1134
Anal. Chem. 1993, 65, 1134-1139
Wet Effluent Denuder Coupled Liquid/ Ion Chromatography Systems: Annular and Parallel Plate Denuders Poruthoor K. Simon and Purnendu K. Dasgupta' Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Design and construction of high-efficiencywetted denuders of annular and parallel plate geometries are described. The silica-coated glass surfaces of these denuders are porous and are wettable with a very small flow of water. The parallel plate design is simpler in construction and offers better collection efficiency and lower particle losses than the annular design. Particle losses are, however, very low for both designs, 10.5% at the intended sampling rates. Collection efficiency for SO2 for two parallel plates (50 X 300 mm active area) separated by 3 mm is essentially quantitative at 10 L/min. Coupled to an ion chromatograph, the detection limit for SO2 for such a system is 500 parts per quadrillion for a 8-min sample. The use of diffusion denuders represents the method of choice in the current practice of the determination of trace atmospheric gases, especially when discrimination from concurrently present particulate matter is essential. Continuously wetted diffusion denuders are especially attractive because the collection surface is constantly renewed and their use also facilitates the development of automated analyzers. In a previous paper,' wet effluent denuder coupled liquid/ion chromatography systems (WEDCLICS) were introduced. In this and a subsequent article,2the denuders used were of the single tube design, with the scrubber liquid flowing down the interior walls of the tube and being collected a t the bottom while the sample air flowed upward in a countercurrent fashion. If it is necessary to maintain near-quantitative collection efficiencies, maximum attainable sampling rates with such denuders are 12 L/min for most analyte gases of interest. In many situations, greater sampling rates would be desirable without sacrificing collection efficiency; a greater sampled air mass generally translates into better limits of detection or better temporal resolution. Often, a diffusion denuder is used to remove acid/basic gases prior to the collection and analysis of particulate matter;3 in such cases, a high sampling rate is essential for the method to be practical. In conventional diffusion-based sampling systems, the annular design of Possanzini et al.4 has largely replaced single tube diffusion denuders. This geometry consists of a pair of concentric tubes where air flows through the annulus and the analyte gases of interest are collected by diffusion onto appropriately coated annular surfaces. Although the theoretical basis of ita operation was not outlined, an annular diffusion denuder with three concentric tubes was described more than 30 years The numerical basis of computing collection efficiencies in annular denuders has been estab( 1 ) Simon, P. K.; Dasgupta, P. K.; Vecera, Z. Anal. Chem. 1991, 63, 1237-42. (2) Vecera, 2.;Dasgupta, P. K. A n d . Chem. 1991, 63, 2210-16. (3) Spengler, J. D.; Brauer, M.; Koutrakis, P. Enuiron. Sci. Technol. 1990, 24, 946-56. (4) Possanzini, M.; Febo, A.; Liberti, A. Atmos. E n w o n . 1983, 17, 2605-10.
lished.6 Multiple annular denuders bearing as many as 12 concentric tubes have been described, and PC-based programs are available for computing the collection efficiency of such denuders.' With increasing diameter of a tubular annular system, the geometry approaches that of two parallel plates. The theoretical background for estimating diffusion-based collection for flow between two parallel plates is wellestab1ished;e diffusion batteries for removing fine particles by diffusion and assessing particle size distribution are commonly designed using flat parallel plate^.^ With the exception of the recent work of Eatough et al.,lo the actual use of flat plate denuder designs has not been much studied, despite their simplicity over the annular tubular designs. A single tube coiled tubular design has also been reported to have efficiencies comparable to the annular design'' but has not as yet been much used. Adapting the high-efficiency high sampling rate denuder designs to the corresponding wet denuders is a nontrivial task. However, a successful adaptation of these geometries to the form of wet denuders is likely to be of significant benefit to automated atmospheric analysis. This paper reports on the design and characterization of wet denuders in the annular and parallel plate geometries.
