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Study of Six Polycyclic Aromatic Hydrocarbons by Chemical Deoxygenation Microemulsion-Stabilized Room Temperature Phosphorimetry
MICROCHEMICAL JOURNAL ARTICLE NO.
57, 294–304 (1997)
MJ961471
Study of Six Polycyclic Aromatic Hydrocarbons by Chemical Deoxygenation Microemulsion-Stabilized Room Temperature Phosphorimetry Xin Yang, Chuan Dong, Jun Zhang, Yan-sheng Wei, Wei-Jun Jin, and Chang-song Liu1 Department of Chemistry, Shanxi University, Taiyuan, Shanxi 030006, People’s Republic of China Received August 10, 1996; accepted December 17, 1996 Six polycyclic aromatic hydrocarbons (PAHs) are studied by chemical deoxygenation microemulsion-stabilized room temperature phosphorimetry with sodium sulfite as an oxygen scavenger and thallous nitrate as a heavy atom perturber in sodium dodecyl sulfate medium. Several factors influencing room temperature phosphorescence such as the concentration of sodium dodecyl sulfate, the heavy atom concentration, the pH, and the concentration of sodium sulfite are discussed and the quenching effect of NO02 on room temperature phosphorimetry of PAHs was compared in the microemulsion and micelle media. q 1997 Academic Press
INTRODUCTION
Room temperature phosphorimetry has useful applications for the determination of polycyclic aromatic hydrocarbons (PAHs) (1–5). Jin et al. studied and compared the analytical features of PAHs by solid-substrate room temperature phosphorimetry (SSRTP) with cellulose membrane and filter paper as substrate (6). Dong et al. simultaneously determined p-triphenyl and m-triphenyl by synchronous SS-RTP technique (7). Cyclodextrin-induced room temperature phosphorescence (CD-RTP) and micelle-stabilized room temperature phosphorescence (MS-RTP) also have many interesting applications for the determination of PAHs with better selectivity (8, 9). Diaz Garcia and Sanz-Medel (10) and Wei et al. (11) improved MS-RTP by using sodium sulfite as an oxygen scavenger, which is more convenient than nitrogen purging. Jin and Liu also determined PAHs by MS-RTP or synchronous MS-RTP with sodium sulfite as an oxygen scavenger (12, 13). Ramos et al. compared the analytical features of PAHs on SS-RTP and RTP in organized media and proved that microemulsion room temperature phosphorimetry (ME-RTP) is an ideal determination method for PAHs (14). Jin et al. further reported the determination of naphthalene and phenanthrene by ME-RTP (15). In this paper, ME-RTP of six PAHs is studied with sodium sulfite as oxygen scavenger. The factors that affect the analytical features are examined in detail. In addition, the kinetic behavior of the NO20 quenching effect is compared in microemulsion and micelle systems. 1
To whom correspondence should be addressed. 294
0026-265X/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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Apparatus All uncorrected luminescence spectra were recorded and measurements of RTP intensities were carried out with a LS-50B luminescence spectrofluorometer (PerkinElmer Co.) equipped with a thermostat cell holder. A xenon arc lamp was used as the excitation light source with pulse width less than 10 ms. The delay time was set at 0.1 ms, which is an important factor for measuring RTP free from the interference of residual fluorescence or scatter light. The gate time was set at 3 ms (fluoranthene), 6 ms (pyrene), 5 ms (chrysene), and 1 ms (fluorene, acenaphthene, p-triphenyl), respectively. Reagents Fluoranthene, pyrene, chrysene, fluorene, acenaphthene, and p-triphenyl were all of HPLC grade from Fluka. Stock solutions of six PAHs were prepared by dissolving a fixed amount of PAH in acetone. The stock solution was transferred into a 25-ml volumetric flask, the acetone was evaporated thoroughly on an electric heat plate, and then 0.2 mol/liter sodium dodecyl sulfate (SDS) solution was added up to the final volume. The concentration of the standard solution for all six PAHs was 1 1 1004 mol/liter in 0.2 mol/ liter SDS solution. SDS, n-heptane, 1-pentanol, thallium (I) nitrate, sodium sulfite, and sulfuric acid were of the analytical reagent grade. Sodium sulfite aqueous solution should be prepared fresh daily. Water was doubly distilled in a subboiling distiller. Procedures The appropriate PAH standard solution was transferred to a 10-ml volumetric flask and a certain amount of 0.2 mol/liter SDS solution was added to maintain the concentration of SDS solution at 0.04 mol/liter. Then n-heptane, 1-pentanol, thallium (I) nitrate, sulfuric acid, and sodium sulfite were added in turn. The mixed solution was diluted with water to 10-ml final volume. If precipitates appear in the flask after adding TINO3 , the flask should be heated gently before adding other reagents until the precipitates disappear. After mixing thoroughly, the flask was placed in a thermostat cell at 25 { 17C for 10 min. A 1-cm fluorescence cell was used for measuring RTP intensity or recording the luminescent spectra. RESULTS AND DISCUSSION
