Anal. Chem. 1985, 57, 1227-1230
chemiluminescence. If coupled to gas chromatography, individual hydrocarbons can be quantified without interference from other species that may also respond to, e.g., the flame ionization detector. MIP allows the controlled production of exotic reactants suitable for selective chemiluminescence processes, by exciting either the analyte or the reagent gas, and establishes a well-defined temperature for the reaction (13). One thus combines chemical selectivity as well as spectroscopic selectivity (both in excitation and in detection) in trace gas determinations. With further work, one may be able to tune the selectivity to a single molecular species, while maintaining good detectability. Registry No. CzH4, 74-85-1; SF,, 2551-62-4.
LITERATURE CITED (1) Kummer, W. A.; Pltts, J. N.; Jr.; Sleer, R. P. Environ. Sc;. Techno/. 1971, 5 , 1045-1047. (2) Turner, G. S. Anal. Instrum. 1978, 74, 1-4. (3) Spurlln, S. R.; Yeung, E. S. Anal. Chem. 1982, 54, 318-320.
(8)
(9) (10) (11) (12) (13)
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Schatz, G.; Kaufman, M. J. Phys. Chem. 1972, 76, 3586-3590. Kolb, C. E.; Kaufman, M. J. Phys. Chem. 1972, 76, 947-953. Rosner, D. E.; Allendorf, H. D. J. Phys. Chem. 1971, 75, 308-317. Bagratashvill, V. N.; Letokhov, V. S.; Makarov, A. A.; Ryabov, E. A. Laser Chem. 1983, I , 211-342. Bagratashvili, V. N.; Letokhov, V. S.; Makarov, A. A,; Ryabov, E. A. Laser Chem. 1984, 4, 311-423. COmita, P. 8.; Berman, M. R.; Moore, C. B.;Bergman, R. G. J. Phys. Chem. 1981, 85, 3268-3276. Orr, B. J.; Keentok, M. V. Chem. Phys. Left. 1978, 47, 68-71. Hudgens, J. W.;McDonald, J. D. J. Chem. Phys. 1982, 76, 173-188. Crlm, F. F.; Kwei, G. H.; Kinsey, J. L. Chem. Phys. Left. 1977, 49, 526-529. Dai, H.-L.; Specht, E.; Berman, M. R.; Moore, C. B. J. Chem. Phys. 1982, 77, 4494-4506.
RECEIVEDfor review December 12,1984. Accepted February 4, 1985. S.R.S. thanks the Phillips Petroleum Co. for a fellowship. The Ames Laboratory is operated by the U.S. Department of Energy by Iowa State University under Contract No. W-7405-eng-82. This work was supported by the Office of Basic Energy Sciences.
Room-Temperature Phosphorescence Lifetimes and Intensities of p -Aminobenzoic Acid Adsorbed on Sodium Acetate-Sodium Chloride Mixtures V. P. Senthilnathan and R. J. Hurtubise*
Chemistry Department, University of Wyoming, Laramie, Wyoming 82071
Some of the Interactions responsible for the roomtemperature phosphorescence (RTP) of the p -amlnobenzolc acld (PABA) anlon adsorbed on sodlum acetate-sodlum chlorlde mlxtures were elucidated by relative lumlnescence lntenslty and phosphorescence lifetime measurements. The luminescence lntenslty and phosphorescence lifetime values varled over a relatlvely wlde range. The results showed that at least two mechanlsrns were Involved for Inducing RTP, with maxlmum RTP belng achieved on pure sodium acetate. Short and long decaying phosphorescent components were detected from the PABA anlon. Both of these components reached constant phosphorescence lifetime values even though the phosphorescence lntenslty contlnued to change as a functlon of the composition of phosphorescence-Inducing solld-surface material.
There have been some reports listing RTP lifetimes in solid-surface luminescence analysis, but there have been essentially no reports of the use of RTP lifetimes to study the interactions in solid-surface RTP. In addition to the RTP lifetime data from solid surfaces, RTP lifetime data have been reported for compounds in poly(methy1 methacrylate), in micelles, and in solutions. West et al. (1) discussed the exponential and nonexponential phosphorescence decay of some polycyclic aromatic hydrocarbons and nitrogen heterocycles in poly(methy1methacrylate) at liquid nitrogen temperature and at room temperature. Schulman and Walling (2) reported the phosphorescence lifetimes for seven sodium salts of organic acids on paper supports at room temperature. The values ranged from 0.080 to 0.74 s. Niday and Seybold (3) investigated the effects of adding various substances to filter paper on the RTP lifetime of adsorbed 2-naphthalenesulfonate.