EXPERIMENTAL SECTION Wet Denuders. Silica Coating. It is necessary to render the denuder active surface highly wettable to form a thin aqueous liquid film on it. This is best done by chemically bonding porous silica to the surface.',* For the annular denuder (AD), made from pyrexglass tubes, the glass surface is prepared by thoroughly washing sequentially with water, 2 N NaOH, and water. The tube is then treated with 10 N NaOH containing 5% (w/w) dissolved Na2Si03.5H20, removed from solution, and maintained vertically for the next 30 min to drain out excess solution. A rudimentary aerosol generator to form a silica spray out of fine thin-layer chromatographicgrade silica (2-25 pm, without binder, Aldrich) is made from a 250-mL-capacitypolypropylene bubbler (Bel-Art, Pequannock, NJ). The cylinder is filled to about 70 mL with the silica. Compressed air is introduced through the sparging frit, and the resulting silica aerosol exits through the top tube of the bubbler. (CAUTION: Use a well-ventilated hood!) The finely powdered silica is sprayed evenly on the treated and still wet tube surface using a delivery tube from the aerosol generator until an apparently even thin coating is formed. The coated tube is next put into a furnace (supported from the inside when the outside surface is the one of interest) and the temperature is raised slowly to 700 "C over a period of 2.5 h, maintained at that temperature for 10-15 min, and then allowed to cool to room temperature over a period of several hours. It is (5) Pack, M. R.; Hill, A. C.; Thomas, M. D.; Transtrum, L. G. ASTM Spec. Tech. Publ. 1959, 281, 27-44. (6) Winiwarter, W . Atmos. Enuiron. 1989, 23, 1997-2002. (7) Coutant, R. W.; Callahan, P. J.; Kuhlman, M. R.; Lewis, R. G. Atmos. Enuiron. 1989,23, 2205-11. (8) Gormley, P. G. R o c . R. Ir. Acad. Sect. A 1938, 45, 59-63. (9) Friedlander, S. K. Smoke Dust and Hare; Wiley: New York, 1977; p 15. (10) Eatough, D. J.; Wadsworth, A.; Eatough, D. A.; Crawford, J. W.; Hansen, L. D.; Lewis, E. A. Atmos. Enuiron., in press. (11) Pui, D. Y. H.; Lewis, C. W.;Tsai, C. J.; Liu, B. Y. H. Enuiron. Sci. Technol. 1990, 24, 307-12.
0003-2700/93/0365-1134$04.00/0 0 1993 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 65. NO. 9, MAY 1. 1993
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IT
.
SN PF
SN
PG SA
IT
SN
(a)
(b)
(C)
Flgure 2. Wet parallel plate denuder: (a)overall appearance. (b) vkw of one plate. and (c)topcross section. GF'. glass plate: LI. hypodermic needle llquld Inlets: PG. Plexlglas spacer: TF, Teflon film: SC. slllca coating: LO, llquld outlet: A I . air Inlet (the outlet. not shown. is similar). A b
INLET
Flgure 1. Wet annular den&. OT. outer tube: LII. llquld Inlet to Inner tube: LIO. llquld Inlet to outer tube; IT. Inner tube: OR. O-rlng: C. caw:LC. Plaxlglas llquld collector cup; LOI. llquld o m t from Inner tube:OF,outerllttlng; LOO, llquldoutletfromoutertube: SN.supporting syrlnge needle: PF, Kynar porous frk ffi. top Plexlglas insert: SA,
silicone adheslve plug.
