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discussion and use of equipment. Reaistm No. HCHO, 50-00-0; OR,10028-15-6; isoprene, 78-79-5.
LITERATURE CITED Monson, R. K.; Fall, R. Plant Physiol. 1989, 90,267-274. Greenberg, J. P.; Zimmerman, P. R. J . Geophys. Res. 1984. 89, 4767-4778. Zimmerman, P. R.; Greenberg, J. P.; Westberg. C. E. J . Geophys. Res. 1988, 93, 1407-1416. Rasmussen, R. A.; Khalii, M. A. K. J . Geophys. Res. 1988, 93, 1417-1421. Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; John Wiley and Sons, Inc.: New York, 1986; p 432. Jacob, D.J.; Wofsy, S. C. J . Geophys. Res. 1988, 93, 1477-1486. Holdren, M. W.; Westberg, H. H.; Zimmerman, P. R . J . Geophys. Res. 1979, 8 4 , 5083-5088. Seila, R. L. Int. Conf. Photochem. Oxid. Poll. Control Proc., 1, PB-264, 232, EPA-600/3-77-001a; US. Environmental Protection Agency, Office of Research and Development: Cincinnati, OH, 1976; pp 41-50. Evans, R . C.; Tingey, D.T.; Gumpertz, M. L.; Burns, W. F. Bot. Gaz. 1982, 743, 304-310. Rasmussen, R. A. Environ. Sci. Technol. 1970, 4 , 667-671. Zimmerman, P. R . Final Report, EPA-450/4-79-004. 1979. Ridley, B. A,; Howlett, L. C. Rev. Sci. Instrum. 1974, 45, 742-746. Ridley, 6. A. A t m s . Tech. 1978. 9 ,27-34. Kiey, D.;McFarland, M. Atmos. Tech. 1980, 72, 63-69. Nelson, J. K.; Getty, R. H.; Birks, J. W. Anal. Chem. 1983, 5 5 , 1767-1770. Glinski, R. J. J . Phys. Chem. 1989, 93, 1913-1915. Getty, R. H.; Birks, J. W. Anal. Lett. 1979, 72, 469-476. Spurlin, S.R.; Yeung, E. S.Anal. Chem. 1982, 5 4 , 320-321. Yamada, M.; Ishiwada, A,; Hobo, T.; Suzuki, S.;Araki, S.J . Chromatogr. 1982, 238,347-356. Warren, G.J.; Babcock, G. Rev. Sci. Instrum. 1970, 4 7 , 280-281. Rao, A. M. M.; Netravalkar, A. J.; Arora, P. K.; Vohra, K. G. Atmos. Environ. 1983, 77, 1093-1097. Bruening, W.; Concha. F. J. M. J . Chromatogr. 1975. 772, 253-265. Schurath, U.; Wiese, A,; Becker, K. H. Staub-Reinhalt. Lutt 1976, 9 , 379-385. Becker, K. H.; Schurath. U.; Wiese, A. US. Environ. Protec. Agen. Off. Res. Dev., [Rep.] E.P.A., 1976, EPA-600/3-77-001a, Int. Conf. Photochem. Oxid. Poll. Control Proc., 1, PB-264. 232, 31-40.