Luminescence Spectra The excitation and emission spectra of six PAHs are shown in Fig. 1 and their spectroscopic characteristics are listed in Table 1. Table 1 shows that the fluorescence Stokes shifts of fluorene and acenaphthene are very small (about 3–4 nm) and their maximum excitation and emission peaks nearly overlap, which results in serious self-quenching. This disadvantage can be avoided in the RTP measurement because of the great Stokes shifts.
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FIG. 1. Excitation and emission spectra of the six PAHs.
Concentration of SDS As in the micelle system, it is essential to provide an appropriate rigid microenvironment to ensure RTP emission in the ME system. The micelle in aqueous solution is ˚ . Once large molecules are a spherical substance with a diameter of 30 to 60 A solubilized in it, the micelle volume is enlarged obviously. The diameter of microemul˚ , which is larger than that of the micelle, so the sion falls in the range 50 to 1000 A oil/water model microemulsion provides a quite hydrophobic inner microenvironment in which many hydrophobic molecules are trapped in each collector. The concentration of surfactant should be great enough to attain optimum emulsion effect. At lower concentrations of surfactant, the oil/water interface adsorbs fewer molecules; its
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ME-RTP OF SIX PAHs TABLE 1 Spectroscopic Characteristics of Six PAHs in the ME System PAH
l exa (nm)
Fluoranthene Pyrene Chrysene Fluorene Acenaphthene p-Triphenyl
360 337 322 302 306 305
lFb (nm)
450 375, 366, 306, 309, 343
385, 394 388, 408 325 341
lPc (nm)
543, 594, 511, 430, 486, 501
591 648 547 457 518
Note. All concentrations for the determination are 5 1 1006 mol/liter. a The same excitation wavelength of fluorescence and phosphorescence. b Emission wavelength of fluorescence. c Emission wavelength of phosphorescence, in boldface, corresponding to maximum RTP peak.
strength decreases, forming an unstable microemulsion system. On the other hand, when the concentration of surfactant increases to a certain degree, more cosurfactant molecules are adsorbed on the interface, the interfacial strength is strengthened, and the stability of the microemulsion system is significantly improved. Once a polar organic solvent that contains groups such as –OH (e.g., n-pentanol), –NH2 , or –COOH is added to surfactants, the interaction of surfactants and polar organic molecules causes the molecules to be arranged more closely on the interface, and interfacial activity and strength are enhanced. As shown in Fig. 2, the phosphorescence intensity of the six PAHs change with the concentration of SDS. As the concentration of SDS increased from 0.02 to 0.05 mol/liter, generally, the RTP intensity showed little change and lifetime (t) tended to increase. When the concentration of SDS was greater than 0.05 mol/liter, the intensity of RTP dropped sharply. Selection of Apolar Solvent and Cosurfactant The microemulsion system consists of apolar solvent, surfactant, cosurfactant (usually a C4–C6 alkyl alcohol), and water. The solubility of the surfactant in apolar solvent is closely related to its aggregation. But the mixing emulsion ability of cosurfactant and surfactant increases the rigidity of the microenvironment in the ME system. Five alkanes—n-pentane, n-hexane, n-heptane, n-octane, n-nonane—and six alcohols—propanol, butyl alcohol, n-pentanol, heptanol, n-octyl alcohol, n-decyl alcohol—were explored. The results show that when five alkanes are used as apolar solvent, the RTP intensity and lifetime of PAHs change inconspicuously. When propanol, butyl alcohol, and n-pentanol are used as cosurfactants, the RTP intensity and lifetime do not show a substantial difference. But when heptanol, octyl alcohol, and decanol are used, precipitates appear in the system and the RTP is quenched for increase in their viscosity with increase in molecular chain length. In these systems RTP signals were hardly detected. Based on the experimental results, heptane and pentanol were selected as apolar solvent and cosurfactant, respectively, in the microemulsion system. The optimum concentrations of heptane and pentanol are listed in
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FIG. 2. Influence of SDS concentration on RTP (a) and lifetime (b) of the six PAHs.