essential that the cool-down procedure be slow. A layer of soft porous glass with a different thermal coefficient of expansion is formedon thesurfaceoftheoriginalglassandcrackscandeveloD at the interface if cooling is not slow. The sheet stock for the parallel plate denuders (PPDs) were Vs-in.-thick picture framing glass-this begins to deform a t ca. 600 OC and melta at ca. 700 "C. In this case, the plate glass was prepared first by covering the edge areas that are not intended to beactive with Parafilmor maskingtape,etchingwithsaturated ammonium hifluoride solution for 1 h to increase surface area and enhance bonding of silica to the surface, and then following the same coating procedure as outlined above. Annular Denuder. The wet annular denuder is shown schematically in Figure 1. It is constructed of a 7-mm-0.d. 30cm-long borosilicate inner glass tube IT placed inside a IO-mmi.d. 40-em-long outer tube OT. Tube IT is drawn to a point and issealed atthe bottom,acupshaped liquidcollectorLC machined from Plexiglas is cemented to it. At the top of IT, another Plexiglas piece PG is machined and affixed in place with a plug of silicone adhesive SA. Pieces LC and PG are designed to promote the development of laminar flow. OT and IT each contain 0.75-mm-diameter holes (drilled with diamond-tipped bits) at120° anglesalignedtoeachother. Three 23-gaugestainlees steel hypodermic needle tubing SN, one set each a t the top and the bottom, are used to concentrically support IT within OT; these are cemented on the outside of OT once the alignment is satisfactory. The top inset shows the placement of the support structure. Wetting liquid is supplied to the inner surface of the annulus by pumping it through one of the support tubes, designated LII. The cavity C is filled and then it overflowsdown the outer surface of IT. To introduce the liquid for wetting the interior surface of OT, couplers OF, equipped with O-rings OR,
similar to those described previously2were used and cemented onto the termini of OT. Air flows inlout through 10-mm-i.d. tubes connected to these couplers. The liquid effluentfrom OT was aspirated through the bottom coupler through outlet LOO and that from tube IT collected in the cavity of LC and was aspirated from there through tube LOI. For either tube OT or ITtheinputflowrateand theaspirationflowrateswereidentical, 151F200pLimin. However,duetoevaporationlosses,theeffluent liquid volume is less than the input and some air is obligatorilv aspirated to ensure that no liquid is lost at the bottom. Parallel Plate Denuder. The inner (fluid wetted) face of one of the plates is shown in Figure 2. Typically each glass plate GP measured 600 X 50 mm (active coated area width 36 mm; in some cases, as specifically stated, the active area width was 50 mm). The silica-coated area SC is represented by the shaded region. The two plates are separated by a 3-mm-thick Plexiglas spacer PG, 600 X 1.5 mm: the spacers completely cover the untreated edges of the plates. Thin PTFE tabe T F is placed around the inner facing edge of each spacer to present an inert surface to thesamplegas. Theassembly is thenclamped together and silicone rubber adhesive isapplied along the edgestosecurely cement the plates and the spacers together. In each plate. ca. 12 cm is left uncoated at the bottom to allow the development of laminar flow before the active collection begins. At the top of the coated region, three evenly spaced holes LI (0.75 mm) are drilled. A t the bottom, one hole LO is provided just above the point of the vee. A rectangular rubber segment (ea. 6 m m thick, 40 X 8 mm, not shown) is cemented on the external face of the plate to cover up the holes LI and LO. Bluntend hypodermic needles (23 gauge) are inserted through each silicone rubber support to just enter the holes LI and LO. Liquid is pumped in parallel through all three needles, typically for a total flow rate of265jtLlmin. Itisaspiratedat thesamerateatthebottomport LO through another hypodermic needle supported in the same manner. The opposing plate has an identical arrangement. Air inlet or outlet connections (AI) to the denuder were made by thermally deforming and stretching thin-walled PTFE tubes of the appropriate diameter (ca. 10 mm for the PPD described) to a rectangular cross section at one end. Analytical System. The analytical system is similar to that described previously.' Three wafer-type sulfur dioxide perme-
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY 1, 1993
y l CLEAN
I
i
R
PULSE DAHPENER
SOURCE
-
?
