Chemiluminometric Measurement of Atmospheric Ozone with

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Anal. Chem. 2003, 75, 5916-5925

Chemiluminometric Measurement of Atmospheric Ozone with Photoactivated Chromotropic Acid Toshio Takayanagi, Xiao-Li Su, Purnendu K. Dasgupta,* Kalyani Martinelango, Guigen Li, Rida S. Al-Horr, and Robert W. Shaw

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409

A highly sensitive, robust, fast, affordable measurement system based on interfacial gas-liquid chemiluminescence (CL) on a wetted transparent screen directly on top of a miniature photomultiplier tube provides the basis of an attractive method for ozone (O3). Alkaline chromotropic acid (CA, 4,5-dihydroxynaphthalene-2,7-disulfonic acid) chemiluminesces upon exposure to ozone. No light emission is observed from exposure of alkaline CA to NO2 or H2O2. However, response to ozone is highly dependent on the age and storage condition of the CA solution. As such, quantitative analysis will require frequent calibration, and the method will not be attractive. We have discovered that photoactivation plays the key role in producing (a) compound(s) from chromotropic acid that appear(s) to be the primary agent(s) responsible for the CL reaction with O3. We thus devised a method wherein a flowing solution of CA (that is stable in neutral/acidic solutions) is rendered alkaline and then exposed for a few seconds on-line to UV radiation. The solution then reacts with ozone on a screen consisting of an “invisible” nylon stocking that provides for low liquid residence time and high light throughput and results in an LOD of 40 pptv, a determination range at least up to 230 ppbv, and 1090% and 90-10% response times of 130 and 80 ms, respectively. Intra- and interday repeatabilities at the same concentration were 0.32 and 3.8% in relative standard deviation. On the basis of aging, CL, chromatography, and chromatography-mass spectrometry studies, we suggest that the primary CL-active species are likely dimeric semiquinone species derived from CA by a series of radical reactions. Ozone is one of the strongest oxidizing agents; it is widely used in water treatment and for sterilization. It is one of the criteria pollutants in the ambient atmosphere as designated by the U.S. Environmental Protection Agency; it is harmful to human health when present in significant concentrations. Tropospheric ozone originates from photochemical reactions. Airborne atmospheric mapping of O3 with good spatial resolution requires very fast response instruments, because the measurement platform is typically moving at a speed of g50 m/s. Eddy correlation measurements also require fast response. Ozone is most commonly measured by UV photometry.1 This approach requires 5916 Analytical Chemistry, Vol. 75, No. 21, November 1, 2003

significant integration time to attain a limit of detection (LOD, S/N ) 3) of 1 ppbv. The oxidation of many compounds by O3 is accompanied by chemiluminescence (CL). CL-based O3 measurement methods have been reviewed.2 The measurement of CL in the ethyleneO3 reaction is the USEPA reference method for O3 and the most commonly used CL-based method for O3.3 Regner was among the first to examine heterogeneous CL between gaseous O3 and luminol on silica gel4 as a means to measure O3; he later substituted Rhodamine B for luminol for greater specificity.5 Bowman and Alexander6 and Hill et al.7 surveyed the heterogeneous CL production from a number of compounds. Rhodamine B can provide an LOD of 1 ppbv O3;8 Phenosafranin,8 Eosin Y,8 Coumarin 47,9 and even surface-adsorbed lubrication oils10 have been used in solid-phase O3 detection systems. Although the use of a solid-phase reagent is convenient, there are several practical problems. Surface saturation leads to limited usable life. Coumarin 47 on silica, probably the most used solid reagent, must be preactivated by prior exposure to ozone. Although the fast response time of the Coumarin 47 based method is a boon, variations in sensitivity make it imperative that the temporal mean be continuously calibrated with a slower monitor that has more response stability.11 The response of solid reagentbased ozone sensors is affected by changes in relative humidity (RH).8,12 This problem is avoided with the use of an aqueous reagent. In eddy correlation measurements, if the sensor is fast enough, the negative interference from water vapor can be eliminated by frequency-domain analysis because water concentration changes much more slowly;13 however, it is desirable not to have this RH dependence to begin with. (1) Methods of Air Sampling and Analysis, 3rd ed.; Method 417; Lodge, J. P., Jr., Ed.; Lewis: Boca Raton, 1988; pp 422-426. (2) Mikuska, P.; Vecera, Z.; Janak, J. Chem. Listy 1992, 86, 407. (3) Methods of Air Sampling and Analysis, 3rd ed.; Method 413; Lodge, J. P. Jr., Ed.; Lewis: Boca Raton, 1988; pp 407-411. (4) Regner, V. H. J. Geophys. Res. 1960, 65, 3975. (5) Regner, V. H. J. Geophys. Res. 1964, 69, 3795. (6) Bowman, R. L.; Alexander, N. Science 1966, 154, 1454. (7) Hill, E. A.; Nelson, J. K.; Birks, J. W. Anal. Chem. 1982, 54, 541. (8) Hodgeson, J. A.; Krost, K. J.; O’Keeffe, A. E.; Stevens, R. K. Anal. Chem. 1970, 42, 1795. (9) Schurath, U.; Speuser, W.; Schmidt, R. Fresenius’ Z. Anal. Chem. 1991, 340, 544. (10) Chisaka, F.; Yanagihara, S. Anal. Chem. 1982, 54, 1015. (11) Hauf, T.; Schulte, P.; Alheit, R.; Schlager, H. J. Geophys. Res. 1995, 100, 22, 957. (12) http://narsto.ornl.gov/Compendium/methods/o3.shtml. 10.1021/ac034723n CCC: $25.00

