Environ. Sci. Technol. 1985, 1 9 , 946-949
Mullin, M. D.; Pochini, C. M.; McCrindle, S.; Romkes, M.; Safe, S. H.; Safe, L. M. Enuiron. Sci. Technol. 1984, 18, 468-476.
Murphy, T. J.; Pokojowczyk, J. C.; Mullin, M. D. In "Physical-Chemical Behavior of PCBs in the Great Lakes"; Mackay, D.; Paterson, S.; Eisenreich, S.;Simmons, M., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; pp 49-58. Wall Street Journal; Jan 21, 1981; p 5. MacLeod, K. E. Environ. Sci. Technol. 1981,15,926-928. Pavoni, J. L.; Heer, J. E.; Hagerty, D. J. "Handbook of Solid
Waste Disposal"; Van Nostrand Reinhold New York, 1975; pp 431-443. (35) Sheppard, J. C.; Westberg, H.; Hopper, J. F.; Ganesan, K. J. Geophys. Res. 1982,87 (C2), 1305-1312.
Received for review May 18,1983. Revised manuscript received March 19,1985. Accepted May I , 1985. this project was supported by the U S . Environmental Protection Agency under Cooperative Agreement CR 807412 and by DePaul University.
Initial Decomposition Mechanisms and Products of Dimethyl Methylphosphonate in an Alternating Current Discharge Mark E. Fraser,+ Harold 0. Eaton, and Ronald S. Shelnson" Chemistry Division, Naval Research Laboratory, Code 6 180, Washington, DC 20375-5000
The efficiency of decomposition of dimethyl methylphosphonate (DMMP) vapor from a helium stream in an atmospheric pressure alternating current capacitive discharge has been determined to range from 50 to 100% depending upon the input concentration and flow rate and is improved with trace oxygen addition. In the absence of oxygen the principal discharge products are methane, ethane, formaldehyde, and carbon monoxide, suggesting an initial decomposition mechanism involving the loss of methyl radicals and formaldehyde. The subsequent chemistry of these species is postulated to produce the observed secondary products. Introduction
Interest in electric discharges as air purification systems has recently been renewed (1-4) with emphasis on the toxic organophosphorus compounds. Material breakthrough, failure in a moist or polluted environment, and contamination during unit disposal are the disadvantages of adsorption techniques that have refocused attention on electric discharges. Although the literature on the behavior of various organic and inorganic compounds in electric discharges is extensive ( 5 , 6), very little is known of the electric discharge induced decomposition chemistry of organophosphorus compounds. A cursory study of the microwave discharge decomposition of the G-agent simulants dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP) has been reported by Bailin et al. (7). In helium, the primary products of the discharge with DMMP were trimethyl phosphite and methanol (7), indicating an initial decomposition mechanism probably involving methyl and methoxy radicals. Mass spectrometric studies performed with real agents (8) and simulants (9) have revealed the electron collison induced fragmentation pattern of the G agents to be by formaldehyde, methyl radical, and methoxy radical loss. The decomposition chemistry of agents (and simulants) in an atmospheric pressure discharge has not yet been examined. The methodology employed in these experiments has been to introduce trace concentrations of an organophosphorus species of interest (dimethyl methylphosphonate or trimethyl phosphate) into a helium bulk carrier gas and monitor the discharge effluent products with GC and GC/MS techniques. In order to study the NRL/NRC Postdoctoral Fellow (1983-1985). Present address: Physical Sciences Inc., P.O. Box 3100, Andover, MA 01810. 946
Environ. Sci. Technol., Vol. 19, No. 10, 1985
discharge-induceddecomposition mechanism, helium bulk carrier gas has been used with and without trace oxygen addition. This was done in order to deconvolute the initial decomposition mechanism from the secondary oxidation reactions. Experimental Techniques
