2417
Anal. Chem. 1985, 57, 2417-2419 (3) Farnsworth. P. B. Eleventh Annual Meeting of the Federation of Analyticai Chemistry and Spectroscopy Societies, Philadelphia, PA, Sept 1984. (4) Farnsworth, P. B. Appl. Spectrosc., in press. (5) Oiesik, J. W., personal communication, 1985. (6) Greenfield, S.; Jones, 1.; Berry, C. Analyst (London) 1984, 89, 7 13-720. (7) Fassei, v. A,; Kniseiey, R. N. Anal. Chem. 1974, 46, 111 0 ~ - 1 1 2 0 ~ . (8) Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, 1155A-1164A. (9) Rezaaiyaan, R.; Hieftje, G. M.; Anderson, H.; Kaiser, H.; Meddlngs, B. Appl. Spectrosc. 1982, 36, 627-631. (10) Rezaaiyaan, R.; Hieftje, 0. M. Anal. Chem. 1985, 57, 412-415.
(1 1) Rezaaiyaan, R.; Oiesik, J. W.; Hieftje, G. M. Spectrochim. Acta, Pelf B 1985, 4 0 8 , 73-83. (12) Ensman, R. E.;Carr, J. W.; Hieftje, G. M. Appl. Spectrosc. 1983, 3 7 , 571-573.
RECEIVED for review April 22,1985. Accepted June 10,1985. Supported in part by the National Science Foundation through Grants CHE 82-14121 and CHE 83-20053 and by the Office of Naval Research.
Preconcentration of Atmospheric Organic Compounds by Heat Resorption and Solvent Microextraction Elliot L. Atlas* and Kevin F. Sullivan Department of Oceanography, Texas A&M University] College Station, Texas 77843
C. S. Giam Graduate School of Public Health, University of Pittsburgh, Pittsburgh] Pennsylvania 15260 To analyze organic compounds in atmospheric samples, it is often necessary to concentrate the campounds prior to analysis. Concentration techniques that have been used include cryogenic trapping, liquid impingers, and preconcentration on solid adsorbents. These techniques are well-established and have been reported widely in the literature (1-3). Among the different methods, the use af a solid adsorbent, such as charcoal (4),Tenax-GC (51, Florisil(6), polyurethane foam (a, and others (8),is perhaps the most used preconcentration technique for atmospheric organic compounds. Desorption of the sample for subsequent analysis of organic compounds will depend on the volatility of the analyte and the nature of the adsorbent. Two methods commonly used to remove the adsorbed compounds from the sorbent trap are solvent extraction and heat desorption directly into a gas chromatograph. Each technique has merits and drawbacks. To achieve high sensitivity, solvent extracts of sorbent traps require evaporative concentration prior to gas chromatographic analysis. This step will permit loss of volatile organic compounds. Procedures which do not include solvent concentration are not subject to volatilization losses, but since the analytes are diluted in solvent, only compounds present in high concentration can be measured. Thus, solvent extraction is most suitable for trace analysis of high molecular weight compounds such as PCBs, pesticides, and various petroleum hydrocarbons, or for analysis of high concentrations of organics in air. Direct heat desorption offers increased sensitivity for the analysis of more volatile organic compounds. However, because the entire sample is desorbed for analysis, there is no chance for replicate or multiple analyses of a single sample. Our objective was to develop a simple and sensitive method to screen atmospheric organics which would complement our normal high-volume sampling system. For example, Florisil adsorbent used in our high volume sampling system retains high molecular weight chlorinated hydrocarbons, alkanes less volatile than n-C15,and other moderately volatile compounds (9). We wished to extend the range of our analyses to include more volatile compounds such as chlorobenzenes, >C, alkanes, and naphthalenes in the ambient atmosphere. Furthermore, we did not wish to modify a gas chromatograph solely for analyses by heat desorption techniques. Still, we wanted higher sensitivity than that which could be obtained by using normal solvent desorption procedures, which dilute the ana0003-2700/85/0357-24 17$0 1.50/0
lytes in 1-2 mL of solvent. Finally, it was necessary to develop a method suitable for field use where full laboratory and gas chromatographic facilities would not be available. In this report we describe a novel application of heat desorption and solvent extraction techniques to preconcentrate organics in air into a small (10-15 pL) solvent volume without evaporative concentration. This technique allows multiple injections of a single sample using a conventional capillary inlet system, offers excellent sensitivity, and allows use of different sizes and geometries of sorbent tubes.
