Environ. sci. Technol. 1992,26,815-816
Trace Analysis of Organobromine Compounds in Air by Adsorbent Trapping and Capillary Gas Chromatography/Mass Spectroscopy Gerard J. Sharp, Yoko Yokouchl," and Hajlme Aklmoto National Institute for Environmental Studies, 16-2, Onogawa, Tsukuba, Ibaraki 305, Japan
w A procedure has been developed for the determination of trace bromomethanes and bromochloromethanes in air. The method is based on adsorption of the organics onto Tenax GC followed by subsequent thermal desorption and analysis by GC/MS with selective ion monitoring. Difficulties accompanying subambient cooling of capillary columns were avoided by the use of a Poraplot Q column. Detection limits for five bromocarbons (CHzBrC1,CH2Br2, CHBrC12,CHBrzC1,CHBr,) were 0.1-0.2 ppt for 4-L air samples. Both land and seashore air were sampled and analyzed. Introduction Adsorptive sampling followed by thermal desorption has been recently reviewed (1)and is a widely used technique for the measurement of organics present in the atmosphere at low levels. Recently there has been interest in the role of organobromine compounds in atmospheric chemistry, and unlike the completely halogenated CFCs (chlorofluorocarbons), these compounds are relatively unstable in the troposphere with the background concentration being in the pptv range. The compounds under consideration in this paper are bromomethane, dibromomethane, bromoform, bromochloromethane, dibromochloromethane, and bromodichloromethane. There has shown to be a seasonal fluctuation in bromoform concentration in the Arctic region and a strong anticorrelation between filterable bromine and ozone concentration in the same region (2,3). Thus, there is an interest in the relative concentrations of these compounds, and with this in mind, an adsorptive technique was developed and the technique was applied to samples taken at both a land and a seashore site. Trace bromochlorocarbons have been analyzed previously using both an adsorptive technique and the grab, or whole air sampling, technique. Class and Ballschmiter et al. (4-6) have measured these organics in marine Atlantic air using the adsorption technique followed by capillary cold trapping and GC/ECD (1)analysis. While Berg et al. and Cicerone et al. (7,8) have measured organobromine compounds in Arctic air using the grab sample technique followed by cryogenic concentration and GC/MS analysis. In this work, we have developed a reliable method for five bromocarbons (CH,BrCl, CH2Br2,CHBrC12, CHBrzC1, CHBr,) based on adsorptive concentration and capillary GC/MS where a Poraplot Q column was used to eliminate the need for subambient cooling of the GC oven. Experimental Section The preconcentration system has been described elsewhere (9). The trap is a stainless steel tube (1/8 in. X 4 cm long) packed with Tenax GC (0.04 g) and set inside a Teflon tube (14 mm 0.d. X 8 cm). The temperature around the trap can be controlled in the range -60 to 300 "C by a temperature regulator which controls both the liquid COz and a heater. All the connections use 1/16-in.SUS tubing that is as short as possible. To avoid any adsorption of 0013-936X/92/0926-0815$03.00/0
the compounds, the valves and the SUS tubes were warmed at 115 "C by wrapping them in a flexible heater. GC/MS work was performed with a Hewlett-Packard 5890A gas chromatograph and a Hewlett-Packard 5970B. GC column was Poraplot Q (10 m X 0.32 mm). Procedure. The air sample (4 L) is collected onto Tenax GC (0.7 g), which has been previously conditioned for 2 h at 250 "C, at a flow rate of 200 mL/min. The adsorbent is contained in a stainless steel tube (20 cm long) fitted with Swagelok caps. A pump with a mass flow control system was used. The SS tube is returned to the laboratory, and the organics are desorbed for 10 min at 230 "C and a flow rate of 20 mL/min of helium against the sampling direction onto the preconcentration trap, which has been previously cooled to 0 "C. The preconcentration trap is subsequently heated to 220 "C, and the organics are desorbed onto a Poraplot Q column maintained at 55 "C. The column is then heated to 220 "C at a rate of 12 "C/min. The retention times and the two ions monitored for each of the compounds are as follows: CHzBrC1(14.27 min; m / z 128, 130), CH2Br2(16.10 min, m / z 172, 174); CHBrC12 (16.62 min; m / z 83, 85); CHBr,Cl (18.31 min, m / z 127, 129); CHBr, (20.01 min, m / z 171, 173). Quantitation is done using peak areas. Quantification. External quantification was done by injecting a methanol solution (0.5 pL) into a 5.0-mL glass vial which was on-line via helium to the preconcentration trap. The vial was heated to 100 "C, and the standards were collected onto the preconcentration trap at 0 "C and then desorbed in the usual way (9). For MeBr, which is a gas at ambient temperature and pressure, a discrete volume of gas (from a standard gas cylinder prepared commercially) was sampled via a sample loop and switching valve.
