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Concave cell design for FTIR measurements
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Chinese Chemical Letters 22 (2011) 1339–1342 www.elsevier.com/locate/cclet
Concave cell design for FTIR measurements Xiang Qin Lin *, Zhi Xiang Zhang, Wan Qun Hu Department of Chemistry, University of Science and Technology of China, Hefei 230026, China Received 10 March 2011 Available online 28 July 2011
Abstract Wedged and V-shaped cells have advantages on depressing strong absorption bands and keeping the photo windows wide open without absorption block. Alternatively, this work presents a concave cell design, which is constructed by a plano-convex CaF2 lens in combination with a flat plate. Mathematical equations have been provided for data treatment with Matlab programs. The result indicates that the concave cell has advantages similar to the V-shaped cell. Reflection–absorption measurements of ethanol have been conducted on the cell in mid-IR region for demonstration. The spectrum obtained from the concave cell can easily be interpreted to regenerate its normal spectrum, which is fitting well with the spectrum taken in a conventional thin-layer cell. The concave cell is easier to construct and more convenient to operate, willing to have popular usages. # 2011 Xiang Qin Lin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Infrared; Concave; Spectroscopic cell; Reflection–absorption; Ethanol
Spectroscopic measurement for liquid samples is usually conducted in cells with uniform light path-length where Bear’s Low can be simply applied. As an alternative design, wedged cells with non-uniform light path-length have been investigated with laser instruments [1–3]. We have also reported a similar design, the V-shaped cells, which can be equipped with a conventional FTIR instruments, showing advantages on depressing strong absorption bands and keeping the photo window wide open without absorption block, benefiting on detection of weak bands and allowing the detection to be carried out in solvents with high absorption bands [4,5]. This ‘‘focusing effect’’ is especially significant for measurements in IR region, where most of organic solvents have strong absorption bands and conventional measurements can only be made at very thin solution layers. However, the V-cell has to be constructed pies by pies by flat plates, which is difficult to produce an integrated cell for reproducible data acquisition. Hence, in this report, we investigated another type of cell design with non-uniform light path-length, which is constructed using a plano-convex lens. The cell can easily be integrated as a tubular cell for popular use. 1. Theoretical As shown in Fig. 1(a), the concave thin-layer solution is formed in-between a convex lens (1) in touch with a flat plate (2). For the incidence light beam (3), a transition light beam (3a) is generated if plat 2 is transparent and a reflection light beam (3b) can be generated if plat 2 is reflective. In general, the path length of the reflection beam 3b is * Corresponding author. E-mail address: [email protected] (X.Q. Lin). 1001-8417/$ – see front matter # 2011 Xiang Qin Lin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2011.05.038
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X.Q. Lin et al. / Chinese Chemical Letters 22 (2011) 1339–1342
Fig. 1. Schematic illustration of a concave spectroscopic cell. (a) Cross section of the concave cell; (b) X–Y illustration of the cell for mathematical treatment. 1, convex lens with a radius of curvature r; 2, light transparent optical plate or a surface reflectance plate; 3, incidence light beam; (3a, the transparent beam; 3b, the reflection beam); 4, solution in the cell; 5, shelter ring that limits the maximal values of opening angle u8, radii of the open window H8 and light path-length b; I8: incidence light (hn) strength, which is perpendicular to the plate 2; y: the distance to the window’s center (0 point).
