Anal. Chem. 1995, 67, 2763-2766
Organic Solvents with Wet Effluent Diffusion Denuder for Preconcentration of 1,4=Dichlorobenzenefrom Air Zbynok Zdrihal,* Pave1 Mikuiika, and Zbyngk V e e k Institute of Analyfical Chemistty, Academy of Sciences of the Czech Republic, VeveW 97, CZ-611 42 Bmo, Czech Republic
Alcohols, glycols, and their aqueous mixhves were screened as absorption liquids for a wet eftluent diffusion denuder (WEDD) for preconcentration of gas-phase organic compounds from air. 1-Propanol proved suitable for the preconcentration of 1,4-dichlorobenzene (1,4DCB), which was chosen as a model substance. The dependences of collection efficiency of the WEDD on flow rates of absorption liquid and air were measured. Under sutticiently high flow rates of absorption liquid (> 150 p L min-l), the experimental collection efficiencies agreed with theoretical values for air flow rates from 0.5 to 4.3 L min-' at adjusted 1,4-DCB concentrations of 48 and 720 pg m-3. The denuder operated without any diBculties for temperatures from 18 to 33 "Cand relative air humidities of 10-100%. The importance of the determination of quantification of the thousands of organic substances emitted into the atmosphere continues to grow. The low concentrations of these organic compounds typically require a preconcentration step before their determination. A convenient preconcentration technique is important for obtaining correct results, mainly for semivolatiles, present both in gas phase and associated with particles in the air. In many cases, diffusion denuders are preferred in atmospheric gas-particle partitioning studies of organic pollutants over commonly used filter-adsorbent technique~l-~ since they remove gaseous analytes from an air sample prior to trapping particles on a filter, avoiding artifacts that filter-adsorbent techniques suffer fr~m.~~~,~ Diffusion denuders with a fixed layer of a sorptive agent have been used for the preconcentration of organic air pollutants such as chlorinated pesticide^,^-^ polychlorinated biphenyls?"O polynuclear aromatic hydrocarbons,10-12 aldehydes,13-15 alkyl sulfates,"jJ7 organic amines,20 nicotine,21 and other vola(1) Pellizzari, E. D. Organic Chemisty ofthe Atmosphere; Hansen, L. D., Eatough D. J., Eds.; CRC Press, Inc.: Boca Raton, FL, 1991; Chapter 1. (2) Hart, K. M.; Isabelle, L. M.; Pankow, J. F. Environ. Sci. Technol. 1992,26, 1048- 1052. (3) Hawthorne, S. B.; Miller, D. J.; Langenfeld, J. J.; Krieger M. S. Enuiron. Sci. Technol. 1992,26, 2251-2262. (4) Kaupp, H.; Umlauf, G. Atmos. Enuiron. 1992,26A, 2259-2267. (5) Zhang, X.; McMuny, P. H. Enuiron. Sci. Technol. 1991,25,456-459. (6) Bidleman, T. F.; Billings, W. N.; Foreman, W. T. Enuiron. Sci. Technol. 1986, 20,1038-1043. (7) Lane, D.A.;Johnson, N. D.; Barton, S. C.; Thomas, G. H. S.; Schroeder, W. H. Environ. Sci. Technol. 1988,22,941-947. (8) Lane, D. A.; Johnson, N. D.; Hanley, M:J. J.; Schroeder, W. H.; Ord, D. T. Enuiron. Sci. Technol. 1992,26,126-133. (9) Krieger, M. S.; Hites, R A Enuiron. Sci. Technol. 1992,26,1551-1555. (10) Krieger, M. S.; Hites, R. A. Enuiron. Sci. Technol. 1994,28, 1129-1133.
