Anal. Chem. 1997, 69, 4283-4285
Determination of Ecgonidine Methyl Ester Vapor Pressure Using a Dynamic Gas Blending System and Gas Chromatographic Analysis Pavel Neudorfl,* Michel Hupe´, Pierre Pilon, and Andre´ H. Lawrence
Laboratory and Scientific Services Directorate, Research and Development Division, Revenue Canada, 79 Bentley Avenue, Ottawa, Ontario, Canada K1A 0L5
Ecgonidine methyl ester (EDME), a product of the thermal decomposition of cocaine, has been identified as a likely candidate for the detection of concealed cocaine by vapor sniffing. Using a continuous-flow vapor generator preconcentration/thermal desorption technique in combination with GC-TSD analysis, the vapor pressure of EDME was measured between -20 and +20 °C, and the results were fitted into the following equation: log P ) 16.6 (3.64 × 103/T), where P is in parts per billion and the temperature is in kelvin. At 20 °C, the vapor pressure of EDME is 16 ppm, approximately 5 orders of magnitude higher than that of cocaine base. The use of marine containers is a well-known smuggling method for large shipments of cocaine. Such containers present an ideal method of smuggling because manual inspection of the containers is time consuming, difficult, and expensive for the importing community. One method being examined to rapidly distinguish between innocent and suspicious cargo is the detection of telltale vapors from air samples of cargo containers. Vapors of cocaine have been observed in the air of a drug vault used to store seized cocaine samples.1 Previous work on the vapor pressure of cocaine base2 indicated that the saturated headspace of cocaine contains approximately 3 ng of cocaine in a liter of air at room temperature (0.11 ppb). Because of the low vapor pressure of cocaine,2 it was of interest to investigate the presence of other compounds in seized cocaine, which would be structurally related to cocaine and would be more volatile, thus making them easier to detect in the vapor phase. Recent work in this laboratory has shown that ecgonidine methyl ester (EDME), also called anhydroecgonine methyl ester, was present in the headspace of all seized cocaine samples analyzed (approximately 20). EDME, shown in Figure 1 along with the structure of cocaine, is a substance which has often been identified in the bulk analysis of seized cocaine samples.3-7 It is a known product of the (1) Pilon, P.; Neudorfl, P.; Hupe´, M.; Lawrence, A. Proceedings of Counterdrug Law Enforcement: Applied Technology for Improved Operational Effectiveness International Technology Symposium, Nashua, NH, 24-27 October 1995; Part 2, pp 16-37-16-50. (2) Lawrence, A. H.; Elias, L.; Authier-Martin, M. Can. J. Chem. 1984, 62, 1886-1888. (3) Casale, J. F.; Waggoner, R. W. J. Forensic Sci. 1991, 36 (5), 1312-1330. (4) Lukaszewski, T.; Jeffery, W. K. J. Forensic Sci. 1980, 25 (3), 499-507. (5) Casale, J. F.; Klein, R. F. X. Forensic Sci. Rev. 1993, 5 (2) 95-107. (6) Schlesinger, H. L. Bull Narc. 1985, 37 (1), 63-78. (7) LeBelle, M.; Lauriault, G.; Callahan, S.; Latham, D.; Chiarelli, C.; Beckstead, H. J. Forensic Sci. 1988, 33 (3), 662-675. S0003-2700(97)00196-0 CCC: $14.00 Published 1997 Am. Chem. Soc.
I
II
Figure 1. Structures of cocaine (I) and EDME (II).
