Resonance light scattering method for the determination of anionic ...

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tion was 5.00 × 10 −3 mol L −1 , which was diluted into the working solution of 5.00 × 10 −4 mol L −1 , Tris(hydroxymethyl) aminomethane buffer solution: 0.1 mol L −1 , which was mixed by Tris and 0.1 mol L −1 HCl. The remaining reagents were all analytic reagent, and experiment water adopted the secondary distilled water. 2.2. Experiment method X. Xiao et al. / Spectrochimica Acta Part A 71 (2008) 398–402 399 2.2.1. Determination of resonance Rayleigh scattering Added 0.50 mL AO solution and 0.50 mL Tris buffer solution (pH2.0) into the 10 mL colorimeter tube, joggled the tube to mix the solutions evenly, and then added certain SDBS standard solution or sample solution, and joggled the tube after the solutions were diluted behind the scale. Put the mixed solution at the place of λ em = λ ex for synchronous scanning, and then the resonance Rayleigh scattering (RRS) spectrum can be obtained. Measured the scattering light intensity at the 537 nm of the RRS peak, marked it as I 1 ; meanwhile measured the scattering light intensity of reagent blank, marked it as I 0; I RRS = I 1 − I 0. Both excitation slit width and emission slit width were all 5.0 nm. 2.2.2. Determination of resonance nonlinear scattering Used the method of above Section 2.2.1 to make the test solution, and used λ em = 1/2λ ex and λ em =2λ ex to measure the intensities I FDs (frequency-double scattering) and I SOS (secondorder scattering) of two resonance nonlinear scattering lights. FDS and SOS spectrograms can be made by plotting the corresponding wavelengths of I FDs and I SOS . Measured the scattering intensities I FDs and I SOS of ion-associated complex at FDS peak and SOS peak as well as the scattering intensities IFDs 0 and ISOS 0 of reagent blank, then ΔI FDs = I FDs − IFDS 0 and ΔI SOS = I SOS − ISOS 0 . Excitation slit width and emission slit width were both 5.0 nm. 3. Results and discussion Fig. 1. Resonance Rayleigh scattering spectra of AO–SDBS system at pH 2.0: SDBS, (2); AO, (3); AO–SDBS ((AO): 1.00 × 10 −5 mol L −1 ; (SDBS): 1.00 × 10 −5 mol L −1 ). ence and accordingly increase the RRS strength. Therefore, this experiment adopted λ em = λ ex = 537 nm as the study wavelength. From the absorption spectrogram, we saw that, with the gradual increase of adding quantity of SDBS, absorbance light of AO at 490 nm constantly reduced, this was because that AO reacted with SDBS to generate the ion-associated complex. The reason that RRS signal increases may be that positive ion dye stuffs AO and SDBS in the water solution generated the ion-associated complex through the reactions such as water repellent, electrostatic reaction or charge transfer complex [16], resulted in the enhancement of RRS signal intensity at 537 nm. Mole-ratio method was used to study the composition of ionassociated complex: fixed SDBS concentration, changed the AO concentration, measured and determined the I RRS of corresponding reagent blanks and various solution groups at 537 nm, plotted the I–V diagram. Result showed that a turning point occurred in case of mole ratio 1.2:1 for AO and SDBS, namely, 3.1. Resonance Rayleigh scattering spectra properties of SDBS–AO system The experimental method was used to measure the RRS spectrum for AO–SDBS system, as shown in Figs. 1 and 2 was the ultraviolet-visible range spectrum for AO–SDBS system. From Fig. 1, we knew that the RRS signals of SDBS and AO were both weaker. AO had a stronger resonance Rayleigh scattering signal near 512 nm, which was corresponding to the wide peak valley of ultraviolet-visible spectrum at 520 nm. With the addition of SDBS, stronger RRS peaks occurred at both 337 nm and 537 nm. The two RRS peaks (337 nm and 537 nm) in the RRS spectra was on the right of corresponding absorption peak, this was a characteristic “absorption-scattering” phenomenon of RRS spectra. Compared to RRS signal of acridine orange, RRS signal at 537 nm was stronger than that at 337 nm. If measuring the signal at the stronger wavelength, it may not only avoid the adverse reaction from the higher radiant energy of short wavelength, but also reduced the background interfer- Fig. 2. Absorption spectra of AO–SDBS system at pH 2.0: (AO), 1.00 × 10 −5 mol L −1 ; (SDBS)/×10 −5 mol L −1 ; (1) 0.00; (2) 0.25; (3) 0.50; (4) 0.75; (5) 1.00; (6) 1.25.
