Detection and analysis of paraquat in confiscated marijuana samples

Sections

Abstract
Introduction
Materials
Methods
Results and discussion
Acknowledgements

Details

Author: Carlton E. TURNER, Ping C. CHENG, Lolita M. TORRES, Mahmoud A. ELSOHLY
Pages: 47 to 56
Creation Date: 1978/01/01

Detection and analysis of paraquat in confiscated marijuana samples

Carlton E. TURNER
Ping C. CHENG
Lolita M. TORRES
Mahmoud A. ELSOHLY
Research Institute of Pharmaceutical Sciences School of Pharmacy, University of Mississippi, Mississippi 38677, U.S.A.

Abstract

A spectrophotometric method used to test for paraquat in 160 confiscated marijuana samples is described. Twenty of these samples (12.5 per cent) tested positive for paraquat. Nine confiscated hash oil samples tested negative.

The identification of paraquat was proven by isolation, chromatography, and spectral methods. The cannabinoids in paraquat positive Cannabis samples were analysed.

Introduction

Paraquat (I) is the common name for l,l'-dimethyl-4,4'-dipyridinium salt, a herbicide developed by Imperial Chemical Company in the 1950s and first marketed for agricultural purposes in the United Kingdom in 1962. Today, paraquat is marketed worldwide and is the most widely used non-selective herbicide.

(I)

Paraquat (1,1 '-dimethyl-4,4'-di-pyridinium dichloride).

Full size image: 5 kB, (I)

Due to its utility in various agricultural functions, paraquat was investigated in the early 1970s and tentatively selected as an agent for chemically controlling Cannabis sativa L. After field evaluations, paraquat was found to be the most effective herbicide in controlling Cannabis(1). If used, paraquat would theoretically reduce the flow of illicit crude drugs made from Cannabis. Thus, the Mexican Government initiated a programme to eradicate Cannabis by spraying illicit fields with aqueous paraquat in late 1975 and intensified the programme in 1976.

In June, 1977, we were requested by the U.S. National Institute on Drug Abuse (NIDA) to investigate the marihuana-paraquat issue and in September we were given a contract for screening 100 confiscated samples of large seizures of allegedly Mexican Cannabis obtained from the Drug Enforcement Administration for paraquat. We now wish to report our findings on the analysis of the original 100 samples of confiscated marijuana, nine samples of hash oil, and 60 additional seizures evaluated after the end of our contract with NIDA.

Materials

Plant material. Most of the samples screened for paraquat were confiscated by the U.S. Department of Justice's Drug Enforcement Administration (160 marijuana kilo bricks and nine hash oil samples). The other samples analysed included Cannabis samples which were sprayed by the U.S. Department of Agriculture (USDA) in a way which simulated an aerial application of paraquat and samples received from sources in Mexico. Paraquat-free samples of Mexican Cannabis grown in Mississippi and one sample from the USDA were used as controls.

Standard paraquat. This was obtained from Chevron Chemical Co., Ortho Division, Richmond, CA.

Methods

Analytical procedure. 1All samples were analysed using the procedure recommended by Chevron Chemical Co. for the analysis of paraquat residue in crops and soils (2, 3). This procedure was modified in order to give results when dried Cannabis plant material was used. A detailed description of the procedure follows.

Extraction of paraquat. 2 Ten gram samples of confiscated marijuana, Cannabis plant material or hashish oil were moistened with 15 ml of water in a one-litre round bottomed flask and then 75 ml concentrated sulfuric acid was added. The mixture was refluxed for 40 minutes, air cooled for 5 minutes, and then 600 ml of distilled water was added. The charred material was filtered and the marc washed water (3 x 75 ml). Then one gram of EDTA was added to the combined filtrate. The pH was adjusted to 9 with 50 per cent NaOH, and the solution cooled with ice water.

Column preparation. A cation exchange resin column was prepared by adding DOWEX 50W-X8 resin to a known volume of water until the resin displaced 5 ml of water. The water was drained from the column and the resin was then washed with 25 ml of saturated NaCl followed by 50 ml of water. The material to be separated was then immediately placed on the column.

1 Melting points were determined on Koffler's hot stage apparatus. The uv spectra were obtained on a Beckman Acta-III spectrophotometer and the ir spectra were determined on a Beckman IR-33 recording spectrophotometer in KBr pellets. 1H nmr spectra were recorded in D 20 on a JeoI-FX60 instrument with DSS as internal standard. Mass spectra were taken with a high resolution DuPont 21-492 mass spectrometer. GLC analyses were run on a Beckman GC-65 instrument according to published procedure (4). HPLC analyses were determined on a Water Associate HPLC, Model 202.