3
- W A Y VALVE
AIR INLET VENT
Flgure 3. (a) Gas flow schematic: MFC, mass flow controliers. (b) Liquid-phase analytical system schematic: CC, preconcentration columns:
GC, guard column: AC, analytlcal column; S, membrane suppressor: D, conductivity detector.
ation devices (VICI Metronics, Santa Clara, CA) thermostated at 30 "C with gravimetrically calibrated respective emission rates of 21.1 i 0.2, 69.8 f 0.5, and 251 f 3.2 ngimin were used singly or in combination to generate gas standards. Mass flow controllers (FC-280,Tylan General, San Diego, CA)were used for flow control throughout. The air flow schematic is shown in Figure 3a; this configuration allows the denuder to sample pure air (generated by a pure air generator, Model 737, AADCO, Clearwater, FL) or SOz (where the concentration sampled and the sampling rate can both be independently varied) as determined by a perfluoroalkoxy Teflon three-waysolenoid valve (Galtek, Chaska, MN). The liquid-phase portion of the analytical system is shown in Figure 3b. The aspirated effluents from each of the plates in the PPD (or each of the tubes from the AD) are combined together and preconcentrated on a AG-5 preconcentrator column (all chromatographic components were obtained from Dionex Corp., Sunnyvale, CA). The eight-port dual-stack valve schematically shown in Figure 3b is configured with two such preconcentrator columns; while one is being loaded with the sample, the other is being eluted. Typically, the valve was switched every 8 min; all performance data cited pertain to this time resolution. As has been previously shown,' the chromatographic system is able to tolerate some injected air without deterioration of performance. Hydrogen peroxide (0.5 mM) was used as the denuder liquid. Following the valve, the chromatographic system (Model QIC) consisted of an AG-SA guard column, an AS4A separator column, an externally resin-packed helical tubular filament-filled Nafion suppressor*2regenerated with 4.5 mM H2S04at 10 mL/min, and a conductivity detector. The chromatographic eluent was 40 mM NaOH pumped at 675 pLimin. Under these conditions, sulfate eluted at ca. 8 min. Collection Efficiency and Evaporation Losses. The collection efficiency of individual denuders as a function of sampling rate was determined by two different methods: (a) comparison of the mass of sulfate collected by the denuder (calculated from the calibration of the chromatographic system with aqueous standards) and (b)operating two identical denuders in series, measuring the relative amounts collected in each, and thereby determining the fraction collected.13 Both methods yielded essentially the same results and are not separately reported. Denuder evaporation losses were determined by gravimetric measurements of the effluent liquid. (12) Gupta, S.; Dasgupta, P. K. J . Chrornatogr. Sci. 1988, 26, 34-38. (13) Dasgupta, P.K.;Dong, S.; Hwang, H.; Yang, H.-C.; Genfa, 2. Atmos. Enuiron. 1988, 22, 949-64.
Aerosol Generation and Evaluation of Particle Deposition. A Lovelace-type nebulizer was used to generate sodium nitrate aerosol from solution. An excess of dry dilution air was used to dry the aerosol before sampling by the denuder. The particle size distribution was determined with a laser-based multichannel optical particle counter (Model A2212-01, MetOne, Inc., Grants Pass, OR). Particle losses in a dry denuder could be determined by the same counter by measuring the inlet and outlet particle concentration. However, this approach is not applicable with a wet denuder. The test aerosol particles grow significantly in the highly humid environment within the denuder. This leads to the paradoxical result that the total outlet particle count is higher than that at the inlet. In reality, the inlet population contains a large number of small particles that are invisible to the counter. These become detectable after deiiquescent growth, and as may be expected, the mean particle size increases upon passage through the denuder. Because of this complication, we chose to measure the mass concentration of the aerosol at the outlet and the inlet of the denuder using collection on a glass fiber filter (Whatman GFIB), aqueous extraction, and ion chromatographic determination of nitrate in the extract. However, since this constitutes a small difference between two large numbers, the uncertainty of such a measurement is large, especially since the aerosol output of the generation source may vary within the time the inlet and outlet concentrations are separately measured. We chose therefore to measure the mass of nitrate in the liquid effluent of the denuder and thus evaluated the extent of deposition that occurs on the active surfaces of the denuder; these data are reported as a fraction of the mean inlet concentration.