© 2003 American Chemical Society Published on Web 09/27/2003

Figure 1. (a) Flow-through gas-liquid CL detection cell; see text for description. (b) Gas/liquid flow and test and instrumental arrangement; see text for details.

Bersis and Vassiliou14 were the first to use a liquid-phase reagent system containing ethanolic Rhodamine B and gallic acid; the LOD was at parts-per-million by volume levels. Others have since reported much better performance with the same reagent.15 Bleaching of indigo derivatives by aqueous ozone is well-known; 16 Takeuchi et al.17,18 discovered that this reaction also produces CL. By using a 20-cm2 photomultiplier tube (PMT) and photon counting, they reported an LOD of 400 pptv O3. Ray et al.19 reported a stable, sensitive, fast response system where the sample gas passes between the PMT and a glass fiber filter continuously wetted by a luminogenic reagent. The reported LOD with Eosin Y in ethylene glycol (for best sensitivity, organic solvents must be used) was 200 pptv. Currently available heterogeneous CLbased commercial instruments use Rhodamine B or Eosin Y.20 In the past, we had investigated the relative CL sensitivities of aqueous solutions of a number of fluorescent compounds for the measurement of dissolved O3,21 the general hypothesis being that the energy released in the oxidation may be transferred to another intact molecule of the fluor (present in much larger excess) and which then luminesces. Chromotropic acid (CA) was the most sensitive, about twice the nearest competitor, Rhodamine B. When faced more recently with the task of fast sensitive atmospheric ozone measurement, we wanted to investigate the reaction with CA in more detail. To our considerable surprise, the CL response of an otherwise optimized CA concoction increased for several days until it reached ∼100× the original sensitivity. When comparing freshly made solutions, technical grade CA produced (13) Gusten, H.; Heinrich, G.; Schmidt, R. W. H.; Schurath, U. J. Atmos. Chem. 1992, 14, 73. (14) Bersis, D.; Vassiliou, E. Analyst 1966, 91, 499. (15) Guicherit, R. Fresenius’ Z. Anal. Chem. 1971, 256, 177. van Dijk, J. F. M.; Falkenburg, R. A. Environ. Pollut. Mgmt. 1979, 9, 42. Takeuchi, K.; Ibusuki, T. Bunseki Kagaku 1987, 36, 311. Mikuska, P.; Vecera, Z. Anal. Chim. Acta 1998, 374, 297. (16) American Public Health Association. Standard Methods for the Examination of Water and Wastewater, 17th ed.; APHA: Washington, DC, 1989; pp 4-162-4-165. (17) Takeuchi, K.; Ibusuki, T. Anal. Chem. 1989, 61, 619. (18) Takeuchi, K.; Kutsuna, S.; Ibusuki, T. Anal. Chim. Acta 1990, 230, 183. (19) Ray, J. D.; Stedman, D. H.; Wendel, G. J. Anal. Chem. 1986, 58, 598. (20) http://narsto.ornl.gov/Compendium/methods/o3.shtml. (21) Chung, H. K.; Bellamy, H. S.; Dasgupta, P. K. Talanta 1992, 39, 593.

more CL with O3 than reagent grade CA. This indicated that it may not be pure CA that is the primary reactive agent. Although the increased sensitivity allowed the use of a small inexpensive PMT to reach sub-parts-per-billion-volume LODs, the continuously variable sensitivity with reagent age made the approach essentially useless for practical purposes. This paper provides an account of how this problem was solved to provide a fast compact inexpensive ozone measurement device, an insight into the CA reaction chemistry, and a novel CL detector design that is well-suited for general application in any gas-liquid heterogeneous CL measurement. EXPERIMENTAL SECTION Instrument and Test Setup. The detector construction is shown in Figure 1a. A layered structure was built on a miniature metal package PMT-based photosensor module (PSM, 22 × 22 × 60 mm, Hamamatsu H5784-00, peak response 320-500 nm) using the screw holes (M-2) in the body of the PMT as the anchoring sites.22 The PSM contained a built-in high voltage supply. At the top, the effective photocathode (8-mm diam) window is recessed 1.5 mm with respect to the top metal surface. The transparent Plexiglas block A was machined to just fit atop the PMT such that its bottom was in contact with the PMT window when it was securely screwed into the metal top of the PMT. The well in this piece was 7 mm in diameter and 1.0 mm deep, with a liquid outlet tube consisting of a 0.36-mm-i.d., 0.56-mm-o.d. stainless steel conduit (24-gauge hypodermic needle tubing, Small Parts Inc., Miami Lakes, FL.). A section of nylon stocking B (most “invisible” type commonly available pantyhose/leggings material appear to be suitable; we have not carried out an exhaustive study), tightly stretched in place, constituted the next layer. A ring-shaped opaque polymeric part C contained a liquid inlet tube, identical to the liquid outlet tube in A and positioned at 180° relative to it, through which the liquid was pumped onto the screen material. Finally, the top piece, D, machined of Al and lined on the underside with reflective Mylar M, brought the black PTFE gas inlet tube I close to the screen surface such that the incoming (22) http://usa.hamamatsu.com/hcpdf/parts_H/H5784-01.pdf.

