Anal. Chem. 1002, 64, 2187-2191
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Determination of Nitrogen Dioxide with a Chemiluminescent Aerosol Detector Pave1 Mikugka’ and Zbyngk Ve6eia Institute of Analytical Chemistry, Czechoslovak Academy of Sciences, VeveFi 97, CS-61142 Brno, Czechoslovakia
INTRODUCTION Nitrogen dioxide is one of the most frequent air pollutanta with a considerable impact on human health.l It plays an important role in the generation of photochemical smog and photochemical oxidants such as ozone and peroxyacetyl nitrate (PAN)2-5and it is a precursor of nitrous acid, nitric acid, and fractions of solid nitrateass which are important in the acid rain chemistry. A number of methods of NO2 measurement have been de~eloped.~ Earlier methods used the conversion of nitrogen dioxide in a liquid medium to nitrite followed by spectrophotometric determination by the Griess reagent. In such cases nitrogen dioxide is f i t absorbed in a suitable absorption solution, most frequently in the Saltzman reagent,10 sodium hydroxide (withoutll or with a contribution of suitable substancesl2-l6),or triethanolamine (with quaiam116or without quaiac01.l~ Besides evident dependence of collection efficiency on experimental factors, the most important disadvantage of those absorption methods is the fact that they do not enable one to follow the changes of nitrogen dioxide concentrations in the air in the real time. We can get only integral information on the atmospheric NO2 concentration. Recently it has been even reported that the results obtained in such a way can be loaded by up to 30% positive error due to the interference of nitrous acid.18 At present nitrogen dioxide is most often determined indirectly after reduction to nitric oxide with a subsequent measurement of the intensity of the chemiluminescent (CL) radiation emitted at the reaction of NO with ozone. This method is specific for NO; the amount of nitrogen dioxide is estimated from the difference of the detector response for normal or reduction mode^.^^.^^ The amount of positive interferences at the NO2 measurement caused with the reduction of nitrogen oxo compounds, such as HN02, HN03, (1) Lee, S.D., Ed. Nitrogen Oxides and their Effect on Health; Ann Arbor Science Publishers: Ann Arbor, MI, 1980. (2) Butler, J. D. Air Pollution Chemistry; Academic Press: London,
1979. (3) Hecht, T. A.; Seinfeld, J. H. Enuiron. Sci. Technol. 1972,6,47-57. (4) Stephens, E. R.; Darley, E. F.; Taylor, 0. C.; Scott, W. E.Znt. J. Air Water Poll. 1961,4, 79-100. (5) Chameides, W.; Walker, J. C. G. J. Geophys. Res. 1973, 78,87518760. (6) Heikes, B. G.; Thompson, A. M. J. Geophys. Res. 1983,88,1088310895. (7) Seinfeld,J. H.Atmospheric ChemistryandPhysics ofAirPollution; Wiley: New York, 1986. (8) Finlayson-Pitta, B. J.; Pitta, J. N., Jr. Atmospheric Chemistry: Fundamental and Experimental Techniques; Wiley: New York, 1986. (9) Mikdka, P.; VeEeia, Z.; Jan&, J. Chem.Listy 1990,84,1249-1265. (10) Saltzman, B. E. Anal. Chem. 1954,26, 1949-1955. (11) Jacobs, M. B.; Hochheiser, S. Anal. Chem. 1958,30,426-428. (12) Nash, T. Atmos. Enuiron. 1970,4, 661465. (13) Christie, A. A.; Lidzey, R. G.; Radford, D. W. F. Analyst 1970,95, 519-524. (14) Huygen, C.; Steerman, P. H. Atmos. Enuiron. 1971,5, 887-889. (15) Nair, J.; Gupta, V. K. Atmos. Enuiron. 1981, 15, 107-108. (16) Mulik, J.; Fuerst, R.; Guyer, M.; Meeker, J.; Sawicki, E.Znt. J. Enuiron. Anal. Chem. 1974,3,333-348. (17) Levaggi, D. A,; Siu W.; Feldstein, M. J.Air Pollut. Control Assoc. 1973,23, 30-33. (18) Pitta, J. N., Jr.; Biermann, H. W.; T w o n , E.C.; Green M.; Long, W. D.; Winer, A. M. J. Air Pollut. Control Assoc. 1989,39,1344-1347. (19) Fontijn, A.; Sabadell, A. J.; Ronco, R. J. Anal. Chem. 1970, 42, 575-579. (20) Sigsby, J. E., Jr.; Black, F. M.; Bellar, T. A,; Klosterman, D. L. Enuiron. Sci. Technol. 1973, 7, 51-54. 0003-2700/92/0364-2187$03.0010