400 X. Xiao et al. / Spectrochimica Acta Part A 71 (2008) 398–402 Fig. 3. Spectra of frequency-double scattering (FDS) for SDBS–AO system: (1) 0.20 mL AO; (2) 1 + 0.20 mL SDBS; (3) 1 + 0.40 mL SDBS ((AO): 5.00 × 10 −4 mol L −1 ; (SDBS): 5.00 × 10 −4 mol L −1 ). the composition ratio of AO and SDBS in the ion-associated complex is 1.2:1. In case of any change of SDBS concentration, its ratio changed too, and this was because that there was a ratio difference between AO monomer and dimer, which was identical to the study of He et al. [17]. 3.2. Resonance nonlinear scattering spectra properties of SDBS–AO system When incident ray of different wavelengths passed through the reagent blank and solution of ion-associated complex, we recorded the scattering light intensity of incident ray at places of 1/2 or tow times wavelength, and plotted the corresponding wavelength diagrams according to scattering light intensity, and then we could get the spectrograms of FDS and SOS (Figs. 3 and 4). From the two figures, we saw that: (1) in FDS spectrum, when λ em = 321 nm and λ ex = 642 nm, I FDs maximized, and it was proportional to the matter concentration in the solution under certain condition. In SOS spectrum, when λ em = 642 nm and λ ex = 321 nm, I SOS maximized, and it was proportional to the matter concentration in the solution under certain condition. (2) FDS and SOS of acridine orange were both weak. When adding the SDBS into acridine orange to generate the ion-associated complex, the FDS and SDBS enhanced greatly. FDS maximum peak of ion-associated complex was at 321 nm and SOS maximum peak was at 642 nm. FDS maximum peak wavelength was 1/2 of SOS maximum peak wavelength. The radiation peak occurred near 542 nm should be the fluorescence peak [18], and therefore, there was a similarity between FDS peak and SOS peak. (3) I FDs > I SOS , FDS had a higher sensitivity and FDS used the long wavelength of low energy as the incident ray, which was more beneficial to determining some systems that are easy to generate the photochemical reaction. As a result, FDS was preferred for the determination study under the general condition. Fig. 4. Spectra of second-order scattering (SOS) for SDBS–AO system: (1) 0.20 mL AO; (2) 1 + 0.20 mL SDBS; (3) 1 + 0.40 mL SDBS ((AO): 5.00 × 10 −4 mol L −1 ; (SDBS): 5.00 × 10 −4 mol L −1 ). FDS and SOS were all nonlinear scatterings caused from the resonance scattering. The above experiment results showed that the resonance nonlinear scattering and resonance Rayleigh scattering of AO itself were all weaker. After adding SDBS, the three scatterings were all greatly enhanced and were in a linear relation with the added quantity. Therefore, there was a correlation between RRS and the two scatterings, which synchronously change with the generation and changes of RRS. The increase of molecular polarizability was an important factor [19] for the enhancement of RRS, and the sharp increase of molecular polarizability however was exactly the important condition for generating the nonlinear scatterings such as frequency-double scattering, second-order scattering, etc. Therefore, a stronger interdependent relationship may be existed among them. 3.3. Experiment of condition optimization The experiment of condition optimization was made on the basis of RRS method. 3.3.1. Influence of acidity Used the experimental method and took “Tris–HCl and Tris–NaOH” buffer solution to adjusted pH value of this solution, we determined the RRS intensity under different acidities and the intensity was among the range of pH 1.8–4.0, and I of this system maximized and kept the constant value. Hence, this experiment selected Tris buffer solution at pH 2.0 to control the acidity of the solution. 3.3.2. Influence of dosage of acridine orange With the SDBS of 1.00 × 10 −5 mol L −1 , we tested the influence of dosage of acridine orange, and the result showed that when AO dosage was 0.50 mL, the resonance light scattering of this system enhanced maximally.
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