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Fluorimetric determination of nitrogen oxides in the air by a novel red-region fluorescent reagent
Talanta 58 (2002) 855 /860 www.elsevier.com/locate/talanta
Fluorimetric determination of nitrogen oxides in the air by a novel red-region fluorescent reagent Xin-Qi Zhan a, Dong-Hui Li b, Hong Zheng b, Jin-Gou Xu b,*, Yi-Qun Zhou c a
b
Department of Chemistry of Zhangzhou Teacher’s College, Zhangzhou 363000, Fujian, People’s Republic of China The Key Laboratory of Analytical Science of MOE, Department of Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China c Xiaman Environmental Protection Bureau, Xiaman 361005, People’s Republic of China Received 8 October 2001; received in revised form 22 July 2002; accepted 25 July 2002
Abstract A sensitive fluorimetric method for the determination of nitrogen oxides (NOx : NO/NO2) in air is described. Nitrogen dioxide (nitrogen monoxide was previously converted to nitrogen dioxide in oxide tubes) was aspirated through a fritted glass bubble at a flow rate of 500 ml min 1 for 120 min and fixed as nitrite, using 0.1 N NaOH as a trapping solution with the empirical absorption efficiency 0.74 and the stoichiometric factor 0.5. The method is based on the fluorescence quenching of a red-region fluorescent reagent, tetra-substituted amino aluminum phthalocyanine (TAAlPc), after being diazotized by nitrite. Under optimal conditions the linear range of the calibration curve for nitrite is 1 /40 ng ml 1 (NO2 0.24 /9.6 ppb, v/v). The detection limit is 0.34 ng ml 1 for nitrite (NO2 0.08 ppb, v/v) and the relative standard deviation for six replicate measurements of 15 ng ml 1 nitrite is 3.2%. The method has been applied to the determination of nitrogen oxides in the air with satisfactory results. Typical gaseous co-pollutants such as SO2, H2S and HCHO did not interference the determination. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen oxides; Tetra-substituted amino aluminum phthalocyanine; Fluorimetry; Determination
1. Introduction The oxides of nitrogen found in the air are consisted mainly by nitrogen monoxide and nitrogen dioxide, most of which in the urban air are from exhausted gases emitted by internal-combustion engines. It has been reported that the con-
* Corresponding author. Tel.: /86-592-218-3398; fax: /86592-218-8054 E-mail address: [email protected] (J.-G. Xu).
centration of nitrogen oxides in the air caused by heavy-automobiles crossing the road can reach above a concentration of 80 mg m 3. Results on laboratory animal tests show that oxides of nitrogen can affect the respiratory organs and cause the extension of bronchial asthma and other diseases. Nitrogen oxides also play an important role in the generation of photochemical smog and photochemical oxidants such as ozone and peroxyacetyl nitrate (PAN) [1].
0039-9140/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 3 9 9 - 5
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A number of methods for the measurement of NOx have been developed in the past decades. NOx is usually absorbed in a suitable solution first and converted into nitrite, followed by the determination of nitrite [2 /7], in which a diazo coupling reaction is often employed, and the spectrophotometric method established by Saltzman [2] is used as one of standard reference methods worldwide. Still some of the above-mentioned methods suffer from a poor sensitivity and need to extract the azo dye formed to improve the detection limits [3,4]. Chemiluminescence (CL) methods are also widely applied, which are based either on the catalytic or photolytic reduction of NO2 to NO and subsequent gas-phase reaction with ozone [8,9], or on the chemiluminescence reaction of NO2 with an alkaline solution of luminol [10,11]. However, some of the methods encounter the interference from species such as SO2, H2S, CO2 and O3. In addition, complex and costly instrumentation is required, and due to the limited stability of the luminol reagent, the systems need to be recalibration rather frequently. Recently an in situ laser-induced fluorescence detection for NO2 was proposed which showed advantages [12], but the high-cost could prevent it from current monitoring use. Direct determination of NOx in the air by fluorimetry is rarely reported although a large volume of methods were developed to detect nitrite ion fluorimetrically [13 /16], in which fluorescent dyes such as 5-aminofluorescein, 2,6-diaminopyridine, 3-amino-4-hydroxycoumarin and 3-aminonaphthalene-1,5-disulphonic acid were used. However, the abovementioned dyes have a fluorescence excitation and emission in the visible region (300 /600 nm), thus being subject to the interference from the background fluorescence and scattering light of the matrix. Metal phthalocyanines compounds have been reported for developing suitable sensors for gasphase NOx detection [17,18], but their applicability is still in its infancy. In this work, we developed a simple, sensitive and selective method for the determination of NOx . This method used a tube filled with chromium trioxide to oxidize NO to NO2, NO2 was then absorbed by sodium hydrox-
ide solution in a fritted glass bubbler with a suction pump. Nitrite was produced and followed by diazotization with tetra-substituted amino aluminum phthalocyanine (named in short as TAAlPc in the following) in 0.48 mol l 1 HCl. The fluorescence of TAAlPc was quenched and the degree of fluorescence quenching was proportional to the concentration of nitrite present. Since the determination is performed in highly strong acidic medium, it is beneficial to the selectivity of the method because of less interference from foreign substances in such a medium. In addition, the application of a fluorescent dye emitting in the red-region is of help to the sensitivity of the detection because scattering light and the fluorescence from background could be reduced obviously. To our knowledge, this is the first report for the use of TAAlPc as a fluorescent reagent for the determination of nitrogen oxides.
