2552
Anal. Chem. 1985, 57,2552-2555
by the weighted least squares method. On the basis of Currie's definition (19) a detection limit of 0.7 pg/g calcium is calculated for measuring times of 200 s at the position of the peak and at each background position.
ACKNOWLEDGMENT Grateful acknowledgement is made to Ir. Lietaert (WTCM) for preparing the samples and to J. Hoste for the interest taken in this research. Registry No. Ca, 7440-70-2; cast iron, 11097-15-7. LITERATURE CITED
1 , 100d m200 0 I'
I
Figure 3. Calibration
I
I
I
300
I
LOO
I
I
A
500 600 XRF l C o u n t s l s l Ca KN
graph.
it is assumed that the intensity can be determined with a negligible standard deviation and that the standard deviation on the calcium concentration determined by charged particle activation analysis i s the same for all measuring points, the best estimate of the linear functional relation between the Ca Ka intensity ( x ) and the calcium concentration (y) is y = -8.27
+ 0.0872~
(2)
The result of 43.1 pg/g for H l l - 3 was not considered, since the chance for such a large deviation to occur was less than 2%. The standard deviation of the regression coefficient is 0.0033 pg s/g. The 95% confidence interval for the calcium concentration deduced from the Ca K a intensity by means of eq 2 is f10.8%, *3.9%, f3.9%, and f4.6% a t 10,20, 30, and 40 pg/g, respectively. The 95% confidence limits for a given Ca Ka intensity are given by the dotted lines in Figure 3. Similar results are obtained when the straight line is fitted
(1) Lux, B. Mod. Cast. 1964, 4 6 , 222-232. (2) Muzumdar, K. M.; Wallace, J. F. AFS Trans. 1973, 81, 412-423. (3) Lalich, M. J.; Hitchings, J. R. AFS Trans. 1978, 8 4 , 653-664. (4) Jacobs, M. H.; Law, T. J.; Melford, D. A,; Stowell, M. J. Met. Techno/. (London) 1976, 3 (March),98-108. (5) Church, N. L.; Schelling, R. D. AFS Trans. 1970, 76, 5-8. (6) Karsay, S.I.; Campomanes, E. AFS Trans. 1970, 78, 85-92. (7) Scholes, P. H. Analyst (London) 1988, 93, 197-210. (8) Taylor, M. L.; Beicher, C. B. Anal. Chim. Acta 1969, 4 5 , 219-226. (9) Headrldge, J. B.; Richardson, J. Ana/yst (London) 1969, 94, 968-975. (IO) Samsoni, 2. Microchim. Acta 1978, 11, 177-190. (11) Yakovlev, P. Y.; Zhukova, M. P. Zavod. Lab. 1970, 3 6 , 1169-1173. (12) Sobkowska, A.; Basinska, M. Microchim. Acta 1975, 11, 227-234. (13) Fu, B.; Ottaway, J. M.; Marshall, J.; Llttlejohn, D. Anal. Chim. Acta 1984, 161, 265-273. (14) Kuemmel, D. F.; Karl, H. L. Anal. Chem. 1954, 26, 386-391. (15) Goto, H.; Ikeda, S.; Klmura, J. J . Jpn. Inst. Metals 1958, 22,
185-187.
(16) Atsuya, I.; Goto, H. Specfrochim. Acta 1971, 26,359-367. (17) Vandecasteele, C.; Strljckmans, K. J . Radioanal. Chem. 1980, 5 7 , 121-136. (18) Ziegler, J. F. "Helium, Stopping Powers and Ranges in all Elemental Matter"; Pergamon: New York, 1977. (19) Currie, L. A. Anal. Chem. 1968, 4 0 , 588-593.
RECEIVED for review April 5, 1985. Accepted June 20, 1985. The investigation is part of a research programme sponsored by IWONL (Instituut voor Wetenschappelijk Onderzoek in Nijverheid en Landbouw). Financial support was received from the IIKW (Interuniversitair Instituut voor Kernwetenschappen) and the NFWO (Nationaal Fonds voor Wetenschappelijk Onderzoek).