RESULTS AND DISCUSSION S u r f a c e Coating and Liquid Film Thickness. A photomicrograph of the active surface is shown in Figure 4. The surface is uneven in texture; roughly hemispherical agglomerates approximately 100-150 l m in diameter are present. If a plane surface is fully covered with hemispheres, the overall surface area will approximately double. The surface is also highly porous, and this enables liquid to wet it easily by capillary action. The thickness of the falling liquid film is likely not uniform along the entire height of a denuder that is oriented vertically during operation. Additionally, the film thickness is dependent on the input flow rate of the scrubber liquid and increases with increasing flow rate. Conversely, it decreases with increasing air flow rate. Nev-
Flpun 4.
Photomicrograph of the silicacoated surface
ertheless, the mean film thickness under any given set of operating conditions can be determined by injecting, for example, some NaN03solution, to the input liquid a t the top of the denuder and measuring the mean residence time with a conductivity detector placed immediately a t the denuder liquid effluentport. Thus, the mean holdup volume can then be computed from the effluent flow rate. The mean film thickness is then obtained from considering the total active area. Experiments with the PPD with an input flow rate of 650 pL/min and no air flow resulted in a calculated mean film thickness of ea. 47 pm. If the actual surface area is double that of the nominal flat surface area, the actual mean film thicheaswill beca.25pm. Inoperation,withairflowpreaent, the film thickness will be even lower. A thin film is essential for the denuder to respond rapidly to changes in the input gas concentration and minimize sample carryover between successive chromatographic runs. With both of the present denuders, when the gas input a t the denuder was abruptly switched, the new steady-state signal was fully established by the second injection cycle after the step change a t the input. Evaporation Losses and Surface Wetting. The reflectivity of the coated surface is markedly different in wet v8 dry conditions. Consequently,dryspotsinthedenuderare readily visible in operation. It is obvious that dry areas muat be avoided to allow the denuders to function with optimum efficiency. Developmentof dry areas is, of courae.more likely a t higher air flow rates and lower liquid flow rates. As a general rule, it is preferable to keep the liquid flow rate as low aa possible-this increases the analyte concentration in the effluent and is advantageous for analytes where preconcentration of the analyte on a column is not possible (e.g., for
HzOz). With the p a r d e l plate denuder, no dry spots develop in the denuder up to air flow rates of 16 standard liters per minute (SLPM) with effluent liquid flow rates as low as 100 pL/min per plate. The hydraulic cross section of the AD is lower, however (by a factor of ca. 2-5 relative to the PPD dimensions shown in Figure 2), resulting in a corresponding higherflowvelocityat thesamevolumetricflowrate. Clearly discernible dry areas develop a t the air entrance end of the AD a t air sampling rates in excess of 10 SLPM (this amounts to a flow velocity of about 31 m/s under our operating conditions of temperature and pressure) at most practical liquid flow rates. Significant amounts of the liquid input are evaporated during transit throughthe denuder. Relative to thesaturation water content of the sample air a t the inlet air temperature, the water content of the exit air can be substantially smaller.
0
0
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0.0
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*ir Flow rote. SLPU
20.0
L.O\
0.w 0.20 0.m 0.-, Recipracol Flow Role (SLPM)
Rgurs 5. (a)Ewaporat!a loss per UnnvohAne oi ah as a hmctbn oi Row rate. (b) Data of (a) plotted with Uw reclprocai Row rate as Uw abscissa; Uw solid line is Um best flt accadlng to eq 1.