2. Experimental
2.1. Apparatus Fluorescence spectra and relative fluorescence intensity were measured on a model 650-10S fluorescence spectrophotometer (Hitachi, Japan) equipped with a xenon lamp, dual-monochromators, a 1 /1 cm quartz cell and a recorder. The band-pass width for excitation and emission were set at 5 and 10 nm, respectively. The absorption spectra were obtained on a DU-7400 Beckman spectrophotometer. Commercial fritted glass bubble with suction pump was used for trapping nitrogen dioxide from air. The airflow rate was measured using a calibrated rotameter.
2.2. Reagents 4,4?,4ƒ,4§-Tetrasubstituted amino aluminum phthalocyanine was synthesized and purified ac-
X.-Q. Zhan et al. / Talanta 58 (2002) 855 /860
cording to the reference [19]. Stock solution of TAAlPc (1.0 /103 mol l 1) was prepared by dissolving solid TAAlPc in dimethyl sulfoxide. A nitrite stock solution (1.0 mg ml1) was prepared by dissolving sodium nitrite which had been dried at 100/110 8C for 2 h in water, and a working standard solution of 1.0 mg ml1 nitrite was freshly prepared by diluting the stock solution with water. A hydrochloric acid solution of 6 mol l 1 was prepared by diluting 500 ml of concentrated HCl (12 mol l1) to 1 l with water. Sodium hydroxide absorber solution (0.1 mol l 1) was prepared by dissolving 4 g of sodium hydroxide in 1 l of water and standardized before use. All of chemicals were of analytical grade and were used directly without further purification. Water distilled by quartz distilling apparatus was used throughout.
857
Fig. 1. Molecular structure of TAAlPc.
2.3. Air sampling For air sampling, a commercial fritted glass bubble fitted with oxidizing tubes was applied. The oxidizing tube designed according to the reference [20] contained about 5 g oxidizing agent, and plugs of cotton wool used as filter was inserted. Oxidizing agent was prepared by making a paste of 5 g of chromium trioxide (CrO3) with a little water. The paste was blended with 95 g of 14 /16 meshes silica, followed by being dried at 105 8C. The oxidation efficiency [5] of the oxidizing tubes can reach 99% when nitrogen monoxide in air is oxidized by chromium trioxide at a relative humidity of 30 /80% ambient temperature and a flow-rate of 500 ml min 1. About 0.1 N NaOH was used as a absorber solution in this work. Other two common trapping solutions, sodium hydroxide-arsenite and triethanolamine, were found unsuitable for use as they interference with the determination. Draw the air through 10 ml of 0.1 mol l 1 sodium hydroxide absorber solution at a flow rate of 500 ml min 1 for 120 min. It was empirically evaluated that the absorption efficiency of such a absorber solution was 0.74 [21] and the NO2: NO2 stoichiometric factor was 0.5 [22].
Fig. 2. Absorption spectra, (1) TAAlPc; (2) TAAlPc/HCl; (3) TAAlPc/HCl/NO2 (0.200 mg ml 1); (4) TAAlPc/HCl/ NO2 (0.400 mg ml 1); (5) TAAlPc/HCl/NO2 (0.800 mg ml 1); (6) TAAlPc/HCl/NO2 (2.000 mg ml 1); [TAAlPc]/ 1.0/10 5 mol l 1, [HCl]/0.60 mol l 1.