Discharge Apparatus. Figure 1 shows the atmospheric pressure discharge apparatus used in these experiments. Technical grade helium was purified by passage through traps of molecular sieve and activated charcoal immersed in liquid nitrogen baths in order to reduce hydrocarbon, oxygen, and nitrogen impurities. DMMP vapor was introduced into the helium stream from a bubbler immersed in a water bath regulated at 15.0 f 0.1 "C. The impurities in commercial-grade DMMP (Aldrich, 97 % pure), primarily methanol and acetone, were reduced by vacuum distillation. Gold label trimethyl phosphate (Aldrich, 99+ % pure) was used without further purification. A needle valve controlled the flow of added oxygen (0-500 ppm (volume)). Copper sampling lines for O2and N2 analyses were attached to points before and after the discharge. Rubber septum ports for syringe sampling were also located at these points to allow DMMP and discharge product analyses. An alternating current (ac) capacitive "ozonizer" type discharge was constructed by coating the outer and inner walls of a 35 cm long Pyrex Liebig condenser with silver conductive paint. Leads were then attached to a 16-kV, 60-H~ transformer. During normal operation the current was 1 mA with no detectable temperature rise. The outside diameter of the discharge tube was 1.9 cm, and the total volume of the discharge was 25 cm3with a wall-wall separation of 0.3 cm. Analytical System. 02, Nz, and CO separation was performed at 50 "C with a 1.83 m by 3.2 mm 0.d. (6 ft by l/s in.) molecular sieve column. A thermal conductivity detector (TCD) was used to detect oxygen and nitrogen while carbon monoxide was detected by a flame ionization detector (FID) after conversion to methane by a heated nickel catalyst with hydrogen flow. Analysis for methane, ethane, ethene, acetylene, propane, propene, acetaldehyde, dimethyl ether, and methanol was performed by gas chromatography using a 0.91 m by 3.2 mm (3 f t by '/a in.) Porapak-S column with temperature programming (10 "C for 4 min followed by an increase of 8 "C/min to 100 "C) and FID detection. The column temperatures were chosen because we have observed decomposition of DMMP on Porapak producing primarily methanol at column tem-
00 13-936X/85/09 19-0946$01.50/0
0 1985 American Chemical Society
Table I. Ozonizer Discharge Products from DMMP/He minor products
maior uroducts unreacted DMMP methane ethane ethene carbon monoxide carbon dioxide propane Flgure 1. Discharge apparatus.
1
3
formaldehyde acetaldehyde dimethyl ether methanol
1. 2. 3. 4. 5. 6. 7.
METHANE ETHENE ETHANE ACETYLENE PROPENE PROPANE DIMETHYL ETHER 8. ACETALDEHYDE 9. METHANOL
methyl ethyl ether ethanol acetone propene acetylene
CH3OCHZCH3
2-butanone
CH3COCHZCH3
dimethoxymethane methyl propyl ether 2-propanol
CH30CHZOCH3
n-propanol
CH3CHZCHzOH
methyl acetate
CH3COOCH3
CH3CHzOH CH3COCHS CHZCHCH, HCCH
CH30CHZCHzCH3 CH3CHOHCH3
water uhosahine
7 2
I
I
0
2
I
5
I
10
I
15
I
20
Product analysis was also performed by using cryogenic trapping at -196 and -131 OC with collection times ranging from 30 min to several hours. A Hewlett-Packard Model 5990A GC/MS equipped with either a 50-m OVlOl capillary column or a 1.83 m by 3.2 mm 0.d. (6 f t by '/s in.) Porapak-S column was used to analyze the samples. Trapping in Tenax was studied, but at the temperatures necessary to desorb (>200 "C), degradation products were observed from DMMP similar to those observed from discharged DMMP (including carbon dioxide, phosphine, acetaldehyde, and methanol).
TIME (MINUTES)
Flgure 2. Sample chromatogram. Shown is a typlcal chromatogram of the organic products from discharged DMMP/He in the absence of oxygen. Analysls was performed on a Porapak-S column with temperature programmlng, 10 OC for 4 mln followed by an Increase of 8 OC/mln to 100 OC, and flame Ionization detectlon. Acetylene and propene were normally unresolved.
peratures exceeding 100 "C. Carbon dioxide was also analyzed with the Porapak-S column and detected by the FID after conversion to methane by the nickel catalyst. Figure 2 shows a sample chromatogram of the organic DMMP discharge products analyzed with the Porapak-S column. Formaldehyde was analyzed on the Porapak-S column at 100 OC with conversion by the nickel catalyst and FID detection. Water was detected on the Porapak-S column isothermally at 90 "C with a TCD (the sensitivity was poor, >20 ppm). DMMP was detected on a 0.61 m by 3.2 mm (2 f t by l/s in.) column packed with 3% 1,2,3-tris(2-cyanoethoxy)propane on Chromosorb G with an FID at 100 "C. Phosphine was detected with a 3-ft length of Porapak-S column with a Perkin-Elmer nitrogen- and phosphorusspecific detector a t 25 OC. Higher molecular weight organics such as acetone and methyl ethyl ketone were analyzed on a capillary column with an FID detector (30-m section of SP2250 merged with a 50-m section of OV101). Standards for each of the analyzed species were prepared and the detectors calibrated. Formaldehyde standards (
