Collection and Analysis of Trace Organic Vapor Pollutants in Ambient Atmospheres Thermal Desorption of Organic Vapors from Sorbent Media Edo D. Pellizzari,* Ben H. Carpenter, and John
E. Bunch
Chemistry and Life Sciences Division, and Environmental Studies Division, Research Triangle Institute, Research Triangle Park, N.C. 27709
Eugene Sawicki Chemistry and Physics Laboratory, National Environmental Research Center, Environmental Protection Agency, Research Triangle Park, N.C. 2771 1
w A thermal desorption gas-liquid chromatograph interface is described for recovering hazardous substances concentrated on sorbents in glass cartridges. The interface consists of a desorption chamber, a six-port two-position high-temperature low-volume valve, a Ni capillary trap, and a temperature controller. The temperature rise time was determined in the center of cartridges for several sorbents. The heating rates were: PCB and BPL activated carbons > oxopropionitrile and Carbowax 400 chemically bonded to Poracil C > Chromosorb 104 > Tenax GC > Chromosorb 101. The heating rate was linear for all sorbents up to 65% of the set desorption chamber temperature (60-90 sec) and required several minutes thereafter to reach the final temperature. The percent recovery of several hazardous vapors adsorbed on Tenax GC using thermal desorption was 290% at the 50 and 100 ng level. The previous paper ( 2 ) alluded to some of the problems associated with analysis of polluted air. Because of the limited sensitivity of currently available detectors, hazardous substances need to be concentrated from highly dilute samples. A step toward the solution of this problem is achieved when cartridges containing an appropriate sorbent is used and large volumes of air are forced or drawn through the sampling device whereupon the pollutants are trapped. The collection efficiency for a selected number of sorbents have been previously reported ( 1 ) . Even when employing the cartridge technique, only trace quantities of hazardous pollutants are accumulated; thus it is imperative that the entire sample be submitted for analysis. Recovery of trapped vapors has been accomplished using thermal (2-21) and vacuum (12-15) desorption that allows for direct introduction of the total sample into an analytical system (GLC, GLC-MS) in the absence of a sorbent as a carrier. These methods are subject to artifactual processes such as pyrolysis, polymerization, isomerization ( I @ , or incomplete recovery. Steam desorption (15, 26) and solvent extraction (17-21) of the sorbent alleviate the above-mentioned problems; however, volatile pollutants cannot be quantitatively concentrated from dilute solutions and since gas chromatographic (GC) analysis is limited to small aliquots of liquid samples, only a fraction of the sample can be examined. As a result, the sensitivity of the overall method is greatly reduced. Because we anticipate the levels of hazardous substances (e.g., epoxides, nitrosamines, sulfonates, sulfites, sultones, aldehydes, ketones, @-lactones, chloroalkyl ethers, and nitro compounds) in ambient air to be very low (possibly ng/m3), we selected thermal desorption as a means of transferring the entire amount of trapped vapors 556
Environmental Science & Technology
to the analytical system. This paper describes the design of an inlet manifold for effecting desorption and efficient transfer of pollutants to a GLC, and the extent of recovery of adsorbed vapors from Tenax GC.
