Laser-induced fluorescence spectroscopy for ... - Optics InfoBase

Laser-induced fluorescence spectroscopy for ... - Optics InfoBase

Fig. 5. Dependence of the peak intensity of the PH component asa function of the partial PH concentration in a pure PH solution, aPH–RE mixture, and a PH–PG mixture. Curve A dotted curvefitted by Eq. 4 with k Q 0 where n Q n 0 within an error of 5.6%is also shown. The absorption cross section 20 for the pure PHsolution is evaluated from the fitting parameter for 20 L to be 1.0 10 16 cm 2 , provided that L 10 cm.coefficient for collisional quenching of levels 1 and 2and the density of the molecules that contribute tothe quenching. We assume that the derivativeterms with regard to time in Eqs. 1 and 2 arenegligible compared with terms on the right-handsides. Hence the LIF intensity I LIF is given byI LIF A 1 n 1 B 20 0 exp 20 Ln 0 n 0k Q n Q A 1 k Q n Q 1 A 2 , (4)Fig. 6. Temporal variations of the LIF intensities of the componentsof PH and others HB during degradation as a function ofthe time after the discharge is started.where is the effective quantum efficiency of thetotal LIF diagnostic system. To examine the validityof Eq. 4 we measured the LIF intensities of PHcomponents of LIF spectra from pure PH solutionsand in solution mixtures of DHB and THB and comparedthe results with values expected from Eq. 4.It is generally accepted that PH is converted mainlyinto hydroxybenzenes HB’s by oxidation. In addition,HB’s are the most probable contributors to thequenching of the laser-excited states because of thesimilarities in their molecular structures. The totalamount of PH plus HB during the degradation processis less than or at most equal to the initial PHconcentration. Hence for the solution mixture wekeep the total concentration constant and change thepartial concentrations of PH and HB. Figure 5shows the dependence of the peak intensity of the PHcomponent as a function of partial PH concentrationin a pure PH solution, in a PH–RE mixture, and in aPH–PG mixture. For the pure PH solution, theplots fit Eq. 4 well, with k Q 0 where n Q n 0 within an error of 5.6%. The absorption cross section 20 for the pure PH solution is evaluated fromthe fitting parameter for 20 L to be 1.0 10 16 cm 2 ,provided that L 10 cm. The fitted curve curve Ais shown as a dotted curve. One can conclude thatquenching of the laser’s excited state is negligible inthe pure PH solution. The peak intensity is proportionalto the PH concentration below 0.4 10 4molL. The effect of the laser absorption is observedfor concentrations higher than 0.4 10 4 molL.Curve A shows the relationship between the PH concentrationand the peak intensity in pure PH solutions.The quenching is more significant for bothPH–RE and PH–PG mixtures when the partial concentrationof HB is increased. The quenching efficienciesof RE and PG are approximately the same.This similarity produces ambiguity in the determinationof PH concentrations from the peak intensities.For example, when a peak intensity of 0.2 arbitraryunits for Fig. 5 is obtained, the expected PH concentrationis 1.2 10 5 molL at minimum and 2.2 10 5 molL at maximum. Hereafter we shall referto the peak intensity of the LIF spectrum as the LIFintensity.B. Phenol DegradationLIF spectroscopy is used for a PH solution that hasbeen exposed to pulsed corona discharges. Figure 6shows the temporal variation of the LIF intensity ofthe PH component during degradation as a functionof the time after the discharge has begun. The peakintensities of decomposed spectra other than PH arealso plotted HB. The decomposed spectra are comparedwith those of pure solutions Fig. 2. The spectraat 10–120 min are approximately identical to thepure RE spectrum. The decomposed spectrum DSat 100 min is plotted in Fig. 7 together with the pureRE spectrum, as an example. The pure RE spectrumis located in the wavelength regions close topure CA and HQ spectra, whereas those of THB’s arenot. The decomposed spectra therefore correspondto those of DHB. The temporal variation of the peakintensity of the DS plausibly reflects the change inthe concentration of DHB. After 120 min, the observedspectra have a weak component near 400–500nm. Figure 8 shows the observed spectrum at 150min together with the pure PH spectrum. The weakcomponent is not identical to any of the pure HBspectra shown in Fig. 2.Finally, the LIF intensities of PH components inFig. 