2.4. Procedure 70 ml of 1.0 /104 mol l1 TAAlPc solution was taken by a pipette and added into a 10-ml volumetric flask, followed by adding 0.80 ml of 6 mol l 1 HCl solution and a suitable aliquot of standard solution of nitrite (or absorber sample solution to be determined). The solution was diluted to 10 ml with water and mixed thoroughly. Relative fluorescence intensities of the reagent blank (F0) and the sample solution (F ) were measured at 686 nm with an excitation wavelength of 610 nm. A calibration curve was obtained by
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Fig. 3. Excitation and emission spectra of TAAlPc, (1) TAAlPc/HCl; (2) TAALPc/HCl/NO2 (8 ng ml 1); (3) TAALPc/HCl/NO2 (15 ng ml 1); (4) TAALPc/HCl/ NO2 (40 ng ml 1); (5) TAALPc/HCl/NO2 (250 ng ml 1); [TAAlPc] /7.0/10 7 mol l 1, [HCl]/0.48 mol l 1.
plotting the fluorescence quenching (DF /F0/F ) versus the concentration of nitrite.
reaction occurred between nitrite and TAAlPc, and also implied the formation of a new derivative of phthalocyanine. From the phenomena described above we suggest that a diazotization reaction occur between TAAlPc and nitrite, and the diazonium salt product in turn reacts with TAAlPc to form diazo coupling compounds with an absorption peak at longer wavelength region. When more amounts of nitrite existed, more of diazonium salt was formed, thus more than one reactive site on TAAlPc were coupled, resulting in the formation of different species of azo derivatives of TAAlPc. The excitation and emission spectra of the system are given in Fig. 3. It can be seen that there are two excitation bands for TAAlPc, a Soret band (at short wavelength region) and a Q band (at long wavelength region). To avoid potential interference from the fluorescence of foreign substances as well as secondary scattering light, the excitation wavelength was chosen at 610 nm (Q band). The emission spectra show that the degree of fluorescence quenching of the system was proportional to the concentration of nitrite.
3. Results and discussion 3.2. Optimization of the general procedure 3.1. Structure and spectral characteristic of TAAlPc The structure of TAAlPc is given in Fig. 1. It can be seen that with the addition of hydrochloric acid to the solution of TAAlPc, the absorption spectrum of TAAlPc changed obviously (Fig. 2). The absorption peak at long-wavelength region (Q band) was blue-shifted for about 60 nm and hyperchromism occurred remarkably. This can be explained by the protonation of amino groups on the phthalocyanine as we have described in the previous work [23]. It was found that when nitrite was added, the Q band absorption of TAAlPc decreased with the increase of the concentration of nitrite. It was also found that a new absorption band at long-wavelength region adjacent to the Q band appeared. With the increase of nitrite, the absorption of the new band increased in company with a decrease in the Q band absorption. When more nitrite was added, the new band even evolved into two peaks. These phenomena indicated a
The fluorescence quenching effect of nitrite was investigated for different concentration of TAAlPc. The results showed that using a lower concentration of TAAlPc would enhance the sensitivity of determination, but would lead to a narrower linear range of determination and a larger determination error since the measurement values of F0 and F were small. Therefore, a concentration of 7.0 /107 mol l 1 of TAAlPc was chosen. Influence of acidity in medium on the fluorescence quenching was also investigated and it was found that the fluorescence quenching reached a maximum in media containing 0.06 / 0.6 mol l 1 (final concentration) of hydrochloric acid. This result is in agreement with the principle that high acidity is beneficial to the diazotization reaction and inhibits the diazo coupling reaction. In this work, 0.48 mol l 1 of hydrochloric acid was chosen. Experimental results of temperature effect indicated that there was no obvious difference in the fluorescence quenching of the system in
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859
Table 1 Analytical characteristic for the determination of nitrite (or NO2 in 60 l air)
Nitrite (ng ml 1) NO2 (ppb, v/v)c a b c
Linear range
Linear regression equation
LODa
rb
R.S.D. (%)
1 /40 0.24 /9.6
DF /16/23c DF /16/96c
0.34 0.08
0.9940 0.9940
3.2 3.2
Limit of detection. Correlation coefficient. NO2 (ppb, v/v) /{NO2 (ng ml 1) /10 (ml)}/{60 (l) /1.845/0.74/0.5}. The density of NO2 (at 31 8C, 101.3 kPa)/0.845 g
l 1. Table 2 Determination of nitrogen oxides in automobile exhaust gases Samplea
1 2 3 a b
NOx found, ppb (v/v) (n/3)b Proposed method
Reference method
11329/48 9579/29 15719/49
11039/20 9089/20 15609/20
NO2 added (ng) (n/3)
Recovery (%)
80 80 80
92 /104 93 /102 90 /101
On different spots. Concentration of NOx (ppb, v/v) /{NO2 (ng)/25}/{0.25/0.74/0.5/60/1.845}, at 31 8C and 101.3 kPa.
the temperature range of 0 /100 8C. So, the determination can be performed at room temperature. The influence of reaction time on the fluorescence quenching was also studied and the results indicated that the sensitivity increased with the reaction time but at the expense of shorter linear range, the fluorescence intensity was, therefore, measured with no incubation time.