0003-2700/95/0367-2763$9.00/0 0 1995 American Chemical Society
tile~?~3*~ Before sampling, the appropriate walls of these denuders were coated with a suitable sorptive agent and concentrated analytes were either eluted by a convenient solvent or thermally desorbed from denuder walls after the sampling period. This type of denuder provides a time-averaged result and requires wall coating and analyte removing for each analysis. Wet effluent diffusion denuders (WEDD) offer advantages of continuously renewed collection surfaces and the possibility of real-time measurement. In its simplest configuration, the WEDD is a tube in which, instead of a fixed layer of a sorptive agent, a suitable absorption liquid flows down the inner wall of the tube (hundreds of microliters per minute) while analyzed air passes through the tube in the opposite direction (liters per minute) under laminar flow conditions. A continuous stream of concentrated analyte is obtained at the bottom of the denuder tube. The ratio of volume flow rates of sampled air and absorption liquid determines the concentration effect of the WEDD under optimum conditions. There have been several reports of the use of WEDDs for the preconcentration of gas-phase inorganic pollutant^.^^-^^ An instrument related to the WEDD, the diffusion scrubber, was (11) Coutant, R W.; Callahan, P. J.; Kuhlman, M. R; Lewis, R. G.Atmos. Environ. 1989,23, 2205-2211. (12) Coutant, R W.; Callahan, P. J.; Chuang, J. C.; Lewis, R. G. Atmos. Enuiron. 1992,26A, 2831-2834. (13) Cecchini, F.; Febo, A.; Possanzini, M. Anal. Lett. 1985,18, 681-693. (14) Possanzini, M.; Ciccioli, P.; Di Palo, V.; Draisci, R. Chromatographia 1987, 23,829-834. (15) Williams, E. L., 11; Grosjean, D. Enuiron. Sci. Technol. 1990,24,811-814. (16) Eatough, D. J.; White, V. F Hansen, L. D.; Eatough, N. L.; Cheney, J. L. Environ. Sci. Technol. 1986,20,867-871. (17) Hansen, L. D.; White, V. F.; Eatough, D. J. Environ. Sci. Technol. 1986, 20, 872-878. (18) Winiwarter, W.; Puxbaum, H.; Fuzzi, S.; Facchini, M. C.; Orsi. G.; Beltz, N.; Enderle, K.; Jaeschke, W. Tellus 1988,40B,348-357. (19) Keene, W. C.; Talbot, R W.; Andreae, M. 0.;Beecher, K; Berresheim, H.; Castro, M.; Farmer, J. C.; Galloway, J. N.; Hoffmann, M. R; Li,S.-M.;Maben. J. R.; Munger, J. W.; Norton, R. B.; Pszenny, A. A. P.; Puxbaum, H.; Westberg, H.; Winiwarter, W. J. Geophys. Res. 1989,94,6457-6471. (20) Possanzini, M.; Di Palo, V. Chromatographia 1990,29,151-154. (21) Koutrakis, P.; Fasano, A. M.; Slater, J. L.;Spengler, J. D.; McCarthy; J. F.; Leaderer, B. P. Atmos. Envzron. 1989,23, 2767-2773. (22) Cobb, G. P.; Braman, R S.; Hua, K. M. Anal. Chem. 1986,58,2213-2217. (23) McEntee, J.; Thomas, C. L. P.; Alder, J. F. Anal. Chim. Acta 1989,226, 145-151. (24) Thomas, C. L. P.; Alder, J. F. Anal. Chim. Acta 1989,217,289-301. (25) Thomas, C. L. P.; Alder, J. F. Anal. Chim. Acta 1993,274,171-177. (26) Simon, P. IC; Dasgupta, P. K.; Vefefa, Z. Anal. Chem. 1991,63,12371242. (27) Vereia, Z.; Dasgupta, P. K Anal. Chem. 1991,63,2210-2216. (28) Simon, P. K; Dasgupta, P. K Anal. Chem. 1993,65, 1134-1139. (29) Taira, M.; Kanda, Y. Anal. Chem. 1993,65,3171-3173. (30) Keuken, M. P.; Schoonebeek, C. A M.; van Wensveen-Louter, A; Slanina, J. Atmos. Environ. 1988,22,2541-2548. (31) Wyers, G. P.; Otjes, R P.; Slanina, J. Atmos. Enuiron. 1993,27A,20852090.
