Methods of residue analysis

Analytical methods

The methods rely on acid hydrolysis to release CS₂, which is then measured colorimetrically or by head-space gas chromatography. They are the same as those for other dithiocarbamates.

In the method of the Dutch Method Manual dithiocarbamates are converted to CS₂ by treatment with hydrochloric acid in the presence of stannous chloride (Ministry of Welfare, Health and Cultural Affairs, The Netherlands, 1988). The CS₂ in the head space is determined by GLC with either an ECD or FPD in the sulphur mode. Wyss-Benz (1994) used a similar head-space method to measure levels of CS₂-generating compounds in apples in the metabolism study.

Wyss-Benz (1994) separated [¹⁴C]ziram from [¹⁴C]CS₂ in the apple metabolism study on a styrene-divinylbenzene column with an EDTA tetrabutylammonium hydroxide aqueous mobile phase. The compounds were detected with a ¹⁴C scintillation detector.

Brielbeck and Marx (1994a) described the CS₂ evolution photometric method used in many of the ziram residue trials. Samples were cut up, treated with a stannous chloride hydrochloric acid mixture and heated for one hour. The evolved gases were carried by a stream of nitrogen from the reaction flask through traps to remove H₂S and the CS₂ was then collected in a methanolic potassium hydroxide trap. Measurement of the UV absorbance of the solution at 302 nm gave the concentration of xanthogenate formed. Standard solutions of CS₂ dissolved in methanol were used to prepare a calibration curve. The LOD for peaches was 0.1 mg CS₂/kg. Recoveries were satisfactory in the range 0.25-4 mg ziram/kg, but tended to be elevated at the lowest level tested.

Ohs (1994b) described the CS₂ evolution colorimetric method used in ziram residue trials. CS₂ was released by reacting samples with a stannous chloride-hydrochloric acid mixture. It was collected in a trap and determined colorimetrically after reaction with copper acetate and diethanolamine. The LOD was 0.05 mg CS₂/kg. The method has been used for dithiocarbamate residue analysis for many years. Ziram recoveries were satisfactory in the 0.05-5.0 mg CS₂/kg range. Balluff (1995g) reported essentially the same method for ziram residues in supervised trials, although the LOD in this case was 0.5 mg ziram/kg. A method based on the same chemistry was used in the goat metabolism study to measure the levels of CS₂-generating compounds in milk and tissues (Bodden, 1993).

Holstege and Westberg (1987) described the CS₂ evolution head-space GLC procedure used in the US trials on ziram. The sample was reacted with stannous chloride-hydrochloric acid reagent at 100°C in a sealed reaction flask. An aliquot of the head-space gas was analysed by GLC and compared with ziram standards similarly reacted and injected. Recoveries were satisfactory over the range 0.05-7 mg ziram/kg. The LOD was 0.05 mg ziram/kg. The same method was used in the apple processing study (Meikle, 1992), where recoveries on apples, juice and pomace were found to be satisfactory. Koch (1996) used a similar method in the frozen storage stability study of ferbam and ziram in apples. Satisfactory recoveries were recorded for apples fortified at 0.2 and 2 mg ziram/kg.

Samples in the apple trial in Belgium (66/09) were analysed by a polarographic method, but no summary or details were available (Vervier and Cigot, 1966).

Stability of pesticide residues in stored analytical samples

The Meeting received information on the frozen storage stability of ziram in apples, peaches, almond kernels and almond hulls.

Koch (1996) tested the stability of ziram in macerated apples fortified at 1 mg/kg and stored in head-space bottles at -20°C for 18 weeks. Samples were analysed by a CS₂ evolution GLC head-space method. Ziram was stable under these conditions for the duration of the experiment.

Table 4. Stability of ziram in macerated apples fortified at 1 mg/kg and stored at -20°C (Koch, 1996).

Storage period	Ziram, mg/kg (as ziram)		Method recovery, %, at time of stored sample analysis
0	0.87	0.86	86	88
2 weeks	0.86	0.82	87	81
4 weeks	0.80	0.83	85	83
18 weeks	1.05	1.05	100	107

Bookbinder (1989j) showed that ziram was stable for limited periods in apples, peaches, almond kernels and almond hulls during freezer storage. The sample (4 g ground with dry ice) was fortified with ziram at 2 mg/kg and stored in glass reaction vessels (160 ml) in a freezer at -20±2°C for intervals up to 6 months (Table 5). Samples were analysed by a CS₂ evolution GLC head-space method. Analytical method recoveries were tested on each occasion for each commodity and the ranges were almond kernels 77-91%, hulls 73-93%, apples 80-98%, and peaches 84-104%.

The data suggest that ziram residues in apples and peaches are not sufficiently stable to allow storage of samples in a freezer longer than 3 months.

Table 5. Freezer storage stability of ziram on almond kernels and hulls, apples and peaches fortified at 2 mg/kg and stored at -20±2°C for intervals up to 6 months (Bookbinder, 1989j).

Storage interval	% of original ziram remaining
	almond kernels		almond hulls		apples		peaches
0 days	76	81	76	79	87	89	96	94
2 weeks	91	86	81	78	90	97	98	100
1 month	91	94	84	82	87	80	97	106
3 months	86	94	74	76	69	69	70	71
4 months					53	45	58	62
6 months	84	91			46	43	56	54

Residue definition

Ziram residues are measured as evolved CS₂ by the methods that are used for the other dithiocarbamates. All samples from the supervised trials on ziram have been analysed by these methods. The Meeting agreed that ziram should be included in the definition of the dithiocarbamate residues: The MRLs refer to total dithiocarbamates, determined as CS₂ evolved during acid digestion and expressed as mg CS₂/kg.

For dietary intake purposes and comparison of calculated intakes with the ADI it is preferable to express the residues as ziram because the ADI is expressed in terms of ziram (ziram = CS₂ x 2.01).