Ana!. Chem. 1990, 62, 338-346
338
(39) Quan, P, M.; Karns, T. K. B.; Quin, L D. J. Org. Chem. 1965, 30, 2769. (40) Frank, R. L; Holley, R. W.; Wlkholm, D. M. J. Am. Chem. Soc. 1942, 64, 2835. (41) Glenn, D. F.; Edwards, W. B., III. J. Org. Chem. 1978, 43, 2860. (42) Seeman, J. I.; Secor, . V.; Chavdarlan, C. G.; Sanders, E. B.; Bassfiek), R. L; Whidby, J. F. J. Org. Chem. 1981, 46, 3040. (43) Seeman, J. I.; Secor, . V.; Armstrong, D. W.; Timmons, K. D.; Ward, T. J. Anal. Chem. 1988, 60, 2120.
(44) Yamamoto, I.; Soeda, Y.; Kamimura, H.; Yamamoto, R. Agrie. Biol. Chem. 1968, 32, 1341.
Received for review May 16,1989. Accepted October 18,1989. Support of a portion of this work by the Smokeless Tobacco Research Council, Inc., to D.W.A. (Grant No. 0154-01) is gratefully acknowledged.
Development of a Method for the Sampling and Analysis of Sulfur Dioxide and Nitrogen Dioxide from Ambient Air Joseph E. Sickles, and Eva D. Estes
II,* Peter
M. Grohse, Laura L. Hodson, Cynthia A. Salmons, Kelly W. Cox, Ann R. Turner,
Research Triangle Institute, Research Triangle Park, North Carolina 27709
most of the other constituents. The nominal 0.1 µg mL™1 detection limit of the IC imposes a nominal 1 ppb detection limit for a 24-h integrated sample assuming a sampling rate of 1 L min™1 and a 20-mL extract volume. This means that acceptable flow rates must be 1 L min™1 or greater. In an effort to identify existing methods that were either suitable or adaptable for our purposes, an extensive literature review was performed (2). Among the several types of samplers available, bubblers and impingers were judged to be unsuitable in view of their limited sampling times (due to evaporation of the collection solution) and their potential for handling, shipping, and freezing problems. These considerations narrowed the focus of our development efforts to sorbents and filters. Sulfur dioxide is relatively easy to collect and has been sampled successfully at stack concentrations using uncoated molecular sieves (MS) (3) and at ambient concentrations using various impregnated filters (4-10). The reported aqueous impregnating solutions usually contained the hydroxide, bicarbonate, or carbonate salts of sodium or potassium, along with glycerin or triethanolamine (TEA) as an humectant. Uncoated silica gel (11, 12), MS (12), activated alumina (12-14), MnG2 (13-15), and charcoal (13) have been used to collect N02 from air streams, but they were found to generate NO, especially under humid conditions (12,13). Sorbents such as firebrick (12) and MS (16-18) coated with aqueous solutions of TEA have been used successfully to sample for N02. Filters impregnated with TEA (19) or p-anisidine (20) have also been used to sample for N02. Only one method has been reported for the simultaneous sampling of S02 and N02 (21). This method uses 13X MS that have been coated with a solution containing primarily TEA (18, 21). The method uses an IC finish, has been reported to be applicable at S02 and N02 concentrations between 1 and 10 ppm, and was demonstrated at sampling rates of 30-200 mL min™1 and sample durations of 5-120 min (21). These conditions are in contrast to the desired working concentration (i.e., ppb), sampling rate (i.e., 1 L min™1), and sample duration (i.e., 24 h) described earlier. The current study was conducted in several phases, and this report is organized similarly. In the first phase, findings of other workers (2) as discussed above were used to guide our selection and subsequent preliminary laboratory testing of sorbents, filters, and coating/impregnating solutions. These tests were performed to identify a method suitable for more
A method has been developed to permit the simultaneous sampling of S02 and N02 from ambient air. Sulfur dioxide and N02 are collected on triethanolamine-impregnated glass fiber filters. After sampling, the filters are extracted, and the extract Is analyzed for sulfate, nitrite, and nitrate using Ion chromatography. Ambient concentrations are computed from the recovered nitrogen and sulfur and the sampled air volume. Laboratory tests were conducted to evaluate the performance of the method under various species concentrations and loadings, sampling rates, humidities, temperatures, flushing conditions, and Interferences. Results of these tests are described. Field measurements using the developed method compared favorably with other established methods. On the basis of the precision of field blanks, S02 and N02 detection limits of 0.3 and 0.2 ppb for 24-h samples and 0.04 and 0.03 ppb for 7-day samples were estimated. Precision estimates of better than ±15% were found, and accuracy within ±10% was observed.
