Methods of residue analysis

Analytical methods

In the supervised trials homogenized samples were extracted with ethyl acetate, cleaned up on a silica gel column and determined by GLC. If necessary, additional clean-up was by acetonitrile-hexane partitioning. Acephate and methamidophos were determined separately in the same sample (Lai and Fowler, 1989). Recoveries of both acephate and its metabolite methamidophos 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 acephate and methamidophos was studied in vegetables, pulses, oilseed, animal products, cereals and grasses for periods ranging from 28 days to more than a year at -20°C. All samples except pinto beans and eggs were taken from crops which had been treated with acephate or from animals which had been dosed with acephate. Pinto beans and eggs were fortified with acephate and methamidophos (Lai, 1987, 1988, 1989a).

In general, acephate was stable in a wide range of macerated or ground commodities when stored at -20°C, even for periods exceeding a year. The results are summarized 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	35
	M	364	0.02-0.29	243-300
Celery	A	94	4.16-4.40	106-116	34
	M	94	0.23-0.29	93-148
Snap beans	A	69	0.30-0.39	73.3-84.6
	M	69	0.12-0.15	75.0-86.7
Snap beans	A	548	0.30-0.39	76.7-82,1	36
	M	548	0,12-0.15	75.0-80.0
Pinto beans (dry)	A	461	0.23-0.243	95.0-95.0
	M	461	0.09-0.103	80.0-90.0
Pigeon peas	A	418	8.11-9.74	104-110	34
	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
Lettuce	A	28	0.29-0.31	84-93
	M	28	0.02-0.02	50-100
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	108-122
	M	61	0.11-0.11	109-117
Bermuda grass	A	60	1.88-2.85	98-102
	M	60	0.31-0.44	102-106.5
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
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
	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 pinto beans and eggs which were added to untreated commodities

³ Fortified separately with acephate and methamidophos