Anal. Chem. 2002, 74, 1256-1268
Field Measurement of Acid Gases and Soluble Anions in Atmospheric Particulate Matter Using a Parallel Plate Wet Denuder and an Alternating Filter-Based Automated Analysis System C. Bradley Boring,† Rida Al-Horr, Zhang Genfa, and Purnendu K. Dasgupta*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Michael W. Martin and William F. Smith
Analytical Technology Division, Kodak Research Laboratories, Rochester, New York 14650-2140
We present a new fully automated instrument for the measurement of acid gases and soluble anionic constituents of atmospheric particulate matter. The instrument operates in two independent parallel channels. In one channel, a wet denuder collects soluble acid gases; these are analyzed by anion chromatography (IC). In a second channel, a cyclone removes large particles and the aerosol stream is then processed by another wet denuder to remove potentially interfering gases. The particles are then collected by one of two glass fiber filters which are alternately sampled, washed, and dried. The washings are preconcentrated and analyzed by IC. Detection limits of low to subnanogram per cubic meter concentrations of most gaseous and particulate constituents can be readily attained. The instrument has been extensively field-tested; some field data are presented. Results of attempts to decipher the total anionic constitution of urban ambient aerosol by IC-MS analysis are also presented. Many instruments exist for the rapid automated determination of gaseous constituents of ambient air. This includes, for example, all the gaseous criteria pollutants. Diffusion-based collection and analysis of atmospheric gases has been reviewed.1 In regard to suspended particulate matter, physical parameters such as optical or aerodynamic size distribution and mass concentration can be relatively readily determined by a number of available commercial instruments. This is not the case for the (near)-real-time determination of chemical composition of the atmospheric aerosol. The quest for instrumentation that can accomplish this objective began some three decades ago and continues today. Crider2 first demonstrated real-time determination of aerosol sulfur with a flame photometric detector (FPD) by switching a SO2-removing filter in and out of line. In many early methods, * Corresponding author: (e-mail)
[email protected]. † Present address: Alcon Laboratories, 6201 S. Fwy., Ft. Worth, TX 76134. (1) Dasgupta, P. K. ACS Adv.Chem. Ser. 1993, No. 232, 41-90. Dasgupta, P. K. In Sampling and Sample Preparation Techniques for Field and Laboratory; Pawliszyn, J., Ed.; Elsevier: New York, in press. (2) Crider, W. L. Anal. Chem. 1965, 37, 1770-1773.
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potentially interfering gases were first removed and the aerosol stream was then thermally decomposed, under controlled temperature conditions, to characteristic gases that were collected by a diffusion denuder and then measured periodically. Much of the effort was directed to the specific measurement of sulfuric acid and the various ammonium sulfates.3 Similar methods were also developed for ammonium nitrate.4 One ingenious method for measuring aerosol acidity involved gas-phase titration of the aerosol with ammonia.5 The flash volatilization (FV) technique of rapid thermal decomposition of a collected analyte6 became widely used for the measurement of aerosol sulfate in conjunction with a FPD.7 Although determination of nitrates by thermal decomposition was originally considered questionable,8 FV NOx detectionbased measurement of nitrate has been shown not only to be viable,9 recent innovations and adaptations by Stolzenburg and (3) Huntzicker, J. J.; Hoffman, R. S.; Cary, R. A. Atmos. Environ. 1978, 12, 83-88. Coburn, J.; Husar, R. B.; Husar, J. D. Atmos. Environ. 1978, 12, 89-98. Tanner, R. L.; D’Ottavio, T.; Garber, R.; Newman, L. Atmos. Environ. 1980, 14, 121-127’ D’Ottavio, T.; Garber, R. L.; Tanner, R. L.; Newman, L. Atmos. Environ. 1981, 15, 197-203. Slanina, J.; Lamoen-Dorrnenbal, L. V.; Lingera, W. A.; Meilof, W.; Klockow, D.; Niessner, R. Int. J. Environ. Anal. Chem. 1981, 9, 59-70. Garber, R. W.; Daum, P. H.; Doering, R. F.; D’Ottavio, T.; Tanner, R. L. Atmos. Environ. 