Chemiluminescence Method for Direct Determination of Sulfur Dioxide in Ambient Air Danian Zhang,' Yasuaki Maeda,* and Makoto Munemori
Laboratory of Environmental Chemistry, College of Engineering, University of Osaka Prefecture, Mom-umemachi, Sakai 591, Japan
Sulfur dloxlde enhances the chemllumlnescence reaction of lumlnol with NO,. The enhanced signal is proportlonal to SO, concentration at a flxed NO, conoentratlon. Based on thls gas/llquld phase chemllumlnescence reactlon, a rapid and sensltlve method for the determlnatlon of SO, Is proposed. A 05% response is obtained wlthln 2 mln. Relatlve standard devlatlons for 10 ppb and 1 ppb of SO, are 0.9% and l o % , respectlvaly, and the detectlon llmlt Is approximately 0.3 ppb. By the present method, real tlme determlnatlon of SO, In amblent alr can be made.
'On leave from East China Institute of Chemial Technology, Department of Environmental Engineering, Meirong Rd, Shanghai, People's Republic of China.
Sulfur dioxide, one of the major air pollutants, is usually determined by the pararosaniline method after absorption of SO2 from air in a solution of potassium tetrachloromercurate (1)or by the conductometric method after absorption of SOz from air in a dilute sulfuric acid solution containing a small amount of hydrogen peroxide (this method is recommended in Japan as a standard method for the determination of SO2 in ambient air). These methods give only the average concentration of SOz during the sampling time (30-60 min) and besides they have following drawbacks: in the former, the toxic mercury compound causes environmental problems, and in the latter, any acid or alkaline gas interferes. Sulfur dioxide in the gas phase can be directly determined by a flame photometric method based on the chemilurnineseence reaction
0003-2700/85/0357-2552$01.50/00 1985 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
1 [
H
Ill
I
Flgure 1. Schematic diagram of the chemiluminescence analyzer: (A,, A2) air compressor; (B,) standard gas cylinder of SO,; (B2) standard gas cylinder of NO,; (C,, C,) silica gel gas purification tube; (C2, C,) active carbon gas Purification tube; (D,, D2)standard gas dllutlon system SDS 201; (E,, E2) flowmeter; (F) photomultiplier tube; (G) reaction vessel; (H) amplifier; (I) recorder; (J) peristaltic pump; (K) reservoir of luminol solution; (L) drain; (M) vacuum pump; (N) sample gas inlet; (0) gas mixer; (V) three-way valve; (W,-W,) needle valve.
of sulfur compounds in a hydrogen-rich flame (2) or by a pulsed fluorescence method ( 3 , 4 ) . However, the flame photometric response is propotional not to the concentration but to the square of the concentration of SO2,and the fluorescence method is influenced by the presence of aromatic hydrocarbons (5). Recently, Stauff and Jaeschke have proposed a new chemiluminescence method based on the oxidation of SO2 in aqueous phase with potassium permanganate (6). This method again required the absorption of SO2 from air in a solution of sodium tetrachloromercurate. In a previous paper (7), we described that sulfur dioxide enhanced the gas/liquid phase chemiluminscence reaction which takes place between nitrogen dioxide in the gas phase in and luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) the aqueous phase. On the basis of this enhancement, the present paper describes a new chemiluminescence method which is capable of making real time measurements of SO2 in ambient air with response times of a few minutes or less. EXPERIMENTAL S E C T I O N , Apparatus. A schematic diagram of the apparatus is shown in Figure 1. An alkaline solution of luminol was fed into reaction vessel G at a constant flow rate with a Tokyo Rikakikai Model MR-3 peristaltic pump J. Sample gas was pulled by pump M into the reaction vessel after mixing it with the reagent gas containing NOz at a constant concentration (5pprn). The mixing ratio (as measured by the flow rate ratio) of the sample to the reagent gas was 1:l unless otherwise mentioned. The reagent gas was supplied by the Seitetsu Kagaku Model SDS 201 standard gas dilution system D2 through which a standard cylinder gas mixture containing NO2was diluted with purified air. For calibration, a standard cylinder gas mixture containing SO2 was diluted with purified air by a Seiktsu Kagaku Model 201 standard gas dilution system D1and the diluted gas was supplied via three-way valve V. The light emitted was detected with Hamamatsu Television R-374 photomultiplier tube F whose photocurrent was amplified and recorded on a Rikakikai R-031 recorder I. The photomultiplier tube was operated at -400 V for subpart-per-million levels of SO2and -750 V for parbper-billion levels of SO2 in the ambient air. The reaction vessel was a 35 mm by 35 mm i.d. stainless steel tube with a conical bottom packed with 1-3 mm glass beads as shown in Figure 2. The reason why the vessel was packed with glass beads will be described in detail later in this paper. Sample gas was introduced through inlet A and reagent gas through inlet B. Two gases were mixed well with the aid of quartz wool J and the mixed gas was sucked into the reaction vessel through H. A, B, and H are Teflon tubings covered with stainless steel. Teflon tubings are used to eliminate the adsorption loss of SOz. The reagent solution was provided at the bottom of the vessel through stainless steel tubing C and sucked out of the vessel through stainless steel tubing D together with waste gas.