INTRODUCTION Assessment of ecosystem acidification resulting from air pollution requires monitoring the airborne concentrations of the contributing acidifying and neutralizing gaseous and
particulate species. The offending constituents include the trace gases S02, N02, HN03, HN02, and NH3 and particulate S042™, N03~, NH4+, and H+. A modular, multiconstituent, integrating sampler known as the transition flow reactor (TFR) has been designed and tested for selected pollutants (1). Although the initial TFR did not sample for S02 and N02, its modular design provided for the addition of a module with that capability. As a result, the object of the current effort was to develop a method for the sampling and analysis of S02 and N02 from ambient air that could be incorporated in a modular form into a multiconstituent sampler such as the TFR. Goals for the method were simultaneous determination of S02 and N02 at their nominal ambient (ppb) levels with ±20% accuracy, sampling durations ranging from
one
to several days,
of ion chromatography (IC) as the analytical finish. Compatability with IC was desirable to streamline laboratory operations for large numbers of samples (e.g., monitoring network applications), since IC was the method of choice for and
use
0003-2700/90/0362-0338S02.50/0
©
1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 4, FEBRUARY 15, 1990
comprehensive laboratory tests and led to the selection of a TEA-based solution for analyte collection. Preliminary testing of uncoated and coated sorbents and impregnated filters led to the selection of TEA-impregnated filters for comprehensive laboratory tests. In the second phase, comprehensive laboratory tests were performed. These tests led to method refinement, the establishment of the bounds of applicability (i.e., species concentrations and loadings, sampling rates, capacities, humidity, temperature, and interferences), and definition of the method. The final phase of method development involved field tests where the method was deployed in a 13-day study, results were compared with accepted methods, and method precision and accuracy were established.
EXPERIMENTAL SECTION General Procedures. Challenge atmospheres containing N02 and/or S02 were prepared by using either permeation tubes or certified cylinders of compressed gas followed by dilution with clean air of controlled humidity. Ambient air was compressed and cleaned with catalytic chemical and sorption scrubbers to remove organics, ozone, nitrogen oxides, and sulfur oxides, but not C02, prior to introduction to the dilution system. A TECO Model 14 B/E chemiluminescence (CLM) NO, analyzer and a Monitor Labs Model 8450 flame photometric S02 monitor were used to analyze the challenge atmosphere upstream and downstream of each sampler. An EG&G Model 880 was used to monitor the dew point (i.e., relative humidity (RH)) of the challenge atmosphere. Tests were performed in the laboratory at temperatures of 25-27 °C or in a temperature-controlled 15-m3 Sherer Environmental Test Chamber. The chamber can operate from 0 to 45 °C and maintain a constant temperature to within ±0.5 °C. Samplers were tested by using challenge atmospheres to evaluate collection efficiency (CE) and/or recovery efficiency (RE). CEs were determined by comparing the sampler inlet and outlet airborne concentrations of the challenge atmosphere as determined by the NO, and S02 monitors. REs were determined by comparing the amount of analyte recovered by extraction and IC analysis of the sample extract with the amount expected based on the gravimetrically or instrumentally determined airborne
input. The TEA solution used in the experiments performed in the current study was prepared by dissolving 25 g of TEA and 4 g of ethylene glycol in 25 mL of acetone and diluting to 100 mL with DI water (18, 21). Samples were normally extracted in 20 mL of DI water, but sometimes IC eluent was used. When S02 was present in the challenge atmosphere, 2 drops of 30% H202 were added to the extract to ensure oxidation of any S032" to S042" and to prevent interference of the S032" with the N03" IC peaks. The addition of H202 was found to have no effect on the N02~ peak at the neutral to basic conditions of the extract. All N02", N03~, and S042" analyses were performed on a Dionex Model 14 ion chromatograph. Analytical conditions were as
follows: 3 X 150 mm anion precolumn; 3 X 500 mm anion separator column; anion fiber suppressor column; 0.003 M NaHC03/0.0024 M Na2C03 eluent composition; 138 mL h"1 flow rate; and 0.1-mL sample size. Standards covering the concentration range of the samples were run daily to prepare linear least-squares calibrations of concentration versus peak height. The nominal detection limit was 0.1 µg mL"1. Preliminary Experiments. Screening of Solutions for Collecting N02. Several candidates for sorbent coatings or filter impregnation solutions were identified in a literature review (2) as having potential for collecting N02. These solutions include: DI water; tri-n-butyl phosphate/water (90/10%); 5 M NaOH; K2C03/luminol (5.0/0.1%); TEA solution (as described previously); 0.2 M Na2S03,0.2 M NaOH; 0.4 M Na^Os; 0.2 M Na^Os, 0.2 M NaOH, 2% dimethyl sulfone; and 0.2 M NaS03, 0.2 M NaOH, 6 x 10"*% Triton X-100. After preparation, these solutions were tested in impingers by bubbling gas streams containing N02 at 0.4, 4.0, 200, and 2300 ppm at 1.2 L min"1 through 20 mL of each solution for approximately 2 h and measuring the CEs at nominal 15-min intervals.