2 For more details see our report to NIDA: ( a) Paraquat-Cannabis Report No. 1: 18 Nov. 1977-Contract No. HSM-42-70-109; ( b) Paraquat-Cannabis Report No. 2. 25 Jan. 1978-Contract No. HSM-42-70-109; ( c) Paraquat-Cannabis Report No. 3:13 Mar. 1978-Contract No. HSM-42-70-109 (Final Report: Contract ended 28 Feb. 1978).

Column clean-up. The alkaline suspension obtained from the original extraction of paraquat was filtered through standard laboratory glass wool and passed through the DOWEX 50W-X8 resin column at a rate of 5-10 ml/minute. The column was then rinsed successively with 25 ml water, 25 ml 2N HCl, 25 ml water and 25 ml 1/10 saturated NH 4Cl solution. All eluates were discarded. Then one gram of NH 4Cl was added to the top of the column followed by elution with a saturated NH 4Cl solution at a rate of 10-12 drops/minute. Complete elution of paraquat was determined by testing the eluate frequently with freshly prepared alkaline sodium dithionite solution. The total volume of eluate was always recorded.

Determination of paraquat concentration in an unknown sample of cannabis preparation. An aliquot of the eluate was reacted with 2 ml of 2 per cent sodium dithionite solution in 0. 1N NaOH and the colour intensity was measured in the uv at 394 ± 4nm. The colour intensity-of the unknown was correlated with the colour intensity produced by standard paraquat in solution at a known concentration. We found that the colour intensity of paraquat follows a linear relationship between 20 and 120 µg. Thus, a standard calibration curve was plotted using concentrations between 20-120 µg; however, due to variable results obtained using the calibration curve, reproducibility was not adequate for our work. A standard solution of paraquat was prepared with each unknown sample so that uv readings of the known and unknown was within ± 0.01 absorbance units. A ten cm uv cell was used for the measurements of both standard and unknown samples. When the paraquat concentration was high, dilutions were carried out in accordance with the procedure described under "Calculations".

Calculations of paraquat concentration in the cannabis samples.

Paraquat = A sa / A st x C st x V st / V sa x dilution of sample x volume of eluate (ppm) / 10

Where:
 
 
A sa = Absorbance of unknown sample.
 
A st = Absorbance of standard sample.
 
C st = Concentration of paraquat in standard sample (μg/ml).
 
V st = Volume of standard solution used.
 
V sa = Volume of unknown sample used.

Table 1 shows the paraquat concentration in confiscated samples, while table 2 shows the cannabinoid analysis of these samples. Table 3 shows the paraquat analysis of the samples obtained from the NIDA-USDA project.

Isolation and identification of paraquat from confiscated cannabis. The identity of paraquat from confiscated samples was confirmed by isolating paraquat from positive samples. The following procedure was used:

One hundred grams of plant material containing an average of 55 ppm of paraquat were placed in a three-litre round bottom flask. The plant material was soaked in 100 ml of water for 10 minutes and 400 ml of concentrated sulfuric acid were then added. The mixture was refluxed for 45 minutes to completely char the plant material. After refluxing, the condenser was slowly rinsed with 1.8 litres of water and the contents of the flask filtered through Whatman No. 1 filter paper in a Buchner funnel. To the aqueous filtrate 10 grams EDTA was added and the pH adjutsted to 9 with 50 per cent NaOH.

Column clean-up. A column packed with cation exchange resin (40 ml of settled resin) was prepared as previously described using DOWEX 50W-X8, 200-400 mesh. The column was rinsed with 100 ml of saturated sodium chloride solution followed by 150 ml of distilled water. The extract obtained from the previous step was then passed over the column at a flow rate of 20 ml/minute with the help vaccum connected to the column outlet. After all the extract passed through the column, the latter was then washed successively with water (100 ml); 2N HCl (100 ml); water (125 ml); and 1/10 saturated ammonium chloride (100 ml): all washings were discarded. Solid ammonium chloride (10 g) was added to the top of the column and paraquat was then eluted with saturated NH 4Cl at a flow rate of 20 drops/minute. The eluate was collected (180 ml) until it showed no blue colour with alkaline Na 2S 2O 4 solution.