1983, 17, 1381-1385. Slanina, J.; Schoonebeek, C. A. M.; Klockow, D.; Niessner, R. Anal. Chem. 1985, 57, 1955-1960. Lindqvist, F. Atmos. Environ. 1985, 19, 1671-1680. Huntzicker, J. J. Anal. Chem. 1986, 58, 653-654. Appel, B. R.; Tanner, R. L.; Adams, D. F.; Dasgupta, P. K.; Knapp, K. T.; Kok, G. L.; Pierson, W. R.; Reiszner, K. D. In Methods of Air Sampling and Analysis, 3rd ed.; Lodge, J. P., Ed.; Lewis: Chelsea, MI, 1988; pp 523-532, Method 713. (4) Klockow, D.; Niessner, R.; Malejczyk, M.; Kiendl, H.; vom Berg, B.; Keuken, M. P.; Wayers-Ypellan, A.; Slanina, J. Atmos. Environ. 1989, 23, 11311138. (5) Dzubay, T. G.; Rook, H. L.; Stevens, R. K. 165th National Meting of the American Chemical Society, 1973; Abstr. WATR-045. (6) Roberts, P. T.; Friedlander, S. K. Proc. Conf. Hlth. Consequences Environ. Controls, Durham, NC, 1974. Roberts, P. T. Ph.D. Dissertation, California Institute of Technology, 1975. Roberts, P. T.; Friedlander, S. K. Atmos. Environ. 1976, 10, 403-408. (7) Husar, J. D.; Husar, R. B.; Stubits, P. K. Anal. Chem. 1975, 47, 20622064. Husar, J. D.; Husar, R. B.; Mascias, E.; Wilson, W. E.; Durham, J. L.; Shepherd, W. K.; Anderson, J. A. Atmos. Environ. 1976, 10, 591-595. Hering, S. V.; Friedlander, S. K. Atmos. Environ. 1982, 16, 2647-2656. (8) Sturges, W. T.; Harrison, R. M. Environ. Sci. Technol. 1988, 22, 13051311. (9) Yamamoto, M.; Kosaka, H. Anal. Chem. 1994, 66, 362-367. 10.1021/ac015643r CCC: $22.00
© 2002 American Chemical Society Published on Web 02/09/2002
Hering10 have made it routine. This technique is also promising for the simultaneous measurement of aerosol S by an FPD and aerosol C by a CO monitor. Thermally speciated elemental versus organic carbon measurements have been demonstrated.11 Direct introduction of an air sample into an air plasma has been shown to be viable for the direct measurement of metallic constituents.12 More recently, Duan et al.13 described a fieldportable low-power argon plasma that tolerates 20% air. Coupled to an inertial particle concentrator,14 such an approach may be practical. For a given analyte, uniquely simple and sensitive solutions may exist; Clark et al.15 reported that a single 100-nmdiameter NaCl particle can be detected, free from matrix interferences, with an FPD. The application of mass spectrometry (MS) to aerosol analysis has had a long and illustrious history.16 Electron and optical microscopic techniques were once believed to be the best route to the analysis of individual particles.17 Single-particle MS can do this today and do so in real time (for an excellent review, see ref 18). MS can provide information on not just specific components such as sulfates and nitrates but on all material present in the particle. While MS may hold the key to the future, the cost, bulk, operator sophistication, and extensions needed to produce reliable quantitative data presently leave room for other, more affordable techniques. (10) Hering, S. V.; Stolzenburg, M. R. U.S. Patent 5,983,732. Stolzenburg, M. R.; Hering, S. V. Environ Sci. Technol. 2000, 34, 907-914. Liu, D. Y.; Prather, K. A.; Hering, S. V. Aerosol Sci. Technol. 2000, 33, 71-86. (11) Turpin, B. J.; Cary, R. A.; Huntzicker, J. J. Aerosol Sci. Technol. 1990, 12, 161-171. (12) Bacri, J.; Gomes, A. M.; Fieni, J. M.; Thouzeau, F.; Birolleau, J. C. Spectrochim Acta 1989, 44B, 887-895. Nore, D.; Gomes, A. M.; Bacri, J.; Cabe, J. Spectrochim Acta 1993, 48B, 1411-1419. Gomes, A. M.; Sarrette, J.-P.; Madon, L.; Almi, A. Spectrochim Acta 1996, 51B, 1695-1705. (13) Duan, Y.; Su, Y.; Jin, Z.; Abeln, S. Anal. Chem. 2000, 72, 1672-1679. AIP Conf. Proc. 2000, 71, 1557-1563. (14) Sioutas, C.; Koutrakis, P.; Olson, B. A. Aerosol Sci. Technol. 1994, 21, 223235. Sioutas, C.; Koutrakis, P.; Burton, R. M. J. Aerosol Sci. 1994, 25, 13211330; Part. Sci. Technol. 1994, 12, 207-221; Environ. Health Perspect. 1995, 103, 172-177. (15) Clark, C. D.; Campuzano-Jost, P.; Covert, D. S.; Richter, R. C.; Maring, H.; Hynes, A. J.; Saltzman, E. S. J. Aerosol Sci. 2001, 32, 765-778. (16) Myers, R. L.; Fite, W. L. Environ. Sci. Technol. 