3.3. Calibration graph The calibration curve for the determination of nitrite was constructed by performing the standard procedure under optimum conditions described above. Good linear relationship was observed between the fluorescence quenching (DF ) and nitrite concentration. The analytical characteristic is given in Table 1. The limit of detection (LOD) was given by the equation, LOD /KS0/S , where K is a numerical factor chosen according to the confidence level desired, S0 is the standard deviation of blank measurements (n /9) and S is the sensitivity of the calibration graph. Here a value of 3 for K was used.
3.4. Interference of foreign substances The effects of various potential interferences were studied. The gases interfering with the determination were incorporated, in the form of ions, into solution containing 15 ng ml1 nitrite and the recovery of nitrite in the range of 95 /105% was considered as quantitative recovery. The results showed that formaldehyde at levels as high as 270-fold in quantity did not interfere. No interference resulted from the presence of a 11 000fold amount of H2S. Sulfur dioxide when present no more than 16 000-fold caused a recovery of nitrite 105%. We have previously reported [24] that the co-existing of formaldehyde and sulfite has a co-quenching effect on the fluorescence of TAAlPc, the interference of co-existing effect of the two species can be overcome by fixing a trapping tube filled with cotton-wool impregnated with lead acetate upstream of the oxidizing tube [5] to remove SO2. Nitrous acid and ammonia, at levels as high as 50 000-fold in quantity, caused a recovery of 107%. The interference due to ozone, if present, can be eliminated by exposing air steam to rubber, which reacts with ozone and not with NO2
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[25]. It showed that the typical interference species with common concentrations in automobile exhaust gas didn’t interfere the determination of NOx . 3.5. Determination of real samples Automobile exhaust gas in the bus station was drawn to a 10-ml 0.1 mol l 1 sodium hydroxide absorber solution at a flow-rate of 500 ml min 1 for 120 min at 31 8C and 101.3 kPa. The absorber solution was neutralized to neutrality with hydrochloric acid. Then the solution was completely transferred to a 25-ml volumetric flask and diluted to the mark with water. The diluted solution of 0.25 ml was taken for the determination by the recommended procedure. In order to evaluate the method, the standard anaphthylamine method [26] was applied as a reference method to determine the samples, and standard nitrite adding experiments were also done. The results are given in Table 2. It can be seen that the proposed method can be used effectively for the determination of NOx in automobile exhaust gas samples with satisfactory.
References [1] X.Y. Tang, Atmosphere Environmental Science, High Education Press, Beijing, 1990. [2] B.E. Saltzman, Anal. Chem. 26 (1954) 1949. [3] M. Pandurangappa, N. Balasubranian, Mikrochim. Acta 124 (1996) 137.