Generally,the phosphorescencedecays were not exponential. In all cases, an increase in the lifetime was observed; however, some compounds caused a greater increase than others. De Lima and de M. Nicola (4) showed that the RTP lifetimes of l,&naphthyridine derivatives on filter paper varied from 0.25 to 0.49 s. With a time-resolved phosphorescence spectrometry designed by Goeringer and Pardue (5), time-resolved RTP spectra and RTP phosphorescence decay curves were obtained for salts of organic acids deposited on filter paper. These authors reported lifetimes in the range of 0.23-0.91 s. Skrilec and Cline Love (6) studied micelle RTP of functionally substituted aromatic compounds and reported phosphorescence lifetimes at room-temperatureand liquid nitrogen temperature for several compounds. The micelle RTP lifetimes ranged from 0.28 to 69 ms. Cline Love et al. (7) considered micelle-RTP lifetimes of selected single-componentsystems and two-componentsystems of polycyclic aromatic hydrocarbons. They were interested in these systems as qualitative indicators, and the lifetimes provided information about molecular dynamics of RTP from micelles compared to low-temperature phosphorimetry. RTP micelle lifetimes in the 0.45-10.2 ms range were reported. Donkerbroek et al. (8) consider the analytical aspects of RTP in liquid solutions and reported phosphorescence lifetimes in the millsecond range for brominated naphthalenes and halogenated biphenyls. Aaron et al. (9) showed the RTP lifetimes of indoles on ion-exchange filter paper with heavy atoms were in the millisecond region. Both long and short decaying species were detected for a given component. In earlier work, the RTP of the PABA anion adsorbed on sodium acetate was studied by reflectance, infrared, and luminescence spectroscopy (IO). In this work, RTP lifetimes and changes in luminescence intensity were used to investigate some of the interactions responsible for RTP of the PABA anion adsorbed on sodium acetatesodium chloride mixtures.
0003-2700/85/0357-1227$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
EXPERIMENTAL SECTION Apparatus. All RTP intensity data were obtained with a SchoeffelSD3000 spectrodensitometer equipped with a SDC300 density computer (Schoeffel Instruments, Westwood, NJ) and a phosphoroscope assembly. The phosphoroscope assembly was discussed earlier (11). Relative RTP and room-temperature fluorescence (RTF) signals were measured with the spectrodensitometer with the inlet and exit slits set at 2 and 3 mm, respectively. A 200-W XeHg lamp (Canrod Hanovia,Newark, NJ) and R928 photomultiplier tube (Hamamatsu Corp., Middlesex, NJ) were used in the spectrodensitometer. A Tektronix 5223 digitizing oscilloscope (Englewood, CO) with a 5B25N digitizer time baselamplifier connected to a Farrand MK-2 spectrofluorimeter (Valhalla, NY) and a MFE 715 M Plotamatic X-Y recorder (Salem, NH) were used in obtaining phosphorescence lifetimes. A Hewlett-Packard HP-86 computer (Corvallis,OR) was used for linear regression calculations. Reagents. Ethanol was purified by distillation. PABA, 99% (Aldrich Chemical Co., Milwaukee, WI),and sodium acetate, reagent grade (Malinckrodt,St. Louis, MO), were used as received. ACS reagent-gradesodium chloride was washed in ethanol before use. Procedures. Sodium acetate-sodium chloride salt mixtures were prepared by grinding in a ball-mill for about 30 min. An aliquot of 1FL of 100 ng/kL of PABA in ethanol was added to a given salt mixture and then the sample was prepared as described previously for sodium acetate (12). Relative intensity measurements were made with the spectrodensitometer. The PABA samples for the RTP lifetime measurements were held in a circular depression of a small blackened brass plate. The samples in the brass plate were supported in the cell compartment of the spectrofluorimeter. All samples for RTP intensity and RTP lifetime measurements were made under ambient conditions. The excitation and emission monochromators were set at the appropriate maximum excitation and emission wavelengths, and the phosphorescence decay signals were obtained in the single sweep mode and stored in the oscilloscope. The saved display was recorded on the Plotamatic 715 X-Y recorder, and intensity and time values were obtained from the recorder tracings. RESULTS AND DISCUSSION Solubility, Solution, and Matrix Effects. Earlier work showed that sodium acetate was a useful surface for selectively inducing RTP from certain compounds (IO). The interactions of PABA with sodium acetate were studied with several spectral techniques, and it was shown that the anion of PABA yielded RTP from sodium acetate. It was also postulated that two sodium acetate molecules anchored one PABA anion. It