6 are converted into absolute concentrations by acalibration with curve A. Figure 9 shows the temporalvariation of absolute PH concentration duringdegradation. As was described in Subsection 3.A,the intermediate products DHB, THB, etc. that existin the degraded solution might cause the de-990 APPLIED OPTICS Vol. 40, No. 6 20 February 2001

Fig. 7. DS of the degraded solution at 100 min, together with apure RE spectrum. The RE spectrum is offset above the horizontalaxis.creases in the LIF intensities that correspond to PHcomponents by means of collisional quenching.Filled circles in Fig. 9 represent the PH concentrationsevaluated with curve A. These are the valuesexpected for a limited case when no quenching occurs.If the quenching results in a decrease in LIF intensity,the PH concentration evaluated with curve A istherefore less than the actual concentration in thedegraded solution. The quenching rate of the LIFintensity depends on the concentration of intermediateproducts. Because the reactor is a closed system,the total concentration of intermediate productsFig. 8. Spectrum observed from the degraded solution at 150 minOS, together with the pure PH spectrum.Fig. 9. Temporal variation of absolute PH concentration duringdegradation. The ambiguity that originates from collisionalquenching with the intermediate products is represented by errorbars that correspond to the differences among the maximum concentrationsexpected from Fig. at most 1.0 1.0 4 molL the initial PH concentrationminus the PH concentration at the time ofobservation. This is the situation equivalent to thatfor Fig. 5. The LIF intensity for a given PH concentrationis at least the intensity that corresponds tothat in the PH–RE or PH–PG mixture in Fig. 5.We can summarize that the LIF intensity for a PHconcentration in a degraded solution is expected tobe, at maximum, the intensity given by curve A and,at minimum, the intensity for the PH–RE or PH–PG mixture in Fig. 5. In return, the PH concentrationfor an observed LIF intensity is expected to be, atmaximum, the concentration derived from thePH–RE or PH–PG data in Fig. 5. The error bars inFig. 9 show the possible deviations in PH concentrationcaused by the quenching effect mentioned above.In the early stage of the degradation 0–50 min, theinaccuracy in determining the concentration is approximately10%, whereas it is 50–100% in the laterstage 100–150 min.C. Laser-Induced Fluorescence versus High-PerformanceLiquid ChromatographyTo examine the advantages and disadvantages of LIFand conventional HPLC, we compared the concentrationsof PH and DHB measured by LIF with thosemeasured by HPLC. Figure 10 shows the temporalvariations of the PH and RE concentrations duringthe process of corona-induced degradation as measuredby LIF and HPLC. 16 The RE concentrationmeasured by LIF is determined, provided that theLIF intensity of HB shown in Fig. 6 is responsible forthe RE. The quenching effect on the LIF intensity ofHB does not need to be taken into account, becausethe quenching of the LIF intensity by existing intermediateproducts is not carefully examined in thiscase. For HPLC measurement, the initial concentrationof the PH solution is 3.0 10 4 molL, avalue that is three times higher than that for LIF. Itis not possible to compare the decay structure of thePH concentration by LIF directly with that by HPLC,because of the difference in the initial concentrations.We discuss here the time and space resolutions andthe accuracy of each diagnostic technique based onFig. 