[4] A. Chaube, A.K. Baveja, V.K. Gupta, Talanta 31 (1984) 391. [5] G.G. Lu, Z.K. He, Y.L. Liu, Talanta 39 (1992) 123. [6] B.S.M. Kumar, T.R. Srikanth, N. Balasubramanian, Fresenius J. Anal. Chem. 345 (1993) 592. [7] N. Gayathri, N. Balasubramanian, Analusis 27 (1999) 174. [8] A. Fontijin, A.J. Sabadelll, R.J. Ronco, Anal. Chem. 42 (1970) 575. [9] J.E. Sigsby, F.M. Black, T.A. Bellar, D.L. Klostermen, Environ. Sci. Technol. 7 (1973) 51. [10] Y. Maeda, K. Aoki, M. Munemori, Anal. Chem. 52 (1980) 307. [11] P. Mikuska, Z. Vecera, Anal. Chem. 64 (1992) 2187. [12] J.A. Thornton, P.J. Wooldridge, R.C. Cohen, Anal. Chem. 72 (2000) 528. [13] H.D. Axelrod, N.A. Engel, Anal. Chem. 47 (1975) 922. [14] L.J. Dombrowski, E.J. Pratt, Anal. Chem. 44 (1972) 2268. [15] T. Ohta, Y. Arai, S. Takitani, Anal. Chem. 58 (1986) 3132. [16] S. Motomizu, H. Mikasa, K. Toei, Talanta 33 (1986) 729. [17] R. Rella, A. Serra, P. Siciliano, A. Tepore, L. Troisi, L. Valli, Thin Solid Films 285 (1996) 870. [18] P.S. Vukusic, J.R. Sambles, Thin Solid Films 221 (1992) 311. [19] Y. Cheng, Chin. J. Appl. Chem. 8 (1991) 14. [20] J.S. Chui, Q.Y. Wang, H.P. Wang, Methods for Atmospheric Pollutant Detection, second ed., Chemical Industry Press, Beijing, 1997, p. 819. [21] J.H. Blacker, R.S. Brief, Chemosphere 43 (1992) 1. [22] W. Leithe. The Analysis of Air Pollutants (R. Kondor, Ann Arbor, Trans.) 1972, pp. 183, 186, 197 (Thirdprinting). [23] D.H. Li, Q.Y. Chen, Q.Z. Zhu, F. Li, H. Zheng, S.Y. Yang, J.G. Xu, Talanta 49 (1999) 745. [24] X.Q. Zhan, D.H. Li, Q.Z. Zhu, H. Zheng, J.G. Xu, Analyst 125 (2000) 2330. [25] J. Nair, V.K. Gupta, Atmos. Environ. 15 (1981) 107. [26] The Writing Group of ‘Standard Analysis Methods for Environment Detection’, Standard Analysis Methods for Environment Detection (Chinese), Beijing, 1980, p. 91.
Fluorimetric determination of nitrogen oxides in the air by a novel red-region fluorescent reagent Xin-Qi Zhan a, Dong-Hui Li b, Hong Zheng b, Jin-Gou Xu b,*, Yi-Qun Zhou c a
b
Department of Chemistry of Zhangzhou Teacher’s College, Zhangzhou 363000, Fujian, People’s Republic of China The Key Laboratory of Analytical Science of MOE, Department of Chemistry, Xiamen University, Xiamen 361005, People’s Republic of China c Xiaman Environmental Protection Bureau, Xiaman 361005, People’s Republic of China Received 8 October 2001; received in revised form 22 July 2002; accepted 25 July 2002
Abstract A sensitive fluorimetric method for the determination of nitrogen oxides (NOx : NO/NO2) in air is described. Nitrogen dioxide (nitrogen monoxide was previously converted to nitrogen dioxide in oxide tubes) was aspirated through a fritted glass bubble at a flow rate of 500 ml min 1 for 120 min and fixed as nitrite, using 0.1 N NaOH as a trapping solution with the empirical absorption efficiency 0.74 and the stoichiometric factor 0.5. The method is based on the fluorescence quenching of a red-region fluorescent reagent, tetra-substituted amino aluminum phthalocyanine (TAAlPc), after being diazotized by nitrite. Under optimal conditions the linear range of the calibration curve for nitrite is 1 /40 ng ml 1 (NO2 0.24 /9.6 ppb, v/v). The detection limit is 0.34 ng ml 1 for nitrite (NO2 0.08 ppb, v/v) and the relative standard deviation for six replicate measurements of 15 ng ml 1 nitrite is 3.2%. The method has been applied to the determination of nitrogen oxides in the air with satisfactory results. Typical gaseous co-pollutants such as SO2, H2S and HCHO did not interference the determination. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen oxides; Tetra-substituted amino aluminum phthalocyanine; Fluorimetry; Determination
1. Introduction The oxides of nitrogen found in the air are consisted mainly by nitrogen monoxide and nitrogen dioxide, most of which in the urban air are from exhausted gases emitted by internal-combustion engines. It has been reported that the con-
* Corresponding author. Tel.: /86-592-218-3398; fax: /86592-218-8054 E-mail address: [email protected] (J.-G. Xu).
centration of nitrogen oxides in the air caused by heavy-automobiles crossing the road can reach above a concentration of 80 mg m 3. Results on laboratory animal tests show that oxides of nitrogen can affect the respiratory organs and cause the extension of bronchial asthma and other diseases. Nitrogen oxides also play an important role in the generation of photochemical smog and photochemical oxidants such as ozone and peroxyacetyl nitrate (PAN) [1].