EXPERIMENTAL SECTION Apparatus and Materials. Sorbent tubes were prepared from amber-coated 5-mL Kimax pipets. The amber coating was applied to inhibit photooxidation of the Tenax-GC sorbent. After Soxhlet extraction with methanol and petroleum ether, Tenax-GC (60-80 mesh) was packed into the volumetric bulb of the pipet and was held in place by plugs of silanized glass wool. Approximately 0.6 g of Tenax was held in the trap, though traps of other sizes could be used in this system. Charcoal traps (1.5 mg) designed for use in a closed-loop stripping apparatus (IO) were purchased from Tekmar, Inc. (Cincinnati, OH), and were used without modification. A tube oven was constructed from 6 in. X 11J4in. cylindrical ceramic heaters (Thermcraft, Inc., Winston-Salem, NC). The heaters were controlled by a variable voltage controller. An aluminum block with a central hole of sufficient diameter for the sorbent tube (0.475 in.) was placed inside the tube oven. Temperature was monitored with a thermocouple inserted in the block. The apparatus is illustrated in Figure 1. (The tube oven was designed around low-cost materials immediately available in our laboratory; a more sophisticated oven with automatic temperature control would be preferred, but the present design worked well and was adequate for our purposes.) All solvent&were high-purity Distilled-in-Glass(Burdick and Jackson, Inc.; Muskegon, MI) or equivalent. Procedures. Prior to sampling, Tenax tubes are conditioned for a minimum of 8 h at 320 "C under nitrogen flow. The charcoal trap is precleaned immediately prior to use by flushing with several milliliters of solvent and drying with a stream of purified purge gas. Samples are obtained by drawing air through the Tenax-filled tube with a metal bellows or oilless vacuum pump at flow rates of 0.9-1.0 L/min. After the sample is taken, the inlet end of the sample tube is connected to a precleaned charcoal trap with a Cajun Ultra-torr adapter. The Tenax trap is then inserted through the bottom of the oven and connected to purge gas with another Ultra-torr fitting. In this configuration compounds sorbed on 0 1985 American Chemical Society
2418
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985 r--
~
5
0
20
15
10
Retention Time (min)
5
Figure 2. Flame ionization chromatogram of I-m3 air sample on 10-m caplllary column [30 O C (4 min), 10 OC/min to 200 OC]: (a)top Tenax trap; (b) bottom Tenax trap; 0 , n-alkanes; N, naphthalene.
Flgure 1. Heat desorption apparatus: (1) purge gas inlet; '/4-in. Cajun Ultra-Torr adapter; (2) aluminum Insert; (3) ceramic tube oven; (4) Tenax sample tube; (5) '/4-in. Cajun Ultra-Torr adapter; (6) 1.5-mg charcoal trap.
Tenax are backflushed into the charcoal trap. Nitrogen or helium purified by a molecular sieve/charcoal trap purges the Tenax for 20 min at 35-40 mL/min. The tube oven is held at 220 "C. To avoid cold spots in the system, the bottom metal connector is placed in thermal contact with the aluminum insert. This configuration produces a strong thermal gradient below the metal fitting and prevents adsorption of compounds prior to the charcoal trap; the charcoal trap itself is at room temperature. After purging is complete, the charcoal trap is removed from the system and is spiked with 1 WL of internal standard. The charcoal is then rinsed three times with a total of 12-15 r L of solvent. Walls of the trap also should be rinsed to ensure complete transfer of the desorbed compounds. Each rinse is carefully withdrawn from the trap by syringe and transferred to a capillary tube. Care must be taken in withdrawing the solvent as the trap is fragile and screens retaining the charcoal can be dislodged with the needle tip. If long-term storage is necessary, the capillary can be heat sealed with a microtorch while the solvent section of the tube is cooled.
