Anal. Chem. 1989, 6 1 , 19-24
Measurement of Atmospheric Sulfur Dioxide by Diffusion Scrubber Coupled Ion Chromatography Per F. Lindgren' a n d P u r n e n d u K.Dasgupta* Department of Chemistry and Biochemistry, Texas Tech University] Lubbock, Texas 79409-1061
A porous membrane dlffuslon scrubber designed to collect soluble atmospherlc gases without obllgatory collection of partlcuiate materlal has been coupled to an ion chromatograph in a continuous automated manner. Wlth 1 mM H202 used as the scrubber ilquld, the collected S( I V ) is oxidlzed to sulfate and determlned as such. The Instrument operates on a 6mln cycle. Durlng most of thls perlod, the loop of the chromatographic valve Is slowly fliied wlth the scrubber effluent. following a brief stay in the InJectposttion, the valve returns to the load position agaln. The desired chromatographic separatlon Is accompllshed wlthln the cycle time. Thus, a new chromatogram is produced every 6 mln. Based on the standard deviation of the blank, the detection llmit is better than 20 parts per trillion by volume (pptrv) SO,; the lowest standard concentration studled was 77 pptrv. Ambleni SO2 determination results are presented. The potentlal of sknuttaneous determlnatlon of several other gases Is obvious.
INTRODUCTION The development of an affordable sensitive technique for the continuous measurement of sulfur dioxide is essential for a full understanding of the background distribution of this species, of key importance in the genesis of atmospheric acidity. It is desirable that such a technique should have a limit of detection (LOD) well below 100 parts per trillion by volume (pptrv) with a time resolution of a few minutes. Utilizing cryogenic preconcentration, GC-MS analysis, and isotopically labeled SOz, Driedger et al. (1) have reported a limit of detection of 1pptrv SOz. Cvijn et al. (2) report an LOD of 200 pptrv obtained with pulsed laser photoacoustic spectrcaopy. The most sensitive of the commercially available SOz detectors, a custom-fabricated pulsed fluorescence instrument, is said to be capable of detecting 100 pptrv SOz (Therm0 Environmental Instruments, Inc., personal communication, 1988). However, the majority of methods used for the collection and analysis of atmospheric gases involve the use of reactive filters, adsorbent-filled cartridges, bubblers/impingers, and a number of devices that utilize turbulent gas/liquid contact (3-6). All of the convenientlyautomatable methods result in obligatory collection of aerosols present in the sample air; as a result, it is impossible to distinguish, for example, between aerosol NH4+ and gaseous NH3, aerosol S O P and gaseous SOz (when the latter is measured as SO?-), etc. The use of a prefilter to exclude the aerosol fraction is rarely permissible; the sample integrity is compromised by on-filter reactions. A more recent device for the collection of atmospheric gases is the diffusion denuder. Originally used to remove undesired gases, a diffusion denuder is a tube whose inside walls are coated with a suitable reagent that displays a high removal efficiency for the analyte gas. Because the diffusion coefficient Permanent address: Department of Analytical Chemistry, University of Umea, Umei S901-87,Sweden.
of a gas is several orders of magnitude higher than that of typical aerosols, the diffusion denuder allows selective sampling of the gas phase in the presence of particles. The collection efficiency is predictable by classical equations (7), assuming laminar flow conditions and walls that are perfect sinks. Despite highly ingenious designs (8), the tedious procedure of coating, washing, and recoating denuder tubes is a major obstacle for the practical use of diffusion denuders. Although a thermally induced sorption/desorption cycle is possible for some gases (9) and computer-controlled thermodenuder systems have been described (lo),the significant thermal mass of any practical denuder system intrinsically limits attainable temporal resolution. The diffusion scrubber (DS), built on the principle of the diffusion denuder, was developed in this laboratory (11) to solve these problems. The present version consists of a narrow-bore, thin-wall inert microporous membrane tube, surrounded by an outer jacket. The sample air flows in the annulus between the outer jacket and the membrane tube while a suitable scrubber liquid flows continuously through the membrane tube in a countercurrent fashion. Atmospheric gases of interest diffuse to and through the membrane tube into the scrubber liquid. A number of applications involving the use of the DS as a collection interface between atmospheric gases of interest and sensitive continuous liquid-phase analysis methods have been developed and shown to compare well with existing alternatives (12-18). The specificity of these techniques is based on the-individual analytical reactions. Many