is important to consider both the initial wet chemistry and the final dried matrix for phosphorescent compounds adsorbed on sodium acetate. For example, adsorption of PABA on sodium acetate is preceded by partial neutralization with dissolved sodium acetate in alcoholic solution (IO). It was calculated that 84% of PABA was converted to the sodium salt of PABA and adsorbed on the sodium acetate surface (IO). If an acetone or ether solution of PABA is adsorbed onto sodium acetate, no RTP is observed from PABA. In this work, we determined that sodium acetate was insoluble in acetone and ether; thus the partial neutralizaton of PABA cannot occur prior to adsorption with these solvents. The results with acetone and ether indicated that PABA (acid form) adsorbed on sodium acetate did not give RTP. The RTP intensities and phosphorescence lifetimes of PABA anion adsorbed on several sodium acetate-sodium chloride mixtures were investigated in this work. It was found that sodium chloride by itself did not induce RTP from PABA. The percentages of sodium acetate in the mixtures were varied from 0.10% to 100%. To more readily compare the wide range of relative intensity data, a log-log graph was prepared as shown in Figure 1. The mole fraction of sodium acetate in the mixtures was used for the abscissa. The RTP relative intensity graph shows a sharp rise in RTP log mole fraction
2.4
t
- 3.0
-2.0
-1.0
0
LOG(mole fraction), N u O A c
Flgure 1. log RTP intensity of PABA anion adsorbed on sodium acetate-sodium chloride mixtures vs. log mole fraction of sodium acetate. Excitation wavelength, 290 nm; emission wavelength, 425 nm.
Table 1. Phosphorescence Lifetimes of PABA Anion Adsorbed on Sodium Acetate-Sodium Chloride Mixtures sodium acetate, 70 0.1 0.4
0.5 1.0 1.4
5.0 50 100
short componentasb lifetime, s 0.002 (0.OOOl) 0.08 (0.05) 0.2 (0.03) 0.2 (0.02) 0.9 (0.2) 1.2 (0.07) 1.1 (0.02) 1.1(0.05)
long componentn,b
lifetime, s 0.004 (0.0008)
0.2 (0.1) 0.4 (0.1) 0.3 (0.03) 1.4 (0.2) 1.6 (0.1) (0.2)
1.5 (0.1)
Average of triplicate determinations. *Standard deviation in parentheses. -2.10 NaOAc (1.2% NaOAc) to about log mole fraction -1.67 NaOAc (3.0% NaOAc), after which there is a gradual rise in RTP until the maximum RTP value is obtained with pure NaOAc. Table I shows the phosphorescence lifetimes for PABA anion adsorbed on several NaOAc-NaC1 mixtures. log RTP vs. time graphs showed both a short and long decaying species. When the component stripping technique described by Demas (13) and least-squares analysis (correlation coefficients generally > 0.99) were used, the phosphorescence lifetimes for the short and long decaying components were obtained (Table I). The lifetime values in Table I were obtained in triplicate, and single lifetime determinations were obtained from several other sodium acetate-sodium chloride mixtures (0.2, 0.3, 1.2, 1.6, 1.8, 2.0, 3.0, 4.0, 10, 30, 70,80, 90, and 95% sodium acetate). The minimum lifetime that could be detected was approximately 2.5 ms. As mentioned earlier, it is important to consider the initial wet chemistry and the dried solid matrix when using sodium acetate as a surface to induce RTP. I t was determined experimentally that the solubility of sodium acetate in pure ethanol was 9.8 mg/mL. Assuming that the effect of sodium chloride on solubility was minimal, a simple calculation showed that about a 1.2% sodium acetate-sodium chloride mixture would yield a saturated solution of sodium acetate. Based on a calculation for the solubility of NaCl in pure ethanol, it was determined that for all the mixtures in Figure 1and Table I, the solutions were saturated with NaCl (14). A 1.2% (log mole fraction, -2.1) mixture of sodium acetate lies near the bottom part of the break in the relative intensity curve (Figure 1). The portions of the RTP intensity curves below about 1.2% sodium acetate in Figure 1reflect the effect of unsaturated solutions of sodium acetate. At percentages below 1.2%
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
sodium acetate, it was thought there might not be enough sodium acetate in solution to react and form a relatively large fraction of the anion of PABA which would be needed for strong RTP. However, experiments showed that the roomtemperature solid-surfacefluorescence of the samples of PABA anion was practically constant from 0.1% sodium acetate to 1.8% sodium acetate. Beyond 1.8% sodium acetate, RTF dropped somewhat, but a t 100% sodium acetate, the RTF signal was about the same as at 0.1% sodium acetate. These results indicated that a large fraction of the anion was on the surface at 0.1% sodium acetate. Another explanation for the lower RTP intensities below 1.2% sodium acetate is the relative amounts of NaCl and sodium