Analytical Chemistty, Vol. 67, No. 17, September 1, 1995 2763
Figure I. Schematic diagram of measuring system: DT, denuder tube; S,inlet subduction zone; OT, outlet tube; TH, top head; BH, bottom head; OR, porous O-ring; B, stock bottle with an absorption liquid; RC, restriction capillary; VG, vapor generator; Tm, thermostat; CT, charcoal trap; T, mixing T-piece; V, glass vial; PP, peristaltic pump; MP, membrane pump; pLC, picochromatograph and W, waste.
employed for the preconcentration of f ~ r m a l d e h y d e . ~ *The ,~~ ability of the WEDD to preconcentrate organics from air was reported for the preconcentration of gaseous 2,4,5trichlorophenol.34 The present paper describes the use of organic solvents as absorption liquids for the preconcentration of less polar gas-phase organic compounds, which are not well-concentrated by water as absorption liquid, from air by the wet effluent diffusion denuder, using 1,Cdichlorobenzene as a model compound. EXPERIMENTAL SECTION
Wet Effluent Diffusion Denuder. The WEDD consisted of denuder tube (DT), inlet subduction zone (S), and outlet tube (OT) assembled by two heads (Figure 1). The tubes were sealed in the heads by Teflon tape to avoid leaking. The untreated glass tube (15 cm x 0.75 cm id.) as the inlet subduction zone was used to adjust the laminar flow of sampled air into the WEDD. The denuder tube was a borosilicate glass tube (50 cm x 0.75 cm id.) of a length chosen to attain satisfactory collection efficiency (>99%). To ensure a wettable surface, the inner wall of the denuder tube was treated with 5 M KOH solution containing 10 g of silica 100 mL-l for 72 h after preliminary washing with water and acetone, respectively.Li An excess of the etching solution was then removed; the tube was washed with an aqueous suspension of silica particles (Aerosil 380, Degussa, Frankfurt, Germany) to create a uniform film of Aerosil particles on the denuder surface. Afte the excess suspension was drained, the tube was heated for 5 min at 700 "C and then slowly cooled to laboratory temperature. After cooling, the inner wall of the denuder tube was brushed to remove any excess of Aerosil particles, obtaining a smooth, wettable surface. The absorption liquid was fed to the denuder tube through the porous PTFE O-ring (OR) (PorexTechnologies, Fairbum, GA) (32) Dasgupta, P. IC;Dong, S.; Hwang, H.; Yang, H.-Ch.;Genfa, Z. Atmos. Environ. 1988, 22, 949-963. (33) Fan, Q.; Dasgupta. P. K. Anal. Ckem. 1994, 66. 551-556. (34) Zdrahal. Z.: Vei'eia, Z.1. Ckromatogr. 1994, 668, 371-374.
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located between the outlet tube (untreated glass tube 9 cm x 0.75 cm id.) and the denuder tube in the top head VH). A compact film of the absorption liquid flowed down continuously, and the analyte concentrate was aspirated at the bottom of the denuder tube through a slit between the end of the denuder tube and the inlet subduction zone at the bottom head (BH). The width of the slit was changed in the range 0.2-0.8 mm to ensure good aspiration for particular liquids. The surfaces of the appropriate tube ends creating the slit had to be carefully cut and polished to assure good aspiration of the absorption liquids. The porous O-ring was not used in the bottom head to avoid possible analyte adsorption. The air passes through the WEDD in the direction opposite to the absorption liquid flow. The WEDD was mounted vertically. Absorption Liquids. Redistilled water, surfactant solutions, and aqueous mixtures of water-miscible organic solvents were used. We tested M solutions of anionic heptanesulfonic acid, dodecyl sulfate (both sodium salts), and nonionic Span-20 (all from Merck, Darmstadt, Germany); aqueous mixtures (usually 50%v/v) of 1-propanol (Merck) 2-propanol, glycerol, and diethylene glycol (all from Lachema, Brno, CR); dipropylene glycol (Fluka Chemie AG, Buchs, Switzerland); and pure solvents 1-propanol and ethylene glycol (both from Fluka Chemie AG). The 30% (v/v) aqueous mixture of glycerol was used because of difficulties with feeding of more concentrated mixtures into the WEDD. Measuring System. The main parts of the measuring system are the WEDD, a generator of gaseous 1,4-DCB, and a picochromatograph with UV detection (Figure 1). The absorption liquid was delivered into the denuder tube from a pressurized stock bottle (B) through PTFE tubing with a restriction capillary (RC). Capillaries of various lengths were employed for different liquids to enable a range of liquid flow rates 60-350 pL min-I. The analyte concentrate (denuder effluent) was aspirated from the bottom of the denuder tube into a glass vial 0 with a peristaltic pump and taken from that vial to HPLC analysis.
