Anal. Chem. 1991, 63, 2210-2216
2210
Measurement of Atmospheric Nitric and Nitrous Acids with a Wet Effluent Diffusion Denuder and Low-Pressure Ion Chromatography-Postcolumn Reaction Detection Zbynek Ve6ei.a' and P u r n e n d u K.Dasgupta* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
The Interlor wall of a borodlkate glass tube b modifled to form a layer of porous Wt glass that Is highly wettable. water b mado to flow down the wan ol this tube whlk uynple air moves upward in countercurrent flow. The effluent liquid k &bubbled, and the anionic constituents are preconcentrated on a 3 mm long anbtbexchange cdunn. Every 11 min the trapped specks are eluted from thk column to an 11 mm long main separation cdumn where NO2- and NO,- are separated. The separated ions flow through a c a d " reductor to convert NO,- Into NO2-, and Qrhss-baltzman reagent Is then added. ForOwlng a 3-min reactlon the, the product dye Is detected at 555 nm. For a 50wet effluent diffusion denuder, ementlally quantitatlve collection eff lclency Is observed for both HONO and HNO, for sampling rates up to 2 Umln at 23 A 2 O C . The system b relatlvdy i n e w e and under m a l operating condnlonr provides IknRs of detection ( S / N = 3) of 80 and 230 parts per trllllon by volume (pptrv) HONO and HNO,, respectively.
INTRODUCTION The importance of both HONO and HNO, in atmospheric chemistry is well recognized (1-5). The bulk of the extant data concerns HNOP However, the role of HONO as a major source of daytime 'OH is well-known, and there is also concern regarding ita role in the formation of carcinogenic nitrosamines (6). The formation of HNO, occurs dominantly via a reaction of NOz, 'OH, and a third body, and formation mechanisms are relatively well understood (1-5). The entire issue of a major journal has recently been devoted to the chemistry of NO,'; the latter also ends up often as HNO, through a variety of reactions (7,8). In comparison, formation mechanisms of HONO are ill-characterized; it has been suggested that daytime formation involves NO and 'OH/H02' and nighttime formation involves NO2 and H 2 0 with or without NO; it is also present as a primary pollutant in auto exhaust (2,8-12). The fact that formation mechanisms for HONO, especially in daytime, are poorly understood is traceable in part to the lack of affordable sensitive measurement methods that can provide good time resolution. Differential optical absorption spectroscopy (13, 141, responsible for most of the extant HONO data, does not have sufficient sensitivity for reliably measuring daytime HONO levels, even in relatively polluted atmospheres (15). A photofragmentation laser-induced fluorescence technique (16)is sufficiently sensitive but is likely to be affordable by few. The long sampling times used with sorbent-coated denuders (I 7-22) make collected NOz- susceptible to oxidation by concurrently present O3(231,while sources for positive errors in this technique include the production of NO; from NOz and peroxyacyl nitrates on alkaline Permanent address: Institute of Analytical Chemistr Czechoslovak Academy of Sciences, Leninova 82,611 42 Brno, &echoSlovakia. 0003-2700/91/0383-2210$02.50/0
denuder surfaces (20,24). Only recently, convenient sensitive techniques based on chemiluminescence measurement (25) and diffusion scrubber-coupled ion chromatography with UV detection (IC-UV, see ref 26) have been introduced; both permit good time resolution. The use of fiiter packs (27,28) for nitric acid measurement typically involves a Teflon prefilter to remove particulate nitrate followed by a filter that captures HN03, typically Nylon. This method has currently fallen into disfavor because of retention of HNO, on the front-filter either directly on the filter material or on concurrently collected particulate matter as well as the possibility that NH4N03collected on the prefilter can either evaporate or release HNO, by reaction with acidic aerosol. Present practice is dominated by the use of various combinations of diffusion denuders and filter packs to individually determine both gaseous and particulate nitrate; the analytical determination in both cases is provided by ion chromatography (IC) subsequent to collection (29-33). The artifact formation of nitrate is always a major concern in this approach and much potential exists for positive error unless sorbents are carefully chosen (34, 35). Thermal sorption/ desorption cycles coupled to gas-phase NO, analyzers constitute an attractive approach first used by Braman et al. and McClenny et al. with tungstic acid coated denuders (36,37). A number of subsequent studies (38-40) have revealed, however, that the method is susceptible to unknown interferences. Another thermodenuder that determines both HN03(g) and aerosol ",NO3 has been described (41);it is substantially more complex. For many water-solubleatmospheric gases, it is convenient to determine the corresponding aqueous-phase analyte after collection. A generally applicable solution to diffusion-based collection of such gases is a continuously wetted, liquid effluent diffusion denuder, hereinafter called a wet effluent diffusion denuder (WEDD).Keuken et al. (42)described a wet annular denuder that is rotated to maintain a continuous water film; the absorber was periodically removed for analysis. This arrangement was undertaken because it proved extraordinarily difficult to maintain a continuously wetted surface on any reasonably inert tube with a small flow of water. More recently, the above was achieved and such a continuously operating WEDD was coupled to an IC for automated analysis (43).