Results and Discussion The method was validated by determining recovery, linearity of detector response, detection limit, and breakthrough volume (BTV). To assess the applicability of the method, samples of air were taken at two sites. The land site was on the roof of the institute at Tsukuba, which is located 60 km north of Tokyo, while the seashore site was Otake Beach, which is approximately 50 km east of Tsukuba. Each of the compounds was monitored by selecting the two most abundant ions. The ratio of the two ions confirmed identification. Breakthrough volumes were determined not by the extrapolation method but by the direct method. Standards (500 pg of each) were trapped onto Tenax GC (0.04 g) contained in the preconcentration trap, and a known volume of helium was passed through the trap. Following desorption and analysis by GC/MS, the peak area was recorded. This process was continued with increasing volumes of gas, and the volume of gas at which the peak area began to decrease was recorded as the BTV. The respective BTVs are outlined in Table I. The BTV for methyl bromide was particularly low even at 0 "C, and so various other adsorbents were tested. The BTVs at 20 "C (L/g) were Carbosieve (25), Chromosorb
0 1992 American Chemical Society
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Table I. Breakthrough Volumes and RSD for Selected Bromocarbons compd methyl bromide bromochloromethane dibromomethane bromodichloromethane dibromochloromethane bromoform
adsorbent (temp, "C) Tenax Tenax Tenax Tenax Tenax Tenax
(0) (25) (25) (25) (25) (25)
BTV, L/g RSD," % 1.0
4.5 11.0 15 30 37.5
3.09 7.35 4.11 4.06 3.05 3.16
Relative standard deviation for 10 iniections. Table 11. Detection Limits and Linearity of Response (Correlation Coefficient, r ) for Selected Bromocarbons
compd bromochloromethane dibromomethane bromodichloromethane dibromochloromethane bromoform
detection limit absolute, concn, Pg pptV 5 2.5 5 5 5
0.20 0.10 0.20 0.15
0.10
linearity of response ( r ) 0.999 0.999 0.999 0.999 0.999
For 4 L of air at STP.
(7.6), and Porapak (0.88). However, in all three cases the desorption recovery was poor (Le., less than 70%). To determine recovery, standards (500 pg) were injected into a glass vial (on-lineto the preconcentration trap), and the vial was then heated. The standards were swept onto one of the sample tubes containing Tenax GC after which 4 L of cylinder air was passed through the tube and the trapped organics were subsequently analyzed to determine recovery. Recovery of all compounds (including MeBr) was quantitative. For the case of MeBr, only 100 mL of helium was passed through the sample tube because of the low breakthrough volume. The relative standard deviation for 10 injections of standards is also included in Table I. For MeBr this figure refers to 10 analyses from the commercially prepared cylinder. Limit of detection and linearity of response (correlation coefficient) data are recorded in Table 11. The range of
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Environ. Sci. Technol.. Vol. 26. No. 4. 1992
standards used for the linearity of response was from 10 to lo00 pg. Detection limit at signal-to-noiseratio of 2 was determined by extrapolation from the analysis of 10 pg of standards. Based on a 4-L sample of air, the relative detection limits in pptv are also recorded. As can be seen from the data, the correlation coefficient was excellent in all cases. The measured bromocarbons in the four air samples taken at Tsukuba in April were bromochloromethane (0.90-1.40 ppt), dibromomethane (0.70-1.28 ppt), bromoform (0.79-1.71 ppt), and methyl bromide. Methyl bromide was not quantitated because of the low BTV for this compound and the volume of air sampled (4 L). Both bromodichloromethane and dibromochloromethane were not detected. Very similar concentration of bromocarbons were found in the seaside air samples (bromochloromethane, 0.52-0.64 ppt; dibromomethane, 0.90-1.31 ppt; and bromoform, 0.91-2.18 ppt). Blank tubes treated in a way identical to the sampling tubes, except that no air was sampled, showed no traces of the selected organics. Registry No. CH2BrCl, 74-97-5; CH2Br2,74-95-3; CHC12Br, 75-27-4; CHBr2C1, 124-48-1; CHBr3, 75-25-2.
Literature Cited (1) Rudolph, J.; Muller, K. P.; Koppmann, R. Anal. Chim. Acta .. 1990,236, 197. (2) Bottenheim, J. W.; Bmie, L. A.; Atlas, E.; Heidt, L. E.; Niki, H.; Rasmussen, R. A,; Shepson, P. B. J. Geophys. Res. 1990, 95, 18555. (3) Barrie, L. A.; Bottenheim, J. W.; Schell, R. C.; Crutzen, P. J.; Rasmussen, R. A. N a t u r e 1988, 334, 138. (4) Class, Th.; Kohnle, R.; Ballschmiter, K. Chemosphere 1986, 15, 429. (5) Ballschmiter, K.; Mayer, P.; Class, Th. Fresenius 2. Anal. Chem. 1986,323, 334. (6) Class, Th.; Ballschmiter, K. Fresenius Z. Anal. Chem. 1986, 325, 1. (7) Berg, W. W.; Heidt, L. E.; Pollock, W.; Sperry, P. D.; Cicerone, R. J. Geophys. Res. L e t t . 1984, 11, 429. (8) Cicerone, R. J.; Heidt, L. E.; Pollock, W. H. J. Geophys. Res. 1988, 93, 3745. (9) Yokouchi, Y.; Ambe, Y.; Maeda, T. Anal. Sei. 1986,2,571.
Received for review April 22,1991. Revised manuscript received August 20, 1991. Accepted December 2, 1991.