Experimental Tenax-GC (60-80 mesh), Chromosorb 101 (100-120), Chromosorb 104 '( 100-120), and Chromosorb W-HP (100-120) were purchased from Applied Science, State College, Pa. Stationary phases chemically bonded to supports which included Carbowax 400/Poracil C (100/120) and oxopropionitrile/Poracil C (80/100) were also obtained from Applied Science. Carbowax 600 and didecyl phthalate stationary phases and the sorbent Porapak Q were from Supelco, Inc., Bellefonte, Pa. Carbon derived from coke (PCB, 12/30) was acquired from Pittsburgh Activated Carbon Division of Calgon Corp., Pittsburgh, Pa. A cocoanut-activated carbon (580-26) was purchased from Barneby-Cheney, Columbus, Ohio. All sorbents were thermally conditioned 10°C below the maximum recommended temperature limit for a t least 12 hr under approximately 20 ml/min of He flow. After sorbents were packed into glass cartridges they were conditioned again for 15 min in the thermal desorption unit prior to use. The standards-ethyl methanesulfonate, @-propiolactone, N-nitrosodiethylamine, 1,2-dichloroethyl ethyl ether, nitromethane, methyl ethyl ketone, and aniline-were from Fisher Chemicals, Pittsburgh, Pa. The source of glycidaldehyde and sulfolane was Aldrich Chemicals, Milwaukee, Wis. The supply of 1,3-propanesultone, maleic anhydride, butadiene diepoxide, and propylene oxide was from Eastman Organic Chemicals, Rochester, N.Y. Styrene epoxide, bis-(chloromethyl)ether, and bis-(2-chloroethy1)ether were acquired from K&K Labs., Plainview, N.Y. The thermal desorption manifold used in this study is depicted in Figure 1. The manifold system consists of basically four main components; a desorption chamber, a six-port two-position high-temperature low-volume valve (Valco Inc., Houston, Tex.), a Ni capillary trap, and a temperature controller. One of the two brass thermal desorption chambers constructed is shown in Figure 2. The tube had an overall length of 13.3 cm and accommodated a Pyrex-sampling cartridge of the dimensions 10.54 mm i.d. x 13.0 mm 0.d. x 10.5 cm in length. The second desorption chamber was of the same external dimensions; however, it accommodated a 1.56 cm i.d. x 1.65 cm 0.d. x 10.5 cm tube. The brass thermal desorption chambers were encased in an aluminum sandwich which served as a heat sink. Two 15-W, 115-V heating cartridges (Varian
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Figure 1. Thermal desorption/high resolution
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Desorption chamber and glass sampling cartridge
Part No. 22-0000-18-00) were used to heat the aluminum sandwich, and the temperature was monitored and controlled with a platinum sensor probe (loon,Varian Part NO.6 4 - ~ 0 0 o o S ) . The desorbed vapors passed via short insulated capillary line (gold plated) through a six-port position valve which was also encased in an aluminum heating bath. Temperature control was identical to the thermal desorption chamber. A nickel capillary (0.020 i.d. x 0.032 x 0.5 m in length) which was gold plated (Atomex Immersion Gold, Engelhard, Ind., Newark, N.J.) constituted one loop of the valve proper and it was cooled with liquid Nz or solid carbon dioxide-isopropanol and served as a trap for collecting and concentrating desorbed vapors for the introduction into high resolution GLC columns. The vapors were released from the capillary trap by rapidly heating to 175°C.
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Figure 2.
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interface manifold for gas chromatography In a typical thermal desorption cycle, a sampling cartridge was placed in the preheated (ca. 225°C) chamber, and He gas was passed through the cartridge (ca. 20 ml/min) to purge the vapors into the liquid No-cooled gold-plated Ni capillary trap; this constituted valve position A (Figure 1). After the thermal desorption step was complete, the six-port valve was rotated to position B (Figure 1) and the temperature on the capillary loop was rapidly raised ( > lO"/min), whereupon the carrier gas carried the vapors onto a GLC column. The temperature rise time in the sorbent bed of the cartridge was measured with a calibrated thermocouple (mV vs. " C ) whose output was directly recorded on a strip chart recorder (Varian Model A-25, Varian Instruments, Walnut Creek, Calif). The rate of temperature increase in the sorbent was determined using preset and isothermal desorption chamber temperatures. The rate of temperature increase was also monitored on the inner glass wall of the cartridge. To demonstrate the efficiency of collection plus thermal desorption of trapped vapors, synthetic air-vapor mixtures were prepared using N-nitrosodiethylamine (100 ng) as an internal standard; the quantity of the other vapors tested was varied from 50-300 ng. An aliquot of this mixture was introduced directly through the thermal desorption chamber, which contained a glass cartridge packed with only glass wool, and the mixture was resolved by GLC (Figure 3). The peak areas for each solute in this calibration mixture was measured by triangulation, and the relative response ratios were calculated:
where A , = area of solute peak and A , T = area of N-nitrosamine. An identical aliquot of the synthetic air-vapor mixture was purged (4.0 l/min) through a cartridge containing a sorbent using the system described in a previous report ( 1 ) . The trapped vapors were desorbed in the thermal desorption chamber followed by GLC analysis (Figure Volume 9, Number 6, June 1975 557
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8
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7
3
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Figure 3. Gas-liquid chromatography of synthetic air/vapor mix-
ture of hazardous substances Peaks A , B, C, D, and E are 300 ng of glycidaldehyde, butadiene diepoxide. N-nitrosodiethyl amine, 1,2-dichloroethyl ethyi ether, and ethyl methanesulfonate, respectively. See text for GLC parameters
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Figure 4. Gas-liquid chromatogram of vapors desorbed from
Tenax G C Thermal desorption temperature at 225°C. see prior figure for peak identification The dashed profile represents the background from Tenax GC
4). The RR, for each constituent was calculated, and the percent recovery was determined as a ratio of the RR, values to those for RRcm. R R 5 ~ r mx
RR,,
100% = percent recovery
Gas-liquid chromatography (GLC) was conducted on a Perkin-Elmer 900 series chromatograph (Perkin Elmer Corp., Nonvalk, Conn.) equipped with dual flame ionization detectors. A 2.5 m m i.d. X 3.6 m silanized glass column containing 2% DEGS on Chromosorb W(HP) 80-100 mesh was used for resolving synthetic air-vapor mixtures. The column was programmed from 55-200°C a t lO"/min with a n initial and final isothermal period of 2 and 10 min, respectively. Carrier gas (Nz), hydrogen, and air flow rates were 45, 30, and 250 ml/min, respectively. The injection port manifold and detector temperatures were maintained a t 250°C.