(25) Finlayson-Pitts, B. J.; Pitts, J. N.. Jr. Atmos. Chem. 443-459. (26) Finlayson, B. J.; Pitts, J. N., Jr.; Atkinson. R. J . Am. Chem. Soc. 1974, 96,5356-5367. (27) Finlayson, B. J.; Pitts, J. N., Jr.; Akimoto, H. Chem. Phys. Lett. 1972, 72. 495-498. (28) Kummer. W. A.; Pitts, J. N., Jr.; Steer, R. P. Environ. Sci. Technol. 1971. 5 . 1045-1047. ~. (29) Hansen, D. A.; Atkinson, R.; Pitts. J. N., Jr.; Steer, R. P. J . Photochem. 1977, 7 , 379-404. (30) Kamens, R. M.; Gery. M. W.; Jeffrles, H. E.; Jackson, M.; Cole, E. I . Int. J . Chem. Kinet. 1982, 74, 955-975. (31) Hills, A. J.; Cicerone, R. J.; Calvert, J. G.; Birks, J. W. J . Phys. Chem. 1987, 9 7 , 1199-1204. (32) Hills, A. J.; Cicerone, R. J.; Calvert. J. G.: Birks, J. W. J . Phys. Chem. 1988, 92, 1853-1858. (33) Stimpfie, R. M.; Perry, R. A.; Howard, C. J. J . Chem. Phys. 1979, 7 7 , 5183. (34) Monson. R. K.: Moore. B. D.;Ku. M. S.B.; Edwards, G. E. Pknta 1988. 768,493-502. (35) Benner, R. L.; Lamb, E. J . Atmos. Ocean. Tech. 1985, 2 , 582-589. (36) Toby, S. Chem. Rev. 1975, 8 4 , 277. (37) Atkinson, R.; Winer. A. M.; Pitts, J. N., Jr. Atmos. fnviron. 1982, 76, 1017-1020. (38) FiniaysonIPitts, B. J.; Pitts, J. N., Jr. Atmos. Chem. 443. (39) Weast, R. C.: Astle. M. J. Handbook of Chemistry and Physics. 57th ed.; CRC Press: Cleveland, OH, 1976; p C-216. (40) Akimoto. H.; Finlayson, B. J.; Pitts, J. N., Jr. Chem. Phys. Lett. 1971, 72,199-202. (41) Kummer, W. A,; Pitts, J. N., Jr.; Steer, R. P. Environ. Sci. Technol. 1971, 5 , 1045-1047. (42) Andreae. M. 0.; Andreae, T. W. J . Geophys. Res. 1988, 93, 1487-1497. (43) Jacob, D. J.; Wofsy, S. C . J . Geophys. Res. 1988, 93, 1477-1486. (44) Kelly, T. J.; Gaffney, J. S.;Phillips. M. F.: Tanner, R . L. Anal. Chem. 1983. 55, 138-140.
RECEIVED for review December 5, 1989. Accepted February 8,1990. This work was supported by the Atmospheric Division and Advanced Study Program of the National Center for Atmospheric Research. A.J.H. thanks the Advanced Study Program of the National Center for Atmospheric Research for providing a Postdoctoral Research Fellowship.
Comparative Study of Sotid-Matrix Luminescence Interactions of p-Aminobenzoate on Two Different Matrices S. M. Ramasamy and R. J. Hurtubise*
Chemistry Department, University of Wyoming, Laramie, Wyoming 82071
The lumbwcence propertles of the anion of p-aminobenzoic acid adsorbed on sodium acetate and filter paper were compared to galn Insights Into the lnteractlons that result In the fluorescence and phosphorescence of p -amlnobenzoate. Fluorescence quantum yields, phosphorescence quantum ylelds, and phosphorescence lifetimes were obtained for the anlon of p-aminobenzoic acid adsorbed on filter paper. Preexponentlal and actlvatlon energy terms were calculated for p-amhwbenzoate on filter paper and compared with similar terms for p-amlnobenzoate adsorbed on sodium acetate. I t was shown that there was a simple relationship between the reciprocal of the phosphorescence lifetlmes and the thermal processes that cause deactlvatlon of the triplet state. I t was concluded that p-amlnobenzoate Is Incorporated into the crystal structure of sodlum acetate and the triplet energy of p-amlnobenzoate was lost prbnarlly via skeletal vibrations In NaOAc. For the anion of p-amlnobenzoic acid on filter paper, some of the triplet energy was lost through vlbratlonal modes In the filter paper, although other factors would also be Involved tn the loss of the triplet-state energy.
INTRODUCTION Room-temperature phosphorescence (RTP) and roomtemperature fluorescence (RTF) analysis from organic compounds adsorbed on solid matrices have been shown to be very effective approaches in organic trace analysis (1-3). New experimental conditions ( 4 ) and solid matrices (5) are still being reported for organic species adsorbed on solid materials. In addition, analytical luminescence figures of merit have been compared for several solid matrices and model compounds (6). Various models for the RTP of adsorbed compounds have been reviewed (1-3). The temperature effects on the solidmatrix luminescence properties of 4-phenylphenol adsorbed on filter paper have been reported (7). Also, the results from varying the temperature from 23 to -180 "C for benzo[flquinoline adsorbed on filter paper indicated that the modulus of filter paper was an important factor in enhancing the phosphorescence quantum yield of benzo[flquinoline (8). In addition, a variety of interactions have been revealed for several compounds adsorbed on 80% a-cyclodextrin-NaC1
0003-2700/90/0362-1060$02.50/0 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
mixtures (9-11). There has been no detailed study in which the interactions in two widely different matrices have been compared with the same adsorbed species. In this work, the interactions of the anion of p-aminobenzoic acid on filter paper and sodium acetate are considered.