RESULTS AND DISCUSSION Tests were performed to evaluate the collection efficiency of Tenax and charcoal traps and to determine the recovery of compounds from the system. We found, in agreement with others (5, II), that Tenax has excellent trapping characteristics for C9-C19 hydrocarbon compounds. Even a t the maximum volume tested (1000 L), we found >89% retention efficiency for compounds less volatile than n-decane (Figure 2). Retention efficiency of more volatile compounds improved with smaller air sample volume. The charcoal traps used to reconcentrate the compounds desorbed from Tenax also worked well. Analysis of a second charcoal trap placed in series downstream of the primary trap was indistinguishable from a blank trap. Recovery was tested by volatilizing 10 ng of individual Cll-C32 hydrocarbons into the Tenax trap and by subsequent desorption using normal procedures. Average recovery of hydrocarbons from CI1to CI9was 92 %. Recovery levels decreased past CI9. Of the solvents used to desorb compounds from charcoal, carbon disulfide and methylene chloride worked best; pentane was least efficient. Benzene also worked well, and it was used for desorbing traps when
0
5
10 15 Retention T h e (min)
20
25
Flgure 3. Electron capture chromatogram of 1-m3 air sample on 25-m capillary column [35 O C (5 min), 8 OC/min -k 200 'C]: (a)top Tenax trap: (b) bottom Tenax trap. an electron-capture detector was used in the analysis (Figure 3). Analytical sensitivity using the combined heat/solvent desorption system is excellent. Because the entire sample is in a small volume of solvent, the sensitivity of the analysis is -50 times better than normal solvent desorption techniques, and it is only a factor of 5-6 less sensitive than heat desorption alone. This is because nearly 20% of the sample can be injected for analysis. For a 1-h air sample (50 L), a typical limit of detection for hydrocarbons is < l o ng/m3. Improved sensitivity can be obtained by using larger sample volumes or injecting a larger volume of this sample. In addition to good sensitivity, the combined heat/solvent desorption system allows three to five replicate injections of the same sample. Thus, different detectors or columns of different polarity can be applied to a single sample or, after analysis, the sample solution may be sealed in its capillary tube and frozen for archival or long-term storage. Problems encountered in this system were associated with artifact formation in the Tenax resin during sampling. Excellent blanks could be obtained with unused Tenax tubes; however, after sampling, artifacts became apparent. In addition to oxidation products normally reported (benzaldehyde, acetophenone, benzoic acid, etc.), we also found high molecular weight material desorbed from the Tenax after long sample times. This high molecular weight material has not yet been characterized.
2419
Anal. Chem. 1985, 57, 2419-2421
Overall, the system for sampling and desorption proved effective and simple t o use. The system described here is a new combination and application of proven analytical technologies. Thus, the basic advantages and limitations of the system are those inherent in the individual trapping materials and desorption techniques. For the materials used in this system (Tenax and charcoal), the range of applicability has been adequately studied ( 4 , 5 , 1 0 , I I ) . With these materials, the combined technique should be applicable to a wide range of organic compounds in the atmosphere. Improvements in the system may come with development of improved sampling resins and sorbent materials. Registry No. C, 7440-44-0;Tenax-GC, 24938-68-9;naphthalene, 91-20-3.
LITERATURE CITED ( 1 ) McClenny, W. A.; Plell, J. D.; HoMren, M. W.; Smith, R. N. Anal. Chem. 1984, 56, 2947. (2) Fox, D. L.; Jeffrles, H. E. Anal. Chem. 1983, 55, 233R. (3) Melcher, R. G. Anal. Chem. 1983. 55, 40R.
National Institute for Occupational Safety and Health “NIOSH Manual Analytical Methods”,2nd ed.; U.S.Department of Health, Education and Welfare: Cinclnnati. OH, 1977; Vol. 1-5. (5) Krost, K. J.; Pelllzzari, E. D.; Walburn, S. G.; Hubbard, S. A. Anal. (4)
of
Chem. 1982, 5 4 , 810. (6) Giam, C. S.;Chan, H. S.;Neff, G. S. Anal. Chem. 1975, 47, 2319. (7) Billings, W. N.; Bldleman, T. F. Atmos. Envlron. 1983, 77, 383. (8) Doskey, P. V.; Andren, A. W. Anal. Chim. Acta 1979, 110, 129. (9) Chang, L. W.; Atlas, E.; Giam, C. S. J . I n t . Envlron. Anal. Chem. 1985, 19, 145. (IO) Grob, K. J . Chromatogr. 1973, 84, 255. (11) Brown, R. H.; Purnell, C. J. J . Chromatogr. 1979, 778, 79.
RECEIVED for review March 29,1985.
Accepted June 5,1985.
