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Anal. Chem. 1986, 58,598-600
Fast Chemiluminescent Method for Measurement of Ambient Ozone John D. Ray,* Donald H.Stedman, a n d Gregory J. Wendell Chemistry Department, University of Denver, Denver, Colorado 80208
An instrument for measuring atmospherlc ozone concentrations has been developed that uses chemiluminescent dyes in various solvents. Soiutlons of eosln Y in ethylene glycol were effective for measuring ambient ozone at a response frequency of 7 Hz and a detection limit of 0.2 ppb ozone. Other atmospherlc species, inciudlng humidity, gave no Interference. The response to ozone Is llnear over the range of 0.2-400 ppb ozone. A successful comparison of the chemliuminescent instrument to a commercial ozone analyzer was made during measurement of ambient ozone concentrations in Denver air.
Chemiluminescent techniques can be used to measure a number of important air pollutants ( I ) . Experiments with an improved version of a previously reported luminol-based NOz detector (2) have indicated that fast, compact, highly sensitive instruments using chemiluminescent solutions can be effectively used for measuring various components of ambient air. The NOz detector, as presently configured, has sensitivity to less than 1pptr NO2in air at 3 Hz response speed and weighs 25 lb. It was the intent of this study to develop a sensitive ozone-measuring instrument suitable for eddy correlation experiments and for aircraft-based sampling with similar characteristics, except that parts per billion response is entirely adequate for studies of ozone in ambient air. Several commercially available instruments can measure ozone based on chemiluminescent reactions. Most automatic instruments utilize the Neberbragt method (3), in which the light-emitting reaction of ozone with excess ethylene gas is used. These are effective and stable instruments with response speeds of 0.1-0.5 Hz and detection limits of about 1 ppb. However, ethylene must be supplied from pressurized cylinders, which increase overall instrument weight and present potential fire hazards. The reaction between NO and ozone is the basis for another chemiluminescence detector (4). The system is similar to the ethylene-ozone instruments but has slightly better sensitivity and better than 1-Hz response speed. Pearson and Stedman (5) have reported response speeds of 12 Hz using such instruments for aircraft sampling. However, NO gas is required, which requires the use of compressed gas cylinders. These limit the portability of the instrument, and the instrument pollutes the neighboring air with NO, which can interfere with other pollutant monitors. Lighter weight chemiluminescence instruments using rhodamine B supported on silica gel have been reported by Regener (6, 7)for balloon sonde and aircraft use. An improved version was reported by Hodgeson et al. (8) with a detection limit of 1ppb and a response time of about 5 s. Although no interference was observed with common air pollutants, moisture caused a considerable decrease in sensitivity. Even so, because of its specificity for ozone and sensitivity, this method offers considerable promise. Present address: Thermetics, Inc., 470 Wildwood St., Woburn,
MA 01888.