E
2553
I)!
J K Flgure 2. Reaction vessel: (A) sample gas inlet; (B) reagent gas (NO,) inlet; (C) luminol solution inlet; (D) luminol solution and waste gas outlet; (E) photomultiplier tube; (F) quartz filter; (G) glass beads; (H) gas inlet; (I) O-ring; (J) quartz wool gas mixer; (K) three-way Teflon joint; (L)
Teflon tube. Reagent. Luminol of chemically pure grade (Wako Pure Chemical Co.) was used without further purification. Other chemicals were of reagent grade and obtained from Wako Pure Chemical Co. A weighed amount of luminol was dissolved in a potassium hydroxide solution. The luminol solution thus prepared was stored in the dark for 10 days before use (8). Although potassium hydroxide was used throughout the present study to adjust the alkalinity of the luminol solution, it was experimentally confirmed that sodium hydroxide was equally useful. A standard cylinder gas mixture containing 144 ppm of SO2 in N2 and a standard cylinder gas mixture containing 435 ppm of NO2 in N2 were obtained from Seitetsu Kagaku Co. Air for dilution of the standard cylinder gas mixtures was purified by passing it through silica gel (C, and C3)and active carbon (Czand C4) columns successively. Carbon monoxide, carbon dioxide, and hydrocarbons of 99.5% purity were obtained from Gasukuro Kogyo. Ozone was generated by a Nihon Ozone Co. ozone generator. Procedure. At first, purified air was introduced from D1and D2 (Figure 1)both at a flow rate of 250 mL/min into the reaction vessel G and the base line was recorded. Then by turning the valve of D2 a stream of reagent gas (5ppm of NO,) was introduced into G after mixing it with a stream of purified air at a mixing ratio of 1:l and the signal produced was reduced to the baseline by applying a negative voltage to the output of the photomultiplier. Finally sample gas was introduced from N by turning the valve V and mixed with the reagent gas and the signal was recorded. R E S U L T S AND DISCUSSION Design of Reaction Vessel. Sulfur dioxide does not produce chemiluminescence when contacted with alkaline luminol solution or when mixed with NOz but enhances the chemiluminescerice which is produced by the reaction of lumino1 with NO2 (7). The enhanced chemiluminescence signal is proportional to the concentration of SO2. This is the principle of the present method. The enhanced signal, however, exhibits a slow response, while the response of chemiluminescence reaction of luminol with NO2itself is fairly rapid. This is probably due to the fact that sulfite ion which is produced by hydrolytic dissolution of SO2 in alkaline luminol solution has an enhancement effect on the N02/luminol chemiluminescence reaction, while the corresponding hydrolysis products of NO2,Le., NOz- and NO3-, have no effect on the chemiluminescence (7). To minimize the effect of sulfite ion in bulk solution and obtain a rapid response, the luminol solution was supplied on a strip of cellulose fiber as suggested by Wendel et al. (8),but the fluctuations in signal were so great that reproducible results could not be obtained. Thus the reaction vessel was packed with glass beads in such a way that a thin layer of the reagent solution was left above the packing. Under this condition, the reaction vessel contained 1 mL of the reagent solution whose surface area was 9.6 cm2 and the signal attained its 95% value within 2 min.
2554
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
flow rate
18
lob
'.4
I
I
0
lurninol concn.
1(5" M
I
I
2
3
luminol flow
Flgure 3. Effect of luminol concentration on signal: SO, 2.3 ppm; KOH, 0.01 M.
1 ppm; NO,
t
4
5
rate rn4min
Flgure 5. Effect of luminol solution flow rate on signal: SO,, NO, 2.3 ppm; lumlnol, 1 X M; KOH, 0.01 M.