TABLE 1

Paraquat concentration in confiscated marijuana samples

Sample

Place confiscated

Date seized

Wt. seized (kg)

Paraquat (ppm)

1
San Diego, CA
24/01/77
794.5
3.3 a
2
San Ysidro, CA
23/10/76
147.0 55.0
3
Chula Vista, CA
16/11/76
100.0 10.0
4
Bisbe, AZ
04/02/77
420.7 11.3
5
Boulevard, CA
16/01/77
610.9 91.2
6
San Ysidro, CA
27/10/76
335.0 415.0
7
Lukeville, AZ
15/03/77
263.6 655.0
8
Campo, CA
08/06/77
319.1 29.0
9
San Ysidro, CA
23/03/77
245.4 2 264.0
10
Tecate, CA
20/06/77
351.4 38.0
11
El Paso, TX
17/11/76
246.8 1 270.0
12
E1 Paso, TX
20/01/77
150.9 87.0
13
Campo, CA
10/06/77
145.0 990.0
14
Sells, AZ
12/05/77
238.6 114.0
15
La Mesa, CA
25/03/77
221.4 2.1
16
Nogales, AZ
27/04/77
164.5
22.0 b
17
Nogales, AZ
22/04/77
145.9
380.0 b
18
Nogales, AZ
20/07/77
144.5
2.0 b
19
San Ysidro, CA
30/11/76
403.0
176.0 b
20
Hildago, TX
17/01/78
249.5
13.0 b

a Average of three Chevron analyses.

b Only one Chevron analysis.

TABLE 2

Cannabinoid a analysis of paraquat positive samples (percentage by dry weight)

Sample

CBC

CBD

Δ9-THC

CBN

THCV

CBL

Δ8-THC

1 0.12
trace b
0.75 0.29 0.03 0.01
trace
2 0.13
trace
0.97 0.24 0.05 0.01 0.02
3 0.11
trace
0.93 0.16 0.08
trace
0.01
4 0.12
trace
0.82 0.28 0.03 0.01 0.02
5 0.05
trace
0.38 0.31 0.01
trace
0.02
6 0.09
trace
0.42 0.28 0.04 0.01
- c
7 0.07
trace
0.54 0.18 0.03
trace
-
8 0.10
trace
0.44 0.21 0.01
trace
-
9 0.08
trace
0.62 0.21 0.02
trace
trace
10 0.08
trace
0.31 0.30 0.02
trace
-
11 0.08
trace
0.76 0.16 0.05
trace
-
12 0.09
trace
0.42 0.22 0.03
trace
-
13 0.07
trace
0.60 0.16 0.02
trace
-
14 0.07
trace
0.73 0.17 0.03
-
trace
15 0.07
trace
0.31 0.18 0.03
-
-
16 0.06
-
0.13 0.17
trace
trace
trace
17 0.06
trace
0.25 0.20 0.02
-
trace
18 0.05
-
0.43 0.37 0.03
-
-
19 0.09
-
0.53 0.21 0.05
trace
trace
20 0.10
trace
0.76 0.20 0.02
trace
-

a CBC-cannabichromene, CBD-cannabidiol, Δ 9-THC-Δ 9- trans-tetrahydrocannabinol, CBN-cannabinol, THCV-Δ 9-tetrahydrocannabivarin, CBL-cannabicyclol, Δ 8-THC-Δ 8-tetrahydrocannabinol.

Less than 0.009 per cent.

None detected.

TABLE 3

Analysis of marijuana samples sprayed with paraquat by the United States Department of Agriculture a

Sample

Concentration paraquat sprayed

Paraquat analysed ppm b

Control
-0-
-0-
2
1/2 lb/acre
215
3
1 lb/acre
500
4
2 lb/acre
250

Plants were sprayed, harvested and shipped to us in dry ice. Samples were allowed to dry after arriving in our laboratory and were then analysed.

Average of three tests using the modified Chevron method.