shorter than the incident beam 3 due to the refraction of light on the convex interface. However, if the radii of curvature r for the lens is large and the opening angle u8 is small, the refraction effect can be ignored and the path-length of the reflection beam can approximately be seen as equal to the incidence beam. For the simplified case as shown in Fig. 1(b), considering a uniform incidence light (I8) goes perpendicularly to the plate 2, the one way path-length is b(y), where y is the distance of the light beam from the center of the window. The opening radius of the shelter ring is H = r sin u8. The maximal path-length is b8 = r(1 cos u8). Considering measurements for light transparent, the light strength at level y is l(y) = l010eb(y)c, where b(y) = r(1 cos u), y = r sin u, Dy = r cos uDu and u8 = a sin(H/r). The average value is H 1 X IðyÞDsðyÞ I¯ ¼ pH 2 0
(1)
where DS(y) = p(y + Dy)2 py2 is the window’s ring area at level y. For An ecb8 we have ecr = An/(1 cos u8) and I(y) = I010(An(1cos u))/(1cos u8). Eq. (1) is expressed as 2
r I0 I¯ ¼ 2 H
u X
10
An ðcos u1Þ 1cos u
ð2 sin u þ cos uDuÞcos uDu
(2)
u0
For numerical calculation, an iteration number N (typically, N = 1000) was selected, Du = u8/N, u = (n + 0.5)Du (n = 0, 1, ..., N 1). The absorption of the concave cell, Ac ¼ logðI¯=I 0 Þ, can be digitally calculated in Matlab ! N 1 An ðcosðnDuÞ1 r2 X I¯ 10 1cos u ð2 sinðnDuÞ þ cosðnDuÞ DuÞcosðnDuÞDu Ac ¼ log ¼ log I0 H 2 n0
(3)
The numerical result gives a Ac vs An plot, which can be converted to the corresponding An vs Ac plot for data interpretation. We found that that the relationship for the concave cell is very much similar to that for the V-shaped cell. Actually, under the assumption of u8/r 1, sin u u, cos u 1, Eq. (1) can be integrated and gives
Ac ¼ log
An ln 10 1 10An
Eq. (4) is exactly the same form as for Av vs An.
(4)
X.Q. Lin et al. / Chinese Chemical Letters 22 (2011) 1339–1342
1341
For reflection–absorption measurements, the path-length can approximately be considered as 2b(y), as described above. Additionally, stray lights generated from the experimental setup should be considered for data interpretation. Assuming Istray = b(l)Io, Eqs. (3) and (4) can be modified as
ATc
ATc
N 1 2An ðcosðnDuÞ1Þ r2 X 1 cos u ¼ log 10 þ bðlÞ ð2 sinðnDuÞ þ cosðnDuÞDuÞcosðnDuÞDu H 2 n0
!
1 102An þ bðlÞ ¼ log 2An ln 10
(5)
(6)
For simplicity, the coefficient b(l) may be considered as an averaged value b over a range of wavelengths. 2. Experimental A CaF2 Plano-Convex Cylindrical Lens (r = 50.0 mm, K = 20.0 mm) and a CaF2 plate (2.0 mm in thickness, K = 20.0 mm) (Changchun Jiusheng Photonics Co., Ltd.) was used for construction of the concave IR cell. Matlab (V7.8, MathWork, USA) was used for numerical calculation with cubic spline interpolation. BRUKER EQUINOX-55 FTIR Spectrometer equipped with a home-made reflection accessory was used. Usually, 100 scans were averaged for a spectrum. Ethanol (AR, Sinopharm Chemical Reagent Co., Ltd.) was used as an IR sample. A typical reflection–absorption cell has been constructed as shown in Fig. 2. The plano-convex lens (1) was fixed on the bottom of the tubular Teflon cell (2) with its convex face up. A straight brass rod (3) with a flat mirror bottom is inserted in the cell, which is hold by two Teflon collimation blocks (4 and 5) to ensure the perpendicular position. In this way, the brass rod touching to the convex surface on its center and generate a concave thin-layer cell between them. A ring light shelter (6) placed in the front of the lens limits the diameter of the light window. The plano-convex lens can easily be replaced by a flat CaF2 window, thus the cell can simply become a conventional thin-layer cell for acquiring normal spectra for comparison. The specific shape of the cell holder (7) allows the cell to sit on the top of the reflection accessory (8) at proper position. So, this cell design is similar to an in situ spectroelectrochemical cell for reflection–absorption measurements, but with different types of solution layers [6–9]. A light transparent concave cell can similarly be constructed in a metal tube with screws and spacers (not shown). 3. Result and discussions For the preliminary investigation, the reflection–absorption spectrum in the concave cell (Ac) for ethanol was taken in comparison with the normal thin-layer spectrum (AN) taken on the same experimental setup except using a flat CaF2 window instead of the convex lens. The result is shown in Fig. 3. As shown in Fig. 3(a), much higher absorption peaks are presented on the Ac spectrum (red) than on the An spectrum (black). However, the factor for enlargement is quite different for strong and weak bands. For example, the strong band at about 3300 cm1 is enlarged for about 1.6-fold, the weak
Fig. 2. (a) Schematic illustration of the concave cell-holder system. (b) Assembly of the cell sitting on the reflection accessory. 1, CaF2 plano-convex lens; 2, tubular Teflon cell body; 3, brass rod with a plane mirror bottom perpendicular to the axis; 4, Teflon collimation block 1; 5, Teflon collimation block 2; 6, ring shelter; 7, metal holder; 8, reflection accessory.