atmospheric gases of interest form, or can be transformed into, characteristic ions in solution. Since the introduction of ion chromatography (IC) by Small et al. in 1975 (19),IC has emerged as the preeminent technique for sensitive ionic analysis, especially for anions (20). A DS coupled to an IC may therefore provide uniquely useful solutions for some of the measurement problems. In this paper, we demonstrate a sensitive continuous method for determination of sulfur dioxide at levels of low- to sub-parts-per-billion by volume (ppbv). EXPERIMENTAL SECTION System Configuration. The arrangement in Figure 1was used for all experiments. Compressed house air was filtered, dried (silica gel, molecular sieves), and cleaned from organic and acid gases (activated carbon, soda lime) before it entered the system (A). A flow of 2.1 standard liters per minute (SLPM) of clean air was introduced through a mass flow controller B (Model FC-280, Tylan Corp., Carson, CA) into a thermostated permeation chamber C, containing a wafer-type SOz permeation device (VICI Metronics, Santa Clara, CA) gravimetrically calibrated to be emitting 21.7 ng of S02/min. A portion of the effluent from the permeation chamber (SO2concentration -3 ppbv) was metered through an all-glass poly(tetrafluoroethylene) (PTFE) needle valve/rotameter E (Gilmont Instruments, Great Neck, NY) and subjected to second-stage dilution; the rest was vented (D). The selected flowstream
0003-2700/89/0361-0019$01.50/00 1988 American Chemical Society
VOL. 61,NO. 1, JANUARY I, 198i9
'11 1 ,
Flgure 1. Configuration of the DS/IC system: A, clean air input; B, mass flow controller; C, permeation device chamber; D, H, vent; E, needle valve/rotameter; F, needle valve; G, mass flow meter; I, diffusion scrubber: J, scrubber liquid reservoir; K, needle valve/rotameter; L, suction pump; M, injection valve; N, peristattic pump; 0, eluent flow; P, downstream chromatographic components Q, sample loop.
(0.1-2.1 SLPM) merged at a tee with the dilution flow (1-5 SLPM) of clean dry air, controlled by needle valve F and measured by mass flow meter G (Model 8111, Matheson Instruments, Horsham, PA). The output of the calibration system was sampled through the DS (I) by a pump L. The DS contains a central microporous membrane tube surrounded by a PTFE jacket. The air sampling rate through the jacket was controlled by a needle valve K and was 2.9 SLPM unless otherwise stated. Excess flow from the calibration source was vented a t a tee-port (H). The sample offering to the DS (calibrant, zero air, ambient air) was changed manually during these experiments; simple automated valving arrangements with inert solenoid valves have been described (14). A solution of HzOz (1mM), the scrubber liquid, was contained in a glass reservoir J. A peristaltic pump N (Minipuls 2, Gilson Medical Electronics, Middleton, WI) aspirated the liquid through the membrane tube in the DS a t a flow rate of 16 pL/min. To avoid the aspiration of any air bubbles through the membrane pores, the solution reservoir was maintained under a small positive pneumatic pressure (1psi). Aspirating, rather than pumping, the scrubber liquid maintains a constant effluent flow rate in the presence of variable evaporative losses. The evaporative loss is a function of inlet air relative humidity and can be a significant fraction of the total flow rate at this air/liquid flow rate ratio (14). Figure 1shows the instrument with the valve M in the load mode; pump N is aspirating the scrubber liquid through a 100-pL Tefzel sample loop Q while eluent flow 0 from a chromatographic pump is directed to the column and downstream chromatographic components (P). When M is in the inject mode, the contents of the sample loop are back-flushed onto the column. Although shown as the functional equivalent of a six-port rotary valve, valve M is a high-pressure dual-stack four-way pneumatically actuated valve with wetted parts made from an inert polymer (P/N 038598, Dionex Corp.). The valve was actuated by two three-way air solenoid valves (MBD 002, Skinner Valve, New Britain, CT) connected to a 100-psi pneumatic source. The solenoids were programmed to be automatically actuated at desired intervals by a ChronTrol timer (Model CD-QS,Lindburg Enterprises, San Diego, CA). The ion chromatograph used was a basic suppressed IC instrument (QIC Ionchrom Analyzer, Dionex Corp.). No modifications were made to it except the substitution of valve M described above for the injection valve supplied with the instrument. The chromatographic conditions were a 28 mM
Figure 2. Construction of the diffusion scrubber: A, B, 30- and 20gauge PTFE tubes; C, 1/4-28male nut; D, 1/4-28female connector; E, nylon ferrule; F, disk segment; G, polypropylene tee; H, 5-gauge PTFE tube; I, glass jacket tube; J, PVC tube segment; K, nichrome wire crimp; L, Celgard microporous membrane tube.
