The stability of tebufenozide in stored analytical solutions was investigated in the USA (Chen et al., 1994e). Standard solutions of tebufenozide at concentrations from 0.01 to 1.0 mg/l in either methanol/water or acetonitrile/water were stored for various periods and compared with freshly made standards by HPLC injection and integration of the peaks. Statistical analysis of the results indicated that working standards of tebufenozide were stable for a minimum of 6 months.
The stability of tebufenozide residues was studied in samples of apples, apple juice, grapes, wine, rice, walnuts and lettuce.
Apples and apple juice. A study of the stability of residues of tebufenozide in apple samples during frozen storage is in progress. Untreated apple samples were homgenized, and control and fortified samples were stored at -15 ± 1°C. The results reported so far (Deakyne and Chen, 1995a) show that tebufenozide is stable for at least six months in apples when stored frozen. The percentage of the residue remaining after six months storage ranged from 90.8% to 108% with no indication of a systematic decrease with time.
Additional information on the stability of tebufenozide residues in apples during frozen storage (-15°C) was obtained by analysing field-treated samples, storing them for two additional years and re-analysing the same samples (Deakyne, 1995b; Burnett et al; 1994). The results are shown below.
|
Sample |
First analysis |
Second analysis | ||
|
|
Storage period days |
mg/kg |
Storage period, days |
mg/kg |
|
2 |
199 |
0.44 |
924 |
0.53 |
|
|
|
|
924 |
0.42 |
|
4 |
190 |
0.5 |
915 |
0.5 |
|
|
|
|
915 |
0.4 |
|
8 |
173 |
0.67 |
917 |
0.61 |
|
|
|
|
922 |
0.52 |
|
6 |
156 |
0.79 |
876 |
1 |
In another study carried out in Japan, untreated apple samples were fortified with a solution of tebufenozide and its metabolites RH-6595 and RH-1788 at a level of 1 mg/kg. Samples were stored at -20°C for 190-202 days. The average recoveries after storage were 94% for tebufenozide, 87% for RH-1788 and 69% for RH-6595 (Komatsu and Yabusaki, 1994).
The stability of residues of tebufenozide in apple juice during frozen storage was investigated by Deakyne and Chen (1995b). Samples of control apple juice (20 g) were fortified with 20 m g of tebufenozide (1 mg/kg) and stored frozen. Samples were removed from storage at intervals and analysed together with control samples stored under the same conditions and freshly fortified. After 6 months storage the mean recovery was 86.7%
Grapes and wine. The stability of tebufenozide in frozen grapes and wine stored for periods up to 12 months was investigated by Schuld and Holzwarth (1994b). Samples were fortified with tebufenozide at 0.2 mg/kg (grapes) or 0.1 mg/kg (wine) and stored at <-18°C. Samples were removed for analysis after approximately 0, 1, 3, 6, 9, and 12 months. There was no loss during the 12 months.
|
% recovery of tebufenozide after period, months | ||||||
|
|
0 |
1 |
3 |
6 |
9 |
12 |
|
Grapes |
|
91 |
82.5 |
88 |
84 |
89 |
|
Wine |
97 |
93 |
118 |
74 |
92 |
94.5 |
Walnuts. Walnut kernels were fortified with 1 mg/kg of tebufenozide and stored frozen at -10°C (Cui and Deakyne, 1994). Controls, freshly fortified controls, and stored samples were analysed after 0, 1, 2, and 3 months storage. No decrease was observed during this period and the study was continued for a total of 18 months. The final residue was 76.6% of the initial level, showing that residues of tebufenozide are adequately stable for at least 18 months of frozen storage.
Lettuce. Control samples of lettuce were homogenized and sub-samples of 20 g each were fortified with 20 m g of tebufenozide in methanol. After evaporation of the methanol the samples were sealed and stored frozen. Samples were removed from storage and analysed at intervals (Chen et al., 1994b). After 128 days the mean recovery was 90%.
Rice. Samples of milled rice spiked with tebufenozide at 2 mg/kg were stored at -20°C for 20-21 days until analysis. Recoveries ranged from 53 to 80% (Komatsu and Yabusaki, 1992c).
The Meeting was informed that the studies on apples, walnuts and lettuce would be continued for a full 3 years of storage.
Residue definition
Tebufenozide was the predominant residue found in all the plant metabolism studies, which included apples, grapes, sugar beet and rice. Although metabolism occurs in plants no single metabolite in any plant metabolism study exceeded 10% of the total residue.
In some residue trials samples were analysed for residues of the metabolites RH-6595 and RH-1788. The levels of these were very low, and in most cases below the limit of determination. Some samples in supervised trials on vegetables in the USA (1991-93) were analysed for the terminal metabolite RH-0897 (Chen et al., 1994c,d). All residues were below the LOD.
The Meeting concluded that the residue should be defined as tebufenozide. The residue is fat-soluble.