X.Q. Lin et al. / Chinese Chemical Letters 22 (2011) 1339–1342
1342
(a)
(b)
1.4 conventional thin-layer cell concave cell
1.2
concave cell, interpreted conventional thin-layer cell
6
1.0
5
Absorbance
Absorbance
7
0.8 0.6 0.4
4 3 2 1
0.2
0 0.0 4000
3500
3000
2500
2000
Wavenumber, cm
1500
1000
-1 4000
3500
-1
3000
2500
2000
Wavenumber / cm
1500
1000
-1
Fig. 3. (a) Reflection–absorption spectrum ðATc Þ of ethanol in the concave cell (H = 2.34 mm, r = 50.0 mm); (b) the AN spectrum after interpretation based on Eq. (5) (b = 0.025), in comparison with the spectrum acquired in a conventional thin-layer cell (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
bands in between 2700 and 1600 cm1 can be estimated for about more than 10-fold. The enlargement ability focused on weak bands and avoiding saturation of strong bands is the basic advantage for such cells with non-uniform path-length. The sensitivity for small bands may have special benefit for in situ spectroelectrochemical studies, in which small structural changes on the molecules due to electron transfer may be revealed directly from the Ac spectrum. In comparison with V-cells, round optical window can be easily arranged and no bottom effect should be taken into count. In addition, the diffusion in the concave cell is obviously much faster than that in V-cells, and much faster than that in conventional thinlayer cells, which increased the ionic conductivity and reduce the resistance in the solution layer and thus is an important advantage for the efficiency of applying electrode potentials, allowing quicker kinetic process may be investigated [10]. Quantitative measurements can also be achieved after simple data interpretation according to Eq. (5) or (6), by properly selecting the calibration parameter b. Fig. 3(b) shows the spectrum resulted after the data treatment with b = 0.025, a perfect fit between the spectrum interpreted and the spectrum taken from the conventional thin-layer cell is achieved, which demonstrated the accuracy of the concave cell. The major fitting error is generated in the region of wavelength lower than 1200 cm1, where serious absorption of CaF2 lens occurs. An additional error may come from the value of b, which was selected as an average value in the whole wavelength region. Because the stray light is a function of wavelength, b(l) should better be used for sophisticated interpretation. 4. Conclusion This communication presents a novel concave IR spectroscopic cell design. Equations for mathematical and numerical data interpretation are provided. It demonstrates that the absorption behavior in the concave cell is also a nonlinear function of concentration, similar to V-shaped cells. The concave cell is easy to construct and convenient to operate especially for reflection–absorption measurements. A constant coefficient b can be selected for stray light correction in a range of wavelength. However, wavelength dependent b(l) may better be used for more sophisticated correction, which is worthy for further investigation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
C.W. Robertson, D. Williams, J. Opt. Soc. Am. 61 (1971) 1316. I.L. Tyler, G. Taylor, M.R. Querry, Appl. Opt. 17 (1978) 960. D.M. Wieliczka, S.S. Weng, M.R. Querry, Appl. Opt. 28 (1989) 1714. X.Q. Lin, K.M. Kadish, Proc. 2nd Conf. Exhib. Instrum. Anal., Beijing, (1987), p. 1311. X.Q. Lin, Z.X. Zhang, W.Q. Hu, Chin. Chem. Lett. 22 (2011) 481. A. Bewick, K. Kunimatsu, B.S. Pons, Electrochim. Acta 25 (1980) 465. D.K. Roe, J.K. Sass, D.S. Bethune, et al. J. Electroanal. Chem. 216 (1987) 293. A.l. Bo, X.Q. Lin, Chin. Chem. Lett. 9 (9) (1998) 851. X.Q. Lin, Z.L. Li, Chin. Chem. Lett. 3 (1992) 1029. X.Q. Lin, K.M. Kadish, Anal. Chem. 58 (1986) 1493.