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gas directly impinged on the screen-supported liquid film. Four screws, S, held the assembly together; ring spacers, R (∼3 mm), between C and D provided the way of escape for the sampled gas. The entire assembly was in a dark housing, and opaque tubes and wires exited out of this housing in a light-proof manner. The complete detection and calibration system is shown in Figure 1b. Compressed house air was dried and purified by sequential columns (C1, C2) containing silica gel/activated carbon/soda lime. This purified O3-free air was metered by individual mass flow controllers (M1-M3, FC-280, Tylan Corp.). The flow rate of air, F1, through the ozone generator was kept constant at 0.25 standard L/min (SLPM). The generated O3 was diluted by flow F2 (g1.5 SLPM) to the desired concentration. Flow F3 provided zero air flow to the instrument. Valve V was an allPFA Teflon solenoid valve (Galtek, Chaska, MN, for 0.25-in.-o.d. tubing) that selected the O3 or the clean air stream for onward transmission to the detector, which sampled at 1.5 SLPM by the miniature air pump AP (Tee-Squared Mfg, Fairfield, NJ) controlled by mass flow controller M4; excess gases were vented. Teflon tubes were used throughout. The following experimental arrangements are not specifically shown. Outdoor air was sampled in a similar manner from a common manifold simultaneously with a UV-photometer type ozone monitor (1003 AH, Dasibi Corp.). For response time experiments, two independent PFA Teflon 2-way solenoid valves were used to bring in zero and ozonated air, respectively; the valves were switched simultaneously. The inlet side of each valve was vented through a restrictor tee to avoid pressure buildup. This arrangement minimizes valve adsorption/desorption processes from influencing the response time. As shown in Figure 1b, alkaline CA solution was prepared by on-line mixing of neutral 0.5 mM CA solution (reagent grade CA solution in water, Aldrich; this is stable over week-long periods when protected from light) and 6 mM NaOH, pumped by peristaltic pump P1 at equal flow rates of 30 µL/min each. Following a short mixing coil (0.8-mm i.d., 300 mm), the stream flows through a thin-walled FEP Teflon tube (0.8-mm i.d., 0.05mm wall, type 20 LW, Zeus Inc.), 10 cm of which was coiled around a low-power UV lamp (Type G4T5, 4 W, Rayovac, Madison, WI), L, and into the detector. The total flow of 60 µL/min result in a mean irradiation time of ∼50 s. In some short-term experiments, an alkaline CA solution protected from light (0.25 mM in 3 mM NaOH, pH 11.5) was directly pumped at 60 µL/min instead of on-line mixing. The liquid was pumped out of the detector continuously by pump P2 at 60 µL/min. A gain control voltage of 0.925 V was applied to the PMT in all experiments. The PMT output was processed with an RC filter (time constant 2 s) and an operational amplifier (TL082, Texas Instruments) which provided up to 25× further amplification. Except for response time measurement experiments (in which the RC filtering was reduced to 7 ms and the data were recorded at 50 Hz), the CL intensity was recorded at 1 Hz on a laptop-type PC with a data acquisition board (PCM-DAS16D12/AO, Computer Boards, Inc., Middleboro, MA). Chemicals. CA was prepared as a 10 mM stock solution in water from the reagent grade chemical and stored refrigerated in the dark. NaOH was used for pH adjustment. Water used was distilled and deionized (18 MΩ‚cm). Gaseous O3 was generated 5918

