Anal. Chem. 1991, 63,1237-1242
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Wet Effluent Denuder Coupled LiquidAon Chromatography Systems Poruthoor K.Simon, P u r n e n d u K.Dasgupta,* a n d ZbynEik Ve6ei.a’ Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
The tHkd Itstrwnentatlon involves an absorber liquid flowing down the inner wails of a tube while sample gas flows countercurrent upward. The liquid h fully aspirated at the bottom and InJected, along with small amounts of air, perk odlcally into a chromatographic system. The presence of the ak In the inJected sample does not degrade chromatographk performance. Four distinct designs of the wet denuder are described and performance data for one type, coupled to a suppressed IC system, are presented for the determination of SO2. Good collection efficiency and excellent reproducIMilty are observed. I n laboratory studies, the estlmated detection limits are In the dngie digit pptrv range for a 7mln sample. Such diffusion-based coiiection/anaiysls systems with continuously renewed collection surfaces are expected to be generally applicable for the measurement of trace atmospheric gases.
Diffusion-based collection, designed to discriminate gaseous analytes from their particulate counterparts, is substantially superior to the use of filters to achieve the same separation; the latter technique is subject to many artifacts. Consequently, diffusion-based collection with ‘diffusion denuders” prior to analysis is commonplace today; some eight out of eighteen instruments deployed in a recent intercomparison study to measure nitric acid used such devices (1). The principles of their operation and various applications have been reviewed (2, 3). Although the operation of diffusion denuder based analyzers can be fully automated for some analytes, typically by cyclic thermal sorption/desorption (4-8),this is not a generally applicable technique. Bos (9) described an automated arrangement to solution-coat a glass denuder tube in situ, sample ambient air, wash to remove the coating, and determine the analyte in the washing with a segmented flow analyzer, the whole cycle being repeated. No efforts have been made by others to duplicate such an apparatus. A membrane-based denuder in which a scrubber liquid flows through a membrane tube and air is sampled outside it has been described (10); termed a diffusion scrubber (DS), this device produces a continuous liquid effluent that can be coupled to continuous or semicontinuous liquid-phase analyzers. Various applications of the DS, coupled to flow injection or chromatographic systems, have been demonstrated (11-141,and the instrumentation has fared well in both ground- and aircraft-based intercomparison studies (15,16). However, the overall collection efficiencies of DS devices are relatively low and as such they cannot be used for nearquantitative removal of gases, which is necessary if the chemical composition of the aerosol fraction penetrating the DS device needs to be determined. Further, the collection efficiency of a porous-membrane DS is subject to change due to deposition of particulate matter, necessitating periodic Permanent address: Institute for Analytical Chemistry, Czechoslovak National Academy of Sciences, Leninova 82,661142 Bmo, Czechoslovakia.
0003-2700/91/0363-1237$02.50/0
cleaning. A continuouslywetted denuder, in ita simplest form a tube in which a suitable scrubber liquid flows down the inside walls and is aspirated off a t the bottom, while air is sampled countercurrent through the device, is widely regarded as a useful solution because it can provide a continuously renewed high-efficiency collection surface. Although “wetted wall columns” resembling the above description have been reported for other applications (13, the liquid/air inlet/outlet geometries in these devices are not compatible with maintaining laminar flow; significant particle deposition is therefore likely to occur. Aside from designing appropriate liquid inlet/outlet geometry, another problem is to successfully maintain a uniformly wetted surface on the inside of a tube of reasonably inert composition with a relatively small flow of an aqueous liquid. One reported wet denuder of annular geometry (18)solves this problem by batch operation, partially charging the annular space with scrubber liquid and continuously rotating the device during operation to maintain an aqueous wall film. The operation is automated but discontinuous, and analysis is carried out off-line. It is possible to form a wall film continuously in small diameter glass coils in which the sampled air velocity is high enough to sustain the film formation. This was introduced 25 years ago for the collection of gaseous samples in connection with the Technicon Autoanalyzers (19)and has been used more recently for the collection of H202 and other gases (see, e.g., ref 