1975, 9, 334-336. Sinha, M. P.; Giffin, C. E.; Norris, D. D.; Estes, T. J.; Vilker, V. L.; Friedlander, S. K. J. Colloid Interface Sci. 1982, 87, 140-153. Marijinissen, J. C. M.; Scarlett, B.; Verheijen, P. J. T. J. Aerosol Sci. 1988, 19, 1307-1310. McKeown, P. J.; Johnson, M. V.; Murphy, D. M. Anal. Chem. 1991, 63, 2069-2073. Kievit, O.; Marijinissen, J. C. M.; Verheijen, P. J. T.; Scarlett, B. J. Aerosol Sci. 1992, 23, S301-S304. Hinz, K. P.; Kaufmann, R.; Spengler, B. Anal. Chem. 1994, 66, 2071-2076. Mansoori, B. A.; Johnston, M. V.; Wexler, A. S. Anal. Chem. 1994, 66, 3681-3687. Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 3540-3542. Carson P. G.; Neubauer, K. R.; Johnson, M. V.; Wexler, A. S. J. Aerosol Sci. 1995, 26, 535-545. Murphy, D. M.; Thomson, D. S. Aerosol Sci. Technol. 1995, 22, 237-249. Reents, W. D. J.; Mujsce, A. M.; Muller, A. J.; Siconolfi, D. J.; Swanson, A. G. J. Aerosol. Sci. 1995, 23, 263-270. Hinz, K. P.; Kaufmann, R.; Spengler, B. Aerosol Sci. Technol. 1996, 24, 233-242. Lui, D.; Rutherford, D.; Kinsey, M.; Prather, K. A. Anal. Chem. 1997, 69, 1808-1814. Gard, E.; Mayer, J. E.; Morrical, B. D.; Dienes, T.; Fergenson, D. P.; Prather, K, A. Anal. Chem. 1997, 69, 4083-4091. Kolb, C. E.; Jayne, J. T.; Worsnop, D. R.; Shi, Q.; Jimenez, J. L.; Davidovits, P. Morris, J.; Yourshaw, I.; Zhang, X. F. 219th National Meeting of the American Chemical Society, March 2000; Abstr. ENVR 100. Song, X.-H.; Hopke, P. K.; Fergenson, D. P.; Prather, K. A. Anal. Chem. 1999, 71, 860-865. Gross, D. S.; Ga¨lli, M. E.; Silva, P. J.; Prather, K. A. Anal. Chem. 2000, 72, 416422. (17) Lodge, J. P.; Ferguson, J.; Havlik, B. R. Anal. Chem. 1960, 32, 1206-1207. Lodge, J. P.; Pate, J. B. Science 1966, 153, 408-410. Lodge, J. P.; Frank, E. R. J. Microsc. 1967, 6, 449-455. Bigg, E. K.; Ono, A.; Williams, J. A. Atmos. Environ. 1974, 8, 1-13. (18) Suess, D. T.; Prather, K. A. Chem. Rev. 1999, 99, 3007-3035.
Since many of the aerosol constituents of interest are ionic, typical present-day practice of aerosol analysis involves gas removal with a denuder, filter collection with subsequent extraction of the filter by an aqueous extractant, and analysis by ion chromatography (IC). This laboratory has had a long relationship with IC,19 and as such, it was of particular interest to us to develop a fully automated IC-based approach to near-real-time aerosol analysis. Continuous impaction is one of the most straightforward approaches to accomplish aerosol collection, but it is difficult to collect very small particles by impaction. This problem was solved by introducing steam into the aerosol flow and allowing the aerosol to grow.20 This general theme has been adapted and refined by others,21 as well as by us,22 and introduced in parallel by a Dutch group.23 Although other approaches to collecting atmospheric aerosols into a liquid receiver coupled to IC analysis have been investigated,24 generally these could not exceed the efficiency of the vapor condensation aerosol collection approach across a large particle size range. The steam introduction approach is, however, not without its shortcomings. A small but measurable artifact is caused by the hydrolytic reaction of NO2, which is not appreciably removed by most denuder systems now in use. The resulting product is measured erroneously as particulate nitrite (and to a much smaller extent, nitrate). Steam introduction requires a condensation chamber that increases the size of the instrument. On the other hand, filter collection also potentially permits differential analysis via sequential extraction with different extractants, not possible with direct collection in a liquid. The present paper describes a new instrument that is a fully automated analogue of manual filter collection, extraction, and analysis. EXPERIMENTAL SECTION The instrument was constructed using a full tower-size personal computer (PC) case as the housing. Various components were (19) Dasgupta, P. K. Atmos. Environ. 