10.It is obvious from Fig. 10 that the time resolution ofLIF is much better than that of HPLC. In principle,the maximum resolution of the LIF technique is determinedby the time required for measuring the fluorescenceif the fluorescence is combined with aconventional pulsed laser for the excitation. Herewe estimate that, from the lifetime of PH relative tospontaneous emission, the time resolution is 10–100ns. 17 The time resolution of HPLC, however, dependsheavily on the chromatography column, theeluent, and the carrier flow. To the authors’ knowledge,it requires at least approximately 10 min toseparate PH and DHB with a liquid chromatographHewlett-Packard 1100 series plus UV absorption detectorwith an ion-exchange column Merck PolyspherOA-HY and a 1-mmtrifluoroacetic acidsolution Merck as an eluent. HPLC can be com-20 February 2001 Vol. 40, No. 6 APPLIED OPTICS 991

umn and the eluent. If a diagnosed molecule is notwell separated from others in the solution, the measurementsof the concentration by a standard methodof HPLC, such as UV absorption, also suffers from thesame problems as does LIF. Hence LIF is inferior toHPLC for determining absolute concentrations.Fig. 10. Temporal variations of the PH and RE concentrationsduring corona-induced degradation measured by LIF and HPLC. 16The RE concentration measured by LIF is determined, providedthat the LIF intensity of HB shown in Fig. 6 is responsible for RE.The quenching effect on the LIF intensity of HB is not taken intoaccount because the quenching of the LIF intensity by existingintermediate products is not examined precisely in this case. ForHPLC measurement, the initial concentration of the PH solution is3.0 10 4 molL, whose value is three times higher than that forLIF.bined with a continuously circulated flow systemfrom a corona reactor to produce a quasi in situ diagnostic.However, there is an additional delay intime between the corona treatment and HPLC analysis.With respect to the space resolution, the spatialresolution of the LIF diagnostic can be as small asthe laser diameter, whereas the HPLC dose is notrelated to the space resolution at all. It is possiblefor a LIF diagnostic to measure in situ twodimensionalimages of the concentrations in acombination of a sheet laser beam with a twodimensionalarray of an ICCD. Hence, with regardto determining time and space resolutions, LIF ismuch superior to HPLC.With respect to accuracy in determining absoluteconcentrations, LIF has serious problems that can beattributed to complications 1–3 mentioned in Section1. The error bars on data points for LIF areobviously longer than those for HPLC, as is shown inFig. 10. The accuracy of the PH concentration isapproximately 90% for LIF in the early phase of thedegradation and approximately 95% for HPLC. Theaccuracy of LIF deteriorates with increasing concentrationof the intermediate products. The accuracyfor HPLC, however, is not influenced by the existenceof intermediate products because of its off-line nature.We mention here that the accuracy of HPLCdepends strongly on the choice of the separation col-4. Concluding RemarksLaser-induced fluorescence spectroscopy has been introducedas an in situ diagnostic for aqueous phenoland intermediate products in corona-degraded solutions.LIF spectroscopy in degraded solutions encountersthree complications in a quantitativeinterpretation of the results: 1 quenching of thelaser-excited states, 2 strong absorption of the laserbeam, and 3 superposition of spectra from the intermediateproducts onto the phenol spectrum.These effects were examined experimentally, and expectederrors were estimated. The temporal variationsof the LIF intensities of PH and theintermediate products were measured as a functionof time after the discharges started. The absolutePH concentration was determined with inaccuraciesof 10% in the early stage of the degradation and of50–100% in the later stage. In the present paper wehave confirmed the applicability of LIF spectroscopyfor monitoring PH concentration during degradation.LIF is superior to