0039-9140/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 2 ) 0 0 3 9 9 - 5
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X.-Q. Zhan et al. / Talanta 58 (2002) 855 /860
A number of methods for the measurement of NOx have been developed in the past decades. NOx is usually absorbed in a suitable solution first and converted into nitrite, followed by the determination of nitrite [2 /7], in which a diazo coupling reaction is often employed, and the spectrophotometric method established by Saltzman [2] is used as one of standard reference methods worldwide. Still some of the above-mentioned methods suffer from a poor sensitivity and need to extract the azo dye formed to improve the detection limits [3,4]. Chemiluminescence (CL) methods are also widely applied, which are based either on the catalytic or photolytic reduction of NO2 to NO and subsequent gas-phase reaction with ozone [8,9], or on the chemiluminescence reaction of NO2 with an alkaline solution of luminol [10,11]. However, some of the methods encounter the interference from species such as SO2, H2S, CO2 and O3. In addition, complex and costly instrumentation is required, and due to the limited stability of the luminol reagent, the systems need to be recalibration rather frequently. Recently an in situ laser-induced fluorescence detection for NO2 was proposed which showed advantages [12], but the high-cost could prevent it from current monitoring use. Direct determination of NOx in the air by fluorimetry is rarely reported although a large volume of methods were developed to detect nitrite ion fluorimetrically [13 /16], in which fluorescent dyes such as 5-aminofluorescein, 2,6-diaminopyridine, 3-amino-4-hydroxycoumarin and 3-aminonaphthalene-1,5-disulphonic acid were used. However, the abovementioned dyes have a fluorescence excitation and emission in the visible region (300 /600 nm), thus being subject to the interference from the background fluorescence and scattering light of the matrix. Metal phthalocyanines compounds have been reported for developing suitable sensors for gasphase NOx detection [17,18], but their applicability is still in its infancy. In this work, we developed a simple, sensitive and selective method for the determination of NOx . This method used a tube filled with chromium trioxide to oxidize NO to NO2, NO2 was then absorbed by sodium hydrox-
ide solution in a fritted glass bubbler with a suction pump. Nitrite was produced and followed by diazotization with tetra-substituted amino aluminum phthalocyanine (named in short as TAAlPc in the following) in 0.48 mol l 1 HCl. The fluorescence of TAAlPc was quenched and the degree of fluorescence quenching was proportional to the concentration of nitrite present. Since the determination is performed in highly strong acidic medium, it is beneficial to the selectivity of the method because of less interference from foreign substances in such a medium. In addition, the application of a fluorescent dye emitting in the red-region is of help to the sensitivity of the detection because scattering light and the fluorescence from background could be reduced obviously. To our knowledge, this is the first report for the use of TAAlPc as a fluorescent reagent for the determination of nitrogen oxides.
2. Experimental
2.1. Apparatus Fluorescence spectra and relative fluorescence intensity were measured on a model 650-10S fluorescence spectrophotometer (Hitachi, Japan) equipped with a xenon lamp, dual-monochromators, a 1 /1 cm quartz cell and a recorder. The band-pass width for excitation and emission were set at 5 and 10 nm, respectively. The absorption spectra were obtained on a DU-7400 Beckman spectrophotometer. Commercial fritted glass bubble with suction pump was used for trapping nitrogen dioxide from air. The airflow rate was measured using a calibrated rotameter.
2.2. Reagents 4,4?,4ƒ,4§-Tetrasubstituted amino aluminum phthalocyanine was synthesized and purified ac-
X.-Q. Zhan et al. / Talanta 58 (2002) 855 /860
cording to the reference [19]. Stock solution of TAAlPc (1.0 /103 mol l 1) was prepared by dissolving solid TAAlPc in dimethyl sulfoxide. A nitrite stock solution (1.0 mg ml1) was prepared by dissolving sodium nitrite which had been dried at 100/110 8C for 2 h in water, and a working standard solution of 1.0 mg ml1 nitrite was freshly prepared by diluting the stock solution with water. A hydrochloric acid solution of 6 mol l 1 was prepared by diluting 500 ml of concentrated HCl (12 mol l1) to 1 l with water. Sodium hydroxide absorber solution (0.1 mol l 1) was prepared by dissolving 4 g of sodium hydroxide in 1 l of water and standardized before use. All of chemicals were of analytical grade and were used directly without further purification. Water distilled by quartz distilling apparatus was used throughout.
857
Fig. 1. Molecular structure of TAAlPc.