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by passing the purified air over a short mercury lamp (Analamp 80-1025-01/430, 2.5-cm length, BHK, Monrovia, CA). The generator was calibrated by a standard method that relied on the oxidation of iodide to iodine in a neutral buffered solution and measuring the absorbance due to I3- at 352 nm.23 Auxiliary Experiments. Several auxiliary experiments were conducted to shed light on the complex chemistry of the aging of chromotropic acid and its ozone-induced CL. Absorption and fluorescence spectra were obtained with Agilent 8453 and Shimadzu RF-540 instruments, respectively. The CL spectrum was acquired with a light conduit-type gas sensor in which the reagent in solution flows through a gas-permeable tube while the sampled gas flows outside and is transmitted through the wall to react with the reagent. Although these devices have generally been used for measurement of light absorption,24 light emission can be followed as well.25 Instead of Teflon AF, we used a 150 mm × 1.5 mm i.d. ePTFE tube (see ref 26 for construction) terminated with a large core (1300-µm) fused-silica fiber that was placed at the focal point of the spectrofluorometer. Although ePTFE is not a waveguide, it does conduct some light,27 and gas transmission through it is far greater than with Teflon AF. This arrangement allowed ready acquisition of the spectrum of the heterogeneous gas-liquid CL generating system with a conventional spectrofluorometer with a parts-per-millioin level concentration of ozone as the test gas and the device shielded from light. Electron spin resonance (ESR) spectra were obtained using an X-band Varian E-109 spectrometer using a quartz flat cell. Microwave bridge power calibrations were accomplished using a 20-dB crossguide coupler (Microwave Development Laboratories, Inc.) and a Hewlett-Packard model 432A microwave power meter equipped with a Hewlett-Packard model 486A temperaturecompensated thermistor mount. Liquid Chromatographic separations were conducted on a semipreparative LUNA 250 × 10 mm 5-µm CN column at a flow rate of 1.5 mL/min under linear gradient conditions from 0 to 100% of solution B (0.75 mL trifiuoroacetic acid in 1 L of acetonitrile) over 50 min with fixed wavelength (254 nm) detection. Eluent A was 1 mL of trifluoroacetic acid in 1 L of water. LC-photodiode array (PDA)-quadrupole mass spectrometry was conducted on a Dionex DX-600 liquid chromatograph coupled to a Dionex PDA detector (operated over 200-750 nm) and a Finnigan AQA mass spectrometer operated in the electrospray mode at a temperature of 80 °C, an ionization voltage of -20 V, and a mass range of 98-776 amu. RESULTS AND DISCUSSION Chemiluminescence of CA by Ozone Oxidation. Typical CL signals monitored with the present setup appear in Figure 2. The CL spectrum consisted of a single broad emission band with a maximum near 500 nm. This is very similar to the fluorescence emission spectrum of an aged CA solution, except for a batho(23) Continuous analyzers for oxidants in ambient air; Japanese Industrial Standard, B-7957-1992; Japanese Industrial Standards Committee: Tokyo, Japan, 1992. (24) Dasgupta, P. K.; Genfa, Z.; Poruthoor, S. K.; Caldwell, S.; Dong, S.; Liu, S.-Y. Anal. Chem. 1998, 70, 4661. Milani, M. R.; Dasgupta, P. K. Anal. Chim. Acta 2001, 431, 169. (25) Dasgupta, P. K.; Liu, S.-Y. Chemical Sensing Techniques Employing Liquid Core Optical Fibers. U.S. Patent 6,011,882; January 4, 2000. (26) Toda, K.; Dasgupta, P. K.; Li, J.; Tarver, G. A.; Zarus, G. M. Anal. Chem. 2001, 73, 5716. (27) Toda, K.; Yoshioka, K.-I.; Ohira, S.-I.; Li, J.; Dasgupta, P. K. Anal. Chem., In press.

Figure 2. Observed chemiluminescence signal as the sample is repeatedly switched between zero air and a relatively low-level ozone sample. The CA reagent solution was photoactivated as described in the Experimental Section.

chromic shift of ∼20 nm, as shown in Figure 3a. The pulsation on the top of the plateau was from pulsations in peristaltic pumping. The temporal profile of the signal shows a short duration peak followed by a plateau. This was an artifact caused by valve switching from zero air to ozonized air flow that resulted in momentary blockage of flow through the ozonizer. Spectral Changes in Alkaline CA during Aging. The alkaline CA reagent is essentially colorless when freshly prepared. However, when stored in a conventional volumetric flask under standard laboratory illumination, it develops a pink tinge within an hour. Over a period of a week, it goes through pink, dark red, brown and finally to yellow (see Supporting Information for a photograph, Figure S1). Figure 3 (see Supporting Information for more easily deciphered colored traces, Figure S2) shows the change in absorption and fluorescence emission spectra over time. In the absorption spectra, note that the absorption at 525 nm increases for 1 day and decreases thereafter, whereas the absorption at ∼370 nm (where CA itself absorbs) decreases continuously. The fluorescence emission maximum, when excited at 350 nm, begins to show a bathochromic shift immediately after preparation, and by 3 days doubles in intensity, decreasing slowly thereafter. Since all of the above changes are dependent on the exact illumination and storage conditions (vide infra), the qualitative patterns rather than quantitative details are of importance here. Change in CL Intensity as a Function of Reagent Age. Figure 4a shows the changes in CL intensity with aging of the solution under different conditions (air purged, N2-purged, and O2-purged while exposed to ambient laboratory light and without light exposure). The data clearly indicate that by far the most important factor is exposure to light. Although some oxygen may be involved (the nature of nitrogen purging would not have removed all traces of oxygen, and activation appears to be slower at low oxygen levels), oxygen does not appear to be stoichiomet-