acetate in the dry state. At 0.1% sodium acetate, the mole ratio of NaCl to sodium acetate is 1400 while at 1.2% sodium acetate the mole ratio is 115. As discussed earlier, with pure sodium acetate, two sodium acetate molecules are needed to anchor one PABA anion. Below 1.2% sodium acetate, the sodium acetate solution is not saturated. Thus, conditions are such that in the dry matrix the sodium chloride ”dilutes” the sodium acetate so that strong interaction of the PABA anion is not possible with sodium acetate under these conditions. In Table I, the short and long decaying species show a substantial jump in phosphorescence lifetime from 1.0%to 1.4% sodium acetate. Also, Table I shows that the phosphorescence lifetimes for the short decaying component becomes approximately constant after 1.4%sodium acetate, and for the long decaying component, the lifetime becomes approximately constant after 1.0% sodium acetate. As mentioned earlier, single lifetime determinations were obtained for PABA anion on several sodium acetate-sodium chloride mixtures from 1.6% to 95% sodium acetate. That data indicated that the phosphorescence lifetimes of both short and long decaying species were essentially constant within the 1.6-95% region. In Table I, the RTP lifetimes of the PABA anion became approximatelyconstant at about the same point at which the ethanol solution becomes saturated with sodium acetate. Prior to adsorption then, PABA has reacted in the saturated solution with dissolved sodium acetate to form the PABA anion. Upon evaporation of the solvent, the PABA anions would have a greater probability of interacting with the closest sodium acetate species, namely, the dissolved sodium acetate. The abrupt increase then in the phosphorescence lifetime near 1.4% sodium acetate is an indication of the fact that no more sodium acetate can dissolve in the solvent. As the undissolved sodium acetate increases in the mixtures, the molecules of sodium acetate would pack the matrix more efficiently and either protect the PABA anion from collisions with oxygen molecules or hold the PABA anion in a more rigid state as indicated by the increase in RTP beyond about 1.4% sodium acetate (Figure 1). This can be thought of as another “dilution” effect; however, instead of sodium chloride diluting the matrix, undissolved sodium acetate dilutes the matrix and can pack the solid matrix more effectively than the sodium chloride. Constant and Changing Lifetimes. Equation 1 defines phosphorescence lifetime (7,) for first-order decay in terms of rate constants, where 12, is the rate constant for phosphorescence, k , is the rate constant for a radiationless tran-
sition, k , is the rate constant for bimolecular quenching such as with oxygen, and [ q ] is the concentration of the quencher. As a first approximation, it is assumed with eq 1 that there is only one radiationless process and one bimolecular quenching process. There may be more than one quenching process; however, sodium acetate contains essentially no im-
1229
purities. Also, the RTP intensity and RTP lifetime of the PABA anion were very constant under normal laboratory conditions and, thus, all sample measurements were obtained under ambient conditions. Additional experimentswould have to be performed to determine the specific effects of changing moisture and oxygen content on the phosphorescence properties of the PABA anion. Niday and Seybold (3) suggested that k , was a measure of the rigidly held mechanism for RTP and k , was a measure of the effect of oxygen on RTP or how efficiently the matrix protects the phosphor from oxygen. However, if there are other RTP quenching processes, the term k,[Q] in eq 1would have to be replaced by the summation Ck,[q](15). In the work with sodium acetate, below 1.4%sodium acetate, k , and k, are most likely large because of the relatively small values of 7, (Table I). It is assumed that similar interactions are occurring for both short and long decaying components. At greater than 1.4% sodium acetate, 7, is almost constant for both short and long decaying components but the RTP increases (Table I and Figure 1). It is assumed that PABA has reacted with the maximum number of sodium acetate molecules near 1.4%sodium acetate because essentially the maximum amount of sodium acetate dissolved in the solvent. For 7, to remain constant, but for the RTP to increase as a function of sodium acetate content, it would seem that both k , and k , would decrease but k , would increase in a proportional fashion as the sodium acetate content increased. This can be postulated by considering eq 2 which shows the relationship between phosphorescence quantum efficiency, $, and k, and triplet formation efficiency, &, and T , (16).