The determination of atmospheric HONO and HN03 is of considerable current interest. We feel that a more elegant solution than denuder collection and analysis by suppressed conductometric IC is available for this problem. Such a solution may be substantially less expensive without a major sacrifice in sensitivity. The determination of aqueous nitrite is sensitively carried out by the Griess-Saltzman colorimetric procedure (44,45). Nitrate can be reduced to nitrite most conveniently by a cadmium reductor; this technique has been thoroughly reviewed (46).Since even a rudimentary IC system should be capable of separating NOz-/N03-,such an ion-exchange separation system can be coupled to the completely 0 1991 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
2211
Flgure 2. Instrument schematic: (D) wet denuder,(MFC) mass flow
controller; (P) air pump; (Pl-P5) peristaltic liquid pumping channels; (DB) debubbler; (0)glassneflon caykent wet valve; (S) Sway soknold valve; (V) sport Injector; (PC) preconcentration cokmn; (C) separation column; (R) Cd reductor; cr) reagent introduction tee; (MC) mixing coil.
Fbure 1. Wet efflent diffusion denuder (WEDD) schematic: (01, G2) air inletloutlet glass tube, 8 mm i.d.; ( L l , L2) liquid inlet/outlet ports; (M) main denuder tube with inside layer of porous wettable glass; (0) O-ring seals; (P) porous PVDF ring; (S) silicone rubber adhesive; (T)
Inlet FEP tube.
specific detection system above. In this report, we describe an inexpensive dedicated analyzer based on these principles, coupled to an improved version of the WEDD.
EXPERIMENTAL SECTION Wet Effluent Denuder. In previous work (43), silica was bonded by tetraethylorthosilicate to the interior of a glass tube to create a water-wettablesurface. The proceduredescribed below results in an inorganically bonded WEDD that shows no loss of wettability over several months. A Pyrex tube (typically 50 cm X 8 mm i.d.1 is thoroughly washed sequentially with detergent, water, methanol, and acetone. A 40-g sample of thin-layer chromatographic grade silica gel (Type 60 HI E. Merck) is dissolved in 340 g of 5 M NaOH by stirring over 2 days. The inside of the clean glass tube is treated with this eolution overnight. After draining the solution f i e silica gel (Type 60 H, E. Merck) is blown into the tube. After drying, loose silica is removed form the interior by tapping and the tube is heated in a flame with gentle rolling. Between 800 and lo00 “C, the interior layer of the soft glass melts and bonds to the Pyrex tube. (CAUTION: the heat treatment must involve half of the tube at a time. Between the two treatments, care needs to be exercised in handling the very hot tube and choosing a suitable insulating surface on which it can be placed for cooling.) A WEDD unit complete with integral polycarbonate end fittings (these do not contact the sampled air) is shown in Figure 1. The male half of each fitting is cemented with silicone sealant to the glass tube. The bottom end of Figure 1 shows the malelfemale halves of the coupling disconnected,and the top end shows them connected. Water is pumped in at the top through the 1/4-28 threaded fitting and the porous ring (cut from 3 mm thick sheets of porous polyvinylidene fluoride (PVDF) with circular cutters; Porex Technologies, Fairburn, GA) uniformly distributes the input liquid. Liquid is aspirated at the bottom with a peristaltic pump. Since the aspiration rate is maintained equal to or greater than the input rate and some evaporation of the input liquid occurs, aspiration of some air occurs as well. The surface roughness of the interior of the W D D is somewhat smaller than the previous silica-coated denuder (43);the latter is estimated to have a root mean surface roughness of 50-100 pm. Particle loss studies were conducted for this denuder, and loss was found to be negligible. Cadmium Reductor. Of the many methods in the literature on the preparation of cadmium reactors, the only approach we have found to produce reproducible results is due to Patton (47); a brief description is given here. Cadmium powder (10 g, -100 mesh, Aldrich) (CAUTION: cadmium dust is extremely toxic; wear gloves and work in a well-ventilated hood) is washed twice with 10 mL of 10% v/v HCl, allowing to soak each time while being stirred with a spatula. It is then washed several times with