·
339
Screening of Uncoated and Coated Sorbents and Impregnated
Filters. Both commercially prepared (SKC, Inc.; Eighty-Four, PA) and laboratory-prepared sorbent tubes were tested. The uncoated sorbents in commercially prepared tubes (nominal dimensions 6 X 70 mm) that were tested include the following: Chromosorb 101 (0.105 g), 102 (0.099 g), 105 (0.112 g), 107 (0.112 g), and 108 (0.112 g); XAD 2 (0.13 g) and 7 (0.12 g); alumina (0.15 g); and 5A MS (0.217 g). Laboratory-prepared tubes were loaded with a nominal 0.25 g of coated sorbent using 50-mm lengths of 6-mm-o.d. glass tubing with glass wool inserted at both ends to secure the sorbent. Several sorbents were coated with TEA solution following the method of Vinjamoori and Ling (21), tubes were loaded, and they were subsequently tested. These sorbents include 5A MS (SKC, Inc.), prewashed silica gel (Supelco, Inc., Bellefonte, PA), 45/60 mesh and 20/40 mesh 13X MS (SKC, Inc.), 20/40 mesh alumina (SKC, Inc.), and Soxhlet extracted (24-h in DI water) 20/40 mesh coconut-based charcoal (SKC, Inc.). Filters used in the study were 47 mm in diameter. They were tested with Mace TFE Teflon filter holders providing an exposed filter diameter of 43 mm. Each filter was held in a separate holder when tests employing multiple filters were performed. Tested filters include Schleicher and Schuell fast flow no. 2 (cellulose) (S&S FF), Pallflex TX40 H 120 (Teflon-coated glass fiber), and Whatman 41 (cellulose), QM-A (quartz fiber), and GF/B, GF/C, and GF/F (glass fiber). In most cases, the TEA solution described previously was used to impregnate filters for testing, but in a few cases, filters were impregnated with aqueous solutions containing K2C03 (25% in 10% glycerin-water), alkaline KMn04 (4% in 2% NaOH), NaAs02 (1% in 1 M NaOH), Pb02 (25% in 10% glycerin-water), guaiacol (10% in 1 M NaOH), and TEA plus 2-naphthyl-3,6-disulfonate (1 %). Filters were prepared in batches of five by rinsing 4 times with DI water and a Büchner funnel. After drying under vacuum at 50-60 °C for 1 h, the filters were transferred to a beaker and covered with an aliquot of the appropriate impregnating solution. After a 5-min soak, they were transferred to the Büchner funnel where excess solution was removed by applying suction for 30-45 s. Filters were dried in a vacuum oven for 1 h at 50-60 °C, placed in a sealed, clean, polyethylene bag, and stored prior to use, in a desiccator containing silica gel and activated charcoal. A second round of screening tests was conducted on selected sampling elements to measure pressure drop versus flow rate, CE versus flow rate at several temperatures, humidities, and analyte concentrations, and stability of collected N02 and S02 in loading and flushing tests. To determine pressure drop for tubes containing TEA-coated charcoal and TEA-impregnated filters, experiments were performed with a Heiss pressure gauge (+2500 to -760 mm, ±1 mmHg), Tylan mass flow controllers, and an air supply/mover at dry (
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