Table 2. The stability of the ME system is based on the amounts of heptane and pentanol. As shown in Table 2, the RTP intensities of PAHs are normally stable and tend to increase slightly in the concentration ranges 0.02–0.1% for heptane and 0.01–
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ME-RTP OF SIX PAHs TABLE 2 Optimum Heptane and Pentanol Concentrations of Six PAHs PAH
Fluoranthene
Pyrene
Chrysene
Fluorene
Acenaphthene
p-Triphenyl
Heptane (%) Pentanol (%)
0.10 0.10
0.02 0.03
0.08 0.10
0.04 0.03
0.02 0.03
0.02 0.03
0.1% for pentanol. No phase separation was observed in the ME system. The phosphorescence lifetime (t) exhibited the same trend. But when the amount of heptane was greater than 0.5% (0.02% pentanol) and that of pentanol was greater than 1% (0.1% heptane), two phases appeared and RTP intensity became weak. Heavy Atom Concentration Heavy atoms with high nuclear charge may cause the spin-orbit coupling action of the solute molecules, which mainly enhances S1 r T1 intersystem crossing probability, and induce the appearance of phosphorescence. Thallous nitrate is an efficient heavy atom perturber for polycyclic aromatic compounds in a chemical deoxygenation micelle system. The experiment suggests that the optimum concentration of thallous nitrate is 0.015 mol/liter for fluoranthene, chrysene, acenaphthene, and p-triphenyl, 0.02 mol/liter for fluorene, and 0.025 mol/liter for pyrene. Generally, RTP lifetime decreased with increase in concentration of thallous nitrate (refer to Fig. 3). Effects of pH In the micelle system, the chemical deoxygenation efficiency of sodium sulfite is greatly influenced by the pH of the bulk solution (10), as proved by the study on five PAHs of micelle-stabilized room temperature phosphorescence (7). Similarly, in the microemulsion system, the pH of the bulk solution heavily influences the intensity and lifetime of RTP. In either strong acidic or alkaline solution, the RTP signals of PAHs are too weak and the phosphorescence lifetime is too short to be detected easily; therefore, it is necessary to select a suitable pH. The experiments showed that when the pH ranged from 6.50 to 7.95, the deoxygenation speed increased, the RTP intensity reached its highest point, and the RTP lifetime increased steadily, responses to a lower oxygen environment. Sulfuric acid 0.02 mol/liter was selected to adjust the pH of the bulk solution (Fig. 4). Concentration of Sodium Sulfite Since the triplet state is quenched quickly by collision with solvent and oxygen molecules, it is essential to remove oxygen thoroughly from the micelle solution. For this purpose, the technique of chemical deoxygenation with sodium sulfite was adopted (10). However, only when enough sodium sulfite exists in a neutral or weak alkaline solution, could oxygen be eliminated thoroughly and could stable RTP be observed. The concentration of sodium sulfite should be maintained in the range 5 1 1003 to 1 1 1003 mol/liter as shown in Fig. 5.
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FIG. 3. Influence of heavy atom concentration on RTP (a) and lifetime (b) of the six PAHs.
RTP Stability The experiment showed that the RTP intensity of PAHs decreases only about 10% in an hour which is stable enough to complete a series of operations. But the RTP
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FIG. 4. Influence of pH on RTP (a) and lifetime (b) of the six PAHs.
signals of pyrene decrease only 1%, which shows that Na2SO3 in the ME–RTP system has greater deoxygenation efficiency. The microemulsion system could provide a stronger hydrophobic microenvironment for pyrene than the micelle system.