Although rhodamine B supported on silica gel was apparently the preferred method, other variations have been studied. Bersis and Vassiliou (9)first reported detecting ozone by bubbling ozonized air through ethanol solutions of rhodamine B and gallic acid. As with most bubble systems, the noise level was high and hence the detection limit was just a few parts per million ozone. Bowman and Alexander (IO) have reported that a number of organic dyes chemiluminesce with ozone. Hodgeson (8)studied some of these dyes adsorbed on silica gel and other substrates but concluded that rhodamine B was the best for ozone-detection instruments. Generally, rhodamine B adsorbed on silica gel plates, often with added gallic acid, is considered the optimum configuration for a chemiluminescence ozone detector. The design has been used in a number of atmospheric research studies, and a commercial instrument is available (11). Our instrument is based on dye solutions and offers several advantages over the dry rhodamine B ozone-detection method. EXPERIMENTAL SECTION Instrument Design. The design of the chemiluminescence detector is shown in Figure 1. Sample gas is drawn through the detection cell at 1-7 L/min by a diaphragm pump. Fluid and air are separated at the reservoir, and the dye solution is recirculated. Sample gas flows across a fiber pad of either paper or glass mat that is saturated with the organic dye dissolved in an alcohol solvent. Figure 2 shows the cell design with air and dye solution entering at the top of the cell through separate holes. Both air and fluid exit at the bottom together. Inserted into the detection cell is an end-on photomultiplier tube (EM19824) that responds to the chemiluminescence as the ozone reacts with the dye. The photocurrent is amplified by an electrometer, and the signal reads out on a meter or a strip-chart recorder. The detection cell is made of black Delrin. Dye solution is pumped to the cell at 1 mL/min. The photomultiplier tube is operated at 1200 V and has a dark current of 0.6 nA. Response to ozone is from 1 to 10 nA/ppb depending on dye type and solvent. Reagents. Dyes tested in ethanol were rhodamine B, safranin 0,eosin Y, fluorescein, and methylene violet; the dyes are available from Eastman Organic Chemicals and from Aldrich Chemical Co. All these dyes emitted light when ozone was introduced into the cell. Solvents other than ethanol that dissolve the dyes, such as water, isopropyl alcohol, tert-butyl alcohol, cyclohexanol, ethylene glycol, or glycerol, can be used. The amount of light emitted is solvent and dye-concentration dependent. RESULTS AND DISCUSSION Chemiluminescent Dyes. Several different organic dyes with similar structure have proven effective for the chemiluminescent reaction with ozone. Rhodamine B dissolved in ethanol is a red solution that emits light with a wavelength peak near 590 nm. A red-sensitive photomultiplier tube (RCA PFlO23A-IPA) was initially used for observation of dye sensitivity to ozone and for calibration. Response to ozone was linear over the range of 5-800 ppb ozone. A Thermo Electron Co. (Model 49P) ozone analyzer provided both a source of ozone for calibration and an accurate reading of the ozone concentration. Based on the noise level and extrapolation of the calibration curve, the detection limit is about 0.2 ppb
0 1986 American Chemical Society 0003-2700/86/0358-0598$01.50/0
ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
air
599
Table 11. Effect of Water on Safranin 0 Response t o Ozone
in
% water
in ethanol 0 10 67 75 80 100 Flgure 1. Diagram of the chemiluminescent ozone analyzer.
air
relative response" 103 M 10-4 M 0.73 0.67 0.18
0.17 0.15
"Response relative to ethanol.
0.39 0.53 0.08 0.06
0.02
M solution of rhodamine B in 95%
in
r-'
Table 111. Solvent Effect on Dye Response to Ozone f IUid
' in
solvent
ethanol isopropyl alcohol tert-butyl alcohol ethylene glycol
I I
water 1:l ethanol/DMPb
L-
Figure 2. Design of the detection cell. The cell Is machined out of Delrin and aluminum. A glass window seals the cell and separates the liquid from the photomultiplier tube (PMT). Both cell and PMT are
a
Relative to
relative response" safranin 0 eosin Y 0.73 5.28 0.11 0.11 0.11
1.00 1.49 0.93
eosin Y in ethanol. Dimethyl phthalate.
encased in a light-tight aluminum housing. Table I. Response of Dyes in Solution to Ozone dye in 95% ethanol" rhodamine B rhodamine B/gallic acid fluorescein eosin Y safranin 0 methylene violet
relative responseb response 0.47 0.44 2.80 1.75 0.53 0.49
1.0 0.9 6.0 3.7 0.5 0.5
emission max, nm 570 570 555 598 560
M solutions. bnA/ppb ozone.