rically involved (with reference to the CA concentration) in generating the most active chemiluminogenic species. With both air or O2 atmosphere, the maximum CL intensity is reached in 7-8 days, and the maximum observed is ∼100 times greater than the CL elicited from a fresh solution. A similar effect can be brought about much more quickly by bubbling ozonized air through the CA reagent. Bubbling 100 ppm O3 through the CA reagent for a few minutes can markedly increase its subsequent CL response to O3. Early experiments with HPLC-PDA showed that all the spectrally detectable compounds in the 200-750 nm range have significant absorption at 254 nm, as would be expected for compounds containing an aromatic ring. Thus, further HPLC analysis of CA solutions as a function of aging time was conducted with detection only at 254 nm. Although there are several other minor peaks present in the chromatograms, the time profile of seven specific peaks (respective tR values: 9.2, 9.7, 10.3, 10.8, 11.6, 12.8, and 15 min) in the chromatograms of alkaline CA solutions aged in room light for 1, 2, 3, 4, 8, and 11 days is presented in Figure 4b. It will be observed that the peak eluting at 10.8 min has a maximum concentration after 1-2 days of aging and decays thereafter. When collected as a fraction and made alkaline, this component turns purple, leaving little doubt that this is the product responsible for the 525-nm absorption. All the other peaks except for the compound eluting at 15 min (note ordinate scaling, this compound is present at lower concentration than the other peaks) increases continuously with time. The compound eluting at 15 min reaches a maximum concentration on the 8th measurement day. Bearing in mind the results shown in Figure 4a, this can very well be the agent responsible for strong CL. Results of Preoxidation. We attempted to determine if the compound responsible for strong CL with ozone can be formed by simple oxidation of alkaline CA. Hydrogen peroxide addition or bubbling air/O2 through an alkaline CA solution did not result in the desired goals. Treatment with ozone, however, resulted in a marked increase of the CL, in much the same way that other reagents have also been reported to have enhanced sensitivity after preexposure to ozone.8,9,11 (In some cases, especially in preexposing Coumarin 47 on silica gel to ozone, the objective may not be to achieve an increase of sensitivity, but rather, stable response behavior; the faster reacting more intense CL producing impurities may be removed during the preexposure.28) Activation of CA for Stable Ozone Measurement with High Sensitivity. Regardless of the fact that properly aged CA has by far the greatest sensitivity compared to any other reagent introduced for similar purposes thus far, the continuously variable sensitivity with time renders the method unusable. We therefore explored the on-line photoactivation of CA solutions. The activation of CA by ambient light proceeds rather slowly. UV irradiation in a flow-through reactor was therefore examined; the photoreactor, a thin-walled FEP Teflon tube, was simply wrapped around a tubular UV lamp source, and the assembly was wrapped with Al foil while alkaline CA solution passed through the tube. Several types of UV (both 254 and 365 nm) as well as visible lamps were examined with different irradiation times. Irradiation at 254 nm was more effective than at 365 nm or with white light. Details are (28) Yushkov, V.; Oulanovsky, A.; Lechenuk, N.; Roudakov, I.; Arshinov, K.; Tikhonov, F.; Stefanutti, L. J. Atmos. Ocean. Technol. 1999, 16, 1345.

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Figure 3. (a) Fluorescence emission spectral characteristics and (b) UV-visible spectral characteristics as a function of solution aging. The solution was aged with air in the headspace under normal laboratory illumination. The CL spectrum is also shown in (a).

Figure 4. (a) CL Signal intensity for 8.7 ppbv O3 with different reagent aging protocol. Storage in the dark produces uniformly low sensitivity, regardless of headspace gas. (b) Areas of different HPLC peaks (254 nm). Note that the 15-min peak area has been multiplied 10-fold for easy visualization.

presented as Supporting Information (Figure S3). These data also indicated that the maximum signal was produced by an optimum amount of radiation exposure; a radiation dose more than this also decreases the signal. For this reason, the inexpensive G4T5type modest power (4-W) lamp was chosen over other more intense pen lamps, for this lamp and a 10-cm photoreactor length (for a 0.8-mm-i.d. tube, at the prescribed flow rate, the mean residence time is 50 s) wrapped around it reproducibly produced 75% of the maximum sensitivity observed in the solution aging experiments. Shorter or longer tube lengths/residence times decreased the sensitivity (Figure S3). 5920 Analytical Chemistry, Vol. 75, No. 21, November 1, 2003

On-line preexposure to ozone was achieved by exposing the alkaline CA solution (60 µL/min) flowing through a 20-cm-long porous PTFE tube (Poreflon TB-0201, 2-mm o.d., 1-mm i.d., Sumitomo Electric, Osaka, Japan), maintained in a 5-mm i.d. PTFE jacket tube through which 25 ppm ozone gas flows co-current at 0.25 SLPM. The observed sensitivity was very comparable to the results obtained from optimum photoactivation experiments; however, on-line ozonation increased the background CL, increasing the baseline noise. A plot that shows the signal and background as a function of the liquid flow rate is presented in the Supporting Information (Figure S4). Given these results, the more

