Analytical methods
This type of compound is particularly suited to determination by HPLC with ultraviolet detection because of its strong absorbance near 254 mm. Teflubenzuron could not be determined by GLC on packed columns, but when GLC with capillary columns became available this procedure was also employed.
Plant material and soil. The HPLC procedure was used for alfalfa, apples, blackberries, broccoli, cabbage, citrus, cotton, cucumbers, grapes, grass, maize, mushrooms, peaches, pears, peppers, potatoes, soya beans and tomatoes, as well as for soils (Celamerk, 1982, 1985b, 1986; Cyanamid, 1995; Shell, 1988a). Teflubenzuron is extracted with acetone or from soil also with an acetone/water mixture. Clean-up is by solvent partition followed by gel-permeation chromatography and/or silica gel column chromatography. The residue is determined by reversed-phase HPLC with ultraviolet detection at 254 nm. Recoveries determined at levels ranging from 0.01 to 2 mg/kg were 70-110%.
Some years ago, an alternative method using capillary gas chromatography was introduced and applied to cherries and plums (Shell, 1992). As in the other methods, the acetone extracts are cleaned up by solvent partition and gel-permeation chromatography. Determination is by both reversed-phase HPLC and gas chromatography with mass-selective detection. Thus GC-MS can be used as a confirmatory method. Validation was at fortification levels between 0.01 and 0.1 mg/kg. Recoveries were 85-96% and the limit of determination (LOD) was 0.01 mg/kg for both procedures.
Animal products. An HPLC method was developed for the residue determination of teflubenzuron in products of cattle and hens (Shell, 1988b). The residue is extracted with methanol or acetonitrile. In the case of high-fat material the fat is separated in a cooling bath. Further clean-up and quantification by HPLC is as in the crop method. The procedure was validated for muscle, liver, kidney, fat, skin, milk and eggs. Fortification levels were 0.01 to 0.2 mg/kg. Recoveries from the various types of sample were in the range 73-110% with an LOD of 0.01 mg/kg.
Water. A modification of the HPLC method was developed for the determination of residues in water (Cyanamid, 1988) to meet the requirements of the EU drinking water directive. Teflubenzuron is extracted from the water sample on a C18 "Bondelut" solid-phase column. After elution, further clean-up on a silica gel column follows if necessary. The compound is determined by reversed-phase HPLC with UV detection at 254 nm. Analysis of water samples spiked at 0.0001 - 0.002 mg/l gave recoveries of 78-100%. The LOD was 0.0001 mg/l.
Air. Air is sucked through a Tenax or XAD column and teflubenzuron is adsorbed. The compound is then eluted and determined by reverse-phase HPLC with UV detection or by GLC with a mass-selective detector as a confirmatory method. The LOD was 56 m g/m3 air. The method was later validated in a separate study and the LOD was lowered to 10 m g/m3 air (Weitzel, 1995). The range of recoveries was 83-110%.
Stability of pesticide residues in stored analytical samples
The stability of teflubenzuron in various crops held under frozen conditions up to 36 months was tested by Thorstenson (1990). Untreated apple, pear, potato and cabbage samples were fortified at 0.2 mg/kg with teflubenzuron and placed in a freezer maintained at -20°C. The frozen samples were analyzed after 3, 6, 12, and 36 months of storage. The results are shown in Table 3.
Table 3. Stability of teflubenzuron in various crops stored under frozen conditions (Thorstenson, 1990).
|
Crop |
Teflubenzuron (%)1 remaining after |
|||
|
3 months |
6 months |
12 months |
36 months |
|
|
Apple |
115.0 |
101.3 |
95.0 |
74.3 |
|
Pear |
90.7 |
113.7 |
90.0 |
82.3 |
|
Potato |
77.0 |
114.3 |
91.3 |
84.7 |
|
Cabbage |
103.0 |
102.3 |
95.0 |
94.0 |
1 Average values
After one and three years storage, 91-95% and 74-94% of the original teflubenzuron remained respectively, showing that teflubenzuron is stable in the crops investigated when stored under deep-frozen conditions.