4, = k p 4 t T p With 7, constant, 4, would increase as the product of kp& increased. With the data from this work, it is not possible to calculate k , and & However, if k , increases, then for 7, to remain constant, k , and k , would decrease (eq 1). Smaller k , and k , values would thus favor a higher RTP quantum efficiency. An alternate explanation for the constant 7, values is that k , reaches a maximum and constant value at about 1.4% sodium acetate and beyond. Then for 7, to remain constant, k , would decrease and .Izp would increase in a proportional fashion as the sodium acetate content increased. However, most likely k , would not reach a constant value near 1.4% sodium acetate because as the sodium acetate content increases, the sodium acetate molecules would pack more effectively which would favor a smaller k , value. In conclusion, two major aspects for the RTP of PABA anion on sodium acetatesodium chloride mixtures and pure sodium acetate have been indicated. PABA anions achieve a certain degree of rigidity at about 1.4%sodium acetate by strong interaction with sodium acetate molecules (IO). However, protective matrix effects and/or increased rigidity of the matrix caused by increasing sodium acetate content allow for enhanced RTP intensity with maximum RTP being observed with pure sodium acetate. It is speculated that inhomogeneous matrix effects are responsible for the slow and fast phosphorescence decay. The solid matrix would not have homogeneous adsorption sites because the experimental procedure did not permit well-ordered crystalline material to be formed. A more extensive investigation is needed to elucidate additional details of the interactions responsibe for RTP. Several of these aspects are being investigated presently. Registry No. PABA anion,2906-28-7; NaOAc, 127-09-3;NaCl, 7647-14-5.
LITERATURE CITED (1) West, M. A.; McCalium, 1970, 66, 2135.
K. J.; Woods, R. J. Trans. Faraday Soc.
(2) Schulman, E. M.; Walling, C. J. fbys. Chern. 1973, 7 7 , 902. (3) Niday, G.J.; Seybold, P. G. Anal. Chem. 1978, 5 0 , 1577.
1230 (4) (5) (6) (7) (8) (9) (10) (11) (12)
Anal. Chem. 1985, 57, 1230-1237 de Lima, C. G.; de M. Nicola, E. M. Anal. Chem. 1978, 5 0 , 1658. Goeringer, D. E.; Pardue, H. L. Anal. Chem. 1979, 51, 1054. Skriiec, M.; Cline Love, L. J. Anal. Chem. 1980, 52, 1559. Cline Love, L. J.; Habarta. J. G.; Skrilec, M. Anal. Chem. 1981, 53, 437. Donkerbroek, J. J.; Elzas, J. J.; Gooijer, C.; Frei, R. W.; Velthorst, N. H. Talanta 1981, 28, 717. Aaron, J. J.; Andino, M.; Winefordner, J. D. Anal. Chim. Acta 1984, 160, 171. Von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1977, 49, 2164. Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1979, 51, 659. Von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1976, 48, 1784.
(13) Demas, J. N. "Excited State Lifetlme Measurements"; Academic Press: New York 1983; pp 39-42. (14) Ramasamy, S. M.; Hurtublse, R. J. Anal. Chem. 1982, 5 4 , 2477. (15) Winefordner, J. D.; Schulman, S. G.; OHaver, T. C. "Luminescence Spectrometry in Analytlcal Chemistry"; Wiley: New York, 1972; p 69. (16) Parker, C. A. "Photoluminescence of Solutions"; Elevler: New York, 1968; p 88.