imidazole buffer (0.1 MI 6.8 g/L, pH adjusted to 7.5 f 0.1 with concentrated HCl, ca. 4 mL) and the surface is then activated by treating it with 10 mL of a Cu2+solution (1:l mixture of the imidazole buffer and 2% w/v CuS04-5HzO). Over several min, with continuous stirring, the blue solution becomes cloudy from the formation of colloidal Cu. This treatment is repeated a second time and the copperized Cd is then repeatedly washed with the imidazole buffer until the decantate is clear. Finally, it is washed twice with 25mL portions of the chromatographiceluent (40mM NazB40,.10Hz0;10 mM NaBr, 10 mM K 8 0 4 adjusted to a pH of 7.7 f 0.1 by the addition of HCI). Cadmium prepared thia way is stable for several months in a closed bottle under the eluent. The packed bed reactor is prepared by using 1.5 mm i.d. Teflon tubing (15 gauge, standard wall, Zeus Industrial Products, Raritan, NJ). Glass wool can be used as the bed supports but frits cut out of porous PVDF are preferred. An 80 mm long column bed is prepared by slurry-packingthe wet Cd suspended in the eluent. Teflon tubing, 1.5 mm o.d., was used for all connections and was readily connected to the reactor by forcible insertion and wire crimps. System Configuration. Figure 2 schematically shows the analytical system. Air or calibrant gas was sampled through the WEDD (D) (at 1L/min unless otherwise stated) controlled by a Model FC-280 mass flow controller (Tylan General, Torrance, CA) and pump P. Five pumping channels (Pl-P5) are necessary and were provided by Minipuls 2 (Gilson Medical Electronics, Middleton, WI) and/or Model XV (Alitea USA, Medina, WA) peristaltic pumps. The rotary six-port injection valve (V) shown in the load mode contains the preconcentration column PC, made from 1.5 mm i.d. tubing and containing a 3-mm bed of a strong base type anion-exchange resin (Dowex 2-X-8,400 mesh). Valve V was either an electropneumatically actuated valve (5020P, Rheodyne, Cotati, CA) or an electrically actuated valve (Type HVXL-6-6, Hamilton Co., Reno, NV). Pure water is pumped at 280 pL/min by P1. At the bottom end, channel P2 aspirates at a rate of 160 pL/min. Either an aqueous calibration standard (0.1 pM each in NOz- and NO3-; when prepared in high-purity water it is stable for 1 week at laboratory temperature) or the denuder effluent is selected by a three-way all-fluorocarbon solenoid valve S (Type 075T3, 12 V, Biochem Valve Corp., East Hanover, NJ) for aspiration by P2. The aspiration is either through PC or bypasses it depending on the position of V. Typically, in field application, the aqueous calibration standard is sampled only once or twice a day, in two sequential replicates each time. Otherwise, during normal operation, P2 aspirates the denuder effluent through a small glass debubbler DB (volume 500 pL). Excess liquid (and air) are aspirated by pump channel P3,this flow rate is not critical as long as it is greater than that of P2; P3 was maintained at 1 mL/min. The chromatographic eluent is pumped by P4 at 80 rL/min through V and chromatographic column C (1.5 mm i.d., 1 0 ” bed of Dowex 2-X-8,400 mesh) and through the cadmium reductor R. Griess-Saltzman reagent consisted of 1 g of N-1-naphthylethylenediaminedihydrochloride, 5.5 g of sulfanilic acid dihydrate, and 100 mL of concentrated HCl per 1 L of solution. In our experience, this solution needs a storage time of 8 h after preparation to avoid bubbles in the flow stream. The reagent is pumped at 50 rL/min by P5 and merges with the reactor effluent R at a tee T. The
2212
0
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991 v A
I ?
\
T Figure 3. Calibration source schematic: (A) glass chamber containing NH,NO,(s)-filled tube GT; (B) bottle containing H,S0,:HN03 with HN03 vapor permeating through Teflon loop TL.