Chinese Chemical Letters 22 (2011) 1339–1342 www.elsevier.com/locate/cclet
Concave cell design for FTIR measurements Xiang Qin Lin *, Zhi Xiang Zhang, Wan Qun Hu Department of Chemistry, University of Science and Technology of China, Hefei 230026, China Received 10 March 2011 Available online 28 July 2011
Abstract Wedged and V-shaped cells have advantages on depressing strong absorption bands and keeping the photo windows wide open without absorption block. Alternatively, this work presents a concave cell design, which is constructed by a plano-convex CaF2 lens in combination with a flat plate. Mathematical equations have been provided for data treatment with Matlab programs. The result indicates that the concave cell has advantages similar to the V-shaped cell. Reflection–absorption measurements of ethanol have been conducted on the cell in mid-IR region for demonstration. The spectrum obtained from the concave cell can easily be interpreted to regenerate its normal spectrum, which is fitting well with the spectrum taken in a conventional thin-layer cell. The concave cell is easier to construct and more convenient to operate, willing to have popular usages. # 2011 Xiang Qin Lin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Infrared; Concave; Spectroscopic cell; Reflection–absorption; Ethanol
Spectroscopic measurement for liquid samples is usually conducted in cells with uniform light path-length where Bear’s Low can be simply applied. As an alternative design, wedged cells with non-uniform light path-length have been investigated with laser instruments [1–3]. We have also reported a similar design, the V-shaped cells, which can be equipped with a conventional FTIR instruments, showing advantages on depressing strong absorption bands and keeping the photo window wide open without absorption block, benefiting on detection of weak bands and allowing the detection to be carried out in solvents with high absorption bands [4,5]. This ‘‘focusing effect’’ is especially significant for measurements in IR region, where most of organic solvents have strong absorption bands and conventional measurements can only be made at very thin solution layers. However, the V-cell has to be constructed pies by pies by flat plates, which is difficult to produce an integrated cell for reproducible data acquisition. Hence, in this report, we investigated another type of cell design with non-uniform light path-length, which is constructed using a plano-convex lens. The cell can easily be integrated as a tubular cell for popular use. 1. Theoretical As shown in Fig. 1(a), the concave thin-layer solution is formed in-between a convex lens (1) in touch with a flat plate (2). For the incidence light beam (3), a transition light beam (3a) is generated if plat 2 is transparent and a reflection light beam (3b) can be generated if plat 2 is reflective. In general, the path length of the reflection beam 3b is * Corresponding author. E-mail address: [email protected] (X.Q. Lin). 1001-8417/$ – see front matter # 2011 Xiang Qin Lin. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2011.05.038
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X.Q. Lin et al. / Chinese Chemical Letters 22 (2011) 1339–1342
Fig. 1. Schematic illustration of a concave spectroscopic cell. (a) Cross section of the concave cell; (b) X–Y illustration of the cell for mathematical treatment. 1, convex lens with a radius of curvature r; 2, light transparent optical plate or a surface reflectance plate; 3, incidence light beam; (3a, the transparent beam; 3b, the reflection beam); 4, solution in the cell; 5, shelter ring that limits the maximal values of opening angle u8, radii of the open window H8 and light path-length b; I8: incidence light (hn) strength, which is perpendicular to the plate 2; y: the distance to the window’s center (0 point).
