Analytical methods
The methods for ferbam are the same as those for other dithiocarbamates: acid hydrolysis to release CS2 which is then measured by head-space gas chromatography.
In the procedure of the Dutch method manual (Ministry of Welfare, Health and Cultural Affairs, The Netherlands, 1988) dithiocarbamates are converted to CS2 by treatment with hydrochloric acid in the presence of stannous chloride. The CS2 in the head-space is determined by GLC with either an ECD or FPD in the sulphur mode.
Westberg and Tufts (1990) described the CS2 evolution head-space GLC procedure used in the ferbam mango trials. 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 ferbam standards similarly reacted and injected. Recoveries were satisfactory at 0.02, 0.3, 0.5 and 7.0 mg ferbam/kg. The LOD was 0.02 mg ferbam/kg. Koch (1996) used a similar method in a study of the frozen storage stability of ferbam and ziram in apples. Satisfactory recoveries were recorded for apples fortified at 0.2 and 6 mg ferbam/kg.
Stability of pesticide residues in stored analytical samples
Koch (1996) tested the stability of ferbam residues in macerated apples fortified at 1 mg/kg and stored in head-space bottles at -20°C for 22 weeks. Samples were analysed as indicated above. Ferbam residues were stable under these storage conditions for the duration of the experiment.
Table 5. Freezer storage stability of ferbam in macerated apples fortified at 1 mg/kg and stored at -20°C (Koch, 1996).
|
Storage period |
Ferbam remaining in stored sample, mg/kg (as ferbam) |
Method recovery, %, at time of stored sample analysis |
|
0 day |
0.93, 0.94 |
109, 104 |
|
2 weeks |
0.79, 0.84 |
81, 74 |
|
4 weeks |
0.83, 0.90 |
76, 84 |
|
12 weeks |
0.96, 0.90 |
82, 94 |
|
22 weeks |
0.78, 0.83 |
72, 89 |
Definition of the residue
Ferbam residues are measured as evolved CS2 by the same methods as are used for the other dithiocarbamates. The samples from the supervised trials on ferbam have been analysed by these methods. The Meeting agreed that ferbam 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) if adequate critical supporting studies become available.