PAN, and solid n i t r a t e ~ ~isl -questionable. ~~ Other inaccuracies of that method result from the fact that a small amount of NO originating from reduction of NO2 is compared with high quantity of NO in the air. The above-mentioned disadvantages are avoided by the direct measurement of NO2. The most frequently used methods are photolytic dissociation combined with chemiluminescent determination of photolytic fragment~~4.25 laser absorption f l u o r e ~ c e n tor ~ ~photoacoustic or also differential optical absorption spectr0scopy.2~ Instrumentation of those methods is mostly complicated, and the purchase and maintenance expenses usually prevent spectroscopicmethods for current monitoring of NOz. There is also a direct determination of NO2 by the wet methods based on the CL reaction of nitrogen dioxide with the luminol alkaline when other nitrogen compounds except PAN do not interfere. The problem of the CL reaction principle is establishment of a constant interface reaction area between the gas and liquid phases.30 Wendel et aL31 are flowing the solution down a fiiter paper, thus providing a defined surface. A similar solution (the air is passed along at a fabric wick wetted with the luminol solution) was used to make a compact commercial detector to measure nitrogen dioxide: LUMINOX LMA-3, Scintrex/Unisearch Associates Inc.32~33 This work describes special modification of chemiluminescent detectors-chemiluminescent aerosol detector (CLAD). The chemiluminescent reaction between the luminoland the nitrogen dioxide resulta from continual spraying of the luminol solution (LMN) by the stream of the analyzed gas, and the intensity of the luminiscent radiation emitted at the reaction between the aerosol of the luminol alkaline solution and nitrogen dioxide in the gas phase is viewed by a photomultiplier.
EXPERIMENTAL SECTION Detector Design. The detector, a modified aerosol enrichment unit,” consists of a reaction cell, a nebulizer, and a photomultiplier (PMT). The analyzed air is sucked into the detector ~~
(21) Winer, A. M.; Peters, J. W.; Smith, J. P.; Pitta, J. N., Jr. Enuiron. Sci. Technol. 1974,8,1118-1121. (22) Joseph, D. W.; Spicer, C. W. Anal. Chem. 1978,50, 1400-1403. (23) Walega, J. G.; Stedman, D. H.; Shetter, R. E.;Mackay, G. I.; Iguchi, T.; Schiff, H.I. Enuiron. Sci. Technol. 1984, 18, 823-826. (24) Kley, D.; McFarland, M. Atmos. Technol. 1980,12,6349. (25) McClenny, W. A.; Hodgeson, J. A.; Bell, J. P. Anal. Chem. 1973, 45,1514-1518. (26) Schiff, H. I.; Hastie, D. R.; Mackay, G. I.; Iguchi, T.; Ridley, B. A. Enuiron. Sci. Technol. 1983,17. 352A-364A. (27) Bradshaw, J.; Davis, D.’D. Opt. Lett. 1982, 7, 224-226. (28) Poizat, 0.;Atkinson, G. H. Anal. Chem. 1982,54, 1485-1489. (29) Platt, V.; Perner, D.; Piitz, H. W. J.Geophys. Res. 1979,84,63296335. (30) Maeda, Y.; Aoki, K.; Munemori, M. Anal. Chem. 1980,52,307311. (31) Wendel, G. J.; Stedman, D. H.; Cantrell, C. A.; Damrauer,L. Anal. Chem. 1983,55,937-940. (32) Schiff, H. I.; Mackay, G. I.; Castledine, C.; Harris, G. W.; Tran, Q.Water Air Soil Pollut. 1986, 30, 105-114. (33) Fehsenfeld,F. C.; Dnunmond, J. W.; Roychowdhury,U. K.; Galvin,
P. J.; Williams, E. J.; Buhr, M. P.; Parrish, D. D.; HQbler, G.; Langford, A. 0.;Calvert, J. G.; Ridley, B. A.; Grahek, F.; Heikes, B. G.; Kok, G. L.; Shetter, J. D.; Walega, J. G.; Elaworth, C. M.; Norton, R. B.; Fahey, D. W.; Murphy, P. C.; Hovermale, C.; Mohnen, V. A.; Demerjian, K. L.; Mackay, G. I.; Schiff, H. I. J. Geophys. Res. 1990, 95, 3579-3697. (34)VeEeia, Z.; Jan&, J. Anal. Chem. 1987,59, 1494-1498. @ 1992 American Chemical Society
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Figure 2.
Figure 1.