The values of absorption liquid flow rate given are measured at the outlet of the WEDD. Evaporative losses of the absorption liquid during passing through the WEDD were determined as the difference between inlet and outlet flow rates of that liquid. The air was sucked into the WEDD by a membrane pump WP) located downstream of the WEDD, usually at flow rate 0.5 L min-I, and cleaned by a charcoal trap (0 The . standard air mixtures were prepared by mixing 1,4dichlorobenzene (1,CDCB) vapors with clean air in a T-piece 0 directly before entering the WEDD. The gas-phase 1,CDCB evolved in the vapor generatop (VG) was delivered to the mixing point in a nitrogen stream (0.2 mL min-1). The concentration of 1,CDCB was kept constant at 360 ng min-l (RSD = 1.3%). To test results for a lower concentration, a capillary dilutor was used to dilute 1,CDCBvapors by 1:15 before mixing with air. Analyte concentrates were analyzed by a liquid picochromatograph PLC-W5 @LC) with optical fiber W detection The detection cell was connected to the W detector LCD 2082 m o m , Prague, CR), which was set to 222 nm. Analyses were performed on a glass column (60 mm x 0.5 mm i.d.) packed with 5pm Silasorb CIS(Lachema, Bmo, CR). Acetonitrile-water (7030) as mobile phase was used at flow rate 5 pL min-l. The injection volume was 1.0 pL for aqueous solutions and 0.5 p L for pure organic solvents. The pure organic solvents were diluted 1:2 with water before the analysis to preserve good chromatographic performance of the column; the picochromatograph allowed this dilution directly at injection of sample. Two impingers with distilled water in series were used for saturation of air at relative humidity measurements. Water drops in the air flow were trapped in a third impinger with glass wool. To obtain air with the required relative humidity, we mixed saturated air with air dried through a silica gel cartridge. A similar procedure was employed for the saturation of air with 1-propanol. The collection efficiency of the WEDD is given by the ratio between analyte amount found in the denuder effluent (analyte concentrate) and its known amount entering into the WEDD.
Table 1. The Absorption Liquids Examined for the WEDD
name
mixture
water heptanesulfonic acid* (M)
dodecyl sulfateb (M)
pure 10-3 10-3
span 20 (MI glycerol (%,v/v) 2-propanol (%,v/v) 1-propanol (%,v/v)
10-3 30 50 50
1-propanol ethylene glycol
pure pure
diethylene glycol (%, v/v) dipropylene glycolb (%,v/v)
50 50
(35) KrejEi, M.; Kahle, V.J. Chromatogr.1987,392, 133-142. (36)JaneEek,M.; Kahle, V.; Krejei, M.1.Chromatogr. 1988,438, 409-413.
nda nd nd nd nd 62 75 96 65 5 3
a nd, 1,4-DCB was not found in denuder effluent (detection limit of HPLC analysis was 9.1 pg, RSD = 4.9%, injection volumes 1.0 and 0.5 p L were for aqueous mixtures and pure solvents, respectively). DifEculties with wetting of denuder wall.