Figure 5. Open symbols refer to dashed axes and filled symbols to solid axes. (a) Variation of log CL intensity with pH (left ordinate bottom abscissa) and with reagent concentration (pH 11.5). (b) The CL signal for flow rates above 60 µL/min is inversely proportional to the flow rate and directly linearly proportional to the residence time.

complex experimental arrangement and the necessity to destroy the high ozone concentration prior to venting, preozonation was not deemed less attractive than the photoactivation method, which was henceforth used. Parametric Optimization. The CA-O3 CL is observed only in alkaline solution. At pH >9, log CL intensity increases linearly with pH, reaching a plateau around pH 11.4. The data are shown in Figure 5a with the best fit trace depicting the ionization of a model monoprotic weak acid. The slope in the linear portion is close to unity, and the composite best fit pKa is 11.2, suggesting the ionization of one or more phenolic protons is needed to generate the most CL active species. A reagent pH of 11.5 was used henceforth. CA concentrations ranging from 25 to 2000 µM (final concentration after adding base) were examined at this pH. As shown in Figure 5a, in our specific experimental system, the maximum response was observed in the range of (2.5-5) × 10-4 M CA. This is likely the result of a combination of two factors: at low concentrations, the amount of the activated material produced is too low, and at high concentrations, the solution in the center of the tube cannot photoconvert efficiently because of self-filtering but subsequently dilutes the active ingredient. Further experiments were conducted with an alkaline CA reagent concentration of 2.5 × 10-4 M. The effect of the flow rate of the CA reagent solution through the reactor is shown in Figure 5b. When a partially aged CA solution was used as the reagent and the light source was turned off, the CL response showed no dependence on the flow rate of the solution in the same range (not shown in the figure). However, under the normal photoactivation conditions, the sensitivity depended strongly on the flow rate; the CL signals approximately fit a straight line when plotted against the residence time in the studied range of flow rates (linear correlation coefficient (r2)

0.9915). As previously stated, a residence time >50 s also decreased the signal (Figure S3). The overall system response also obviously depends on the PMT gain control voltage. Within the range of 0.80-0.95 as the PMT gain control voltage VC (maximum for VC is 1.0), the signal increased linearly with the control voltage (r2 ) 0.9970). The sensitivity of the PMT is reported to vary in a log-log fashion with the gain control voltage, increasing 10-fold within as VC changes from 0.7 to 1.0.22 However, noise increased nonlinearly, and best results were obtained with VC ) 0.925 V, which was henceforth used. Performance: Determination Range, Response Linearity, Detection Limit, Reproducibility, and Interferences. In the present experiments, we tested up to 234 ppbv ozone. Over 1 order of magnitude concentration range, the response is acceptably linear (e.g., the linear r2 value for measurement in the 0-10 ppbv concentration range is 0.9975). However, over an extended range, there is perceptible downward curvature in the response at higher concentrations. Quadratic fits match the response very well (r2 ) 0.9990) over an extended range (Figure 6a). It is possible that the nonlinearity arises from an inadequate amount of the most active ingredient being made during photoactivation. This would likely be solved if we used a higher CA concentration with a longer photo reactor length; however, with the present practice of computerized data reduction or the use of embedded microprocessors, this small degree of nonlinearity is not a significant concern. On the basis of repeated measurements of zero air and the response from 3 ppbv O3, we calculate the S/N ) 3 limit of detection (LOD) to be 40 pptv (S/N ) 3). This LOD is superior or comparable to the best extant O3 detectors. Considering that this LOD is obtained with an inexpensive photosensor module Analytical Chemistry, Vol. 75, No. 21, November 1, 2003

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Figure 6. (a) Calibration for ozone extending to relatively high levels. No secondary amplification was used. Filled and open symbols represent data obtained several weeks apart, with a small overlap in the range studied to check reproducibility. The calibration equation is for the combined data. Lack of response to NO2 is shown. (b) Response signal from 0 to 110 ppb ozone and vice versa, no secondary amplifier, PMT circuit time constant 6.6 ms.