shorter than the incident beam 3 due to the refraction of light on the convex interface. However, if the radii of curvature r for the lens is large and the opening angle u8 is small, the refraction effect can be ignored and the path-length of the reflection beam can approximately be seen as equal to the incidence beam. For the simplified case as shown in Fig. 1(b), considering a uniform incidence light (I8) goes perpendicularly to the plate 2, the one way path-length is b(y), where y is the distance of the light beam from the center of the window. The opening radius of the shelter ring is H = r sin u8. The maximal path-length is b8 = r(1 cos u8). Considering measurements for light transparent, the light strength at level y is l(y) = l010eb(y)c, where b(y) = r(1 cos u), y = r sin u, Dy = r cos uDu and u8 = a sin(H/r). The average value is H 1 X IðyÞDsðyÞ I¯ ¼ pH 2 0
(1)
where DS(y) = p(y + Dy)2 py2 is the window’s ring area at level y. For An ecb8 we have ecr = An/(1 cos u8) and I(y) = I010(An(1cos u))/(1cos u8). Eq. (1) is expressed as 2
r I0 I¯ ¼ 2 H
u X
10
An ðcos u1Þ 1cos u
ð2 sin u þ cos uDuÞcos uDu
(2)
u0
For numerical calculation, an iteration number N (typically, N = 1000) was selected, Du = u8/N, u = (n + 0.5)Du (n = 0, 1, ..., N 1). The absorption of the concave cell, Ac ¼ logðI¯=I 0 Þ, can be digitally calculated in Matlab ! N 1 An ðcosðnDuÞ1 r2 X I¯ 10 1cos u ð2 sinðnDuÞ þ cosðnDuÞ DuÞcosðnDuÞDu Ac ¼ log ¼ log I0 H 2 n0
(3)
The numerical result gives a Ac vs An plot, which can be converted to the corresponding An vs Ac plot for data interpretation. We found that that the relationship for the concave cell is very much similar to that for the V-shaped cell. Actually, under the assumption of u8/r 1, sin u u, cos u 1, Eq. (1) can be integrated and gives
Ac ¼ log
An ln 10 1 10An
Eq. (4) is exactly the same form as for Av vs An.
(4)
X.Q. Lin et al. / Chinese Chemical Letters 22 (2011) 1339–1342
1341
For reflection–absorption measurements, the path-length can approximately be considered as 2b(y), as described above. Additionally, stray lights generated from the experimental setup should be considered for data interpretation. Assuming Istray = b(l)Io, Eqs. (3) and (4) can be modified as
ATc
ATc
N 1 2An ðcosðnDuÞ1Þ r2 X 1 cos u ¼ log 10 þ bðlÞ ð2 sinðnDuÞ þ cosðnDuÞDuÞcosðnDuÞDu H 2 n0
!
1 102An þ bðlÞ ¼ log 2An ln 10
(5)
(6)
For simplicity, the coefficient b(l) may be considered as an averaged value b over a range of wavelengths. 2. Experimental A CaF2 Plano-Convex Cylindrical Lens (r = 50.0 mm, K = 20.0 mm) and a CaF2 plate (2.0 mm in thickness, K = 20.0 mm) (Changchun Jiusheng Photonics Co., Ltd.) was used for construction of the concave IR cell. Matlab (V7.8, MathWork, USA) was used for numerical calculation with cubic spline interpolation. BRUKER EQUINOX-55 FTIR Spectrometer equipped with a home-made reflection accessory was used. Usually, 100 scans were averaged for a spectrum. Ethanol (AR, Sinopharm Chemical Reagent Co., Ltd.) was used as an IR sample. A typical reflection–absorption cell has been constructed as shown in Fig. 2. The plano-convex lens (1) was fixed on the bottom of the tubular Teflon cell (2) with its convex face up. A straight brass rod (3) with a flat mirror bottom is inserted in the cell, which is hold by two Teflon collimation blocks (4 and 5) to ensure the perpendicular position. In this way, the brass rod touching to the convex surface on its center and generate a concave thin-layer cell between them. A ring light shelter (6) placed in the front of the lens limits the diameter of the light window. The plano-convex lens can easily be replaced by a flat CaF2 window, thus the cell can simply become a conventional thin-layer cell for acquiring normal spectra for comparison. The specific shape of the cell holder (7) allows the cell to sit on the top of the reflection accessory (8) at proper position. So, this cell design is similar to an in situ spectroelectrochemical cell for reflection–absorption measurements, but with different types of solution layers [6–9]. A light transparent concave cell can similarly be constructed in a metal tube with screws and spacers (not shown). 3. Result and discussions For the preliminary investigation, the reflection–absorption spectrum in the concave cell (Ac) for ethanol was taken in comparison with the normal thin-layer spectrum (AN) taken on the same experimental setup except using a flat CaF2 window instead of the convex lens. The result is shown in Fig. 3. As shown in Fig. 3(a), much higher absorption peaks are presented on the Ac spectrum (red) than on the An spectrum (black). However, the factor for enlargement is quite different for strong and weak bands. For example, the strong band at about 3300 cm1 is enlarged for about 1.6-fold, the weak
Fig. 2. (a) Schematic illustration of the concave cell-holder system. (b) Assembly of the cell sitting on the reflection accessory. 1, CaF2 plano-convex lens; 2, tubular Teflon cell body; 3, brass rod with a plane mirror bottom perpendicular to the axis; 4, Teflon collimation block 1; 5, Teflon collimation block 2; 6, ring shelter; 7, metal holder; 8, reflection accessory.