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FIG. 5. Influence of concentration of sodium sulfite on RTP (a) and lifetime (b) of the six PAHs.
Under optimum conditions, the RTP lifetimes of six PAHs in the ME system are 2.65 ms (fluoranthene), 4.39 ms (pyrene), 4.05 ms (chrysene), 0.71 ms (fluorene), 1.08 ms (acenaphthene), and 3.23 ms (p-triphenyl).
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ME-RTP OF SIX PAHs TABLE 3 ME–RTP Analytical Features of Six PAHs LOD (mol/liter) PAHs Fluoranthene Pyrene Chryene Fluorene Acenaphthene p-Triphenyl
LDR (mol/liter) 2 1 1 3 3 3
1 1 1 1 1 1
1007 –5 1007 –1 1007 –3 1007 –1 1007 –1 1007 –1
1 1 1 1 1 1
1005 1005 1005 1004 1004 1005
Slope (log–log)
Relative coefficient
0.785 0.781 0.857 0.824 0.877 0.747
0.998 0.996 0.997 0.997 0.999 0.997
3sn01a 2.0 4.3 5.4 2.5 4.4 3.5
1 1 1 1 1 1
1009 10010 1009 1009 1009 1009
S/N Å 3b 1.4 4.4 4.7 8.6 2.0 2.5
1 1 1 1 1 1
1009 1009 1008 1008 1007 1007
RSDc (%) 4.4 2.7 1.5 3.3 2.5 4.2
a
Calculated on the basis of three times the SD criterion. Measured with a signal-to-noise ratio of 3. c Seven measurements. b
Analytical Features The analytical features are listed in Table 3. Comparison of the Quenching Kinetic Behavior of Fluoranthene in the ME and MS Systems Sodium nitrite is a strong RTP quencher and its quenching effect on the RTP of fluoranthene was compared in the two systems. The experiments demonstrated that the quenching effect in the range 1 1 1005 to 1 1 1004 mol/liter NO20 follows the dynamic quenched Stern–Volmer equation. The quench rate constants in these two systems are 1.22 1 107 liters/molrs (micelle system), 6.84 1 106 liters/molrs (microemulsion system), respectively. The quenching rate constant in the micelle system is 60% greater than that in the microemulsion system; i.e., the quenching effect of NaNO2 on phosphorescence in the microemulsion system is less than that in the micelle system, which shows that the microemulsion system provides stronger protection for the phosphor than does the micelle system. CONCLUSION
The experimental results demonstrate that the methods established for determination of the six PAHs have wider LDRs, nearly reaching three amount levels; lower LODs, reaching 1009 or even 10010 mol/liter; and higher-precision RSDs (less than 5%). The method of extracting organic solvents of PAH samples can be successfully applied to prepare microemulsions. The ME system has a more rigid microenvironment than the MS system for the phosphor. ACKNOWLEDGMENTS This work is supported by the NNSF of China and the Youth Science Foundation of Shanxi Province.
REFERENCES 1. Vo-Dinh, T. Room Temperature Phosphorimetry for Chemical Analysis. Wiley, New York, 1984. 2. Hurtubise, R. J. Phosphorimetry: Theory, Instrumentation, and Applications. VCH, New York, 1990.