Anion exchange resin column. Paraquat was then precipitated by adding excess Mayer's Reagent to the above eluate. The orange precipitate was separated by centrifugation; washed three times with water; and then dissolved in 10 ml of acetone/water (1:1). The resultant solution was exchanged on an anion exchange resin column (packed with 10 grams of Bio-Rad, analytical grade Anion exchange resin, AG21K, 40-100 mesh, chloride form) and elution was carried out by acetone-water mixture. The eluate (50 ml) was collected and concentrated in a rotary evaporator to a minimum volume, the concentrate transferred to a Brinkman concentrator and evaporated to dryness. The brownish residue (about 3.8 mg) was crystallized from 95 per cent ethanol to give white needle crystals, mp. dec.>300 °C; ir ? KBr / max 3060, 3000, 2860, 1650, 1565, 1510, 1180 and 1105 cm -1; uv ? MeOH / max 260 nm (? 4.3 x 10 4); 1H nmr (D 2O) ? 4.53 (6H, s), 8.55 (4H, d, J = 6.5Hz) and 9.08 (4H, d, J=6.5Hz); MS, m/e (per cent intensity) 186 (1), 171 (3), 157 (23), 156 (100) and 103 (20). These data were identical with those of authentic paraquat.

Thin layer chromatography of paraquat. Thin layer chromatography (5) of isolated paraquat was carried out on silica gel plates with methanol-6N hydrochloric acid (20:30) as the solvent. With this system paraquat shows a spot at R f 0.55. Visualization was by spraying the plate with Dragendorff's reagent. The isolated paraquat was identical to the standard.

Reduction and GC determination of paraquat. Isolated paraquat (2.0 mg) was dissolved in 5 ml of 95 per cent ethanol and 15 mg of sodium borohydride was added (6). After 5 minutes, the solution was evaporated to dryness and the residue was partitioned between ethyl ether and water. The ethyl ether layer was concentrated and 1 μl was injected into a Beckman GC-65 fitted with a 3 per cent OV-17, 8-ft column, operated isothermally at 160°. A major peak with a retention time of 6.5 minutes was observed with both isolated and standard paraquat analysed by this procedure. The retention time observed is that of hexahydroparaquat.

High pressure liquid chromatography of paraquat.
 
Instrument:
Water Associate HPLC (Model 202).
Column:
μ-Bondapak C 18Column 30 cm x 3.9 mm I.D.
Mobile Phase:
0.025 M NaH 2PO 4 in 40 per cent MeOH or water pH = 5.0 adjusted with 5 per cent NaOH.
Flow rate:
2 ml/min.
U. V. detector at 254 nm and 280 nm.
 

The retention time for standard and isolated paraquat was 3.5 minutes using the buffer in 40 per cent MeOH and 1.8 minutes using water as the solvent. The eluate corresponding to the peak for isolated paraquat was collected and tested with sodium dithionite which gave the characteristic blue colour.

Results and discussion

The intensive use of paraquat as a herbicide to control illicit fields of Cannabis in Mexico and the possibility of contaminated marihuana reaching the U.S. illicit market prompted the National Institute on Drug Abuse to initiate a programme to screen confiscated marijuana from Mexico for paraquat. The analytical procedure used was a modified Chevron Chemical Company procedure which involves extraction with sulphuric acid followed by cation exchange resin column purification and spectrophotometric determination of paraquat using sodium dithionite reagent. It should be mentioned that the modified Chevron procedure, although tedious, can detect paraquat down to 0.1 ppm. However, we found that the neutralization step as well as the addition of EDTA could be eliminated with marijuana without affecting the sensitivity of the method which is in agreement with Claderbank and Yuen's findings (7) on certain food crops.

Analysis of 160 kilo brick samples of confiscated marijuana showed that 20 samples or 12.5 per cent contained paraquat (table 1). However, by observing the date of seizure, it was found that no paraquat positive samples were seized before 20 October 1976. Using this date and considering only those samples seized after this date, it can be said that 20 out of 115 samples or 17.4 per cent were positive for paraquat. The concentration of the herbicide in these samples ranged between 2.0 to 2264 ppm. The average level of contamination is 331 ppm; however, normalizing all seizures the concentration of paraquat would be 279 ppm or 0.28 mg per gram cigarette.

Table 2 shows the cannabinoid analysis of these samples. Although it is impossible to absolutely assign geographical origin of any Cannabis product by cannabinoid analysis, some points can be gleaned from the data. Cannabis grown in Mexico and other geographical locations contain little if any cannabidiol (CBD). These 20 positives samples contain only trace amounts of CBD. Thus, this combined with the fact that they were confiscated on the border or near the border with Mexico, strongly suggest Mexican origin. Moreover, the ratio of Δ 9-THC to CBC is within the range of Mexican Cannabis (8). The analyses shown in table 2 are similar to our most recent analysis of 412 confiscated marijuana samples: Δ 9-THC-0.83 per cent; CBC-0.10 per cent; CBD-0.04 per cent; CBN-0.35 per cent. It should be noted that CBL, Δ 8-THC and CBN are artifacts. These cannabinoids do not exist in fresh Cannabis plant material. Thus, care must be exercised in using these cannabinoids for identification purposes in an attempt to assign geographical origin.