mixture then proceeds through a 1.5 m length 0.55 mm i.d. mixing coil that provides 2.75-min residence time at the cited P4/P5 flow rates to a visible absorbance detector set at 555 nm (Model 757, Applied Biosystems, Ramsey, NJ). Some experiments were conducted with a homemade detector based on a high-intensity green light emitting diode (LED, Stanley HBG5566, Component Technology, Richardson, TX) and a log ratio amplifier (557N, Analog Devices, Norwood, MA) serving the reference and detector photodiodes to provide the absorbance output. Detector outputs were recorded on strip chart recorders (Knauer TY-2 or (Omniscribe B-2000). System timing was controlled by a programmable timer (CD-4S, Chrontrol, San Diego, CA). Gas-Phase Calibration Sources. A source for HONO(g) based on the NH4N02(s)= NH,(g) + HONO(g) equilibrium was recently developed (26). The output of this source in direct flow-through mode is too high for convenient generation of very low levels of HONO; very small flow-throughrates, operation at low temperatures, and high dilution flows are required. We adapted this source to a diffusion-tube format: a 3-mm bed of NH4N02(s)was packed into a bottom-sealed 1.5 mm diameter X 18 mm high poly(viny1 chloride) tube, and the open end was sealed by a porous PVDF frit 3 mm in thickness. The nitric acid source is permeation based; a solution of composition 3:l v/v concentrated H2S04:concentratedHN03 (the latter boiled first to remove oxides of nitrogen) is used, following a recipe provided by Stedman (48). As shown in Figure 3, the HONO and HN03 sources are connected in parallel with 10 mL/min Mg(C104)2-driedN2 flowing through each source; the entire assembly is thermostated at 23 f 0.1 "C. The active length of the permeation tube TL (2.3 mm 0.d.; 1.7 mm i.d.) was 5 cm. The combined source output (20 mL/min) is admitted into the principal sampled stream through glass-Teflon valve G (Figure 2). The source output was calibrated by collection in aqueous bubblers followed by IC analysis either on conventional suppressed IC equipment or by the liquid-phase portion of the analytical system described here. For interference experiments, SO2and NO2were generated from wafer-type permeation-based sources (VICI Metronics, Santa Clara, CA) and NO was obtained as a certified standard in N2 (Scott Specialty Gases, Houston, TX).
RESULTS AND DISCUSSION Calibration Source Stability, Collection Efficiency. The calibration sources, after 1 week of initial equilibrium time, provided stable output. Under the cited conditions the mean source output was 116 and 2.42 ng/min of HN03 and HONO, respectively. Source stability was assessed by continuous measurement with the present system. The relative standard deviation of the NO3- signal was 4.6% (11h, n = 30), and that for the NO2- signal was 4.0% (16 h, n = 43). These variations not only include any changes in the source output but those in the collection efficiency of the WEDD and sensitivity of the analytical system as well. At the concentrations involved, the stability of the calibration source was therefore considered acceptable. Collection efficiencies for the WEDD were determined for both HONO and HN03 by using the devices of 35 and 50 cm
active length. Strictly, these data refer to percent recovery of the known source output in the WEDD rather than the true collection efficiency which would also include any irrecoverable losses. For HONO sampled at 0.5-2.5 standard liters per minute (SLPM) (atmospheric pressure and temperature during the experiments were 680 mmHg and 23 "C,respectively), the recovery efficiency was not statistically distinguishable from unity, either for the 35 cm (97.8 f 3.9% recovered) or for the 50 cm (99.5 f 4.3% recovered) WEDD. The recovery was also quantitative (102.2 f 2.6%) for HN03 with the 50-cm WEDD. The flow rate dependence of the collection of HN03 was clearly discernible only for the 35-cm WEDD; the penetration fractions measured 0.2888 f 0.0089, 0.2128 f 0.1002,0.1446 f 0.0000,0.0974 f 0.0068, and 0.1164 f 0.0239 a t 2.5, 2.0, 1.5, 1.0, and 0.5 SLPM, respectively. These and other similar data collected with short-length WEDD devices indicate an interesting behavior. At the higher flow rates, the Gormley-Kennedy equation (49) is closely followed. For the three highest flow rates above, if the natural logarithm of the penetration fraction is plotted against reciprocal flow rate (corrected for temperature and pressure; see ref 50), a linear plot (r > 0.997) is obtained with a y intercept corresponding to a penetration fraction of 0.791, in reasonable agreement with the theoretical value of 0.819. The slope of the plot yields 0.129 cm2/s as the diffusion coefficient of HN03, in comparison to values of 0.118-0.122 cm2/s (36, 51, 52) observed in previous studies. At lower flow rates, however, the WEDD shows a lower recovery than theoretical predictions. This is probably not due to a real decrease in collection efficiency but due to losses in the inlet region that increase at decreased flow rates due to increased residence times. Film Thickness and Evaporation Losses in the WEDD. This thickness of the falling film in the WEDD is of interest and was determined by taking the difference in weight of the WEDD dry and under operational water flow rates (rapidly removed and weighed after removing excess water at the termini). Repeated determinations show that the film thickness is 33 pm at a liquid flow rate of 235 pL/min; the film thickness increases with increasing flow rate. The input liquid flow rate to the WEDD is 280 pL/min. It is easily calculated from the known vapor pressure of water that if the sampled air is completely dry and the exit air is completely humid, up to 25 pL/min of water can evaporate per L/min sampled air at 25 "C and 1 atm. During field sampling with 1 L/min sample flow, we arbitrarily assume an evaporation loss of 50% of this maximum value, which should lead to an effluent liquid flow rate of 262 pL/min. Since only 160 pL/min is put into the chromatographic system, the mass of NO2- and NO3- determined by the liquidphase analytical system is multiplied by the factor 267/160. The error of variable evaporation losses is maximally f12 pL/min, this amounts to