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3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Vo-Dinh, T. Environ. Sci. Technol., 1985, 19, 997–1003. Bello, J.; Hurtubise, R. J. Appl. Spectrosc., 1986, 40, 790–794. Perry, L. M.; Campiglia, A. D.; Winefordner, J. D. Anal. Chem. 1989, 61, 2328–2330. Jin, W. J.; Liu, C. S. Fenxi Shiyanshi, 1993, 12(1), 115–119. Dong, C.; Liu, C. S.; Feng, K. C. Guangpuxue & Guangpufenxi, 1994, 14(1), 43–48. Cline Love, L. J.; Skrilec, M.; Habarto, J. G. Anal. Chem., 1980, 52, 754–759. Scypinsxi, S.; Cline Love, L. J. Anal. Chem., 1984, 56, 322–327. Diaz Garcia, M. E.; Sanz-Medel, A. Anal. Chem., 1986, 58, 1436–1440. Wei, Y. S.; Liu, C. S.; Zhang, S. S. Fenxi Huaxue, 1990, 18(3), 228. Jin, W. J.; Liu, C. S. Microchem. J., 1993, 48, 94–103. Jin, W. J.; Liu, C. S. Fenxi Huaxue, 1993, 21(5), 509–513. Ramos, G. R.; Khasawneh, I. M.; Garcia-Alvarez-Coque, M. I.; Winefordner, L. J. Talanta, 1988, 35, 41. 15. Jin, W. J.; Wei, Y. S.; Duan, W. S.; Liu, C. S.; Zhang, S. S. Anal. Chim. Acta, 1994, 287, 95–100.
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MJ961471
Study of Six Polycyclic Aromatic Hydrocarbons by Chemical Deoxygenation Microemulsion-Stabilized Room Temperature Phosphorimetry Xin Yang, Chuan Dong, Jun Zhang, Yan-sheng Wei, Wei-Jun Jin, and Chang-song Liu1 Department of Chemistry, Shanxi University, Taiyuan, Shanxi 030006, People’s Republic of China Received August 10, 1996; accepted December 17, 1996 Six polycyclic aromatic hydrocarbons (PAHs) are studied by chemical deoxygenation microemulsion-stabilized room temperature phosphorimetry with sodium sulfite as an oxygen scavenger and thallous nitrate as a heavy atom perturber in sodium dodecyl sulfate medium. Several factors influencing room temperature phosphorescence such as the concentration of sodium dodecyl sulfate, the heavy atom concentration, the pH, and the concentration of sodium sulfite are discussed and the quenching effect of NO02 on room temperature phosphorimetry of PAHs was compared in the microemulsion and micelle media. q 1997 Academic Press
INTRODUCTION
Room temperature phosphorimetry has useful applications for the determination of polycyclic aromatic hydrocarbons (PAHs) (1–5). Jin et al. studied and compared the analytical features of PAHs by solid-substrate room temperature phosphorimetry (SSRTP) with cellulose membrane and filter paper as substrate (6). Dong et al. simultaneously determined p-triphenyl and m-triphenyl by synchronous SS-RTP technique (7). Cyclodextrin-induced room temperature phosphorescence (CD-RTP) and micelle-stabilized room temperature phosphorescence (MS-RTP) also have many interesting applications for the determination of PAHs with better selectivity (8, 9). Diaz Garcia and Sanz-Medel (10) and Wei et al. (11) improved MS-RTP by using sodium sulfite as an oxygen scavenger, which is more convenient than nitrogen purging. Jin and Liu also determined PAHs by MS-RTP or synchronous MS-RTP with sodium sulfite as an oxygen scavenger (12, 13). Ramos et al. compared the analytical features of PAHs on SS-RTP and RTP in organized media and proved that microemulsion room temperature phosphorimetry (ME-RTP) is an ideal determination method for PAHs (14). Jin et al. further reported the determination of naphthalene and phenanthrene by ME-RTP (15). In this paper, ME-RTP of six PAHs is studied with sodium sulfite as oxygen scavenger. The factors that affect the analytical features are examined in detail. In addition, the kinetic behavior of the NO20 quenching effect is compared in microemulsion and micelle systems. 1
To whom correspondence should be addressed. 294
0026-265X/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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EXPERIMENTAL