2.3. Air sampling For air sampling, a commercial fritted glass bubble fitted with oxidizing tubes was applied. The oxidizing tube designed according to the reference [20] contained about 5 g oxidizing agent, and plugs of cotton wool used as filter was inserted. Oxidizing agent was prepared by making a paste of 5 g of chromium trioxide (CrO3) with a little water. The paste was blended with 95 g of 14 /16 meshes silica, followed by being dried at 105 8C. The oxidation efficiency [5] of the oxidizing tubes can reach 99% when nitrogen monoxide in air is oxidized by chromium trioxide at a relative humidity of 30 /80% ambient temperature and a flow-rate of 500 ml min 1. About 0.1 N NaOH was used as a absorber solution in this work. Other two common trapping solutions, sodium hydroxide-arsenite and triethanolamine, were found unsuitable for use as they interference with the determination. Draw the air through 10 ml of 0.1 mol l 1 sodium hydroxide absorber solution at a flow rate of 500 ml min 1 for 120 min. It was empirically evaluated that the absorption efficiency of such a absorber solution was 0.74 [21] and the NO2: NO2 stoichiometric factor was 0.5 [22].
Fig. 2. Absorption spectra, (1) TAAlPc; (2) TAAlPc/HCl; (3) TAAlPc/HCl/NO2 (0.200 mg ml 1); (4) TAAlPc/HCl/ NO2 (0.400 mg ml 1); (5) TAAlPc/HCl/NO2 (0.800 mg ml 1); (6) TAAlPc/HCl/NO2 (2.000 mg ml 1); [TAAlPc]/ 1.0/10 5 mol l 1, [HCl]/0.60 mol l 1.
2.4. Procedure 70 ml of 1.0 /104 mol l1 TAAlPc solution was taken by a pipette and added into a 10-ml volumetric flask, followed by adding 0.80 ml of 6 mol l 1 HCl solution and a suitable aliquot of standard solution of nitrite (or absorber sample solution to be determined). The solution was diluted to 10 ml with water and mixed thoroughly. Relative fluorescence intensities of the reagent blank (F0) and the sample solution (F ) were measured at 686 nm with an excitation wavelength of 610 nm. A calibration curve was obtained by
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X.-Q. Zhan et al. / Talanta 58 (2002) 855 /860
Fig. 3. Excitation and emission spectra of TAAlPc, (1) TAAlPc/HCl; (2) TAALPc/HCl/NO2 (8 ng ml 1); (3) TAALPc/HCl/NO2 (15 ng ml 1); (4) TAALPc/HCl/ NO2 (40 ng ml 1); (5) TAALPc/HCl/NO2 (250 ng ml 1); [TAAlPc] /7.0/10 7 mol l 1, [HCl]/0.48 mol l 1.
plotting the fluorescence quenching (DF /F0/F ) versus the concentration of nitrite.
reaction occurred between nitrite and TAAlPc, and also implied the formation of a new derivative of phthalocyanine. From the phenomena described above we suggest that a diazotization reaction occur between TAAlPc and nitrite, and the diazonium salt product in turn reacts with TAAlPc to form diazo coupling compounds with an absorption peak at longer wavelength region. When more amounts of nitrite existed, more of diazonium salt was formed, thus more than one reactive site on TAAlPc were coupled, resulting in the formation of different species of azo derivatives of TAAlPc. The excitation and emission spectra of the system are given in Fig. 3. It can be seen that there are two excitation bands for TAAlPc, a Soret band (at short wavelength region) and a Q band (at long wavelength region). To avoid potential interference from the fluorescence of foreign substances as well as secondary scattering light, the excitation wavelength was chosen at 610 nm (Q band). The emission spectra show that the degree of fluorescence quenching of the system was proportional to the concentration of nitrite.