pyrolysis and thermal degradation of cocaine,8-10 and its presence from the decomposition of cocaine at high temperature in the presence of quartz or glass fibers has been used as a confirmatory method in the detection of concealed cocaine.11 EDME has also been detected in the hair and urine of crack smokers12-15 and in the bile of intravenous cocaine users.16 In this work, a dynamic gas blending system was employed to produce controlled vapor concentrations of EDME. The technique involves the saturation of a gas stream with EDME vapors by passage of the stream over the test sample. The vapors are collected in a solid adsorber, thermally desorbed, and analyzed by gas chromatography (GC). Similar procedures were used to measure the vapor pressure of amphetamine, cocaine, and heroin.2 EXPERIMENTAL SECTION Materials. EDME was prepared at this laboratory using the method of Lukaszewski and Jeffery.4 The purity of the material was estimated to be >99% by gas chromatography. Vapor Generation. Controlled concentrations of EDME vapors were generated using a dynamic gas blending system (8) Poziomek, E. J.; Chen, S.; Wohltjen, H.; Jarvis, N. L. Proceedings of Contraband and Cargo Inspection Technology International Symposium, Washington, DC, 28-30 October 1992; pp 425-432. (9) Brown, S.; Bothe, C.; Landstrom, D. Proc. SPIE-Int. Soc. Opt. Eng. 1994, 2276, 340-351. (10) Martin, B. R.; Lue, L. P.; Boni, J. P. J. Anal. Toxicol. 1989, 13, 158-162. (11) Jadamec, J. R.; Su, C-W.; Rigdon, S.; Norwood, L. Third International Workshop on Ion Mobility Spectrometry, Proceedings, Galveston, TX, 16-19 October 1994; NASA Conference Publication 3301, pp 229-244. (12) Kintz, P.; Cirimele, V.; Sengler, C.; Mangin, P. J. Anal. Toxicol. 1995, 19, 479-482. (13) Jacob, P.; Jones, R. T.; Benowitz, N. L.; Shulgin, A. T.; Lewis, E. R.; EliasBaker, B. A. Clin. Toxicol. 1990, 28 (1), 121-125. (14) Jacob, P.; Lewis, E. R.; Elias-Baker, B. A.; Jones, R. T. J. Anal. Toxicol. 1990, 14, 353-357. (15) Cone, E. J.; Hillsgrove, M.; Darwin, W. D. Clin. Chem. 1994, 40 (7), 12991305. (16) Lowry, W. T.; Lomonte, J. N.; Hatchett, D.; Garriott, J. C. J. Anal. Toxicol. 1979, 3 (May/June), 91-95.
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Figure 2. Continuous trace vapor source with dynamic gas blending system. (1) Jacketed vessel with a circulating constant-temperature bath. (2) Glass wool soaked with EDME. (3.) Glass chips to promote mixing. Table 1. Sample Collection Conditions source volume time F f room temp, °C drawn (mL) (s) (mL/min) (mL/min) temp, °C -20.0 -10.0 -0.1 10.0 20.0 -15.0 15.0
80 20 15 10 10 10 80
20 5 3 1 1 1 20
615 620 630 1275 1800 1800 1175
7.2 7.4 3.4 3.4 3.2 3.4 6.8
25.6 24.1 23.3 23.7 24.3 24.2 24.2
n 10 5 5 5 5 5 5
shown in Figure 2. A piece of glass wool was soaked with EDME (a liquid at room temperature) and placed in a U-tube, which was kept at constant temperature using a Grant (Cambridge, UK) circulating bath (methanol or water was used as the circulating medium). When the temperature of the EDME source was changed, the system was left to equilibrate for at least 24 h before samples were taken. The temperature of the circulating bath was varied between -20 and +20 °C. A small gas stream (f) was saturated with EDME by passing the air over the soaked glass wool. Stream f was subsequently diluted by admixing a large flow of purified air (F); both streams were adjusted so that the amount of EDME sampled from the vapor source was approximately 1 or 2 ng. The gas chromatograph was calibrated with a corresponding amount of EDME in hexane. Sample Collection. The samples from the vapor source were collected in a glass tube (i.d. 0.25 in.) containing a plug of platinum mesh. A 100 mL (for larger volumes) or a 20 mL (for smaller volumes) syringe to which the sampling tube was attached was used to collect a certain volume from the vapor source. For proper sampling, the flow rate of the vapor exiting from the system (f + F, where f is the carrier flow rate from the source and F is the diluting flow rate) was maintained at least twice as large as the sampling rate. The conditions used for sample collection are summarized in Table 1. After the vapor sample was collected, the adsorber was attached to the gas chromatograph for analysis. Gas Chromatography Method. A Varian Star 3400 Cx gas chromatograph equipped with a thermionic specific detector (TSD) was used for the analytical work. The GC was modified to accommodate a two-stage desorber system described by Lawrence and co-workers.2 A six-port switching valve provided the various functions involved in the desorption/adsorption steps. 4284
Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
A 9 m Ultra-2 column (Hewlett Packard Co., 5% diphenyl-95% dimethyl polysiloxane, 0.20 mm i.d., 0.33 µm film thickness) was employed. Helium was used as the carrier gas with a head pressure of approximately 50 psi. The oven temperature was programmed as follows: initial temperature, 85 °C, hold for 1 min; temperature program rate 50 °C/min; final temperature, 200 °C. The total run time was 4.3 min. The retention time of EDME under these conditions was 2.14 min. The detector temperature was set at 300 °C. The following temperatures were set in the two-stage desorption system: desorber heater block, 225 °C; valve heater, 190 °C; interface, 190 °C. The signal from the detector was recorded on a Hewlett Packard 3392A strip chart reporting integrator. For the calibration of the GC, 1 or 2 µL of a 1.08 ng/µL solution of EDME in hexane was deposited on the platinum mesh in the glass tube. The tube was placed in a flow of nitrogen (30 mL/ min). After evaporation of the solvent, desorption of the trapped material was achieved by heating the tube for 2 min at a temperature of 225 °C. The vapors were adsorbed on a second adsorber, kept at room temperature, consisting of a 6 cm nickel loop (0.53 mm i.d.) coated with OV-1. The adsorber was resistively heated via a capacitor discharge; the vapors were desorbed and subsequently analyzed by GC. The tube used for sample collection was heated in the same manner as the tube used in the calibration. RESULTS AND DISCUSSION The collection efficiency of the sampling tube was tested at various times by ensuring that an increase in the volume sampled from the vapor source (at approximately the same sampling rate) showed a corresponding increase in the GC signal. In addition, the vapor source was tested by measuring the equilibrium vapor concentration (C0) as a function of the carrier flow, to ensure that saturation of the EDME vapor was maintained. No change in C0 was obtained with carrier flows up to 17.5 mL/min. Under conditions where the static equilibrium vapor pressure above the sample is maintained (very low carrier flow over the sample, f ) 3.4-7.4 mL/min), the concentration C in the air stream at the sampling port is simply the equilibrium vapor concentration C0 of EDME reduced by the appropriate dilution factor ratio according to eq 1, where f is the flow rate of the carrier
C ) C0(f/(f + F))
(1)
air and F is the flow rate of the diluting air entering the mixing manifold. The diluted vapor concentrations were measured experimentally by analyzing the adsorber tube using gas chromatography as described above. From the vapor concentration obtained and from the volume collected, the vapor pressure data can be calculated using the ideal gas law. The vapor pressure and the corresponding temperatures are shown in Table 2. The data were plotted as log P vs 1/T and treated by the unweighted least-square method to yield eq 2. The
log P ) 16.6 - 3.64 × 103/T
(2)
following statistical parameters were calculated: correlation coefficient, r ) -0.998; σslope ) 31 K-1; σintercept ) 0.1; n ) 40. P is the vapor pressure expressed in parts per billion (ppb), and T is the temperature in kelvin.
Table 2. Table of Data, log P (ppb) vs 1/T (K-1) T (K)
P (ppb)
1/T (K-1)
log P
253.0 258.0 263.0 272.9 283.0 288.0 293.0
156 305 614 1972 5710 7428 15960
0.00395 0.00387 0.00380 0.00366 0.00353 0.00347 0.00341
2.193 2.484 2.788 3.295 3.757 3.871 4.203
low concentration in the bulk sample, the high vapor pressure of this compound makes EDME a possible candidate for detection of concealed cocaine by air sampling. ACKNOWLEDGMENT The authors acknowledge the participation of Drs. Mohinder Chauhan and Karel Muzika in the preparation of EDME.
Received for review February 18, 1997. Accepted July 18, 1997.X The vapor pressure of EDME at 20 °C is 16 ppm, equivalent to 120 µg/L, which is approximately 5 orders of magnitude higher than that of cocaine base. Thus, even when EDME is present in
AC970196U X
Abstract published in Advance ACS Abstracts, September 1, 1997.
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