X.Q. Lin et al. / Chinese Chemical Letters 22 (2011) 1339–1342
1342
(a)
(b)
1.4 conventional thin-layer cell concave cell
1.2
concave cell, interpreted conventional thin-layer cell
6
1.0
5
Absorbance
Absorbance
7
0.8 0.6 0.4
4 3 2 1
0.2
0 0.0 4000
3500
3000
2500
2000
Wavenumber, cm
1500
1000
-1 4000
3500
-1
3000
2500
2000
Wavenumber / cm
1500
1000
-1
Fig. 3. (a) Reflection–absorption spectrum ðATc Þ of ethanol in the concave cell (H = 2.34 mm, r = 50.0 mm); (b) the AN spectrum after interpretation based on Eq. (5) (b = 0.025), in comparison with the spectrum acquired in a conventional thin-layer cell (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
bands in between 2700 and 1600 cm1 can be estimated for about more than 10-fold. The enlargement ability focused on weak bands and avoiding saturation of strong bands is the basic advantage for such cells with non-uniform path-length. The sensitivity for small bands may have special benefit for in situ spectroelectrochemical studies, in which small structural changes on the molecules due to electron transfer may be revealed directly from the Ac spectrum. In comparison with V-cells, round optical window can be easily arranged and no bottom effect should be taken into count. In addition, the diffusion in the concave cell is obviously much faster than that in V-cells, and much faster than that in conventional thinlayer cells, which increased the ionic conductivity and reduce the resistance in the solution layer and thus is an important advantage for the efficiency of applying electrode potentials, allowing quicker kinetic process may be investigated [10]. Quantitative measurements can also be achieved after simple data interpretation according to Eq. (5) or (6), by properly selecting the calibration parameter b. Fig. 3(b) shows the spectrum resulted after the data treatment with b = 0.025, a perfect fit between the spectrum interpreted and the spectrum taken from the conventional thin-layer cell is achieved, which demonstrated the accuracy of the concave cell. The major fitting error is generated in the region of wavelength lower than 1200 cm1, where serious absorption of CaF2 lens occurs. An additional error may come from the value of b, which was selected as an average value in the whole wavelength region. Because the stray light is a function of wavelength, b(l) should better be used for sophisticated interpretation. 4. Conclusion This communication presents a novel concave IR spectroscopic cell design. Equations for mathematical and numerical data interpretation are provided. It demonstrates that the absorption behavior in the concave cell is also a nonlinear function of concentration, similar to V-shaped cells. The concave cell is easy to construct and convenient to operate especially for reflection–absorption measurements. A constant coefficient b can be selected for stray light correction in a range of wavelength. However, wavelength dependent b(l) may better be used for more sophisticated correction, which is worthy for further investigation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
C.W. Robertson, D. Williams, J. Opt. Soc. Am. 61 (1971) 1316. I.L. Tyler, G. Taylor, M.R. Querry, Appl. Opt. 17 (1978) 960. D.M. Wieliczka, S.S. Weng, M.R. Querry, Appl. Opt. 28 (1989) 1714. X.Q. Lin, K.M. Kadish, Proc. 2nd Conf. Exhib. Instrum. Anal., Beijing, (1987), p. 1311. X.Q. Lin, Z.X. Zhang, W.Q. Hu, Chin. Chem. Lett. 22 (2011) 481. A. Bewick, K. Kunimatsu, B.S. Pons, Electrochim. Acta 25 (1980) 465. D.K. Roe, J.K. Sass, D.S. Bethune, et al. J. Electroanal. Chem. 216 (1987) 293. A.l. Bo, X.Q. Lin, Chin. Chem. Lett. 9 (9) (1998) 851. X.Q. Lin, Z.L. Li, Chin. Chem. Lett. 3 (1992) 1029. X.Q. Lin, K.M. Kadish, Anal. Chem. 58 (1986) 1493.