EXPERIMENTAL SECTION Apparatus. All quantum yield values were determined with a Farrand MK-2 spectrofluorometer (Valhalla, NY). A 150-W xenon lamp (Hanovia, Inc., Newark, NJ) and a R928 photomultiplier tube (Hamamatsu Corp., Middlesex, NJ) were used in the spectrofluorometer. A Bascom-Turner recorder, Model 4120 (Newton, MA), was used to store and integrate the spectral areas in the quantum yield measurements. The instrumental setup and the linear regression analysis of phosphorescence lifetime decays were described earlier (12). A Cygnus 100 Fourier transform infrared spectrometer (Mattson Instruments, Inc., Madison, WI) which was purged with dry nitrogen and equipped with a triglycine sulfate detector was used to obtain infrared spectra. A "praying mantis" diffuse reflectance accessory (Harrick Scientific Corp., Ossining, NY) was used to obtain diffuse-reflectance infrared spectra. Reagents. Absolute ethanol was purified by distillation. The sodium salt of p-aminobenzoic acid (NaPABA) and sodium salicylate (Gold Label, Aldrich) and anhydrous sodium acetate and sodium chloride (Analyzed Reagent, Baker Chemical Co., Phillipsburg, PA) were used as received. Whatman filter paper No. 1 was developed in ethanol prior to use. The details of nitrogen purification, cell compartment, sample holder, and cryogenic system were described earlier (12).
Procedure. Sample Preparation. A solution of NaPABA was prepared by dissolving it in a saturated solution of sodium chloride in ethanol-water (80:20). Two microliters of the above solution containing 200 ng of NaPABA was spotted onto the top of a pack of five filter paper circles (diameter 13/32 in.), which were contained in a Delrin sample holder. The sample and its blank were dried at 80 " C for 30 min. Ethanol was used to spot NaPABA without NaCl onto the filter paper circles. Powdered samples of sodium acetate (blank) and sodium salicylate/sodium acetate (1:150) (standard) were prepared as reported previously (12). Measurement of Quantum Yield Values. The procedure used to determine the quantum yield values was the same as described earlier (7,12). A sodium salicylate-sodium acetate mixture was employed as a standard. The important aspects in the quantum yield determination are that the differences in the reflectance band areas of the blank and the sample at the excitation wavelength of the phosphor are related to the quanta of radiation absorbed. Also the difference in the emission areas of the sample and the blank are related to the quanta of radiation emitted. The standard and analyte and their blanks were excited with 280-nm radiation. The fluorescence quantum yield, phosphorescence quantum yield, and phosphorescence lifetime data reported earlier for NaPABA adsorbed on sodium acetate were used in this work (12). RESULTS AND DISCUSSION Fluorescence Quantum Yields. Figure 1 compares the fluorescence quantum yield (4f)values for the anion of PABA adsorbed on filter paper and sodium acetate as a function of temperature. As shown in Figure 1, the Gf values were essentially constant with temperature for the anion of PABA adsorbed on sodium acetate as was reported earlier (12). However, the values for the anion of PABA on filter paper were approximately constant from about 300 to 230 K, then the quantum yield at 193 K increased and the remaining two cbf values at 153 and 93 K were approximately the same. The +f
o'60
I
3' k.
1061
0.10
0.00
90
125
160
195
TEMPERATURE.
230
265
300
K
Figure 1. Fluorescence quantum yield values as a function of temperature for the anion of PABA adsorbed on filter paper (M) and sodium acetate (X). Data for sodium acetate were taken from ref 12.