Capillary Tube Sealing for Microcell Nuclear Magnetic Resonance Spectrometry Ben V. Burger* a n d Hendrik S. C. Spies Department of Chemistry, University of Stellenbosch, Stellenbosch 7600, South Africa Microcell NMR is used as a routine analytical technique in our research on insect and mammalian chemical communication ( 1 , 2 ) . Although the capillary tube sealer developed by Skrabalak and Henion (3)is considered to be a handy piece of equipment in the NMR laboratory, we have found the sealing of micro NMR samples under a slight vacuum to result in a higher success rate, especially if such samples are to be used for temperature studies, if very long data acquisition periods are required, or if highly volatile solvents are employed. The application of vacuum during the sealing process eliminates the formation of a fine capillary which is normally formed when a glass tube is drawn out. For this purpose we have been using the extremely simple sealing aid described herein for more than 7 years without a single failure.
EXPERIMENTAL SECTION Apparatus. The device which is used for the sealing of sample tubes is shown in Figure 1. It consists of a gastight 500-pL syringe (B) and a connecting capillary tube holder (C) made of Teflon (15 X 4 X 4 mm). This connection is provided with a hole in one end to fit the tip of the syringe needle as tightly as possible and a connecting hole in the opposite end, the size of which is selected so as to give a vacuum-tight connection with, for example, a 1.7 mm diameter precision micro NMR sample tube (A). Procedure. The open end of the sample tube is fire-polished to facilitate proper insertion into the holder and to avoid damaging the Teflon surface. The sample tube is then purged with dried (molecular sieve) and filtered ultra-high-puritynitrogen or treated with D20 to remove adsorbed moisture ( 4 ) , if required. The sample is introduced into the tube by rapidly ejecting 5-8 pL of a concentrated solution of the material from a 10-pL syringe into the capillary. This procedure is repeated with more of the sample or with solvent until the required sample size (for example, 15 pL,corresponding to ca. a 15 mm column in a 1.7 mm sample tube) has been reached. The upper half of the sample tube is then carefully purged for 10-15 s with a slow stream of ultra-high-purity nitrogen from a short piece of uncoated fused silica tubing or a drawn out melting point capillary, so as to remove residual solvent and solvent vapor from this part of the sample tube, care being taken not to disturb the surface of the sample itself. The sample tube is then immediately connected to the 500-rL syringe by means of the Teflon connector, the plunger drawn back to the 100 pL mark, and the sample tube, held at an angle of 45”, cooled yith solid COP During the cooling process the plunger is gradually drawn back to the 250 pL mark. Degassing of the sample takes place. The rising air bubbles are an indication of a proper vacuum-tight connection between syringe, Teflon connector, and
sample tube. As soon as the sample is frozen solid, the plunger is drawn back to the 400 p L mark, the syringe is rotated with the right hand, and the sample tube is sealed and drawn out in the side of the flame from a small Bunsen burner, care being taken to grasp the sample tube well above the surface of the frozen sample (Figure 1). If necessary, the sealed tip of the sample tube can be trimmed or straightened in the flame while the sample is still frozen.
RESULTS AND DISCUSSION Two problems are often encountered with the conventional sealing of NMR capillary sample tubes with a microburner or with the sealing apparatus of Skrabalak and Henion (3). The first is the formation on the inside capillary wall of a black carbon deposit by pyrolysis of the solvent, preventing the glass from forming a gastight seal. Furthermore, DC1 produced by the pyrolysis of a halogen-containing solvent, such as CDC13, could lead to the catalytic decomposition or rearrangement of the sample. Even with solvents, such as ethanol-c16which do not readily produce such a carbon deposit, pinholes are often formed, and at a probe temperature of, for example, 34 “C aJl the solvent can be lost during overnight data acquisition. We have found that these problems are eliminated by the procedure described herein. Care is taken to avoid getting the sample on the capillary wall at the point where it is to be sealed. If a sample is ejected slowly into an empty capillary, it will invariably form a plug and the vapor pressure of the solvent will push this plug upward. The plug could even be pushed out of the sample tube, and although this can be avoided by quickly shaking the plug down into the bottom of the tube, some material may be left on the upper parts of the tube wall. When a sample is ejected slowly, a film of liquid will also be trapped between the needle and the glass of the tube by capillary forces. When the needle is afterward withdrawn, some material will adhere to the needle and will be lost. The rest of the trapped liquid will be deposited at the upper end of the tube, forming a plug which, in most cases, will be pushed out of the tube before it can be shaken down. If, however, the solution is “shot” into the sample tube, the formation of a plug and the trapping of solvent between the needle and the glass surface are totally eliminated, and if care is taken to ensure that the needle of the syringe is clean and dry before it is introduced into the sample tube, no solvent or sample should come into contact with the glass at the point where it is to be sealed. Should
0003-2700/85/0357-2419$01.50/00 1985 American Chemical Society
