a) assembled the validation data for these methods. Dithiocarbamate recoveries were tested with celery, snap beans, dry beans, frozen corn and potatoes fortified with mancozeb at 0.02, 0.2 and 2.0 mg/kg, tomatoes fortified at 0.005, 0.01 and 0.02 mg/kg, and meat and milk at 0.002, 0.005 and 0.02 mg/kg. The work was distributed among three laboratories. Recoveries exceeded 70% except from dry beans (55-62%) and frozen corn (67-73%), both analysed in the same laboratory.

ETU recoveries were tested with celery, snap beans, dry beans and corn fortified with ETU at 0.01, 0.1 and 1.0 mg/kg, potatoes, tomatoes and tomato paste fortified at 0.002, 0.005 and 0.01 mg/kg, and meat and milk fortified at 0.001, 0.003 and 0.01 mg/kg. Recoveries exceeded 70% except from meat (67-74%).

There was some evidence that ethylenebis(dithiocarbamate) residues could be converted to ETU during analysis, with estimated conversion rates of 0.22-8.5%. Experimental techniques which minimize the time taken to perform critical steps and ensure that reagents such as HPLC-grade water do not degrade the dithiocarbamates are needed to reduce the conversion.

Bulb onions (Pennwalt study BR-88-15) and broccoli (Pennwalt study BR-89-09, and Rohm and Haas data) were shown to contain endogenous CS2 or

compounds which produced CS2 in the dithiocarbamate analytical method.

Twelve samples of bulb onions (10 varieties, from 10 sites in the USA) certified not to have been treated with dithiocarbamates showed, on analysis, CS2 residues ranging from undetectable (<0.03 mg/kg) to 0.13

mg/kg, with a median of 0.05 mg/kg. The CS2 in eight samples of broccoli (6

varieties, from 6 sites in the USA), certified as not treated with dithiocarbamates, ranged from undetectable (<0.01 mg/kg) to 0.79 mg/kg, median 0.32 mg/kg.

Kallio and Salorinne (1990) reported carbon disulphide as one of the 27 volatile compounds identified by headspace GC-MS of onions.

Larese (1988a) analysed bananas for dithiocarbamate residues by boiling the sample with dilute acid to release CS2, which was carried by an

air stream into an ethanol trap at dry-ice temperature. The CS2 was

measured by GLC with flame-photometric detection in the sulphur mode. This method was used in the US supervised residue trials on bananas and wheat.

An earlier method (Keppel, 1971) measured the trapped CS2

colorimetrically with a cupric acetate/diethanolamine reagent. It was used in the US supervised residue trials for the analysis of almonds, asparagus, bananas, carrots, celery, cucumbers, oranges, peanuts, potatoes, summer squash, tomatoes, wheat and winter squash.

Larese (1988b) extracted ETU from bananas with methanol, and cleaned up the extract on an aluminium oxide column. The ETU was derivatised with bromobutane to form butyl-ETU, which was determined by GLC with flame- photometric detection in the sulphur mode. The method was used in the US supervised trials to analyse almonds, asparagus, bananas, celery, cucumbers, oranges, peanuts, potatoes, tomatoes, and wheat.

Australian residue analyses were by methods for dithiocarbamates (Shields, 1990e) and ETU (McCarthy, 1990), similar to those described by Westberg (1989a) and Rogers et al., (1989a).

Shields (1990e) described a GLC method for measuring the carbon disulphide evolved from dithiocarbamate residues. Samples were cut up and representative portions (100 g) taken for analysis. The maceration of crop samples was not recommended because contact between plant acids and dithiocarbamates may cause loss of residues.

Carbon disulphide was generated in a hydrolysis flask by treating the sample with 40% stannous chloride in hydrochloric acid under reflux. The evolved carbon disulphide was swept by a current of air into an ethanol trap maintained at a low temperature in a dry-ice/acetone trap. The ethanol solution was then analysed for CS2 in a gas chromatograph equipped with a

flame-photometric detector (S filter).

Recoveries of mancozeb from the trial crops were in the range 55- 115%, mean 84% (n = 38).

McCarthy (1990) described an HPLC method for ETU residues in plant material. The sample was mixed with the anti-oxidant cysteine hydrochloride and extracted with water (adjusted to pH 11-12 using concentrated ammonia) and methanol. The extract was filtered and the filtrate reduced in volume by rotary evaporation. Clean-up was effected by absorption of the aqueous concentrate into 10 g of GLC column support material followed by elution of the ETU from this material with methanol/chloroform through a small alumina column.

The solvent was removed and the residue taken up in water for HPLC analysis with UV detection. The ease of oxidation of ETU and danger of loss of residues were stressed. Precautions such as the use of silanized glassware and the addition of the anti-oxidant were needed. Recoveries were in the range 44-137%, mean 81% (n = 9).

Mellet (1993a, and related reports) described the method used for measuring the dithiocarbamate residues in the French trials. The analytical sample was treated with stannous chloride in hydrochloric acid under hot conditions to liberate carbon disulphide, which was swept with a current of air into an absorption trap containing a colorimetric reagent (diethanolamine and cupric acetate). The absorbance of the coloured solution was measured at 435 nm. Known amounts of mancozeb were run through the procedure to establish the calibration.

Recoveries were determined and controls analysed with each crop in the residue trials. Some types of sample can give a false response if they contain sulphur compounds which generate CS2 during the hydrolysis step, or

if they give a false colour with the reagent. Some examples are discussed in the section on supervised trials.

A UK Panel on the Determination of Dithiocarbamate Residues (1981) examined the headspace method for dithiocarbamate residues in lettuce. The Panel drew attention to the loss of residues which can occur between beginning to cut the sample and inserting it into the reaction bottle. Vegetables and fruits must be analysed as soon as possible after cutting or picking, and any further cutting or dicing of the whole commodity should be carried out immediately before placing in the reaction flask, and should be kept to a minimum. Foodstuffs should be frozen whole, when this becomes necessary, and chopped and mixed in the frozen state immediately before taking the analytical samples.

It should be noted that the previously described freezer storage studies on spiked homogenised samples showed that mancozeb residues were stable under freezer conditions, but the evidence suggests that if storage is necessary samples should be frozen whole.

Onley et al., (1977) reported a method for ETU residues in crops and food, which minimized the conversion of ethylenebis(dithiocarbamate) residues to ETU. Extracts were cleaned up by adsorption on GLC column support material and alumina. The final determination of ETU was by HPLC or GLC (as the S-butyl derivative). TLC was used for additional identification. Collaborative testing of the method was reported by Onley (1977). The HPLC method was used to determine the ETU residues in US supervised trials on carrots, celery, summer squash and winter squash.

Krause (1989) extracted ETU with a methanol/aqueous sodium acetate solution and cleaned up the extract on a diatomaceous earth column. The final analysis was by HPLC on a graphitized carbon column with electrochemical detection. The limit of determination was 0.01-0.02 mg/kg. Celery samples showed low recoveries.

Doerge and Miles (1991) extracted and cleaned up ETU residues in crop samples by the method of Krause (1989), and used particle beam liquid chromatography/mass spectrometry for quantitative determination and positive identification of ETU down to 5 ìg/kg.