ozone. Addition of antioxidants (such as gallic acid) to rhodamine B solutions decreased response and was unnecessary for stable operation. Other dyes chemiluminesce with ozone and can replace rhodamine B. Several of the more promising dyes were tested in ethanol solutions containing 10% water. At a concentration of M each dye emitted light in response to ozone. Response was measured for five different dyes (Table I); eosin Y has the best response in ethanol. Sensitivity was dependent on the spectral response of the photomultiplier tube, the emission maximum of the dye solution, and the dye efficiency for light emission. In general, a shift of the emission maximum toward the blue increases sensitivity and allows for use of a more blue-sensitive photomultiplier tube. Overall, eosin Y has proved to be the most sensitive dye tested up to now. We chose 1 g/L eosin Y in ethylene glycol for further studies. Effects of Solvent. The type of solvent can greatly affect the amount of light emitted from the dye solutions. Since sensitivity varies with dye concentration, evaporation of solvent can lead to calibration changes with time. Addition of water reduced evaporative losses somewhat but also affected sensitivity. Table I1 compares response to ozone for different water-ethanol ratios and clearly shows that large amounts of water are undesirable. Most of the dyes dissolve in water alone and can be used in water solution for detection of ozone, but with loss of signal. A number of solvents were tested with the goal of retaining sensitivity to ozone while decreasing evaporation. Alcohol solvents generally retained the ability to dissolve the dyes and give reasonable light emission. Table I11 compares some of
the better solvents tested. The best response was obtained with isopropyl alcohol, yet the solvent with the least evaporation that still had reasonable sensitivity was ethylene glycol. Eosin Y in ethylene glycol was chosen as a compromise for an instrument that had good sensitivity and needed a minimum of attention. Instrument Responses. The response of the system to species other than ozone was also investigated. No response was seen when rhodamine B in ethanol was exposed to 60 ppb NO2, 50 ppb NO, or 20 ppb PAN. Air samples made up with NO2 of approximately 500 ppb likewise did not produce a signal. Tests with eosin Y and safranin 0 gave similar results. This lack of interference from other species is consistent with the experience of other researchers with rhodamine B adsorbed on silica gel. In addition, the dye solutions have shown no sensitivity to atmospheric humidity in laboratory experiments. Thus, we do not know of any interferences affecting this method of ozone measurement. Response speed to changes in ozone is important, especially for aircraft-borne instruments and for dry deposition studies. Both the design of the detection cell and the type of dye solution determine the limits of response speed. Factors such as air-flow rate, internal cell volume, fluid pumping speed, and type of fiber pad inside the cell also affect the response speed. In general, changes that increase the air velocity across the chemiluminescent surface also increase the response speed. Initial measurements using a high-speed data collection system and air flow of 1L/min yield a response frequency of 1.5Hz at 95% of full signal. With further reductions in cell volume and at greater than 9 L/min air flow, the response frequency was found to be faster than 7 Hz. Ambient Air Measurements. Measurements of ozone in Denver air were made between April 19 and April 22,1985, using both the chemiluminescence instrument and a commercial ozone analyzer (TECO, Model 48P). During the measurement period the weather was mostly sunny with temperatures between 4 and 27 "C. Data were collected with a microcomputer system that took 3-min averages for each instrument. Switching for a periodic calibration sequence was provided by using a Chrontrol timer. Every 6 h the calibration sequence of zero air, 15ppb ozone in air, zero air was repeated. This procedure provided a periodic check of both instruments in case any sensitivity or zero drift might have occurred.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH
1986
will be reported later along with other data from the PRECP program.
50
I
0
40
20
60
Tlmelhrr)
Ambient ozone measured by two instruments between and 4 / 2 2 / 8 5 in Denver, CO. Upper trace is data from the chemiluminescence instrument; lower trace (inverted)is data from the commercial UV absorption ozone analyzer. A half-hour cycle of zero air and a calibration level of ozone followed by zero air occurs every Flgure 3. 4/19/85
6 h.