Nebulizer: for key, see the text.
by a gas pump, and an ejection effect of the streaming air sucks the solution of LMN into the detector and disperses it here to form a fine aerosol with a large interfacearea. The formed LMN aerosol is directed toward the bottom wall of the reaction cell, and the PMT position is perpendicular to the aerosol direction. The aerosol strikes on the bottom wall, and the formed liquid film is removed by the influence of both gravity and the flow of the gas to the lowest part of the detector cell and is continuously sucked out by the underpressure to the liquid trap. The detector cell is made of a stainless steel tube of 29-mm i.d. and 95-mm length. One end of the tube, in the direction of the longitudinal axis, is closed with a photomultiplier tube separated from the inside space of the detector cell by a fused silica window of the diameter 10mm. The reaction cell is oriented in the space aslant so that the detector longitudinal axis is at an angle of 40° to the horizontal support. Such an orientation facilitates spontaneous accumulation of the condensed LMN solution at the lower end of the detector cell and enables the easy drainage the condensate from the detector. The nebulizer body is screwed in the upper wall of the reaction cell in the distance of 45 mm from the fused silica window. Description of the Nebulizer. The nebulizer (Figure 1) consists of a stainless steel body (1) (20-mm 0.d.) screwed into the upper wall of the reaction cell (2). Into the nebulizer body (1)a stainless steel front (3) is screwed to which a stainlesssteel cone (4) with four ribs (4a)is inserted. The cone ensures centric position of the jet (5) (stainleas steel capillary, 0.86-mm o.d., 0.55-mm i.d.). In the outlet opening of the nebulizer front the longitudinal position of the jet is ensured with a Teflon O-ring (6) fixed by a stainless steel thrust screw (7). The solution of LMN is carriedto the jet from the reservoir viaa hydraulicresistor by a stainless steel capillary (8) (1.28-mm o.d., 0.36-mm i.d.) connected to the nebulizer by a cap nut (9) and the thrust screw (7). The gas is carried into the nebulizer by a stainlesssteel tube (10) (2.0-mm i.d.), it streams between the ribs (4a) of the concentric cone and through the outlet opening of the nebulizer front (5), and it is sucked to the detector reaction space. The tightnessbetween individual parts of the nebulizer is ensured by Teflon o-rings (11, 12). Apparatus. Basic arrangement of the measuring apparatus is represented schematically in Figure 2. It consists of three basic parts: dilution system of nitrogen dioxide, CLAD, and measuring electronics. The analyzed air passes through a dust filter (Fl), the zero air is purified by passage through activated charcoal (F2) and silica gel. A three-way valve (V) regulates the ambient air or zero/calibration air entering into the detector. The emitted CL radiation is detected by a photomultiplier
Schematic diagram of the experimental arrangement.
(Special Designing Bureau, Estonian Academy of Sciences, Tallin) with no wavelength discrimination. PMT is operated at 600 V at the laboratory temperature of 22-25 O C . Reagents. Luminol and other chemicals of analytical grade (LachemaBrno) were used without any further purification. The primary NO2 source used for calibration was a permeation tube with a 1371 ng of NOdmin emission rate. The produced NO2 was diluted by nitrogen purified by passing through silica gel and a molecular sieve. Ozone was generated by UV photolysis of oxygen.% PAN was generated from acapillarydiffusion system containing PAN dissolved in an n-tridecane solvent.%
RESULTS AND DISCUSSION Optimization of Reagent Solution Content. Concentration of individual components-potassium hydroxide and luminol-were optimized individually from the point of view of signallnoisemaximal ratio. With increasing concentrations of luminol (in the range 0.oooO5-0.04 mo1.L-l) and hydroxide (0.05-2.0 mol.L-'), the detector response first increases monotonously, reaches ita maximum, and then decreases. Maximal response to nitrogen dioxide is exhibited by an aqueous alkaline solution of 0.002 M LMN in 0.5 M potassium hydroxide. The so-defined solution of the reaction agent is further referred to as the LMN basic solution (LMN B solution). Substituting potassium hydroxide by sodium hydroxide of equal concentration decreased the detector response to NO2 to 65 % of the response of the solution with KOH. Such a decrease is difficult to explain without further detailed study. The possible hypotheses are (1) optimal concentrations of the components of the LMN-NaOH solution for the CL aerosol detector of NO2 differ from the optimal concentrations of the components of the LMN-KOH solution and (2) the NaOH solution may inhibit the NOzLMN reaction or even quench the emitted CL radiation. This effect has not been further investigated in spite of the fact that a different action of the above-mentioned hydroxides to the CL response of NO2 detectors has not been published so far. On the contrary it was mentioned36 that application of KOH or NaOH leads to equal results. However, in that case the reaction between NO2 and LMN did not take place on the gas-aerosol interface boundary of the reagent solution. Effect of Liquid Flow. Our determination of nitrogen dioxide in the gas phase is based on the reaction of the aerosol of the reaction agent with the NO2 molecules in the gas phase. From the viewpoint of the quantity of the emitted radiation, it is necessary to ensure the liquid-gas interface boundary to be as large as possible and the aerosol concentration and its distribution curve to be stable in the wide range of the ratio (35)Hodgeson, J. A,; Stevens, R.K.;Martin, B.E.ZSA Trans. 1972, 11, 161-167.
(36)Gaffney, J. S.;Fajer, R.;Senum, G. I. Atmos. Enuiron. 1984,18, 215-218.
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