Results and Discussion Prior to defining the parameters (temperature, heating time) for effecting thermal desorption of vapors trapped on sorbents, the rate of heat transfer from the heat source to the sorbent bed was measured using the desorption unitGLC interface depicted in Figure 1. Operating the desorption unit under isothermal conditions, the temperature increase in the sorbent bed was monitored immediately after inserting a cartridge. Figure 5 depicts the thermocouple response time and the heating rate in the center of a cartridge containing Tenax GC. Because the response of the thermocouple significantly contributed to the temperature rise profile (8 sec required to reach 66% of the upper temperature limit), its contribution was subtracted from all measurements. These temperature rise times (Figure 5) indicate a crosssectional gradient was produced immediately after inserting the cartridge into the chamber. The periphery of the 558
Environmental Science & Technology
I
2
3
I
4
5
6
TIME (MINI
Figure 5. Differential heating rate in a cylindrical tenax bed Curve A represents the measured thermocouple response to the thermal desorption chamber set at 175°C. Curve B is the heating rate on the periphery of the Tenax bed; curve C is the temperature increase in the center for the sorbent bed. The cartridges were (1.0 m m wall thickness) Pyrex glass tubes which contained a sorbent bed of 1.0 c m i.d. X 3.0 c m in length
sorbent bed reached the temperature maximum in approximately 4 min, while the center of the sorbent bed required a n additional 2 min. Furthermore we also observed that upon increasing the cartridge diameter from 1.0 to 1.5 cm, the temperature differential increased by a factor of 1.3. The heat transfer coefficient for sorbents and similarly their observed differential temperature gradient can vary
considerably. The relative temperature rise times for several sorbents, which previously were shown to have good collection efficiencies ( I ) , were compared. The order of their heating rates were observed to be: PCB and BPL carbon (12 x 30) > oxopropionitrile and Carbowax 400 chemically bonded to Poracil C (100/120) > Chromosorb 104 (100/120) > Tenax GC (60/80) and Chromosorb 101
2Mr
3 4 TIME ( M I N . )
2
I
5
7
6
Figure 6. Comparison of heating rates for some sorbents Curves A , B , C correspond to PCB carbon (12/30). oxopropionitrile, and Tenax GC. respectively, with t h e thermal desorption unit at 210°C
/0 ’
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I
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TIME (MINI
Figure 7. Temperature rise time for Tenax G C at different thermal desorption chamber temperatures Curves A , 6.C, were obtained in t h e center of t h e bed at preset c h a m b e r temperatures of 180, 205, and 228”C, respectively.