Apparatus All uncorrected luminescence spectra were recorded and measurements of RTP intensities were carried out with a LS-50B luminescence spectrofluorometer (PerkinElmer Co.) equipped with a thermostat cell holder. A xenon arc lamp was used as the excitation light source with pulse width less than 10 ms. The delay time was set at 0.1 ms, which is an important factor for measuring RTP free from the interference of residual fluorescence or scatter light. The gate time was set at 3 ms (fluoranthene), 6 ms (pyrene), 5 ms (chrysene), and 1 ms (fluorene, acenaphthene, p-triphenyl), respectively. Reagents Fluoranthene, pyrene, chrysene, fluorene, acenaphthene, and p-triphenyl were all of HPLC grade from Fluka. Stock solutions of six PAHs were prepared by dissolving a fixed amount of PAH in acetone. The stock solution was transferred into a 25-ml volumetric flask, the acetone was evaporated thoroughly on an electric heat plate, and then 0.2 mol/liter sodium dodecyl sulfate (SDS) solution was added up to the final volume. The concentration of the standard solution for all six PAHs was 1 1 1004 mol/liter in 0.2 mol/ liter SDS solution. SDS, n-heptane, 1-pentanol, thallium (I) nitrate, sodium sulfite, and sulfuric acid were of the analytical reagent grade. Sodium sulfite aqueous solution should be prepared fresh daily. Water was doubly distilled in a subboiling distiller. Procedures The appropriate PAH standard solution was transferred to a 10-ml volumetric flask and a certain amount of 0.2 mol/liter SDS solution was added to maintain the concentration of SDS solution at 0.04 mol/liter. Then n-heptane, 1-pentanol, thallium (I) nitrate, sulfuric acid, and sodium sulfite were added in turn. The mixed solution was diluted with water to 10-ml final volume. If precipitates appear in the flask after adding TINO3 , the flask should be heated gently before adding other reagents until the precipitates disappear. After mixing thoroughly, the flask was placed in a thermostat cell at 25 { 17C for 10 min. A 1-cm fluorescence cell was used for measuring RTP intensity or recording the luminescent spectra. RESULTS AND DISCUSSION
Luminescence Spectra The excitation and emission spectra of six PAHs are shown in Fig. 1 and their spectroscopic characteristics are listed in Table 1. Table 1 shows that the fluorescence Stokes shifts of fluorene and acenaphthene are very small (about 3–4 nm) and their maximum excitation and emission peaks nearly overlap, which results in serious self-quenching. This disadvantage can be avoided in the RTP measurement because of the great Stokes shifts.
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FIG. 1. Excitation and emission spectra of the six PAHs.
Concentration of SDS As in the micelle system, it is essential to provide an appropriate rigid microenvironment to ensure RTP emission in the ME system. The micelle in aqueous solution is ˚ . Once large molecules are a spherical substance with a diameter of 30 to 60 A solubilized in it, the micelle volume is enlarged obviously. The diameter of microemul˚ , which is larger than that of the micelle, so the sion falls in the range 50 to 1000 A oil/water model microemulsion provides a quite hydrophobic inner microenvironment in which many hydrophobic molecules are trapped in each collector. The concentration of surfactant should be great enough to attain optimum emulsion effect. At lower concentrations of surfactant, the oil/water interface adsorbs fewer molecules; its
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ME-RTP OF SIX PAHs TABLE 1 Spectroscopic Characteristics of Six PAHs in the ME System PAH
l exa (nm)
Fluoranthene Pyrene Chrysene Fluorene Acenaphthene p-Triphenyl
360 337 322 302 306 305
lFb (nm)
450 375, 366, 306, 309, 343
385, 394 388, 408 325 341
lPc (nm)
543, 594, 511, 430, 486, 501
591 648 547 457 518
Note. All concentrations for the determination are 5 1 1006 mol/liter. a The same excitation wavelength of fluorescence and phosphorescence. b Emission wavelength of fluorescence. c Emission wavelength of phosphorescence, in boldface, corresponding to maximum RTP peak.