1 ppm;
Table I. Optimum Values for Experimental Parameters luminol solution luminol concn KOH concn flow rate reagent gas NOz concn flow rate
300
'7d 0 0
I
1
5
15
10 qconcn
p ~ o
Flgure 4. Effect of NOp concentration on signal: SO, 0.01 M; luminol, 1 X M.
1 ppm; KOH,
Effects of Luminol and Potassium Hydroxide Concentration. The effect of luminol concentration on the signal was examined at a potassium hydroxide concentration of 0.01 M for 1ppm of SOzin the presence of 2.3 ppm NO2 As shown in Figure 3, the signal (curve A) increased with the increase in luminol concentration and the maximum signal was obtained for the luminol concentration from 1X lV5to 2 X At the luminol concentration of M, however, the dynamic range was limited to SO2 concentration lower than 100 ppb and, therefore, lo4 M ltuninol solution was used in subsequent experiments. With this luminol concentratioh, which is out of the optimum range for NOz chemiluminescence (7), SOz gives an enhanced signal about 10 times higher than that given by NOz itself (curve B). The effect of potassium hydroxide concentration on the signal was examined at a luminol concentration of 10"' M. In the absence of potassium hydroxide (pH 5.5), no light emission was observed. When pH value was higher than 9, the chemiluminescence was detected. The signal increased with the increase ih potassium hydroxide concentration and reach the maximum at a potassium hydroxide concentration of 0.01 M and then decreased at the higher concentration. Effect of NOz Concentration, As shown in Figure 4, the signal increased with the increase in NOz concentration even for a constant concentration of SOz. Noise also increased with the increase in NOz concentration. Therefore, NOz concentration must be rigidly controlled at an appropriate value. In
10-4 M
M 2.5 mL/min 5 PPm 250 mL/min
the subsequent study, 5 ppm was chosen as the appropriate NO2 concentration, because this concentration is 250 times higher than that of NOz in ambient air and the effect of atmospheric NO2 can be completely neglected without loss of the sensitivity. As a cylinder gas mixture kontaining 5 ppm of NO2 was not available in the present work, a cylinder gas mixture containing 435 ppm of NOz was diluted by a Seitetsu Kagaku standard gas dilution system SDS 201. By this dilution system, the desired rigid control of the concentration could be achieved. At this condition 20 ppb variation in NOz concentration affected the response of the monitor less than a few parts-per-billion variation in SOz concentration. Effects of Flow Rate. With the increase in the flow rate of luminol solution at various sample gas flow rates, the signal changes as shown in Figure 5. A t the gas flow rates above 1L/min, the signal was gradually decreased with the increase in the luminol flow rate and a t the gas flow rates below 0.5 L/min, a minimum appeared in the signal. The reason for this change is not yet clear. Calibration Graph for SOz. Optimum values for the experimental parameters are summarized in Table I. Under this optimum condition, the calibration graph gives a straight line from 1ppb to 1 ppm of SOz. The correlation coefficient was 0.997. A typical example of the signals recorded on chart paper is shown in Figure 6. The relative standard deviations (n = 7) were 0.9% and 10% at 10 ppb and 1 ppb of SOZ, respectively. The detection limit ( S I N = 3) estimated from the standard deviation at 1 ppb of SOz was about 0.3 ppb. Although this detection limit may be improved, some difficulties are encountered for preparing and manipulating such dilute gas mixture at parts-per-trillion level. Reaction Mechanism. The spectra of chemiluminescence produced by the reaction of luminol with NOz in the absence and in the presence of SOz, being quite similar each other, coincide fairly well with the spectrum of chemiluminescence produced by the reaction of luminol with oxidants in the presence of metal ion catalysts. Accordingly, it seems likely that the chemiluminescenceof the SOz/NOz/luminol system