First,dueto thesignificant latent heatofevaporationof water and near-adiabatic cooling. the temperature of the wetted surface is substantially below ambient. For example, with the PPD operating a t an air sampling rate of 15 SLPM and an inlet humidity and temperature of 530% and 23 "C, respectively, within a few centimeters of the inlet, the temperature of the PPD, as measured from the externalglass aurface,dropato 11.8'C. Figure5ashowatheexperimentdy observed amounts of water evaporated per standard liter of air as a function of air flow rate. Aside from the temperature, a t high flow rates, the equilibration of the air flow through thedenuder with thewatersurface becomesadominantfactor. This consideration is the same as the efficiency of mass transport to the wall used in calculating the efficiency of denuders for collecting analytegases and should be governed hy an equation of the type l-aW=be*'4
(1)
where a, b, and c are constants, Q is the volumetric flow rate, and W is the amount of water evaporated per liter of air. Figure 5b shows W plotted against 1/Q with the solid line representing the best fit to eq 1 (a = 0.0259, b = 0.735, c = 2.855). Collection Efficiency. Ali etal.I4havecarefullyevaluated the available literature on annular and parallel plate denuder geometries. On the basis of the original work of Gormley,B they concluded that the collection efficiency f for a PPD is (14) Ali, Z.; Thomas, C. L.P.; Alder, J. F.Analyst 1988,114,75949.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY 1, 1993
i
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1
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Best Fit to Eq. 2 - - Theoretical fit to Eq. 2
0
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AD
J
om0Experimental Dota
assl,Experimental Data
0
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-
-
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4
6
0
best expressed by 1- f = 0.91e-? i 7 n D L B
(2) where D is the diffusion coefficient of the gas, L is the length of the tube, and Q is the volumetric flow rate. For a PPD, the parameter a is given by a = bia (3) where a and b are the short and long dimensions of the denuder cross section, respectively. Ali et al.14 have further argued that consideration of the AD as an extension of the parallel plate geometry suggests that eq 2 is also valid for the AD with the provision that a be defined as a = * ( d o + d,)/(d, - d,)
(4)
where do and d, are the outer and inner diameters defining the annulus, respectively. The applicability of eq 2 assumes, of course, that the uptake coefficient a t the wall is essentially unity; this assumption is not always valid.15 We chose SO2 as a test gas to determine collection efficiencies because it is a well-behaved analyte gas that does not suffer from major adsorption problems on the transmission conduits, etc. Further, the diffusion coefficient of SO2 is at the low end of the common atmospheric gases of interest. Therefore, it may be expected that the collection efficiency of other analyte gases, e.g., NH3, HC1, HONO, etc., will be a t least as good. The experimental data for two parallel plate denuders (PPD-1, 36 X 300 mm; PPD-2, 50 X 300 mm, each with a 3-mm gap) and the annular denuder are shown in Figure 6. In each case, the experimental data, obtained typically at low to sub parts per billion (ppbv) SO2concentrations, are shown as the points with error bars spanning fl standard deviation. The uncertainty in the collection efficiency values computed is dependent on the accuracy and stability of the calibrant concentration, however, and can potentially be higher than the uncertainty of the measured response shown in Figure 6. At low sampling rates, the influence of the impurity levels of sulfate contained in the H202scrubber liquid may also become nontrivial and calculated collection efficiency values tend to marginally exceed unity. These are shown as unity, however, for consistency. In each case, the dashed line represents the theoretically calculated collection efficiency from eq 2 with the value of a being computed from the known physical ~~
(15)Murphy, D. M.; Fahey, D. W . Anal. Chem. 1987, 59, 2753-9
Figure 7. Triplicate chromatograms resulting from sampling (leftsue) clean air or (right side) 0.019 ppbv SOn. Peak A is due to an unknown analyte that Is leached from poly(vinyichloride) pump tubing, B Is from Con,and C is sulfate.
dimensions of the denuders. The diffusion coefficient of sulfur dioxide was assumed to be 0.13 cm2/s(value a t 20 OC).14The solid lines shown for the two PPDs are best fits to eq 2, with a being treated as an adjustable parameter. These best-fit values of a are 2-3 times higher than the value of a computed from physical dimensions. Earlier, Tanner et a l . l 6 suggested that corrections for increased or decreased surface area relative to a conventional annular geometry could be made by applying an appropriate multiplier to the exponential term. This explanation would clearly be attractive in the present case because the active area of the coated surface is much higher than the nominal area, as previously noted. However, placing a thin, flat, wettable polysulfone membrane (Supor 800, Gelman Sciences, Ann Arbor, MI) on the coated surface did not markedly change the observed collection efficiency. The surface of the membrane is smooth, and if increased surface area due to surface roughness is the reason for collection efficiencies greater than those calculated, we would have expected a significantly lower collection efficiency. Moreover, the behavior of the annular denuder much more closely approximates theoretical expectations-the less than theoretically expected collection efficiency at the higher sampling rates may be due to (a) the effective temperature of the denuder being significantly lower than 20 OC, and thus the diffusion coefficient of SO2 being lower than that assumed, and (b) the development of (not obviously visible) dry spots on the wetted surfaces that result in a decreased collection (16) Tanner, R. L.; Markovits, G. Y.; Ferreri, E.M.;Kelly, T. J. Anal. Chem. 1986, 58, 185745.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 9, MAY 1, 1993
1
jo0
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0
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0
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24 6 12 18 Time (hours)
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Flgurr 8. Ambient air levels of SO2, Lubbock, TX, for a 4 8 4 period beginning in the morning of May 1, 1991. 3
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Flgurr 0. Partlcle losses in a parallel plate and an annular denuder. The inset shows the sire dlstrlbution of the tnlet NaN03 aerosol.