strength decreases, forming an unstable microemulsion system. On the other hand, when the concentration of surfactant increases to a certain degree, more cosurfactant molecules are adsorbed on the interface, the interfacial strength is strengthened, and the stability of the microemulsion system is significantly improved. Once a polar organic solvent that contains groups such as –OH (e.g., n-pentanol), –NH2 , or –COOH is added to surfactants, the interaction of surfactants and polar organic molecules causes the molecules to be arranged more closely on the interface, and interfacial activity and strength are enhanced. As shown in Fig. 2, the phosphorescence intensity of the six PAHs change with the concentration of SDS. As the concentration of SDS increased from 0.02 to 0.05 mol/liter, generally, the RTP intensity showed little change and lifetime (t) tended to increase. When the concentration of SDS was greater than 0.05 mol/liter, the intensity of RTP dropped sharply. Selection of Apolar Solvent and Cosurfactant The microemulsion system consists of apolar solvent, surfactant, cosurfactant (usually a C4–C6 alkyl alcohol), and water. The solubility of the surfactant in apolar solvent is closely related to its aggregation. But the mixing emulsion ability of cosurfactant and surfactant increases the rigidity of the microenvironment in the ME system. Five alkanes—n-pentane, n-hexane, n-heptane, n-octane, n-nonane—and six alcohols—propanol, butyl alcohol, n-pentanol, heptanol, n-octyl alcohol, n-decyl alcohol—were explored. The results show that when five alkanes are used as apolar solvent, the RTP intensity and lifetime of PAHs change inconspicuously. When propanol, butyl alcohol, and n-pentanol are used as cosurfactants, the RTP intensity and lifetime do not show a substantial difference. But when heptanol, octyl alcohol, and decanol are used, precipitates appear in the system and the RTP is quenched for increase in their viscosity with increase in molecular chain length. In these systems RTP signals were hardly detected. Based on the experimental results, heptane and pentanol were selected as apolar solvent and cosurfactant, respectively, in the microemulsion system. The optimum concentrations of heptane and pentanol are listed in
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FIG. 2. Influence of SDS concentration on RTP (a) and lifetime (b) of the six PAHs.
Table 2. The stability of the ME system is based on the amounts of heptane and pentanol. As shown in Table 2, the RTP intensities of PAHs are normally stable and tend to increase slightly in the concentration ranges 0.02–0.1% for heptane and 0.01–
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ME-RTP OF SIX PAHs TABLE 2 Optimum Heptane and Pentanol Concentrations of Six PAHs PAH
Fluoranthene
Pyrene
Chrysene
Fluorene
Acenaphthene
p-Triphenyl
Heptane (%) Pentanol (%)
0.10 0.10
0.02 0.03
0.08 0.10
0.04 0.03
0.02 0.03
0.02 0.03
0.1% for pentanol. No phase separation was observed in the ME system. The phosphorescence lifetime (t) exhibited the same trend. But when the amount of heptane was greater than 0.5% (0.02% pentanol) and that of pentanol was greater than 1% (0.1% heptane), two phases appeared and RTP intensity became weak. Heavy Atom Concentration Heavy atoms with high nuclear charge may cause the spin-orbit coupling action of the solute molecules, which mainly enhances S1 r T1 intersystem crossing probability, and induce the appearance of phosphorescence. Thallous nitrate is an efficient heavy atom perturber for polycyclic aromatic compounds in a chemical deoxygenation micelle system. The experiment suggests that the optimum concentration of thallous nitrate is 0.015 mol/liter for fluoranthene, chrysene, acenaphthene, and p-triphenyl, 0.02 mol/liter for fluorene, and 0.025 mol/liter for pyrene. Generally, RTP lifetime decreased with increase in concentration of thallous nitrate (refer to Fig. 3). Effects of pH In the micelle system, the chemical deoxygenation efficiency of sodium sulfite is greatly influenced by the pH of the bulk solution (10), as proved by the study on five PAHs of micelle-stabilized room temperature phosphorescence (7). Similarly, in the microemulsion system, the pH of the bulk solution heavily influences the intensity and lifetime of RTP. In either strong acidic or alkaline solution, the RTP signals of PAHs are too weak and the phosphorescence lifetime is too short to be detected easily; therefore, it is necessary to select a suitable pH. The experiments showed that when the pH ranged from 6.50 to 7.95, the deoxygenation speed increased, the RTP intensity reached its highest point, and the RTP lifetime increased steadily, responses to a lower oxygen environment. Sulfuric acid 0.02 mol/liter was selected to adjust the pH of the bulk solution (Fig. 4). Concentration of Sodium Sulfite Since the triplet state is quenched quickly by collision with solvent and oxygen molecules, it is essential to remove oxygen thoroughly from the micelle solution. For this purpose, the technique of chemical deoxygenation with sodium sulfite was adopted (10). However, only when enough sodium sulfite exists in a neutral or weak alkaline solution, could oxygen be eliminated thoroughly and could stable RTP be observed. The concentration of sodium sulfite should be maintained in the range 5 1 1003 to 1 1 1003 mol/liter as shown in Fig. 5.