(active PMT area 0.50 cm2), we deem this performance notable and ascribe it to the extraordinary sensitivity provided by photoactivated CA. Intraday and interday reproducibility were examined with 8.7 ppbv ozone. Short-term reproducibility, as shown in Figure 2, was excellent. The relative standard deviation (RSD) for 10 sequential measurements was 0.32%, within-day reproducibility was typically 0.9% in RSD. On the basis of measurement on seven disparate days, the day-to-day reproducibility showed an RSD of 3.8%, which included the intrinsic variation in ozone generation and uncertainties involved in the calibration of the ozone source. This performance was all the more remarkable when one considers the great variability in response when CA measurements are conducted simply with a prepared solution. Generally, the CL methods based on organic dyes are highly specific for ozone;5,8,9,17,19,29 other common oxidizing atmospheric trace gases, such as NO2, H2O2 ,and peroxyacetyl nitrate (PAN), do not interfere with the determination. We had ourselves determined that alkaline CA does not produce detectable CL with aqueous H2O2 or Cl2 (as OCl-) even at a relatively high level of 10 mg/L (0.3 mM H2O2).21 During this work, we studied potential interference of gaseous NO2, up to 137 ppbv in concentration, under conditions identical to that used for measuring O3. The data shown in Figure 6a indicate that there is no significant interference. Even at 137 ppbv NO2, the observed response was below that equivalent to 0.8 ppbv O3. Response Time. One primary motivation behind this work was to devise a truly fast ozone measurement method. The UV absorption instrument, the workhorse for O3 measurement, usually does not provide a response time better than 30 s15 although one new instrument achieves a “measurement interval” of 10 s.30 Specialized UV monitors have been described for aircraft applica(29) Mikuska, P.; Vecera, Z. Anal. Chim. Acta 1998, 374, 297. (30) http://www.twobtech.com/o3monitor.htm.

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tions that can operate with a response time of 1 s,31 but this is achieved by essentially operating multiple monitors out of phase with each other. Most response time specifications also give little information as to the method of response time measurement and generally ignore the fact that in virtually all instruments, the rise time and the fall times are not the same. Another characteristic of extant instruments is that the response time is very sample flow rate-dependent. Ray et al.19 reported a response time of 700 ms at a sampling rate of 1 L/min and 140 ms at 9 L/min. They correctly argue that the sweep time of the chamber itself is the principal factor. To our knowledge, the Coumarin 47-based solidstate CL sensor developed by Schurath et al.9,13 has the fastest response time reported thus far. But this response time of 47 ms is obtained at a sampling rate of 105 L/min and is reported to increase significantly as the sample flow is reduced. In that context, the present flow cell has very little swept volume and should provide good response time without excessively high sampling rates. The results are shown in Figure 6b with a data acquisition rate of 50 Hz. The 10-90% and 90-10% rise/fall times are 130 and 80 ms, with the corresponding 1/e times being 60 and 35 ms. Considering that this response was obtained with a sampling rate of 1.5 L/min and the PMT used is not particularly fast, the merit of the transparent screen-supported liquid-gas reactor for measuring CL is clear not only for the present application but in general for any CL measurement efforts involving gas-liquid contact. Comparative Ambient Air Measurement. Figure 7 shows simultaneous ozone measurement by a commercial UV absorptionbased instrument and the present instrument as ambient air is drawn into a common Teflon manifold and sampled by the individual instruments. The signal from the present instrument (31) Tsutsumi, Y. J. Meteorol. Soc. Jpn. 1997, 75, 969.

Figure 7. Collocated measurement of ozone using the present instrument (top trace) and a commercial UV absorption instrument in Lubbock, TX.

was converted to parts-per-billion-volume O3 using a calibration function, as shown in Figure 6a. The two instruments tracked each other well. Compounds/Intermediates Formed during Aging/Photoactivation of Alkaline Chromotropic Acid. The reactivity and the use of chromotropic acid and its derivatives have been reviewed within the past decade by Duda.32 The degradation of CA and its derivatives by photocatalyzed oxidation has also been studied.33,34 Although it is known that the halogenated derivatives of CA form 8-hydroxy-1,4- and 1,2-naphthaquinones in which the sulfonate groups are still retained,33 in CA itself, partial or complete desulfonation occurs, leading to p-quinone derivatives that possibly polymerize.32,34 Desulfonation reactions of this type often proceed by a free radical mechanism.35 Although no ESR signal was observed in the absence of light exposure or when exposed in situ to tungsten light, 10 min of UV irradiation at 254 nm gave an obvious ESR signal, and subsequent in-situ radiation and acquisition of ESR signals led to the spectrum shown in Figure 8. (While the CA solution was continuously irradiated by the UV light, the signal increased for 3 h and remained detectable for some time even after the removal of the light.) The ESR spectrum shows two groups of signals centered at g ) 2, with each signal resolved into 12-15 hyperfine or superhyperfine lines. This is consistent with the steady-state existence of one or more radical species, as shown in Figure 9. Individual fractions were collected from the LC effluent, which were then injected into a water stream that merged with a 0.1 M NaOH stream; this further merged with an ozone-saturated (32) Duda, J. Chem. Soc. Rev. 1994, 425. (33) Duda, J. Chem. Anal. (Warsaw) 1993, 38, 405. (34) Duda, J. Pol. J. Chem. 1991, 65, 67. (35) Mori, Y.; Shinoda, H.; Nakano, T.; Kitagawa, T. J. Photochem. Photobiol. 2003, 157A, 33. Patterson, D. A.; Metcalfe, I. S.; Xiong, F.; Livingston, A. G. Ind. Eng. Chem. Res. 2001, 40, 5517.

