Methods of residue analysis

Analytical methods

In the supervised trials homogenized samples were extracted with ethyl acetate or a water-acetone mixture. The water-acetone extract was filtered and, after the addition of sodium chloride, partitioned with chloroform or methylene chloride. The samples were cleaned up on a silica gel column and analysed by GLC (Leary, 1971; Möllhoff, 1971; Luke, 1975; Lai and Fowler, 1989; Blass, 1994).

Recoveries of both methamidophos and acephate were generally >70% and the limit of determination was 0.01-0.02 mg/kg.

Stability of pesticide residues in stored analytical samples

The stability of methamidophos was studied in vegetables, pulses, oil seed, animal products, cereals and grasses as part of the stability studies on acephate. All samples except pinto beans and eggs were from crops or animals which had been treated with acephate. Pinto beans and eggs were fortified. Storage was for periods ranging from 28 days to more than a year at -20°C (Lai, 1987, 1988, 1989).

The results of the trials did not establish the stability of methamidophos in the treated commodities, because they contained substantially higher residues of acephate than of methamidophos and there was a possibility that acephate was degraded to methamidophos during storage. Methamidophos was stable in pinto beans and eggs at -20°C for periods of 461 and 175 days respectively, but was unstable in cow kidneys and cotton seed. The results are given in Table 1.

Table 1. Stability of acephate and methamidophos in samples stored at -20°C.

Commodity	Compound1	Storage period, days	Initial concentration2, mg/kg	% of initial residue remaining	Reference
Celery	A	364	0.26-4.40	87-97	56
	M	364	0.02-0.29	243-300
Celery	A	94	4.16-4.40	106-116	55
	M	94	0.23-0.29	93-148
Snap beans	A	548	0.30-0.39	76.7-82.1	57
	M	548	0.12-0.15	75.0-80.0
Snap beans	A	69	0.30-0.39	73.3-84.6	55
	M	69	0.12-0.15	75.0-86.7
Pinto beans (dry)	A	461	0.23-0.243	95.0-95.0	57
	M	461	0.09-0.103	80.0-90.0
Pigeon peas	A	418	8.11-9.74	104-110	55
	M	418	0.94-1.07	108-111
Bell peppers	A	386	3.67-3.83	103-112
	M	386	0.51-0.53	131-136
Brussels sprouts	A	272	1.61-2.06	84-88
	M	272	0.03-0.03	100-100
Cotton seed	A	48	0.38-0.82	73.2-86.8
	M	48	0.02-0.03	0.0-0.0
Grass	A	269	0.52-0.70	78.6-100
	M	269	0.10-0.14	78.6-90
Bermuda grass	A	61	0.62-0.72	108.1-122.2
	M	61	0.11-0.11	109.1-116.7
Bermuda grass	A	60	1.88-2.85	98.2-101.6
	M	60	0.31-0.44	102.3-106.5
Fresh hay	A	58	6.95-7.36	72.0-85.8
	M	58	0.49-0.54	75.5-83.3
Spent hay	A	58	2.81-2.91	96.2-96.4
	M	58	0.33-0.36	90.9-91.7
Lettuce	A	28	0.29-0.31	84-93
	M	28	0.02-0.02	50-100
Rice grain	A	506	1.09-1.19	81-126
	M	506	0.21-0.23	96-124
Rice straw	A	507	0.17-0.21	90-94
	M	507	0.06-0.06	83-83
Eggs	A	175	0.15-0.163	96.8-103	55
	M	175	0.07-0.083	93.3-93.3
Cow milk	A	202	0.04-0.79	98.7-150
	M	202	0.02-0.12	58.3-100
Cow kidneys	A	172	0.26-0.73	71.2-73.1
	M	172	0.02-0.07	50.0-60.0
Cow muscle	A	193	0.11-0.40	90.5-112
	M	193	0.01-0.03	100-100

¹ A: Acephate M: Methamidophos

² Initial concentrations were the residues found in the commodity at harvest or collection, except in pinto beans and eggs in which acephate and methamidophos were added to the untreated commodities

³ Fortified separately with acephate and methamidophos