Table 2. The WEDD Collection Efficiency vs Concentration of 1-Propanol in Aqueous Mlxture.
vol % of 1-propanol
collecn
effic (%) >1
20 30 40 50 60
13
42 76 85
vol % of
collecn
1-propanol
effic (%)
70 80 90 pure
86 87 89 96
a Air and absorption liquid flow rates were 0.5 L min-' and 330 pL min-l, respectively. The concentration of 1,CDCB was 720 pug m3.
100 1
............... -.....e....o
f'
RESULTS AND DISCUSSION
Absorption Liquid Evaluation (Table 1). If redistilled water is taken as an absorption liquid, concentrations of 1,4DCB in the denuder effluent are below the analytical detection limit for entrance concentration, 360 ng min-' (720 pg m-3). Aqueous solutions of tested surfactants showed no improvement in collection efficiency. In addition, poor wetting of the denuder tube by anionic surfactant solutions was observed. Aqueous mixtures of organic solvents (except 30%v/v glycerol) had a positive innuence on 1,CDCB collection efficiency in the WEDD. Aqueous mixtures (50% v/v) of glycols increased the collection efficiency only slightly, but the collection efficiency was 75% for a 50% (v/v) 1-propanol mixture at flow rates of air and absorption liquid of 0.5 L min-l and 330 pL mir-', respectively. The evaporative losses in a 50% (v/v) aqueous mixture of 2-propanol were twice those of the same 1-propanol mixture. 1-Propanol and its mixtures provided the best results. The collection efficiency was less than 1%at 20% mixture but rose to 75%at 50% 1-propanol mixture and 96%for pure 1-propanol (Table 2). The latter efficiency is close to the expected theoretically calculated value (99.5% at 25 "C). The RSD of these experimental data was 3.9%. On the basis of these results, 1-propanoland ethylene glycol were further tested.
collection efficiency (%)
0
d
"0
!00 200 300 absorptaon liquid flow rate [uL/rn.an]
Flgure 2. Dependence of collection efficiency of the WEDD on absorption liquid flow rate for pure 1-propanol (O),50% ( v h ) aqueous 1-propanol mixture (0),and pure ethylene glycol (m) at a 1,4-DCB concentration of 720 pg m-3. Air flow rate was 0.5 L min-I.
Absorption Liquid Flow Rate Dependence. Pure 1-propanol, its 50% (v/v) aqueous mixture, and pure ethylene glycol are compared in F i r e 2. For each liquid, the collection efficiency of the WEDD starts to decrease with decreasing absorption liquid flow rate at a particular given value: 150, 330, and 200 pL min-l for pure 1-propanol, 50%(v/v) propanol mixture, and pure ethylene glycol, respectively. The maximum collection efficiencies for pure 1-propanol and its aqueous mixture were 97 and 75%,respectively, while that for pure ethylene glycol was 65%, indicating that ethylene glycol is not a good absorption liquid for preconcentration of 1,CDCB in terms of Gormley-Kennedy equation. Air Flow Rate Dependence. At constant temperature, the flow rate of air streaming through the denuder is the only factor Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
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100-
\
b
m
'
,
o ! 0 0
0 5
1 0
15 UZT
20
25
flow rate
3 0
3 5
4 0
45
50
[i/mzn]
Figure 3. Dependence of the WEDD collection efficiency on air flow rate: solid line, theoretical curve according to Gormley-Kennedy; dashed line, theoretical curve according to Cooney-Kim-Davis (Sherwood number, Nshw -0.4, sticking coefficient y = 1.5 x 10-5); ( 0 )pure 1-propanol; (B)ethylene glycol. Liquid flow rates were 170 and 200 pL min-I for 1-propanol and ethylene glycol, respectively, at 1,4-DCB concentration of 720 pg m-3.