NaOH eluent (prepared from Baker "carbonate-free" NaOH stock solution with distilled deionized water of resistivity >12 MQ) at 0.6 mL/min and a Dionex HPIC AS5A column (250 x 4 mm, 5-pm packing). A homemade helical filament-filled externally resin-packed suppressor, containing a 1m long, 400 pm i.d. Nafion fiber (21),was used as the suppressor, with 4.5 mM HzS04being used as regenerant at -5 ml/min (5-psi penumatic pressure). Intrusion of COz into the NaOH eluent during operation was prevented by using a solid NaOH trap. Scrubber solution (1mM HzOz)was prepared fresh every week by diluting 120 pL of 30% HzOzto 1 L; the solution was stored refrigerated. Diffusion Scrubber Construction. A new design of the DS, which allows rapid replacement of the membrane tube or cleaning of the interior walls, was used in this work and is described below. A detailed view of one side of the DS is shown in Figure 2. It consists of a microporous polypropylene membrane tube L (40 cm long, Celgard X-20,400-pm i.d., 25-pm wall thickness, 40% porosity, 0.02-pm mean pore size, Hoechst-Celanese Corp., Charlotte, NC) suspended concentricallywithin a PTFE tube H (63 cm, 5 AWG), which in turn is maintained in a rigid linear configuration by the outer glass tube I (60 cm X 6 mm i.d.). Each end of the membrane tube L is connected to a 30-AWG PTFE tube A (ca. 35 cm) by 1-cm segments of 0.015 in. i.d. poly(viny1 chloride) (PVC) tubing J (Elkay Products, Shrewsbury, MA) and crimped with 30-gauge nichrome wire K. The insertion of the membrane tube L in connecting tubing J is facilitated by a 21-gauge hypodermic needle. The PVC tube is covered with PTFE tape. The membrane tube with the PTFE tube end connections, after testing for leaks by injecting water through the lumen, is then pulled into the glass-jacketed PTFE tube by a suitable device (e.g., a length of 29-gauge needle tubing). Tee connections G (3/le-in.polypropylene tees, Nalgene Type 6151-0187) are then pushfitted to each side of the jacket. The ends of two 1/4-28 threaded male nuts C (P/N 37626, Dionex Corp., Sunnyvale, CA) are countersunk with a 7/61-in. drill bit, so that the tapered end of a 1/16-in.nylon ferrule E goes halfway into each nut. A 5-mm-thick, disk-shaped segment containing the countersinked end F is then cut off from each nut with a rotary cutter. These segments are then screwed from the flat end, with a Phillips screwdriver, halfway into each of two female 1/4-28 threaded connectors (P/N 24447, Dionex Corp.). The inset in Figure 2 shows this arrangement, along with the eventual deployment of the nylon ferrule. The 1/4-28 female connectors D screw onto the tees G, with the countersunk disk end outward and the 30-gauge PTFE tubes protruding through the disks. A small length of a 20gauge PTFE tube B is inserted about 1cm into the tee as a sleeve on the 30 gauge tube A. The membrane tube is then centered in the jacket and held in position by the nylon ferrule E and male nut C. The membrane tube is stretched taut before affixing the second side. To allow laminar flow to develop and thus minimize particle deposition, the membrane
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tube begins at least 5-7 cm after the tee inlet. Finally, the PTFE inlet and outlet tubes (A, B) are cut to the desired length (minimum permitted by the location of the other components). All PTFE tubes used are of standard wall thickness (type SW, Zeus Industrial Products, Raritan, NJ). The total construction time for a DS is -30 min, and the membrane tube can be replaced in -5 min. To minimize particle deposition due to gravitational settling, the DS is always deployed in a verticle configuration. Maintenance. The DS was washed daily with 1-mL injections of 75% (v/v) methanol/water repeated four or five times. Following this, the membrane was allowed to dry completely in a flow of clean dry particle-free air for 20 min before use. This procedure removes deposited particles or dissolved solids evaporated from the scrubber liquid and cleans the membrane pores. Before overnight and longer term storage, the DS should be cleaned in this manner. For shorter term system shutdown, the scrubber liquid flow should be left on.