As shown in Figure 3, a characteristic cyclic pattern was observed with an ozone maximum in late afternoon and values of nearly zero occurring at about 9:00 p.m. Both instruments gave the same variation in ozone levels. Agreement between the two ozone instruments was excellent throughout the sampling period. A linear regression for the correlation between the two instruments had a slope of 0.9599, intercept of -0.0631, and R2 of 0.9880. Standard deviation for the correlation was 2.199. The periodic calibrations showed that the eosin Y dye solution in ethylene glycol was stable and consistent in its response. Rain and high humidity during the later part of the monitoring period did not change the instrument response. Likewise, temperature variations of 25 "C did not have an effect. With proper calibration and periodic solution changes, the chemiluminescent ozone instrument proved to be an effective and reliable ambient ozone monitor. Measurements of ambient ozone were also made during a month-long field study as part of the PRECP program in Oklahoma City. The objective of that program was to measure how convective storm clouds processed air pollutants. A revised instrument package weighing 22 kg and designed to operate on 28 V dc at 1 A was mounted on a KingAir aircraft. Measurements of ozone concentration were made during several flights in June of 1985. Preliminary data indicate only a 20% loss in ozone sensitivity at reduced atmospheric pressures at heights up to 4 km. Solutions of eosin Y in ethylene glycol gave acceptable results for 12-15 h of instrument operation. Detailed results of the ozone measurements
CONCLUSION Our studies show that the chemiluminescence of organic dye solutions with ozone can be used as the basis for a continuous monitoring system for ambient ozone. An instrument designed and built in our laboratories, using eosin Y in ethylene glycol, gave a detection limit of 0.2 ppb and a very fast response speed of 7 Hz, making it especially suitable for eddy-correlation studies or aircraft-based air sampling. Further advantages of the instrument are its low noise level, the ease with which the liquids are handled, its lightness and compactness, and the fact that the exhaust air is the same as the input air except for the removal of some of the ozone content. No new pollutants are introduced to the surrounding air. Although eosin Y in ethylene glycol is effective and makes for a practical instrument, further study of the dye-solvent combinations may well lead to additional improvements in response and detection limit. ACKNOWLEDGMENT Our thanks to Bob Lynch and Kevin Nepsund for their help building the final instrument and to Joey Boatman and the crew of the NOAA KingAir for their help during the PRECP program in Oklahoma City. Registry No. Rhodamine B, 81-88-9; safranin 0, 477-73-6; eosin Y, 17372-87-1; fluorescein, 2321-07-5; methylene violet, 8004-94-2;ozone, 10028-15-6. LITERATURE CITED (1) Hodgeson, J A,; McClenny, W. A.; Stevens, R K "Analytical Methods Applied to Air Pollution Measurements": Stevens, R. K , Herget, W F., Eds.; Ann Arbor Science: Ann Arbor, MI, 1974,p 43. (2) Wendel, G. J.: Stedman, D. H.; Cantrell, C. A ; Damrauer, L. Anal Chem. 1983,5 5 , 937. (3) Nederbraat, G. W.: Van Der Horst. A.; Van ? Duiin, . J. Nature (London)
' 1g65,205,a7. (4) Fontijn, A.; Sabadell, A. J.; Ronco, R. J. Anal. Chem. 1970,42 , 575. (5) Pearson, R., Jr.; Stedman, D. H. Atmos. Techno/. 1980, 12, 5 1. (6) Regner, V. H. J . Geophys. Res. 1060, 6 5 , 3975 (7) Regner, V. H. J . deophys. Res. 1964, 6 9 , 3795. (8) Vodgeson, J. A.; Krost, K. J.; O'Keeffe, A. E.; Stevens, R. K. Anal. Chem. 1970. 4 2 . 1795. (9) Beisis, D.; Vassilibu, E. Ana/yst(London) 1966,9 1 , 499. Bowman, R. L.: Alexander, N. Science 1966, 154, 1454. (10) (11) Ballard, L. F.; Tommerdahl, J. B.; Decker, C. E.; Royal, T. M.; Nifong, D. R . EPA Publication APTD0775, 1971.
RECEIVED for review September 10, 1985. Accepted October 3, 1985. This work was supported by AMAX Corp., Battelle Northwest Laboratories (Subcontract l%H6560-A-E), and Department of Commerce/NOAA (Contract NA84-WC-C06 137).