controlling its collection efficiency, according to the GormleyKennedy e q ~ a t i o n , which ~ ~ - ~can ~ be written for the used denuder tube in simplified C/C, = 0.819 exp(-3.6568xDL/F)
used for the theoretical curve. It is evident that 1-propanol complies with the assumptions of the Gormley-Kennedy equation. The collection efficiency for ethylene glycol does not reach theroetical values for the Gormley-Kennedy equation and strongly decreases with increased air flow rate (Figure 3), but the CooneyK i m D a v i ~equation, ~~ which accepted a sticking coefficient less than unity (Le., no perfect sorption), fitted these data with greater precision (dashed line in Figure 3). Data were taken also at a lower 1,4DCB concentration, with a diluted 1,CDCB standard air mixture (48 pg m-3); the results were consistent with those in Figure 3 for both solvents. Range of Operation Conditions. The detection limit of the measuring system was 5.8 p g m-3 (RSD = 4.9%) at air and liquid flow rates of 0.5 L min-l and 160 pL min-', respectively. The analysis time was 9 min, and the WEDD with 1-propanol worked without any difficulties in the temperature range 18-33 "C if correction for evaporative losses (25-40 p L min-l at an air flow rate 0.5 L min-') was performed to keep liquid flow rate above 150 pL min-'. The calculated decrease of collection efficiency with temperature due to the temperature dependence43of 1,4 DCB diffusion coefficient was 1%per 5 "C (calculated for 25 and 20 "C). The WEDD operated without a change of collection efficiency over a relative humidity range from 10 to 100%. Collection efficiency was also unchanged if the sampled air was saturated with 1-propanol.
(1) CONCLUSION
where C is mean concentration of the gas-phase trace analyte leaving the denuder tube, C, is concentration of the gas-phase trace analyte entering the denuder tube, D is diffusion coefficient of the gaseous analyte in sampled gas (air), L is denuder tube length, and F is volume flow rate of sampled gas. In view of the results in Figure 2, the absorption liquid flow rate has to be higher than 150 p L min-l for 1-propanol in the WEDD for the GormleyKennedy equation to be valid. Experimental values of collection efficiency for 1-propanol (Figure 3) and a range of air flow rates, 0.5-4.3 L min-l (at liquid flow rate 170 p L min-l, the 1,CDCB concentration was 720 pg m-3), agree with values calculated from the Gormley-Kennedy equation (solid line in Figure 3). The 1,4 DCB diffusion coefficient (7.29 x cm2 s-l) calculated according to the method of Fuller, Schettler, and Giddings41was (37) Gormley, P. G.; Kennedy, M. Proc. R. Ir. Acad. Sect. A 1949,52, 163-169. (38) Slanina. J.; de Wild, P. J.; Wyers, G. P. Gaseous Pollutants: Characterization and C y c h g Wiley: New York. 1992; pp 129-154. (39) Adam, K M.; Japar, S. M.; Pierson. W. R Atmos. Enuiron. 1986,20,12111215. (40) Ai, Z.; Thomas, C. L. P.; Alder, J. F. Analyst 1989, 10, 759-769. (41) Fuller, E. N.; Giddings, J. C. J. Gas. Chromatogr. 1 9 6 5 , 3, 222-227. (42) Murphy, D. M.; Fahey, D.W. Anal. Chem. 1987,59, 2753-2759. (43) Lugg, G . A; Anal. Chem. 1968, 40, 1072-1077.
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Organic solvents for preconcentration of the model organic compound were proved. 1-Propanol was found to be the most suitable absorption liquid for the preconcentration of 1,4dichlorobenzene from air in terms of the Gormley-Kennedy equation. Although ethylene glycol was not as good as 1-propanol for the preconcentration of 1,4dichlorobenzene by the WEDD, it is good wetting agent with low evaporative losses (up to 20 pL m i x 1 at air flow rate 0.5 L min-l). It is supposed to be a suitable absorption liquid for preconcentration of other organic compounds. ACKNOWLEDGMENT
This work was supported by Grant Agency of the Czech Republic under Project 203/93/2157. The authors thank Dr. V. Kahle for helpful technical assistance. Received for review February 20, 1995. Accepted May 30, 1995.B AC950189A Abstract published in Advance ACS Abstracts, July 15, 1995