Table 3 shows the herbicide concentration in samples sprayed with different amounts of paraquat by the United States Department of Agriculture. These sprayings were designed to simulate aerial applications and were carried out in Beltsville, MD. Data show that at 2 lb/acre the paraquat concentration was less then that at 1 lb/acre. There was no linear relationship between the amount sprayed and the concentration in the plant material. Why no linear relationship was observed may be explained in several ways. The plant material sprayed at a rate of 2 lb/acre was yellow in colour and the dessicant effect of paraquat had caused severe necrosis. In the other samples reported in table 3 necrosis was not observed or was minimal. It is possible that as plant cells were being destroyed by the dessicant action of paraquat, paraquat itself was utilizing the plant water to undergo decomposition as described by Calderbank (9).

If the observation that the paraquat concentration will be lower in necrosed plant material is valid, then the longer Cannabis plant material is exposed to paraquat the lower the paraquat concentration. To evaluate this observation, Mexican Cannabis plants growing "in Mexico" were "sprayed from a helicopter" with a 5 per cent aqueous paraquat solution. 3 Samples were collected immediately after spraying, 3 hrs after spraying, 48 hrs after spraying and 72 hrs after spraying. Also, a sample supposedly not sprayed with paraquat was collected (see table 4). Analytical data of these samples showed the "control" sample received over 1,000 ppm of paraquat as an overspray. Also, the sample taken immediately after spraying contained 4,651.2 ppm, whereas after 3 hrs, and the setting of the overspray, the paraquat concentration was 5,546.7 ppm. After 3 hrs the plant material began to show signs of necrosis. The concentration of paraquat dropped significantly as the necrosis became more severe. Although these data do not totally validate our observations, they do add support to the proposed degradation of paraquat in Cannabis.

3

3 Samples were provided through contacts in Mexico.

TABLE 4

Paraquat determination of Mexican samples sprayed by helicopter with 5 per cent aqueous paraquat solution in Mexico in a field experiment

Sample

Harvested

Paraquat determination

1
Immediately after spraying
4 651.2 ppm
2
3 hours after spraying
5 546.7 ppm
3
48 hours after spraying
4 176.0 ppm
4
72 hours after spraying
2 932.5 ppm
5
Not sprayed
1 093.3 ppm

The possible role of photochemical degradation is also a distinct possibility. This type of decomposition requires oxygen, aqueous solution and ultra-violet light (10).

It is our observation that once the paraquat is absorbed into the plant tissue and the tissue dried, the paraquat becomes stable to light. This stability is probably due the strong ionic bonding properties of paraquat. Marijuana containing paraquat has been exposed to a heat lamp (400 watts, sample temperature 60-65°) for 15 days and to direct sunlight (a total of 15 hours) and no significant change in the paraquat concentration was found.

During the course of our work, nine hash oil samples confiscated by the U.S. Drug Enforcement Administration were analysed for paraquat (table 5). Hash oil is produced by extracting Cannabis with an organic solvent, thus, paraquat would not be expected to be soluble in many organic solvents except ethanol, methanol and some aqueous mixtures of these two solvents. No paraquat was found in the nine samples analysed. These samples were seized in New York, NY and from the cannabinoid analysis (table 6) it is impossible to state geographical origin. However, it is possible to state they are not of Mexican origin. Mexican Cannabis plant material does not contain significant quantities of cannabidiol (8). Also from the cannabinoid analysis, there is a strong possibility that samples 1-8 were grown, harvested and processed by the same source. This is also supported by refractive index and specific gravity data. Although the refractive index of sample 9 is very similar to the others, cannabinoid and specific gravity data show this is not the case.