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FIG. 3. Influence of heavy atom concentration on RTP (a) and lifetime (b) of the six PAHs.
RTP Stability The experiment showed that the RTP intensity of PAHs decreases only about 10% in an hour which is stable enough to complete a series of operations. But the RTP
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FIG. 4. Influence of pH on RTP (a) and lifetime (b) of the six PAHs.
signals of pyrene decrease only 1%, which shows that Na2SO3 in the ME–RTP system has greater deoxygenation efficiency. The microemulsion system could provide a stronger hydrophobic microenvironment for pyrene than the micelle system.
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FIG. 5. Influence of concentration of sodium sulfite on RTP (a) and lifetime (b) of the six PAHs.
Under optimum conditions, the RTP lifetimes of six PAHs in the ME system are 2.65 ms (fluoranthene), 4.39 ms (pyrene), 4.05 ms (chrysene), 0.71 ms (fluorene), 1.08 ms (acenaphthene), and 3.23 ms (p-triphenyl).
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ME-RTP OF SIX PAHs TABLE 3 ME–RTP Analytical Features of Six PAHs LOD (mol/liter) PAHs Fluoranthene Pyrene Chryene Fluorene Acenaphthene p-Triphenyl
LDR (mol/liter) 2 1 1 3 3 3
1 1 1 1 1 1
1007 –5 1007 –1 1007 –3 1007 –1 1007 –1 1007 –1
1 1 1 1 1 1
1005 1005 1005 1004 1004 1005
Slope (log–log)
Relative coefficient
0.785 0.781 0.857 0.824 0.877 0.747
0.998 0.996 0.997 0.997 0.999 0.997
3sn01a 2.0 4.3 5.4 2.5 4.4 3.5
1 1 1 1 1 1
1009 10010 1009 1009 1009 1009
S/N Å 3b 1.4 4.4 4.7 8.6 2.0 2.5
1 1 1 1 1 1
1009 1009 1008 1008 1007 1007
RSDc (%) 4.4 2.7 1.5 3.3 2.5 4.2
a
Calculated on the basis of three times the SD criterion. Measured with a signal-to-noise ratio of 3. c Seven measurements. b
Analytical Features The analytical features are listed in Table 3. Comparison of the Quenching Kinetic Behavior of Fluoranthene in the ME and MS Systems Sodium nitrite is a strong RTP quencher and its quenching effect on the RTP of fluoranthene was compared in the two systems. The experiments demonstrated that the quenching effect in the range 1 1 1005 to 1 1 1004 mol/liter NO20 follows the dynamic quenched Stern–Volmer equation. The quench rate constants in these two systems are 1.22 1 107 liters/molrs (micelle system), 6.84 1 106 liters/molrs (microemulsion system), respectively. The quenching rate constant in the micelle system is 60% greater than that in the microemulsion system; i.e., the quenching effect of NaNO2 on phosphorescence in the microemulsion system is less than that in the micelle system, which shows that the microemulsion system provides stronger protection for the phosphor than does the micelle system. CONCLUSION
The experimental results demonstrate that the methods established for determination of the six PAHs have wider LDRs, nearly reaching three amount levels; lower LODs, reaching 1009 or even 10010 mol/liter; and higher-precision RSDs (less than 5%). The method of extracting organic solvents of PAH samples can be successfully applied to prepare microemulsions. The ME system has a more rigid microenvironment than the MS system for the phosphor. ACKNOWLEDGMENTS This work is supported by the NNSF of China and the Youth Science Foundation of Shanxi Province.
REFERENCES 1. Vo-Dinh, T. Room Temperature Phosphorimetry for Chemical Analysis. Wiley, New York, 1984. 2. Hurtubise, R. J. Phosphorimetry: Theory, Instrumentation, and Applications. VCH, New York, 1990.
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