(100/120). The largest differences in heating rates (as measured in the center of the sorbent bed), exemplified by the activated carbons and Tenax GC, are shown in Figure 6. The temperature rise time for Tenax GC was also examined using three isothermal conditions on the desorption unit (Figure 7 ) . The heating rate (slope) was only slightly increased by increasing the desorption unit temperature; however, it was evident that the rate was linear up to 6590 of the final temperature or during the first 7 5 sec. Thereafter, an additional several minutes was required to reach a plateau. These data, therefore, indicate that the desorption unit should be set a t a temperature which allows the attainment of the required desorption temperature in 60-90 sec after insertion of the cartridge sampler. By use of a very short preheat period, the desorbed vapors can be introduced directly onto a conventionally packed GLC column since the carrier gas flow rate is satisfactory for purging the desorption chamber. The desorption chamber can be “in-line” with the GLC column long enough to desorb the substituents including those with the greatest adsorption affinities and then returned to the “by-pass” mode during the remainder of the chromatographic period. Under these conditions, the background introduced during heating of the sorbent, as well as the artifactural processes resulting from decomposition and polymerization of solute vapors, can be minimized. For introduction of desorbed vapors onto high-resolution capillary columns, the vapors were concentrated in a small carrier gas volume to prevent excessive band spreading and a decreased column efficiency. This was achieved by retrapping desorbed vapors in a Ni capillary (0.020 in i.d. x 0.5 m length) using liquid N2 as the coolant. After the desorption period, the carrier gas was routed through the capillary trap (Figure 1) and the trap rapidly heated. Sorbents which exhibited good collection efficiencies ( I ) were evaluated for background contribution and their ability to release trapped vapors during thermal desorption, using the maximum recommended temperature limit for each. Of the sorbents tested, the lowest background was observed for the activated carbons, Tenax GC and Chromosorb 101. Tenax GC was superior to Chromosorb 101. The worst offender was Porapak Q . Prior treatment of the polymeric beads by using solvent extraction (acetone, methanol, or benzene for 18 hr in a Soxhlet apparatus) and thermal conditioning was only effective in reducing the background for Tenax GC and Chromosorb 101. Because the desorption experiments indicated that the background contribution from Tenax GC was least of the sorbents tested, and it exhibited excellent collection efficiencies for selected hazardous substances ( I ) , we examined the thermal recovery of vapors adsorbed to this polymer. Ten compounds, which represent a broad spectrum of chemical properties and are of particular interest in air pollution studies, were chosen. Each substance was introduced through a cartridge of Tenax GC (1.0 cm i.d. x 3.0 cm in length) a t levels of 50, 100, 200, and 300 ng as a synthetic air-vapor mixture. The GLC analysis of an aliquot of the mixture and the vapors desorbed from Tenax GC were compared (Figures 3 and 4 ) . The percent recoveries are given in Table I. Except for nitromethane, all of the substances examined were quantitatively desorbed. It was also apparent from the chromatographic analysis that decomposition, polymerization, or incomplete desorption did not occur. Chromosorb 101gave similar results. In contrast to these results, desorption of these substances from activated carbons could not be achieved using temperatures up to 320°C.
Volume 9, Number 6, June 1975
559
Table I. Percent Recovery of Vapors Adsorbed on Tenax GC Cartridges Using Thermal Desorption. Quantity adsorbed, ngb
50
Compound
N.nitrosodiethylaminec p-Propiolactone Ethyl rnethanesulfonate Nitromethane Glycidaldehyde Butadiene diepoxide Styrene epoxide Aniline Bis.(chloromethyl)ether
100 105 105 100 100 100 95 100 95 i.d. x 30
Bis-(2-chloroethy1)ether
100
100 100 100
-
100 100 100 95 100 90
200
300
-
-
100 95 70 95 100 105 95 100 90
100 100 70 80 100 90 60 90
-
Tenax G C cartridge, 10.5 m m m m in length. Synthetic airvapor mixtures were introduced onto a Tenax GC bed a t 4 Ifmin. Desorption U n i t was a t 225°C. Represents theoretical a m o u n t in synthetic air-vapor mixture. CValues an average of duplicate r u n s were calculated on basis of a ratio of pbak areas for calibration mixtu;e and f r o m thermal desorption.
*
Acknowledgments The authors are grateful to L. Retzlaff for his assistance in the machining and construction of experimental devices used in this study, R. L. Marquard and C. Cleary for the design and assembly of the temperature controller, and M . E. Wall for his interest in this program. Literature Cited (1) Pellizzari, E . D., Carpenter, B., Bunch, J., Sawicki, E., Enuiron. sei. Tech., 9,552 (1975). (2) Williams. I. H.. Anal. Chem. 37. 1724 (1965). (3) Leggett, D . C., Murrmann, RT P., Jenkins, T . J., Barriera, R., CRREL, SR176, l(1972).