high-performance liquid chromatographyfor investigating the dynamic reaction insolutions by in situ, time- and space-resolved diagnosticsand is inferior to HPLC for determining theabsolute concentration of each species as well as themolecular structure of the species. The LIF techniquepresented here can be applied for time- andspace-resolved measurements of the concentrationsof molecules in any solution mixture. However, toexamine the quenching effects with the moleculesthat exist in the solutions, it is necessary to determinetheir molecular properties a priori by use of anoff-line analytical chemical diagnostic such as HPLC.In combination with LIF and HPLC, the dynamicreaction processes in solutions can be investigatedwith sufficiently high time and space resolution andhigh accuracy to define the reaction chemistry.The authors acknowledge technical support fromLoek Baede and Hans Freriks.References1. P. Sunka, V. Babicky, M. Clupek, P. Lukes, M. Simek, J.Schmidt, and M. Cernak, “Generation of chemically active speciesby electrical discharges in water,” Plasma Sources Sci.Technol. 8, 258–265 1999.2. A. K. Sharma, B. R. Locke, P. Arce, and W. C. Finney, “Apreliminary study of pulsed streamer corona discharges for thedegradation of phenol in aqueous solutions,” Hazard. WasteHazard. Mater. 10, 209–220 1993.3. A. A. Joshi, B. R. Locke, P. Arce, and W. C. Finney, “Formationof hydoxyl radicals, hydrogen peroxide and aqueous electronsby pulsed streamer corona discharge in aqueous solution,” J.Hazard. Mater. 41, 3–30 1995.4. J. S. Clements, M. Sato, and R. H. Davis, “Preliminary investigationof prebreakdown phenomena and chemical reactions992 APPLIED OPTICS Vol. 40, No. 6 20 February 2001

using a pulsed high-voltage discharge in water,” IEEE Trans.Ind. Appl. IA-23, 224–235 1987.5. M. Sato, T. Ohgiyama, and J. S. Clements, “Formation ofchemical species and their effects on microorganisms using apulsed high voltage discharge in water,” IEEE Trans. Ind.Appl. 32, 106–112 1996.6. B. Sun, M. Sato, and J. S. Clements, “Optical study of activespecies produced by a pulsed streamer corona discharge inwater,” J. Electrost. 39, 189–202 1997.7. B. Sun, M. Sato, and J. S. Clements, “Use of a pulsed highvoltagedischarge for removal of organic compounds in aqueoussolution,” J. Phys. D 32, 1908–1915 1999.8. W. F. L. M. Hoeben, E. M. van Veldhuizen, W. R. Rutgers, andG. M. W. Kroesen, “Gas phase corona discharges for oxidationof phenol in an aqueous solution,” J. Phys. D 32, L133–1771999.9. K. Muraoka and M. Maeda, Laser-Aided Diagnostics for Plasmasand Gases Sangyo-tosho, Tokyo, 1995.10. D. Hayashi, W. F. L. M. Hoeben, G. Dooms, E. M. v. Veldhuizen,W. R. Rutgers, and G. M. W. Kroesen, “LIF diagnostic forpulsed-corona-induced degradation of phenol in aqueous solution,”J. Phys. D 33, 1484–1486 2000.11. A. Oikawa, H. Abe, N. Mikami, and M. Ito, “Solvated phenolstudied by supersonic jet spectroscopy,” J. Phys. Chem. 87,5083–5086 1983.12. H. Abe, N. Mikami, and M. Ito, “Fluorescence excitation spectraof hydrogen-bonded phenols in a supersonic free jet,” J.Phys. Chem. 86, 1768–1772 1982.13. G. Berden, W. L. Meerts, M. Schmitt, and K. Kleinermanns,“High resolution UV spectroscopy of phenol and the hydrogenbonded phenol–water cluster,” J. Chem. Phys. 104, 972–9821996.14. T. Burgi and S. Leutwyler, “O—H torsional vibration in the S 0and S 1 states of catechol,” J. Chem. Phys. 101, 8418–84291994.15. S. J. Humphrey and D. W. Pratt, “High resolution S 1 4 S 0fluorescence excitation spectra of hydroquinone. Distinguishingthe cis and trans rotamers by their nulear spin statisticalweights,” J. Chem. Phys. 99, 5078–5086 1993.16. W. F. L. M. Hoeben, Pulsed Corona-Induced Degradation ofOrganic Materials in Water Eindhoven University of Technology,Eindhoven, The Netherlands, 2000.17. A. Gilbert and J. Baggott, Essentials of Molecular PhotochemistryBlackwell, Oxford, 1991.20 February 2001 Vol. 40, No. 6 APPLIED OPTICS 993

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