1982, 16, 1265-1268. Dasgupta, P. K. In Ion Chromatography; Tarter, J. G., Ed.; Marcel Dekker: New York, 1987; pp 191-367. Dasgupta, P. K. J. Chromatogr. Sci. 1989, 27, 422-448. Dasgupta, P. K. Anal. Chem. 1992, 64, 775A-783A. Sjo ¨gren, A.; Boring, C. B.; Dasgupta, P. K.; Alexander, J. N., IV. Anal. Chem. 1997, 69, 13851391. Al-Horr, R.; Dasgupta, P. K.; Adams, R. L. Anal. Chem. 2001, 73, 4694-4703. (20) Blatter, A.; Neftel, A.; Dasgupta, P. K.; Simon, P. K. In Physico-Chemical Behavior of Atmospheric Pollutants; Angletti, G., Restelli, G., Eds.; Proceedings of the 6th European Symposium, Report EUR 15609/2 EN, Luxembourg, 1994. pp 767-772. (21) Loflund, M.; Kasper-Giebl, A.; Tscherwenka, W.; Schmid, M.; Giebl, H.; Hitzenberger, R.; Reischl, G.; Puxbaum, H. Atmos. Environ. 2001, 35, 28612869. Weber, R. J.; Orsini, D. J.; Daun, Y.; Lee, Y.-N.; Klotz, P. J.; Brechtel, F.; Okuyama, K. Aerosol Sci. Technol. 2001, 35, 718-727. Zellweger, C.; Ammann, M.; Hofer, P.; Baltensperger, U. Atmos. Environ. 1999, 33, 11311140. (22) Simon, P. K.; Dasgupta, P. K. Environ. Sci. Technol. 1995, 29, 1534-1541. Simon, P. K.; Dasgupta, P. K. Anal. Chem. 1995, 67, 71-78. Poruthoor, S. K.; Dasgupta, P. K.; Genfa, Z. Environ. Sci. Technol. 1998, 32, 1147-1152. Poruthoor, S. K.; Dasgupta, P. K. Anal. Chim. Acta 1998, 361, 151-159. Ito, K.; Chasteen, C. C.; Chung, H.-K.; Poruthoor, S. K.; Genfa, Z.; Dasgupta, P. K. Anal. Chem. 1998, 70, 2839-2847. (23) Slanina, J.; ten Brink H. M.; Otjes, R. P.; Even, A.; Jongejan, P.; Khlystov, A.; Waijers-Ijpelaan A.; Hu, M. Atmos. Environ. 2001, 35, 2319-2330. Khlystov, A.; Wyers, G. P.; Slanina, J. Atmos. Environ. 1995, 29, 22292234. (24) Buhr, S. M.; Buhr, M. P.; Fehsenfeld, F. C.; Holloway, J. S.; Karst, U.; Norton, R. B.; Parrish, D. D.; Sievers, R. E. Atmos. Environ. 1995, 29, 2609-2624. Liu, S.; Dasgupta, P. K. Talanta 1996, 43, 1681-1688; Anal. Chem. 1996, 68, 3638-3644. Karlsson, A.; Irgum, K.; Hansson, H. J. Aerosol Sci. 1997, 28, 1539-1551. Liu, S.; Dasgupta, P. K. Microchem. J. 1999, 62, 50-57.
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anchored or attached directly to the PC chassis. Fully assembled, the particle collection and extraction instrument had dimensions of 55 cm × 76 cm × 76 cm (L × W × H; including instrument components outside the case). Gas Removal and Analysis. Soluble gas collection is accomplished with a parallel plate wet denuder (PPWD). The current PPWD differs from previous designs as follows. The denuder is composed of Plexiglas plates with Teflon spacers. Nonglass construction eliminates fragility problems. The desired area of each Plexiglas plate is microstructured to render it wettable. The denuder is bolted to a stand consisting of a support base to which threaded pipe flanges are secured by screws. The threaded ends of 3/8-in. (9.5-mm)-i.d. steel piping, used as the support stands, are secured thereto. For the measurement of gases and aerosols with the highest temporal resolution possible, it is necessary to dedicate individual IC units to the gas system and the aerosol system. There are two potential arrangements: (a) a PPWD supplying its liquid effluent to an IC dedicated to gas analysis and a second independent PPWD the gas-phase effluent of which is directed to the particle collection system (PCS), which is coupled to its own IC, and (b) a single PPWD connected to the PCS, the liquid effluent from the PPWD and the PCS each going to separate IC units. Even though the latter arrangement may at first seem to be the simpler, in all field experiments to date,25 we have chosen the first option. Among others, HNO3 and HCl are two gases that are of interest and both are known to be “sticky”; to prevent adsoptive losses, high sampling rates or the very minimum of an inlet line must be used.23,26 On the other hand, it is generally desired to measure the aerosol composition in the e2.5-µm-size fraction; this necessitates both a cyclone and a gas removal denuder prior to the aerosol collector. The cyclone cannot be placed after a wet denuder because of the growth in size of hygroscopic aerosols during passage through the denuder. Placing the cyclone before the denuder would entail loss or undesirable integration of the “sticky” gases.26 The general arrangement we suggest thus involves the deployment of the gas analysis denuder in open air (typically immediately on the roof of the shelter where the analytical