Figure 8. ESR spectrum of an alkaline solution of 2.5 mM CA irradiated with a 4-W G4T5 254 nm lamp (from a distance of 70 cm; closer placement interfered with the measurement) for ∼4 h at room temperature. The microwave power level was 1 mW, the magnetic field modulation frequency was 100 kHz, and the magnetic field modulation amplitude was 0.1 G.

aqueous solution directly in front of a phototube (cf. ref 21). The ability of the different LC fractions to produce CL were thus studied. Although the ability to produce CL was not unique, the later eluting peaks, particularly the peak eluting at 15 min, was especially CL-active relative to its 254-nm absorbance. On the basis of the prior literature and the LC/MS results, we thus suggest the mechanism and products in Figure 9. In a fresh solution, both the monoanion (M - H)- at m/e 319 and especially the dianion (M - 2H)2- at m/e 159 from CA are prominent. There are low-intensity peaks at m/e 189, which we believe is due to 12 and at m/e 167 that we are presently unable to assign. The radical 2 and with more intense light exposure, the diradical 3 are formed. From 2, either 7 or 8 can be formed by H or OH abstraction. Similarly, 4-6 are formed from 3. It is Analytical Chemistry, Vol. 75, No. 21, November 1, 2003

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Figure 9. Aging and photoactivation products of alkaline chromotropic acid: proposed scheme.

facile to form a radical at either one of the peri-hydroxyl groups. Thus, 4 forms 9 and, thence, the semiquinone radical 10. This leads to 11, which is readily oxidized to 12. Similarly, 13 can be formed from 6. Compounds 12 and 13 are similar to naphthopurpurine; indeed, these and similar compounds together likely constitute the initially formed colored compound (yellow in acid, purple in base; see Figures 3a and 4, tR ∼ 10.8 min).36 This fraction does not produce significant CL. We attempted a synthesis of naphthopurpurine.37 Although we did not attempt complete purification, the crude product exhibited no CL in our test system. The primary CL producing products are believed to be the dimeric products 14 which are formed by radical-radical recombination reactions. Two molecules of 10 for example, will form 14a with WdZdOH and XdYdH. There are strong peaks at both m/e 349 (M - H)- and 174 (M - H)2-. From the mass spectra, we cannot distinguish between 14a and, for example, WdZdH and XdYd OH. However, there is clear evidence for other related compounds. The strongest signal of these dimeric compounds is with m/e peaks at 333 (M - H)- and at 166 (M - 2H)2-that would arise from 14b, in which only one substituent among W, X, Y, and Z is OH, the rest being H. There are also clear m/e peaks that would indicate the presence of compounds similar to 14a and 14b, except that one of the reacting radicals was derived from 2, such that one sulfonate group is present in the final molecule. It is not clear if com(36) Singh, I.; Ogata, R. T.; Moore, R. E.; Chang, C. W. J.; Scheur, P. J. Tetrahedron 1968, 24, 6053. (37) Bekaert, A.; Andrieux, J.; Plat, M. Bull. Soc. Chim. Fr. 1986, 314.

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pounds such as 14 directly react to form CL or via 15, which would readily form by air (or ozone) oxidation of 14. Ozone is expected to cleave the central connecting double bond in 15 with considerable release of energy that would be available for excitation. CONCLUSIONS We have presented here a very fast CL-based ozone measurement procedure using a unique reaction cell design that permits a thin stable renewable liquid film supported on a nearly transparent screen to be directly formed on top of a PMT window. The small flow-through volume of the cell enables high response speed without high flow rates, and the use of inexpensive components results in a small and affordable package. We also presented a novel reagent photoactivation procedure that results in heretofore unequalled sensitivity and provides stable sensitivity from a CA-based reagent that was previously irreproducible. Inasmuch as pretreatment by O3 is also beneficial with other reagents, it could be of interest to study how prior irradiation affects response in other cases. Although the aging/activation chemistry of CA is complex, we have provided an enhanced understanding of the chemistry of what may be the primary CL-active species involved in this reaction. This may hopefully lead in the future to deliberate synthetic efforts to make pure compounds that will produce intense CL with ozone. ACKNOWLEDGMENT We gratefully acknowledge the experimental assistance of Li Xiang and Jianzhong Li. This research was supported in part by

Paul Whitfield Horn Professorship funds at TTU, by the SOS/ Supersite Research Program of the USEPA, and by NSF Grant CTS-0088198.

results of preozonation as a function of solution flow rate through a membrane ozonator. This material is available free of charge via the Internet at http://pubs.acs.org.

SUPPORTING INFORMATION AVAILABLE Change in color of alkaline CA solutions, absorption spectroscopic changes of an alkaline CA over several days, effect of different lamps and photoreactor lengths on sensitivity, and the

Received for review July 1, 2003. Accepted August 22, 2003. AC034723N

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