20). However, these coils are subject to significant particle deposition; the exact extent depends on the particle size and flow rate (21). In this paper, we report a continuous wet effluent denuders coupled liquid/ion chromatography (WEDCLIC) system and demonstrate the efficient collection and ion chromatographic determination of SO2 as an example. EXPERIMENTAL SECTION Wet Denuder Designs. Four separate designs are described here and are shown in Figure 1. Active lengths in all cases are 30-40 cm. Device a is a glass-filled PTFE tube (A) originally 6.3 mm i.d. that has been provided with internal threads (7 mm, 1 thread/mm). Air inlets/outlets are PTFE tubes (D, E), and liquid inlet/outlet connectionsare made with push-fitted 25-gauge tubw (B,C). Device b contains a porous polypropylene membrane tubing (5.5 mm i.d., 1.5” wall, 0.2-fimpores, Accurel PP, Enka, Wuppertal, Germany) inside a rigid polymericjacket tube G (9.7 mm i.d.1 with polyethylene spacers H. Air inleta/outlets were PTFE tubes (I, J), and liquid inlet/outlet tubes were 25-gauge tubing (K,L). The air inlet tube at the bottom is put in with spacer M such that liquid collects in the resulting well. Design c contained a rolled sheet of very thin polycarbonate membrane N (Nucleopore sheet stock, Nuclepore Corp., Pleasanton, CA) put inside rigid polymer tube 0 (9.5 mm i.d.). Liquid enters through four symmetricallyplaced inlets P fed from a common source and is drawn off at the bottom at a single outlet Q where air inlet tube R is put in with a spacer S. Air leaves through T. Design d is based on an 8 mm i.d. soft-glass tube etched inside with a commercially available etching solution (Armour Etch, Armour Products, NJ). (CAUTION the etchant contains HF and appropriate care should be taken to avoid skin contact.) After thorough washing and drying, the tube is treated with tetraethylorthceilicate(E.Merck), which bonds to the surface. Before the above is completely cured, thin-layer chromatographicgrade silica gel (type 60 H, E. Merck, approximate particle size 2-25 0 1991 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 13, JULY 1, 1981
i-
w
I I-”-
E
AA
K
A
e:
X-
r--U L
1. Test wet denuder deslgns: (a)internallythreaded gless-fllledPTFE denuder; (b) wws-wa~denuder; (c) wettable ” - I i n e d (d) silica-coated denuder. See text for the identification of the components.
denuder;
Fburr 2. Complete WEDCLIC system: (A) peristaltic pump; (Ll, L2) injection loops each containing a preconcentratlon column: (V) dual-stack slider valve; (C) chromatographic column; (S) suppressor; (D) detector.
pm) is blown into the tube and bonds to the surface. After overnight curing at room temperature, excess silica was blown out with compressedair and the tube also copiously washed with water. PTFE air inlet/outlet tubes (U,V) to denuder tube W are put in place with spacers X and Y. At the top, a ring of filter paper Z is present between the denuder tube and the air inlet and ensures uniform distribution of liquid put in through finegauge needle AA; the liquid is aspirated at the bottom through BB. Denuder Liquid. Dilute HzOzis an attractive scrubber liquid for the simultaneous determination of acid gases by ion chromatography (IC). It oxidizes SOz to sulfate completely but the oxidation of NOz- to NO3- can be negligible (13). For denuders a-c, an aqueous solution of a nonionic fluorocarbon surfactant (Zonyl FSN, 0.3% w/v, DuPont) is paseed through a mixed-bed ion-exchange resin column and sufficient H202added thereafter to yield a concentration of 1 mM. The ion-exchange resin treatment is conducted to remove ionic impurities present in the surfactant that otherwise presents a significant chromatographic
background in zero-air chromatograms. For denuder d, the denuder surface could be wetted without added surfactant and the H2OZsolution was therefore used without added surfactant. The solutions are pumped at a constant rate either peristaltically or pneumatically with a precision-pressureregulator. Aspiration of the effluent is conducted with a variable-speed peristaltic pump (Minipuls 2, Gilson Medical Industries, Middleton, WI). System Configuration. The complete analysis system is shown in Figure 2. Ambient air, calibrant gas, or zero air is supplied to the denuder inlet with the switching accomplished by manual means or by appropriate valving (12) controlled by a programmable timer. Calibrant gas is generated from one of two SO2permeation wafer devices (VICI Metronics, Santa Clara, CA) with permeation rates of 74 and 21 ng/min respectively,with mass flow controllers (ModelFC-280,Tylan Corp., Torrance, CA) providing the necessary dilution. The denuder liquid effluent is pumped by peristaltic pump A into either loop L1 or L2 of a dual-stack slider valve V (Dionex Corp., Sunnyvale, CA). Each of these loops contains a concentrator column (AG-5, Dionex).