efficiency. Note that no satisfactory fit of the experimental data for the annular denuder to eq 2 was observed; the solid line shown is a fit to an equation where the preexponential constant 0.91 is not a constant but is also an adjustable parameter. Previously we have speculated' that, with countercurrent liquid and air flow, oscillating wave patterns may be set up in the liquid film and this surface undulation may be responsible for a nonstagnant boundary layer, leading to enhanced collection efficiencies. However, extensive experiments with cocurrent and countercurrent flow were conducted with the PPD devices during the present experiments and no significant differences in collection efficiencies were found. The reason for the supertheoretical collection effi(17)Lindgren, P.F.;Dasgupta, P.K.Anal. Chem. 1989, 61, 19-24.
1150
ciencies exhibited by the PPDs therefore remains an open question. Response Linearity and Detection Limits. The overall analytical system showed excellent linearity of response. For a PPD device operated a t a sampling rate of 10 SLPM, the observed response in a concentration range of 0.01940 ppbv could be described by a linear equation (r2 = 0.9996, uncertainty of slope 0.3%). The slope of a log-log plot was 0.996f 0.003, essentially indistinguishable from unity. Figure 7 shows triplicate chromatograms resulting from sampling clean air and 0.019 ppbv S02. Reproducibility of the sulfate peak (C) a t this level of SO2 was 0.8% in relative standard deviation. The precision level of the blank (clean air) signal was equal or better. These data lead to a computed limit of detection of O.OOO5 ppbv, well below any previously reported technique. The results with the AD are marginally worse; a detection limit of 0.0023 ppbv was computed for a sampling rate of 8 SLPM. Peaks B and A in Figure 7 are due respectively to COz and an unknown component that leaches from the PVC pump tubing used to pump the sample from the wet denuders to the preconcentrator tubing. The latter peak decreases in intensity with continued use of the tubing. The ambient concentration of SO2 at our location over a 48-h period measured with the PPD-IC analyzer is shown in Figure 8. The low levels observed are consistent with previously reported measurements a t this location.lJ7 Particle Losses i n t h e Wet Denuders. Efficient transmission of the particles is a vital performance parameter of a denuder. Sodium nitrate aerosol (density 2.26) with a concentration of 28 pg/m3 and a mass median diameter of 0.51 pm was used for these experiments. The inset of Figure 9 shows the particle count data fitted to a lognormal size distribution; the calculated value of the geometric standard deviation a, was 1.66for this distribution and the mass median aerodynamic diameter was 0.80 pm. The resulta of the deposition experiments are shown in Figure 9. The overall particle losses are quite low for both denuder types, especially at higher flow rates where they are intended to be operated. The present wet denuder designs are obviously also applicable to nonaqueous liquids as wetting agents. In particular, the parallel plate design is simple and represents an efficient diffusion-based collection system with low particle losses that is readily coupled to an automated liquid/ion chromatography system.
ACKNOWLEDGMENT This work was supported by the Office of Exploratory Research, US. Environmental Protection Agency through Grant R815928-01-0. However, the manuscript has not been subject to review by the EPA and no endorsement should be inferred. David Purkiss is thanked for the photomicrograph. RECEIVED for review June 3, 1992. Accepted December 30, 1992.