3. Results and discussion 3.2. Optimization of the general procedure 3.1. Structure and spectral characteristic of TAAlPc The structure of TAAlPc is given in Fig. 1. It can be seen that with the addition of hydrochloric acid to the solution of TAAlPc, the absorption spectrum of TAAlPc changed obviously (Fig. 2). The absorption peak at long-wavelength region (Q band) was blue-shifted for about 60 nm and hyperchromism occurred remarkably. This can be explained by the protonation of amino groups on the phthalocyanine as we have described in the previous work [23]. It was found that when nitrite was added, the Q band absorption of TAAlPc decreased with the increase of the concentration of nitrite. It was also found that a new absorption band at long-wavelength region adjacent to the Q band appeared. With the increase of nitrite, the absorption of the new band increased in company with a decrease in the Q band absorption. When more nitrite was added, the new band even evolved into two peaks. These phenomena indicated a
The fluorescence quenching effect of nitrite was investigated for different concentration of TAAlPc. The results showed that using a lower concentration of TAAlPc would enhance the sensitivity of determination, but would lead to a narrower linear range of determination and a larger determination error since the measurement values of F0 and F were small. Therefore, a concentration of 7.0 /107 mol l 1 of TAAlPc was chosen. Influence of acidity in medium on the fluorescence quenching was also investigated and it was found that the fluorescence quenching reached a maximum in media containing 0.06 / 0.6 mol l 1 (final concentration) of hydrochloric acid. This result is in agreement with the principle that high acidity is beneficial to the diazotization reaction and inhibits the diazo coupling reaction. In this work, 0.48 mol l 1 of hydrochloric acid was chosen. Experimental results of temperature effect indicated that there was no obvious difference in the fluorescence quenching of the system in
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Table 1 Analytical characteristic for the determination of nitrite (or NO2 in 60 l air)
Nitrite (ng ml 1) NO2 (ppb, v/v)c a b c
Linear range
Linear regression equation
LODa
rb
R.S.D. (%)
1 /40 0.24 /9.6
DF /16/23c DF /16/96c
0.34 0.08
0.9940 0.9940
3.2 3.2
Limit of detection. Correlation coefficient. NO2 (ppb, v/v) /{NO2 (ng ml 1) /10 (ml)}/{60 (l) /1.845/0.74/0.5}. The density of NO2 (at 31 8C, 101.3 kPa)/0.845 g
l 1. Table 2 Determination of nitrogen oxides in automobile exhaust gases Samplea
1 2 3 a b
NOx found, ppb (v/v) (n/3)b Proposed method
Reference method
11329/48 9579/29 15719/49
11039/20 9089/20 15609/20
NO2 added (ng) (n/3)
Recovery (%)
80 80 80
92 /104 93 /102 90 /101
On different spots. Concentration of NOx (ppb, v/v) /{NO2 (ng)/25}/{0.25/0.74/0.5/60/1.845}, at 31 8C and 101.3 kPa.
the temperature range of 0 /100 8C. So, the determination can be performed at room temperature. The influence of reaction time on the fluorescence quenching was also studied and the results indicated that the sensitivity increased with the reaction time but at the expense of shorter linear range, the fluorescence intensity was, therefore, measured with no incubation time.
3.3. Calibration graph The calibration curve for the determination of nitrite was constructed by performing the standard procedure under optimum conditions described above. Good linear relationship was observed between the fluorescence quenching (DF ) and nitrite concentration. The analytical characteristic is given in Table 1. The limit of detection (LOD) was given by the equation, LOD /KS0/S , where K is a numerical factor chosen according to the confidence level desired, S0 is the standard deviation of blank measurements (n /9) and S is the sensitivity of the calibration graph. Here a value of 3 for K was used.
3.4. Interference of foreign substances The effects of various potential interferences were studied. The gases interfering with the determination were incorporated, in the form of ions, into solution containing 15 ng ml1 nitrite and the recovery of nitrite in the range of 95 /105% was considered as quantitative recovery. The results showed that formaldehyde at levels as high as 270-fold in quantity did not interfere. No interference resulted from the presence of a 11 000fold amount of H2S. Sulfur dioxide when present no more than 16 000-fold caused a recovery of nitrite 105%. We have previously reported [24] that the co-existing of formaldehyde and sulfite has a co-quenching effect on the fluorescence of TAAlPc, the interference of co-existing effect of the two species can be overcome by fixing a trapping tube filled with cotton-wool impregnated with lead acetate upstream of the oxidizing tube [5] to remove SO2. Nitrous acid and ammonia, at levels as high as 50 000-fold in quantity, caused a recovery of 107%. The interference due to ozone, if present, can be eliminated by exposing air steam to rubber, which reacts with ozone and not with NO2
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[25]. It showed that the typical interference species with common concentrations in automobile exhaust gas didn’t interfere the determination of NOx . 3.5. Determination of real samples Automobile exhaust gas in the bus station was drawn to a 10-ml 0.1 mol l 1 sodium hydroxide absorber solution at a flow-rate of 500 ml min 1 for 120 min at 31 8C and 101.3 kPa. The absorber solution was neutralized to neutrality with hydrochloric acid. Then the solution was completely transferred to a 25-ml volumetric flask and diluted to the mark with water. The diluted solution of 0.25 ml was taken for the determination by the recommended procedure. In order to evaluate the method, the standard anaphthylamine method [26] was applied as a reference method to determine the samples, and standard nitrite adding experiments were also done. The results are given in Table 2. It can be seen that the proposed method can be used effectively for the determination of NOx in automobile exhaust gas samples with satisfactory.
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