Table I. Phosphorescence Quantum Yields and Lifetimes for the Anion of PABA Adsorbed on Filter Papera temperature, K
@P
r p , s-'
296 273 233 193 153 93 83
0.22 0.19 0.25 0.27 0.37 0.44
1.28 1.47 1.70 1.94 2.31 2.76 2.88
a The pooled standard deviation for the $p values was 0.029, and the pooled standard deviation for the r p values was 0.045 s.
pooled standard deviation for the 4f values for the anion of PABA on filter paper was 0.03. The graphs in Figure 1 can be considered to indicate the differences in the fibrous characteristics of filter paper and the crystalline nature of sodium acetate. It should also be noted that all of the values were higher with filter paper compared to the dfvalues obtained on sodium acetate. This is primarily due to the higher intersystem crossing quantum yield for the anion of PABA adsorbed on sodium acetate (12). At all temperatures, the $t values with sodium acetate were greater than the 4t values with filter paper. For example, at room temperature the & value was 0.66 for the anion on filter paper and at room temperature the C#I~ value was 0.81 for the anion on sodium acetate. The &values were obtained earlier for the anion of PABA on sodium acetate, and the $t values for the anion of filter paper were calculated in this work as described previously (12). Phosphorescence Quantum Yields and Lifetimes. Table I gives the phosphorescence quantum yields (+J and lifetimes (7p) for the anion of PABA adsorbed on filter paper as a function of temperature. Throughout the temperature range investigated, the dP values were smaller than those obtained for the anion of PABA adsorbed on sodium acetate (12). For example, the ratios of @p values for the anion of PABA on sodium acetate to the anion on filter paper at 296 and at 93 K were 1.8 and 1.4, respectively. The values reached essentially a constant value in the temperature region of 153-93 K for the anion of PABA adsorbed on sodium acetate (12). However, this did not occur for the anion adsorbed on filter paper. As indicated in Table I, the 4pvalues continued to increase at the lower temperatures. The phosphorescence lifetimes in Table I were very similar to the average phosphorescence lifetimes acquired earlier for the anion of PABA adsorbed on sodium acetate (12). However, the phosphorescence lifetime with sodium acetate reached a constant value at low temperatures. In the previous work with +f
+,,
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
Table 11. Parameters Related to the Triplet State for the Anion of PABA Adsorbed on Filter Papera temperature, K
Q~
k,, s-'
296 273 233 193 153 93
0.66 0.68 0.66 0.55 0.55 0.52
0.25 0.19 0.22 0.26 0.23 0.31
k,,
SKI
0.50 0.50 0.37 0.26 0.20 0.05
The 95% confidence limits for dt and k , were f0.07 and f0.04, respectively. The pooled standard deviation for k , was 0.03.
sodium acetate, both a short and long decaying component were obtained a t each temperature. Thus, the average lifetimes mentioned above refer to the average value of the short and long decaying components at a given temperature for the anion of PABA adsorbed on sodium acetate. In this work, the anion of PABA showed one decaying component on filter paper for the temperatures investigated. Parameters Related to the Triplet State. Table I1 gives the triplet formation efficiencies, phosphorescence rate constants (k,), and the rate constants for the radiationless transition from the triplet state (k,) for the anion of PABA adsorbed on filter paper. The 4t values were calculated from the equation C#It = 1 - 4p The use of this equation has been discussed earlier (12, 13). The main criterion used in calculating C#It is the approximate constancy of C#I,/T, with temperature (12, 13). In this work, from 296 to 93 K, the range of 4 , / ~ , values was 0.13-0.17. The rate constants for phosphorescence were calculated from, 4, = &7,kp, and the k , values were calculated from T~ = ( k , + k,)-' (12). In this work, it was assumed that quenching processes were not significant. Because the measurements were made with the samples protected by a nitrogen atmosphere and under dry conditions, quenching effects due to water and oxygen would be minimal. If quenching impurities were present in filter paper, the term xk,[q] would have to be added to the expression, k , + k,, and then included in the reciprocal sum, ( k , + kJ1, where k , is the rate constant for bimolecular quenching, and [q] is the concentration of quencher. However, the values of k,, as calculated in this work, do give a measure of nonradiative loss of energy from the triplet state. Other factors that would favor minimal bimolecular quenching are that the solid matrix protects the luminescent species from quenchers, and for true bimolecular quenching to occur, the quenching molecules would have to undergo diffusion to the luminescent species. Diffusion would be