instruments are located) without a cyclone and with a very short inlet (e5 cm of a perfluoroalkoxy (PFA) Teflon tubing). The air sample enters the denuder at the bottom. A peristaltic pump located in the instrument shelter pumps the liquid to and from the denuder. The transit time in typical deployment is ∼2 min, and temporal gas analysis data are corrected for this transit delay. The denuder stand is sufficiently tall to allow the inlet to be ∼60 cm off the support base. To minimize interaction of the inlet air sample with the stand components, especially in still air, the iron support stand from the base to the bottom of the denuder is wrapped with Teflon tape. (25) Atlanta 1999: http://www-wlc.eas.gatech.edu/supersite/. Houston 2000: http://www.utexas.edu/research/ceer/texaqs/. Philadelp.hia 2001: http:// www.cgenv.com/Narsto/. (26) Appel, B. R. ACS Adv. Chem. Ser. 1993, No. 232, 1-40. Koch, T. G.; Fenter, F. F.; Rossi, M. J. Chem. Phys. Lett. 1997, 275, 253-260. Neumann, J. A.; Huey, L. G.; Ryerson, T. B.; Fahey, D. W. Environ. Sci. Technol. 1999, 33, 1133-1136. Li-Jones, X.; Savoie, D. L.; Prospero, J. M. Atmos. Environ. 2001 35, 985-993. Komazaki, Y.; Hashimoto, S.; Inoue, T.; Tanaka, S. Atmos. Environ., in press.
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Figure 1. Wetted denuder shown schematically. Key: AI/AO, air in/out apertures; LI/LO, liquid in/out apertures; LR, porous poly(vinylidene fluoride) element acting as a liquid flow restrictor; WA, wetted area; S, PTFE spacer; SH, screw holes for affixing the two denuder plates and the spacer together.
The denuder is shown schematically in Figure 1. Each denuder plate is 10.0 × 55 cm (3/8 in. (9.5 mm) thick) with the active wettable area of 6.5 × 42 cm, starting 7.5 cm from the top and 1.75 cm from each edge. The denuder liquid is forced through a fritted PVDF barrier to allow even flow down the plate and is aspirated from the apex of the V-groove, 4.5 cm from the bottom edge. The two plates are spaced by a 3-mm-thick PTFE spacer. The air inlet/outlet holes, circular at the termini, are machined with a contour that becomes elliptical as they approach the interior of the denuder to allow for a smooth entrance/exit of the airflow. PFA Teflon tubing (1 gauge, 8.3-mm o.d., 7.5-mm i.d.) fit tightly into these apertures. The overall airflow arrangement and gas system liquid flow arrangement is shown in Figure 2a. Typically the air-sampling rate is 5 standard liters per minute (SLPM), controlled by a mass flow controller (MFC-C, Aalborg instruments AFC 2600D, Orangeburg, NJ). A diaphragm pump (P2, Gast DOA-P120-FB) provides the sample flow, the same pump is used for flow aspiration on a filter FC (vide infra). Hydrogen peroxide (0.5 mM) is used as the denuder liquid at ∼0.5 mL/min on each plate, each stream pumped through disposable mixed-bed ion-exchange resin columns MB (0.67 cm i.d. × 15 cm, PTFE column filled with Dowex MR-3 resin) located immediately before the PPWD liquid entrance ports. The effluent streams are aspirated at ∼1 mL/min from each plate (using same peristaltic pump but larger tubing, 0.89- vs 1.29mm-i.d. Pharmed tubes are used for input vs aspiration, peristaltic pump speed fixed at 6 rpm) to ensure all liquid is aspirated from the bottom of the PPWD. The aspirated flow streams are combined and sent to the IC analysis system consisting of alternating TAC-LP1 anion preconcentrator columns, AG11HC guard, and AS11HC separation columns and an electrodialytically
Figure 2. (a) Total system airflow and gas analyzer liquid flow schematic. Key: PPWD, gas system wet denuder; MB, mixed-bed resin deionizer columns; IC1, gas analysis system ion chromatograph (uses 10-port dual concentrator column injector as in PCS IC in Figure 3; FA, FB, glass fiber filters; T, trap bottles; MFC-A, -B, -C, -D, mass flow controllers; C, cyclone; FC, 47-mm filter for MS analysis; P1, P2, air sampling pumps; PP, peristaltic pump; F, filter; P, purifer; H, heater. The dotted section including the denuder is on the roof and the air pumps are either below the instrument shelter or in a modified doghouse with forced air ventilation. V1, aerosol switching valve, shown in detail in (b).
