Analytical methods
Thier (1979) described a regulatory analytical method for dithiocarbamate residues based on the generation of CS2 by heating with stannous chloride and hydrochloric acid. The evolved CS2 is collected in an ethanolic solution of cupric acetate and diethanolamine to form a colour which is measured with a spectrophotometer. Sample blank problems occur with some crops. With a 500 g sample the lower limit for reliable determination is generally around 0.1 mg/kg.
Dithiocarbamate residues can be measured by GLC head-space determination of the CS2 evolved by treating the sample with stannous chloride and hydrochloric acid (Netherlands Ministry of Welfare, Health and Cultural Affairs, 1988). The method has been tested on many fruits and vegetables. In thiram trials on apples the limit of determination was 0.05-0.1 mg/kg (as CS2) and recoveries were 85-121%.
Roland et al. (1992h) described an HPLC method for thiram residues in crops that measures thiram as the intact molecule and distinguishes it from other dithiocarbamate compounds. The residue is extracted from crop samples with dichloromethane or acetonitrile, and after clean-up on a C18 microcolumn with methanol/water and acetonitrile/water as eluting solvents, it is analysed on a C18 reversed-phase HPLC column with UV detection at 280 nm. Quantitative recoveries were achieved from strawberries and apples down to 0.2 mg/kg.
Weber (1993) validated the method on grapes and wine. Quantitative recoveries were achieved on grape samples fortified with thiram down to 0.1 mg/kg. To improve recoveries whole grapes were extracted in the frozen state and wine was ice-cooled before extraction. Low recoveries occurred in wine fortified at 0.1 mg/kg because thiram was being degraded. Weber (1994b) also validated the method for wet and dry grape pomace and must. The results are summarized in Table 22. The LOD for the method with the substrates tested was 0.1 mg/kg.
Table 22. Analytical recoveries of thiram from fortified samples by the method of Roland et al. (Weber, 1993, 1994b).
|
Commodity |
Fortification level, mg/kg |
Recovery, % |
|
Grapes |
0.1 |
86-110, n=4 |
|
10 |
92-110, n=4 |
|
|
Wine |
0.1 |
54-69, n=4 |
|
10 |
95-99, n=4 |
|
|
Wet pomace
|
0.1 |
110 |
|
0.2 |
95 |
|
|
0.5 |
108 |
|
|
1.0 |
120 |
|
|
Dry pomace
|
0.1 |
90 |
|
0.2 |
75 |
|
|
0.5 |
72 |
|
|
1.0 |
74 |
|
|
Must
|
0.1 |
80 |
|
0.2' |
90 |
|
|
0.5 |
88 |
|
|
1.0 |
100 |
Gatti (1992a) described a method very similar to that of Roland et al. (1992), and (1992b) a GLC head-space (CS2 evolution) method which he tested on peaches, pears, apples, plums and cherries. Recoveries from spiked samples fell in the range 88-99% by HPLC and 93-105% by GLC, with each fruit tested once at 0.1 mg/kg and once at 4.0 mg/kg.