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
2555
Table 111. Removal of Ozone Interference gas (concn, ppb) SO2 (160) SOz (160) + O3 (110) SOz (160) O3 (350) SOz (160) + O3 (580)
+
re1 intens without precolumn with precolumn 1.0 -0.9 0.9 3.6
1.0 1.0 1.0
0.8
Table IV. Comparative Analytical Results
gas species (concn, ppb)
so2 (10)
*?" +-I Table 11. Interference from Other Gasesa other gas
concn, ppm
none
eo
COZ
propylene ethylene NZO
NO 0 3 3"
HZS methyl sulfide ethyl mercaptan
80 100 500 1000 100 100 0.3 1.1 0.3
0.1 0.6 1.0 1.0
1.0 1.0
re1 intens
1.00 1.02 0.75 0.64 0.44 1.03 1.01 0.98 0.99 1.00 -0.85 3.64 0.98 9.20 1.59 15.60
"SOz concentration, 160 ppb; luminol concentration, 1 X loa M;
and KOH concentration, 0.01 M. is also produced from the exited state of aminophthalate ion (9). Sulfur dioxide probably acts as a catalyst. Interference from Other Substances. The interference from other air pollutants with the determination of SO2 by the present method was examined. As shown in Table'II, CO, propylene, ethylene, NzO, NO, and NH3 do not interfere. Ambient NOz does not interfere as mentioned above. Carbon dioxide exhibits the concentration-dependent interference. This interference is probably due to the pH change in the surface layer of luminol solution by the absorption of carbon dioxide. The interference of carbon dioxide up to 0.05% was completely removed by buffering the luminol solution with 0.1 M phosphate buffer solution (pH 12). Ozone exhibits a strong positive interference with the determination of NOz by the luminol method (7). With the
conductometric method,
9.7
(20) (40) SO2 (10) + NH3 (100) SO2 (20) + NHa (100)
Figure 6. Signal of SO2: total gas flow rate, 500 mL/min; measuring time, 1.5 min.
present method, ppb 20.4
39.9 10.6 19.8
PPb
9.8 20.3 39.9 7.1
17.0
present method, however, ozone interfered negatively at lower concentration (ZOO ppb). Ozone interference, however, could be removed as shown in Table I11 by using a precolumn (200 mm long and 5.5 mm i.d.) packed with glass beads (1.0-1.5 mm 0.d.) on which ferrous sulfate was coated (IO). The present method was compared with the conductometric method. As shown in Table IV, in the case of sulfur dioxide alone both methods gave satisfactory results, but in the presence of ammonia the conductometric method gave low results as expected, while the present method gave satisfactory results. Hydrogen sulfide and ethyl mercaptan produced strong chemiluminescence and intefered with the determination of SOz by the present method (Table 11). These interferences, the elimination of which is now under study by using an Ag wool scrubber ( I I ) , are not so important in the determination of atmospheric sulfur dioxide.
ACKNOWLEDGMENT The authors thank Seitetsu Kagaku Kogyo for making a Standard Gas Dilution System SDS 201 available for the present work. Registry No. SOz, 7446-09-5.
LITERATURE CITED West, P. W.; Gaeke, G. C. Anal. Chem. 1958, 2 8 ,
1816-1819.
Stevens, R. K.; O'Keefe, A. E.; Ortman, G. C. Environ. Sci. Technol. I g W . 3. 652-655.
Okabe, H.; Splitstone, P. L.; Ball, J. J. J . Air Pollut. Control Assoc. 1973. 2 3 . 514-516. Schwarz, F. P.; Okabe, H.; Whittaker, J. K. Anal. Chem. 1974, 4 6 , 1024- 1028.
Smith, W. J.; Buckman, F. D. J . AirPollut. ControlAssoc. 1981, 3 1 , 1101-1 103.
Stauff, J.; Jaschke, W. Afmos. Environ. 1975, 9 , 1038-1039. Maeda, Y.; Aokl, K.; Munemori, M. Anal. Chem. 1980, 52, 307-311. Wendei, G. Y.; Stedman, D. H.; Cantrell, C. A. Anal. Chem. 1983, 55, 937-940.
White, E. H.; Zafiriou, 0.;Kagi, H. H.; Hill, J. H. M. J . Am. Chem. SOC. 1984, 86, 940-941. Anderson, H. H.; Moyer, R. H.; Sihbett, D. J.; Sutherland, D. C. U. S. Patent No 3 659 100, Aug 1970. Huntzicker, J. J.; Cary, R. A.; Ling, C. S. Environ. Sei. Technol. 1980, 14. 819-824.
RECEIVED for review February 7,1985. Accepted July 1,1985. This work is partly supported by Grants-in-Aid from the Japanese Ministry of Education.