regenerated suppressor (ASRS, operated at 100 mA). The chromatographic system itself consisted of a DX-100 pump and detector with 22.5 mM NaOH eluent flowing at 1 mL/min. In more recent work, we used an IS-25 chromatographic pump, coupled to an EG-40 electrodialytic eluent generator (15.5 mM KOH, 1.5 mL/min, LC-30 oven at 29 °C), and an ED40 detector used as a conductivity detector (CD). Chromatography is conducted either on a 10- or a 15-min cycle. A four-channel peristaltic pump (Rainin Dynamax) is used for all liquid pumping. All chromatographic equipment and columns (both above and in the following) were from Dionex Corp. Particle Collection System. A Teflon-coated aluminum cyclone (10 L/min; University Research Glassware, URG, Chapel Hill, NC) is used as the first element of the inlet system to remove particles larger than 2.5 µm. The cyclone exhibits the desired size cut point only at the design flow rate. Referring to the overall airflow arrangement in Figure 2a, the air sample passes through the cyclone at 10 SLPM and is divided by an Y-connector into two flow streams of 5 SLPM each. One is drawn through a 47-
mm glass fiber filter F1 (Whatman type GF/B, filters were changed at ∼12-h intervals or corresponding to daylight and nighttime hours and were used for archival purposes and IC-CDUV-MS analysis of the filter extract in home laboratory) via mass flow controller MFC-D (Aalborg AFC2600D). The cyclone and the filter holder are mounted on a modified camera tripod. The feet of the tripod are bolted to the roof of the instrument shelter; the air inlet is maintained ∼2 m above the roofline. The second flow stream from the cyclone exit proceeds through a copper conduit or aluminized PFA Teflon tube to a PPWD located within the instrument shelter. The metal is electrically grounded to minimize aerosol loss. The PPWD is fed with ∼1 mL/min streams of 10 mM Na2HPO4 (adjusted to pH 7) containing 0.5 mM H2O2, on each plate that serves to remove both acidic and basic gases; the denuder effluent (aspirated at ∼1.5 mL/min per plate) is sent to waste. The gaseous effluent from the denuder bearing the aerosol proceeds to the PCS. The first element of the PCS is a specially constructed rotary valve V1 that directs the ambient air stream to either filter A or Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
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Figure 3. Particle system setup: (a) state 1; (b) state 2. Key: V5, V6, all-PTFE solenoid valves; V7, V8, pinch-type solenoid valves; FTF, fiber trap filter; IV, 10-port injection valve bearing preconcentration columns; PC1 and PC2, IC2, PCS ion chromatograph; V2-V4, on/off solenoid valve for airflow; T1, acid-washed silica gel; T2, soda lime (T1 and T2 together constitute the purifier P in Figure 2); PPWD, PCS gas denuder; WB, water bottle; W, waste. The top and bottom portions respectively indicate liquid and gas flow.
filter B. This valve must provide a straight passageway for the sample stream to one of the two sample filters without aerosol loss. The valve is shown in functional detail in Figure 2b. The stator plate has three holes, the central port is connected to the sample air stream (from the PPWD), while the two other ports are connected in common through a Y-connector to a sequential trap containing a particle filter (F) and acid-washed silica gel (T1, 6-8 mesh, which removes NH3) followed by a soda lime trap (T2, 4-8 mesh, that removes acid gases) and a heater (H) that thus provides a hot dry clean air source (Figure 2a). The rotor plate has two holes, connected to filter A (FA) and filter B (FB), respectively, and is rotated by a spring-return rotary solenoid (TRW/Ledex, Vandalia, OH, 30° rotation angle). The air transmission tubes to the valve are 7.5-mm-i.d., 8.75-mm-o.d. PFA tubing, push fit into the stator and rotor plates of the valve. With the solenoid unenergized, ambient air is sampled on filter A, and with the solenoid energized, ambient air is sampled on filter B; flow is thus switched without aerosol loss. Other air valves V2-V4 (see Figure 3) are 1/2-NPT large-orifice, low-power, on-off-type solenoid valves (Skinner A10, Parker/Hannifin, 12 VDC) that govern airflow in the PCS. Plexiglas filter holders were machined to hold 25-mm-diameter filters. Atop a stainless steel screen are placed a paper filter (Whatman grade 5) and a glass fiber filter (Whatman GF/B). Two 10-32 threaded ports on opposite sides of the top half of the filter holder provide entry of wash liquids. The bottom half of the filter holder is designed as a shallow cone with the air outlet at the center. The liquid exit port is a 10-32 threaded aperture, located equidistant from the inlet apertures such that the inlet/outlet apertures constitute an equilateral triangle in top view. Air/liquid separators, constructed using 3-in. transparent poly(vinyl chloride) (PVC) pipe with PVC caps cemented to each end, constituting 500-mL-capacity reservoirs, were incorporated below each filter holder in the air exit path. These contained air in and exit ports, as well as a port to remove accumulated water 1260
Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
(periodically, e.g., every 24 h) using a syringe. These separators serve to keep any wash liquid from entering the respective mass flow controllers (MFC-A, B; 0-10 L/min; UFC-1500A, Unit Instruments, Inc., Chaska, MN). The diaphragm pump (P1, same as P2) used for sampling is capable of aspirating at g8 L/min through each filter holder simultaneously. Standard wall PFA Teflon tubes (1SW, Zeus Industrial Products, Orangeburg, SC) were used for connecting PCS components upstream of the filter holders. This tubing was externally wrapped with electrically grounded Al tape and then with bare Cu wire. This served the dual purpose of improving its structural strength and reducing electrostatically induced aerosol loss. Instrument components were machined to provide a leak-free push-fit with this size tubing. Flexible PVC tubing (3/8-in. i.d.) was used for component connections downstream of the filter holders. Filter Extraction System. A six-channel peristaltic pump (Dynamax RP-1, Rainin) provides liquid pumping. Valves V5-V8 (Figure 3) are low-power miniature liquid-handling solenoid valves. Valves V5 and V6 are subminiature all-PTFE wetted part valves (161T031, NResearch, W. Caldwell, NJ) that direct the flow of deionized water to the filter holders. Prior to the filter holders, the pumped water (1 mL/min total flow) is split into two flow streams. A 2-cm length of PEEK tubing (0.010-in. i.d., Upchurch Scientific, Oak Harbor, WA) was placed immediately prior to the filter holder at each water entrance to provide flow resistance. This served to distribute the flow from both inlets evenly on to the filters. Valves V7 and V8 (161P091, NResearch) handle the filter extract in which stray glass fibers may be present. Therefore, these valves are pinch-type valves that can tolerate such fibers without valve malfunction. A low-volume fiber trap filter (FTF, Acrodisc CR, 5 µm, 25 mm) placed prior to the injection valve prevents glass fiber intrusion to the preconcentration columns. Such intrusion can result in high-pressure drops, resulting in decreased sample loading on the columns. Injection valve IV is a 10-port electrically actuated valve (Rheodyne) that contains two
Table 1. Four States of the Instrument, Programmed Chromatograph TTL Outputs, and Outputs of Integrated Circuit Chips U1 and U2 status begin sampling on FA, start washing FB, water being aspirated through FB and being loaded on to PC1 sampling still on FA, washing FB complete dry FB, water purge PC1 FB dry, switch sampling to FB Put PC1 in injection mode begin washing FA sampling still on FB, begin drying FA water purge on PC2
IC using
V1
V2
V3
V4
V5
V6
V7
V8
TTL1
TTL2
U1
U2
PC2
off
off
off
on
off
off
off
off
lo
lo
4-lo
2-hi
PC2
off
on
on
on
on
off
off
on
lo
hi
5-lo
4-hi
PC1
on
off
on
off
off
on
on
off
hi
lo
6-lo
6-hi
PC1
on
on
on
on
on
off
off
on
hi
hi
7-lo
8-hi
low-pressure-drop anion preconcentration columns (TAC-LP1) PC1 and PC2. PEEK peristaltic pump tubing adapters (PF-S, VICI) terminating in 1/4-28 fittings were used. Male nuts (1/4-28 threaded) and ferrules were used to connect tubing to the pump adapters. Pharmed tubing (1.29- and 1.52-mm i.d., respectively) was used for pumping water to and from the filter holders (∼1 and 1.5 mL/ min); larger aspiration flow is used to prevent water backup at the filters. Similarly, 1.29- and 1.52-mm-i.d. Pharmed pump tubes were used for pumping and aspirating liquid to and from each wall of the PPWD. All liquid-transfer lines were 20-gauge standard wall PTFE tubing (20 SW, Zeus Industrial Products). For connections, PTFE tubes were butt-joined with Pharmed pump tubing as sleeves. The chromatographic columns and suppressor were identical to that for the gas analysis system. The chromatographic system itself used either a DX-120 IC and detector with a 22.5 mM NaOH eluent at 1.0 mL/min or a DX-600 system with an electrodialytically generated (EG 40) 14.75 mM KOH eluent flowing at 1.5 mL/min with columns thermostated at 31 °C and a CD 20 conductivity detector. Under either operating conditions, chloride, nitrite, nitrate, sulfate, and oxalate were analyzed in less than 15 min. Occasionally the system was operated with 30-min sample collection and 30-min gradient elution runs. Instrument Operation. Table 1 shows the air and liquid valves and their respective on/off status. Figure 3 illustrates the four states of the instrument cycle. The first state depicted in Figure 3a is 8.5 min in duration. In the particle collection system (Figure 3a, bottom), the soluble gas denuded aerosol flow stream is directed to filter A by valve V1. Air passes through filter A though mass flow controller A (MFC-A), which regulates the airflow to 5 SLPM, and finally through valve V4, which is on during state 1. Valves V2 and V3 are off, and filter holder B (FB) is under airlock. In the liquid extraction portion of the instrument (Figure 3a, top), deionized water is contained in a 2-L bottle (WB). The air entrance to the water bottle is equipped with a soda lime trap to minimize acid gas intrusion into the bottle. Water from WB is aspirated and then pumped at 1 mL/min by the peristaltic pump (PP) through a mixed-bed ion-exchange column (MB1; packed with Dowex MR-3 resin, Sigma) to remove any trace impurities present in the deionized water. Valve V5 directs flow to valve V6,