ANALYTICAL CHEMISTRY, VOL. 63, NO. 13, JULY 1, 1991
0.ring Porous Element
Liquid Out To Analysis Syslem Coupling Body
Male Nul
I
Air Inlet Tube
t
Figure 3. End fittings used for connecting the sliica-coated denuder to a second denuder or air In/out tubes. The top and bottom end Mngs are shown on the left and d@t respectively; the porous element Is not essentlal for the top end fitting. In any given position of the valve, one column is loaded while the other is flushed onto the analytical column C by pump P. The chromatographic system is a basic ion chromatograph (QIC, Dionex). The column/eluent/suppressor operating conditions have been previously described (13);sulfate elutes under these conditions within -7 min, and therefore valve V was programmed to spend -7 min in each position (ChonTrol CD-4S, Lindburg Enterprises, San Diego, CA). Quantitation was accomplished with an integrator (3394A, Hewlett-Packard). If response time is defined to be the time for the wet denuder effluent to attain 90% of its final analyte concentration as the sample stream is abruptly switched from zero air to the test gas stream (and vice versa), this is significantly lower in some cases than the chromatographic cycle time. The intrinsic response time is therefore determined directly by pumping the wet denuder effluent to the conductometric detector and switching the inlet gas from calibrant to zero and vice versa. Air-sampling rates through the denuder were maintained by mass flow controller F and suction pump Q. Determination of Collection Efficiency. Collection efficiency is determined by putting a second wet denuder or a bubbler in series after the test denuder and analyzing that absorber liquid in batch-mode experiments. Additionally,the collection efficiency is calculated from the known concentration of the calibrant gas, its flow rate, and the liquid-phase calibration of the chromatographic system with standard solutions of sulfate. If the collection efficiency determined by the second method falls substantially short of the value produced by the first, irrecoverable losses in the system are indicated. A particularly extensive set of measurements was carried out with the silica-coated denuder (type d). In this case, end fittings machined from poly(viny1edene) fluoride (PVDF) and containing apertures for liquid in/out connections were used to couple the wet denuder tubes to each other and to air inlet/outlet tubes. The bottom end fitting is shown in Figure 3 (right); liquid flowing down the denuder tube collects in a well aided by a ring of a porous material (analytical grade filter paper or porous polypropylene/PVDF; the porous polymer sheets are from Porex Technologies, Fairburn, GA) and is aspirated from there. The top fitting (left, Figure 3) is essentially the same as the bottom fitting except that the porous element is not essential. The pumped liquid fills the well and then overflows across the lips along the interior walls of the silica-coated
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tube. The bores of the air in/out tubes, the end fittings, and the denuder tube are maintained the same to preserve laminar flow. In this mode, experiments were conducted with a wet denuder, 9 or 38 cm in length, as the first denuder and a 30 cm long wet denuder as the second denuder to measure the collection efficiency of the first set (11). The collection efficiency of a 38-cm NaOHcoated denuder (coated from methanolic NaOH) was also determined for comparison, using a S c m wet denuder as the backup collector to measure the penetration. Particle Deposition Measurements. We generated NaNOs aerosol by nebulizing a 0.1 g/L solution of this salt with a Lovelace type nebulizer (22),modified for continuously feeding fresh solution into the reservoir. Filtered compressed air was used to generate the aerosol and resulted in a flow rate of 3.5 standard liters per min (slpm). This flow traveled up a glass tube (4 X 50 cm) to a 2-Lvolume large-diameter polyethylene chamber. The aerosol was sampled through the type d wet denuder via side porta in this chamber. Through an access port at the top of this chamber, microscope slides, appropriately treated with a heavy sticky grease (Fluorolube, Fisher Scientific) to retain impacted particles, could be placed to obtain particle samples. For photomicrographic measurement of particle size, the aerosol generation solution was doped with Rhodamine 6G to facilitate observations by fluorescence microscopy. A research grade microscope with photographic and epifluorescence attachments (Olympus Corp.) was used for microscopy, and the results presented are based on approximately 40 photomicrographs from 10 individual sample slides. For measurement of particle transmission through the wet denuder, 47-mm glass fiber filters (Whatman GF/B) were used as the backup collectors following the wet denuder. Nitrate concentration was measured in the aqueous extract of these fitera by IC; the sampled amount of nitrate was high enough to render corrections using blank filter extracts unnecessary. The nitrate deposited on the active areas of the wet denuder was measured directly by the denuder coupled chromatograph in the same way it is used for gas measurement. The mass of nitrate collected w i t h a given period in the denuder and fiter were then compared to obtain the fraction of aerosol deposited in the denuder. RESULTS AND DISCUSSION Wettability and Liquid Flow Rates. The active surface of denuders a-c cannot be maintained reliably wet with a purely aqueous liquid. Studies with various surfactants indicated that the nonionic fluorocarbon surfactant used is best suited for this purpose. It is effectively cleaned by the mixed-bed resin and does not affect the performance of the chromatographic column significantly. It does, however, have some effect on the fluorocarbon membrane based suppressor, a vital component of the ion chromatograph. The baseline drift and noise increases 2-3 times, resulting in a proportionate deterioration in detection