(4) Williams, F . W., Umstead, M. E., Anal. Chem., 40, 2232 (1969). ( 5 ) Krumperman, P. H., J . Agr. Food Chem., 20,909 (1972). (6) Hollis, 0. L., Anal. Chem., 38, 309 (1966). (7) Zlatkis, A,, Bertsch, W., Lichenstenstein, H . A., Tishbee, A., Shumbo, F., Liebich, H. M., Coscia, A. M., Fleischer, N., ibid., 45,763 (1973). (8) Zlatkis, A,, Lichenstein, H . A,, Tishbee, A , , Chromatographia, 6,67 (1973). (9) Bertsch. W.. Chane. R. C.. Zlatkis. A.. J . Chromatoe. Sei.. (1974). (11) Saalfeld, F. E., Williams, F. W., Saunders, R. A., American Lab., 3 , 8 (1971). (12) Saunders, R . A,, Umstead, M . E . Smith, W. D., Gammon, R. H.. “The atmosDheric trace contaminant Dattern of SEALABII,” ‘Proc. 3rd Ahn. Conf. Atmos. Contahination Confined Spaces, AMRL-TR-67-200, (1967). (13) Duel. C. L.. et al.. “Collection and Measurement of Atmospheric Trace’ Contaminants,” Aerojet Electrosystems Co., Azusa, Calif., Final Report, Contract NAS 1-9814, NASA Doc. No. 71-19636. (14) Saunders, R. A,, “Analysis of Spacecraft Atmospheres,” NRL Rept ,5316 (1962). (15) Turk, A,. Morrow, J . I.. Stoldt. S. H., Baecht, W., J . Air Pollut. Contr. Assoc., 16, 383 (1966). (16) Turk, A,, Morrow, J . I., Kaplan, B. E . , Anal. Chem., 34, 561 (1962). (17) Chiantella, A. J., Smith, W. D., Umstead, M . E., Johnson, J . E., “Aromatic Hydrocarbons in Nuclear Submarine Atmospheres,” A m . Ind. Hyg. Assoc. J., 27, 196 (1966). (18) Saunders. R. A.. “AtmosDheric Contamination in SEA-LAB I,” Proc. Conf. Atmos. Contamination Confined Spaces, AMRL-TR-65-230 (1965). (19) Grob. K.. Grob. G.. J . Chromatop.. 62. l(1971). (20) Jennings, G., Nursten, H . E., A&/. Chem., 39,521 (1967). (21) Herbolsheimer, R., Funk, L., Drasche, H., Staub-Reinhalt. Luft., 32,31 (1972).
Received for review Juls 11, 1974. Accepted Dec 2, 1974. Work supported by EPA Contract Rio. 68-02-1228 from the Enuironmental Protection Agency, Health, Education, and Welfare.
Aerosol-Size Spectra by Means of Nuclepore Filters Qctavio T. Melo and Colin R. Phillips* Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto M5S 1A4, Canada
The size and chemical composition of aerosols are of great importance in air pollution, industtial hygiene, atmospheric physics, and other fields. The health hazard posed by a particulate pollutant depends on the size and chemical composition of the particles involved while various atmospheric phenomena, such as condensation and photochemical smog production, involve or lead to particles of various sizes and chemical composition. While sampling and size distribution determination of aerosols above 1 pm are straightforward tasks ( I ) , the measurement of the size spectrum of submicron aerosols, as well as the chemical characterization of aerosols of all sizes, is still a very active area of research. For a long time the electron microscope was the only means of obtaining information in the submicron range through direct counting. More recent developments include diffusion batteries (21, electronic particle counters ( 3 ) , and the scintillation spectrometer ( 4 ) that have made possible the size characterization of aerosols down to about 0.05 pm. Recently Spurny et al. ( 5 ) noticed the possibility of using membrane filters to build an inexpensive and fully portable aerosol sampler. Nuclepore filters were used 560
Environmental Science 8 Technology
rather than the more traditional membrane filters for reasons that include the reduced thickness of the Nuclepore filters (about 10 micrometers) and the uniformity of shape (circular), size (geometric standard deviation 51.10), and orientation (normal to the surface) of the pores. These structural properties ensure the existence of pronounced minima in the efficiency characteristics, and this allows the use of these filters in the size classification of aerosols. The work of these authors has been expanded upon to deduce a size spectrum from filter loadings (6). The present study is an experimental application of that theory both to aerosols consisting of a single substance and to aerosols consisting of two chemical substances. Experimental Aerosol Generation. Test aerosols were generated from dye solutions with a Collison atomizer built according to the specifications of the British Standards Institution, 1955. The coarse spray from the atomizer was removed with an impactor similar to that described by Whitby et al. (7). A separation of 2.54 mm was used between the ori-