inhibited by the solid matrix, and the luminescent molecules are held rather tightly to the solid matrix. The dt values in Table I1 are somewhat smaller at the lower temperatures compared to the higher temperatures. The 4t values for the anion of PABA on sodium acetate were essentially the same as a function of temperature, a typical value being 0.81 at room temperature (12). The k , values in Table I1 show some variability with temperature, but the variability is generally within experimental error (see footnote in Table 11), and the k , values are considered constant with temperature, although the k , value at 93 K is slightly higher than the other k , values in Table 11. As indicated in Table 11, the k , values changed by a factor of 10 with temperature from 0.50 at 296 K to 0.05 at 93 K. Luminescence Parameters from the Anion of PABA Adsorbed on Filter Paper without NaCl. Luminescence data similar to that in Figure 1, Table I, and Table I1 were obtained a t 296 and 93 K for the anion of PABA adsorbed on filter paper which did not contain adsorbed NaC1. Table I11 gives the data obtained. By comparison of the corresponding 4f values at both room temperature and at low
Table 111. Luminescence Parameters for the Anion of PABA on Filter Paper without NaCP temperature, K
q+
6,
296 93
0.10 0.40
0.03 0.31
T,,
s
1.28 3.13
& 0.15 0.61
k,, 0.16 0.16
k,, s-' 0.62 0.16
"Duplicate values were obtained for @f, #, and 7,. The ranges for $f, b,, and 7, values were 0.04, 0.03, and 0.12 s, respectively, a t 296 K. At 93 K the ranges were 0.01,0.05, and 0.00 s, respectively.
temperature in Figure 1 and Table 111, it is clear that the df values were greater on filter paper treated with NaCl. A similar conclusion can be made for the 4, values by comparing the 4, data in Tables I and 111. At 296 K the 4, value is 7.3 times greater with the NaC1-treated paper. At 296 K, the 7, values do not differ significantly for the two types of filter paper, but at 93 K the T , values differed by 0.37 s (see Tables I and 111). At room temperature, the 4t value is 4.4 times greater on the NaC1-treated filter paper relative to the c $ ~ value obtained from untreated filter paper (Tables I1 and 111). However, at 93 K the dt value is 1.2 times greater on the untreated filter paper than on the NaC1-treated filter paper. In Table 111,the cbt values were calculated as described in ref 15 because 4,/7, was not constant a t room temperature and 93 K. Also, by comparing the data in Tables I1 and 111, it can be concluded that the k , values are greater on NaC1-treated filter paper, but the k , values are smaller on the NaC1-treated filter paper at 296 and 93 K compared to the corresponding temperatures on the untreated filter paper. One of the more important conclusions from the data in Figure 1 and Tables I and I11 is that higher 4f and 4, values can be obtained with NaC1treated filter paper. A similar conclusion was reached from the RTP intensities of 4-phenylphenol adsorbed on untreated and NaC1-treated filter paper (14). It should be mentioned that earlier work with the anion of PABA adsorbed on sodium acetate-NaC1 mixtures showed that at room temperature the maximum 4f and 4, values were achieved with 100% sodium acetate (15). NaCl can serve mainly two functions. For example, when the sample solution is applied to the filter paper, the filter paper is wet and the NaCl in solution can break intermolecular hydrogen bonds in the filter paper matrix and permit more effective entry of the phosphor into the cellulose matrix. After the evaporation of the solvent, the NaCl would help increase the rigidity of the solid matrix and thus increase the 4, value. Equations for 4, as a Function of Temperature. Equation 1 defines 4, in terms of &, k,, and k , (13). As considered earlier, it is assumed that quenching processes are minimal, and the term k,[q] does not appear in eq 1 4p
= 4-t kp
k, = + km
4tkpTp
(1)
For the anion of PABA adsorbed on sodium acetate, & and k , values were essentially constant with temperature (12); however, the k , values changed considerably with temperature. The data in Table I1 for the anion of PABA on filter paper indicate that is approximately constant from 296 to 233 K, then drops to a somewhat smaller & value and remains practically constant from 193 to 93 K. The change in dt values occurs because of the change in Gf values at 193 K (Figure 1). The k , values in Table I1 are considered constant within experimental error. However, the k , value at 93 K is somewhat higher relative to the other k , values in Table 11. By far, the k , values change to the greatest extent with temperature (Table 11). From the data for the anion of PABA adsorbed on sodium acetate obtained earlier (12) and the data obtained
ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990 -0.m
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