which in turn directs the water to filter FB. The water enters FB through the two ports in the top of the holder and is simultaneously aspirated from the bottom of FB through valves V7 and V8 by the peristaltic pump. Since FB is under airlock, water does not enter the air outlet tubing at the bottom of the filter holder. The extracted material from the filter is pumped through the fiber trap filter to remove glass fibers from the flow stream before passing to the appropriate preconcentration column. Valve IV is configured such that while one preconcentration column is chromatographed, the other preconcentration column is loaded with sample or washed with water. In the present case, preconcentration column PC1 is loaded with sample. Following 8.5 min, state 2 begins (Figure 3b). During state 2 in the PCS, ambient air continues to be sampled on FA, just as in state 1. Valves V2 and V3 are activated in state 2, allowing clean hot air to pass through filter FB for the duration of this state. Clean (ammonia/acid gas- and particle-free) air, produced by passing ambient air through F, T1, and T2 is heated to ∼75 °C by passing it over a siliconized resistance heater (Watlow, St. Louis, MO) contained in a PVC cylinder housing that is powered by 110 VAC power (∼20 W) via a dc relay that is switched in parallel with valve V2. This clean, hot air is aspirated through the previously extracted filter FB, to dry it prior to state 3. [Safety Device: Within the PVC cylinder housing the heater, a thermal cutout device is located in close proximity to the heater and is connected in series with the heater such that the heater shuts off in the event of overheating (t g 143 °C).] Note that at the time the instrument enters state 2 from state 1, although all the analyte has been extracted from filter FB and preconcentrated, the last portion of the wash water is still contained in the filter housing. This water is aspirated into the trap bottle ahead of MFC-B. Water that enters into the trap bottle is generally of the order of 1 mL/cycle. This volume may be used to monitor the filter extraction process; excessive water accumulation in the water trap bottle indicates flow problems through the filter or through the relevant preconcentration column. In the liquid extraction system, valves V5 and V8 are activated. Valve V5 now directs water used to wash filter FB in state 1 back into the water bottle. This recycling procedure helps maintain the purity of the water in WB. As a result of liquid being aspirated faster from the filter housing than it is pumped in, air bubbles inevitably enter into the preconcentration column. To remove the Analytical Chemistry, Vol. 74, No. 6, March 15, 2002
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air bubbles before the sample is injected, valve V8 is activated and water is aspirated by the pump through a mixed-bed ion-exchange column (MB2) through V8 and pumped through the preconcentration column PC1. The duration of state 2 is 6.5 min. After state 2 ends, state 3 (8.5 min) and state 4 (6.5 min) follow. States 3 and 4 are identical to states 1 and 2, respectively, except that the roles of filters A and B are interchanged relative to those in states 1 and 2. States 1-4 constitute an instrument cycle; state 1 starts at the end of state 4 and this continues until deliberately shut down. The chromatographic system is calibrated by a valve-loop combination in which each side of the valve is separately calibrated volumetrically by filling the loop with an alkaline solution of bromthymol blue of known absorbance, injecting, collecting all the effluent into a 5-mL volumetric flask, making up to volume, and measuring the absorbance. Such a calibration takes into account the internal volumes of the valve ports, etc. Standards containing chloride, nitrite, nitrate, sulfate, and oxalate are then injected using the loop, keeping the concentrator column ahead of the guard column to match actual experimental dispersion. Multipoint calibration curves are constructed in terms of absolute amount injected in nanograms versus peak area. Electrical. The main ac power to the instrument goes to a PC-style power supply (that comes with the PC chassis) providing +5 and (12 V power, of which only the +12 V supply is used (rated at 8 A,