TABLE 5

Refractive indices and specific gravity of hash oil samples analysed for paraquat (total weight of all samples analysed was 6,224.7 grams)

Sample

Where confiscated

Date seized

Paraquat attalysis

N 20D

Specific gravity

1
New York, NY
23/5/76
-
1.5432 a
1.04419 b
2
New York, NY
23/5/76
-
1.5440 1.04887
3
New York, NY
23/5/76
-
1.5438 1.02314
4
New York, NY
23/5/76
-
1.5444 1.04668
5
New York, NY
23/5/76
-
1.5432 1.04492
6
New York, NY
23/5/76
-
1.5452 1.06107
7
New York, NY
23/5/76
-
1.5450 1.07476
8
New York, NY
23/5/76
-
1.5440 1.07135
9
New York, NY
12/8/76
-
1.5423 1.15187

a Refractive index determination: Instrument-Abbe Refractometer; Results-average of three readings.

b Specific gravity determination: Temperature-25 °C ± 1°; Pycnometer-15 ml volume weighing bottle, direct comparison with distilled water.

TABLE 6

Cannabinoid analysis a of hash oil samples (percentage by dry weight)

Sample

CBDV

THCV

CBL

CBD

CBC

Δ8-THC

Δ9-THC

CBN

1 0.68 1.66 0.15 19.55 1.27 0.73 36.27 3.05
2 0.67 1.43 0.16 19.04 1.00 0.70 33.58 3.50
3 0.65 1.70 0.15 20.10 1.16 0.64 33.71 3.29
4 0.69 1.72 0.14 20.59 1.12 0.33 37.86 3.36
5 0.71 1.69 0.14 20.93 1.22 0.78 36.47 3.49
6 0.68 1.72 0.14 20.34 1.15 0.48 37.80 3.13
7 0.69 1.72 0.25 20.31 1.04 0.31 36.92 3.36
8 0.69 1.55 0.13 19.92 1.05 0.60 33.20 3.82
9 0.08 0.37 0.43 6.37 1.53 0.24 20.88 3.16

a Duplicate cannabinoid analyses were run on 4 per cent of 6 1/2 per cent Phenyl/Methyl Silicone Ratio Column. For more details on methodology see Marihuana: Chemistry, Bio-Chemistry and Cellular Effects, G.G. Nahas, ed., Springer Verlag, New York, NY 1976.

Due to the cost of the modified Chevron method an alternate method for extracting paraquat from marijuana would be desirable. The following methods for extracting paraquat from confiscated Cannabis plant material were tried.

  1. Water, cold extraction and hot extraction.

  2. Saturated NaCl, cold and hot.

  3. Saturated NH 4Cl, cold and hot.

  4. 50 per cent HCl, cold and hot.

  5. 50 per cent H 2SO 4, cold and hot.

In all cases, paraquat recovery was very poor, the extracts were coloured and thus direct testing for paraquat by colour development with sodium dithionite alkaline solution was practically impossible, especially at concentrations below 220 ± 20 ppm.

However, in our laboratory a new procedure for the detection of paraquat in Cannabis has been developed and is sensitive down to less than 1 ppm. Details of such procedure will be reported elsewhere (11).

Acknowledgements

Supported by the Research Institute of Pharmaceutical Sciences and paraquat analyses addendum to contract No. HSM-42-70-109 from the National Institute on Drug Abuse.

We wish to thank the Drug Enforcement Administration of the U.S. Department of Justice; U.S. Department of Agriculture; U.S. State Department; Imperial Chemical Industry, Mexico City; Chevron Chemical Company, Richmond, CA; and the Directors of the Drug Enforcement Administration Laboratories, for assistance in obtaining samples and raw materials.

References

001

M. Horowitz. Bulletin on Narcotics, XXIX : 1, 75, 1977.

002

A.M. Carlstorm. JAOAC, 51, 1306, 1968.

003

S.H. Yuen, J.E. Bagness and C. Myles. Analyst. 92, 375, 1967.

004

C.E. Turner, K.W. Hadley, J.H. Holley, S. Billets and M.L. Mole. J. Pharm. Sci., 64, 810, 1975.

005

S. Tsunenari. J. Forens. Sci., 61, 1975.

006

G.H. Draffan, R. A. Clare, D. L. Davies, G. Hawksworth, S. Murray and D. S. Davies. J. Chromatography, 139, 311, 1977.

007

A. Calderbank and S.H. Yuen. Analyst, 90, 99, 1965.

008

J.H. Holley, K.W. Hadley and C.E. Turner. J. Pharm. Sci., 64, 892, 1975.

009

A. Calderbank, in Advances in Pest Control Research, vol. 8, R.L. Metcalf, ed., Intersciences Publisher, New York, 1968, p. 127.

010

P. Slade. Nature, 207, 515, 1965.

011

M.H. Elsohly and C.E. Turner. U.S. Patent Ser. No. 927, 852. Patent pending.