limits. The amount of surfactant minimally needed to maintain an uniformly wet surface (this is readily apparent, upon careful visual examination, especially if a dye is incorporated in the liquid) is dependent on the liquid flow rate. With 0.3% surfactant (v/v), liquid input flow rates as low as 50-75 pL/min are able to maintain a wet surface in denuders a and b while dry air is sampled at 1 L/min. These denuders were operated at these low liquid flow rates. Under the same air flow conditions, denuder c required minimally 60 pL/min, it was operated at 60-100 pL/min liquid imput rates. If a sheet of tissue paper is substituted for the polycarbonate membrane, it is possible to dispense with the surfactant but an increased flow rate (- 200 pL/min) is necessary to maintain satisfactory operation. For denuder d, 100 pL/min is minimally necessary (no surfactant used) and 100-800 pL/min was utilized. Liquid Inflow/Outflow Balance: Considerations on Injection Strategies. A major difference between the operational requirements of a membrane-based DS vs the wet denuder is that the liquid flow conduit of the DS, starting at the input end and ending at the injection valve, is essentially a closed system. As such, maintaining a balanced liquid in-
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flow/outflow is simple. With the present denuders, this is not the case. Only if the temperature and the humidity of the sample air remain invariant is it possible to choose a liquid aspiration rate (for some fEed liquid input rate) that will result in neither any waste of the input liquid nor aspiration of any air at the liquid suction end. This cannot be attained in real sampling situations where the temperature and/or humidity of the sampled air varies on a temporal basis, causing variable evaporation losses in the denuder. Assuming that the relative humidity (rh) of the inlet air can vary from 10 to 100% (at 760 mm and 25 "C), theoretically, it is possible to lose 0-20 pL/min of influent water through evaporation per L/min of sampled air. Experimentally, we find that a maximum loss of 15 pL/min per L/min sampled air occurs up to sampling rates of 2 L/min and the loss rate greatly decreases at higher sampling rates; complete air liquid equilibration does not occur. Initially, we attempted to solve the problem of variable evaporation loss by active feedback. Aside from the principal liquid input flow, an auxiliary input flow channel, controlled through an on/off solenoid valve, was provided. A pair of 0.3 mm diameter stainless steel electrodes, 1 mm apart, were inserted a few millimeters above the liquid aspiration point and functioned as a rudimentary conductivity-based level detector. Through a simple voltage comparator circuit, the auxiliary flow valve was turned on when the sensor indicated that the liquid level had fallen below the sensor. Reproducible performance on a long-term basis was not obtained, whether this is due to any intrinsic defect in the scheme itself or the failure of the sensors to perform reliably with variable amounts of wet film clinging to them. Fiber optic sensors to replace their conductometric counterparts were contemplated, but it was fortuitously discovered that the performance parameters of the chromatographic system are quite immune to the injection of moderate amounts of air mixed with the denuder liquid effluent. The operating pressure of our chromatographic system is -11300 psi; this is equal to or less than the typical operating pressure encountered in most high-performance liquid/ion chromatography systems. Even under our operating pressure, it was found that the degassed eluent easily redissolves 100-pL (at ambient temperature and pressure) quantities of air. With the usual extent of flow restriction placed after the detector, no bubble formation problems in the detector are encountered. Although we chose to use concentrator columns in each injection loop, it is possible that simply large loop lengths will suffice. It has been shown that direct injection volumes can be as large as 2 mL without deterioration of chromatographic performance in both ion exchange and reversed phase liquid chromatography (23-27). As long as the sample solvent has a very low eluting strength, no problems, with the possible exception of accurate quantitation of very poorly retained analytes, are reportedly encountered. In operation, the aspiration pump rate was adjusted to take in virtually all of the influent liquid with no air flowing through the denuder. As such, during actual sampling the system is overaspirated and some air is taken in along with the denuder effluent liquid and pumped into the concentrator columns. This arrangement can be generally applicable for the determination of analytes that can be effectively retained on a preconcentrator column. Response Times and Other Considerations. Fast response times are desirable in many applications. Largely, the response time is a function of the liquid holdup volume within the denuder. For design a, thw is substantial, 1.4-1.5 mL, and results in very long response times at 1100 pL/min liquid flow rates. It is obvious that unless a much finer thread can be used,the response time will remain relatively large at practical
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Table I. Reproducibility of the WEDCLIC System at Low Concentrations of SO2 SOz concn, ppbv
re1 std dev, %
absolute std dev, pptrv SOz
0 0.700 0.875 0.994 1.575 1.750
1.00 0.36 0.46 0.15 0.07 0.17
1 2.3 3.2 1.5 1 2.9
liquid flow rates. The holdup volume in device b is ill-defined. A t the very low flow rates used, this denuder also produces a very long response time, -30 min. It appears that diffusive mixing in the pores of the relatively thick membrane effectively results in a thick f i and increases the residence time. In contrast to the above devices, the response times for both devices c and d were excellent, substantially below the cycle time of the chromatographic system. At the operative liquid flow rates, this was