RESULTS AND DISCUSSION System Description. Chromatographicconditions chosen must result in satisfactory resolution of the desired analyte species. In typical ambient air sampling, one may expect to see the corresponding anions derived from COz, SO2,HNO2, "03, organic acids, etc. To obtain the best possible limit of detection, we wished to confine ourselves to a hydroxide eluent. Under the chosen chromatographic conditions, the analytes of particular interest, nitrite, sulfate, and nitrate, are well separated and elute in the stated order with a retention time of -4.2 min for nitrate. Organic acids and carbonate also elute within this time. The minimum interval between consecutive injections in the IC system is dictated by the time required for the last anion to completely elute from the column. Therefore, a six-minute interval between injections is more than enough to avoid overlapping consecutive chromatograms and was chosen for this work. Of this interval, the injection valve must remain in the inject position for a period of time long enough for the sample to be completely flushed from the loop. At a flow rate of 600 pL/min through a 1 W r L volume loop, 30 s represents three residence times and was judged sufficient for complete injection. The valve remains in the load position during the remaining 5.5 min, to fill the loop with the scrubber effluent. It is obvious that the scrubber liquid flow rate should be such that the loop is not overfilled; i.e., little or none of the collected sample should go to waste. On the other hand, if the loop is partially filled, the response should remain essentially independent of the degree to which the loop is filled. This is because the collection efficiency of the DS for the analyte gas of interest is largely independent of the scrubber liquid flow rate. At any given concentration of the analyte gas, a decrease in the scrubber liquid flow rate results in a correspondmgincrease of the concentration of the collected analyte in the scrubber effluent liquid. Thus, if at a certain scrubber liquid flow rate, the loop is filled to 80% of its capacity in the allotted time, halving that flow rate will result in filling the loop to 40% of its capacity with twice as concentrated a sample. The above example would be totally valid if plug flow existed, but some dispersion is unavoidable and will accordingly affect the results. The effect of varying the scrubber liquid flow rate from 12 to 48 pL/min upon the response of the DS/IC system is shown in Figure 3 with fl standard deviation (SD) indicated as the error bar for each point. The highest signal is obtained at a flow rate of 16 pL/min (within the resolution of the data shown); the flow rate necessary to completely fill a loO-pL loop in 5.5 min, assuming plug flow, is -18 pL/min. At high flow rates, not only does the magnitude of the signal decrease in
10 30 50 S c r u b b e r Liquid Flow R a t e ( v L / m i n )
Figure 3. Effect of the scrubber liquid flow rate on the sulfate peak height (fl SD shown as an error bar): sulfur dioxide concentration is 2.5 ppbv. 300
I 1 oa
4 6 8 1'0 lnlection Number
Flgure 4. Decay of sulfate height to background value as the sample gas is switched from 300 pptrv SO2 to blank.
the predicted manner, but precision deteriorates as well. The minor decrease in the signal at flow rates below 16 pL/min appears to be real. A possible explanation is stagnation of the boundary layer at the membrandiquid interface, resulting in the onset of surface saturation and a decrease in collection efficiency. In any case, one should choose the highest possible flow rate within the plateau region for an entirely separate reason. The holdup volume associated with the scrubber and the tubing connecting it to the injection valve is unavoidable. A higher scrubber flow rate minimizes carryover. Carryover can be measured by allowing the DS/IC system to sample the analyte gas and then switching to zero gas just as the collected sample is injected into the chromatographic system. The decay of the signal to the base-line value occurs quite rapidly in consecutive injections, as shown in Figure 4. A carryover of -11% to the next sample can be estimated from these data. It should be noted, however, that the decay can be considerably slower if adsorption/desorption of the gaseous analyte on the system components is the limiting
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factor; we have observed this, for example, with HN03. We conducted a set of experiments in which a low-holdup-volume inert three-way valve was utilized in such a manner that immediately after switching the injection valve to the load position, a brief burst of compressed gas removed the eluent held up in the loop. The three-way valve then switched, allowing sample loading to proceed. This arrangement was intended for avoiding dispersive loss of the sample. However, some overfilling had to be present to avoid injecting gas into the chromatographic system. The results, particularly in terms of precision, did not justify this more complex arrangement. Choice of a Scrubber Liquid. Use of water as the scrubber liquid was found to result in both sulfite and sulfate peaks in the chromatogram. To avoid unnecessary complications in the interpretation of the resulting data, we deemed it desirable to completely oxidize the collected SOz to sulfate. Hydrogen peroxide is a particularly convenient nonionic oxidant; its concentration was chosen with the ultimate goal of performing simultaneous measurement of SO2, HNOz and HN03 Based on published rate data on the oxidation of S ( N ) and NOz- by Hz02(22,23),within the pH range of 4-6,1 mM HzOzshould result in 100% oxidation of S(1V) to SO?- while 300 pptrv produced the best fit equation signal (ns) = 0.7107 X SO2 concentration (pptrv) 35.4 (1)
with a correlation coefficient better than 0.999. The computed intercept compared well with the measured blank value over the same period, 43.2 f 1.5 ns. The standard error of estimate for the above linear relationship for ail of the pooled data was 32 pptrv SOz. However, for SOzconcentrations below 300 pptrv, negative deviation from eq 1 was observed; i.e., the observed signal height was lower than that predicted by the equation. The deviation from linearity is not large, but increases with decreasing SO2 concentration. At the lowest concentration studied (77 pptrv SOz),the response was 23% lower than that
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predicted by the above linear relationship. Because the RSDs of the determinations at any of these concentrations were