RECEIVED for review September 25,1984. Accepted February 14, 1985. Financial support for this project was provided by the Department of Energy, Division of Basic Energy Sciences, Contract DE-AC02-80ER10624,
Investigations of a New Sample Compartment Assembly for Low-Temperature and Room-Temperature Solid Substrate Phosphorimetry Gad Scharf,' B. W. Smith, and J. D. Winefordner*
Department of Chemistry, University of Florida, Gainesville, Florida 32611
A novel sample compartment for use In roomtemperature (25 "C) and in low-temperature (-65 "C) phosphorlmetry (and fluorometry) of samples held In solid substrates Is described. The sample compartment assembly deslgned for the PerklnElmer LS-5 fluorometer contalns four posltlons for samples and blank and allows convenlent subtraction of the blank from the sample spectra. Excltatlon and emlsslon phosphorescence background spectra and phosphorescence llfetlmes of several solid substrates, Including varlous fllter papers and a TLC plate, In the presence and absence of strong base and lodlde are given. Slmllar studies were performed wlth two model compounds 6-chrysenecarboxyllc acid and p -aminobenzoic acid. The background Increased wlth the presence of heavy atom and at low temperatures, and the background Is llkely a result of a comblnatlon of phosphors. The roomtemperature phosphorescence (RTP) detectlon limits for 6chrysenecarboxyllc acid (6-CHRA) and p -amlnobenrolc acid (PABA) were 0.2 ng, the percent relatlve standard devlatlons for 5 pg/mL concentrations were about l o % , and the analytlcal calibration curves were linear.
Solid surface room-temperature phosphorescence (RTP) has recently been developed as an analytical technique for quantitative determination of a variety of organic compounds (1-23). Several reviews have been published (24-29). Parker et al. (25,26) have reviewed the analytical and physical aspects of RTP. In a recently published monograph, Vo-Dinh (29) has given an extensive survey of RTP. In several studies (4,9,12,30),it was found that an inert, dry atmosphere is essential for producing intense and stable phosphorescence signals at room temperature. The drying period reported for each sample was in the range of a few minutes to 2 h (22). Therefore, the drying stage can be fairly time-consuming. On leave from Institute of Chemistry, Tel-Aviv University, Ramat-Aviv, Tel-Aviv, Israel.
Since the main advantage of RTP is its simplicity and speed, we decided to design a suitable system in which up to four samples could be simultaneously dried and later successively measured. Moreover, it was found by us and by others that many analytes and support materials photodecompose during the RTP measurement (11, 31). The UV radiation used for excitation may cause photochemical changes in the samples and thus influence the accuracies of the lifetime and/or intensity measurements. Therefore, the ability to measure a new sample after a relatively short excitation period has a considerable advantage for improving the accuracy of the measurements. Several researchers have shown that in certain cases lowering the temperature of the analyte adsorbed on the filter paper increases the phosphorescence intensity and thus enables an improvement in the limit of detection (12, 32,33). McCall and Winefordner (33) found that low-temperature measurements affect each compound in a different way, e.g., the intensity enhancement for trytophane was 2.4X and the improvement of the limit of detection (LOD) at low temperature was 1.7X; however, in the case of 6-methylmercaptopurine the intensity enhancement was 84X, and the LOD at 90 K was 8X better than the LOD at room temperature. Bower et al. (12) found that a cold N2 stream caused an increase in the phosphorescence intensity of several analytes. Schulman (9) showed that lowering the temperature from 311 K to 286 K accounted for a 15% increase in RTP intensity of sodium naphthoate adsorbed on sucrose. Recently, Nishikawa et al. (34,35) have shown the analytical application of thermally activated delayed fluorescence (TADF) on filter paper. They found that in several cases intense delayed fluorescence signals were maintained only at elevated temperatures, e.g., 30-60 OC. These results indicated that an intensity enhancement was not always obtained while lowering the temperature. Thus cooling or heating of the analytes might serve as an additional tool to increase the selectivity and the versatility of solid surface luminescence. We therefore, have designed a system which would enable us to easily measure solid surface luminescence at different temperatures.
0003-2700/85/0357-1230$01.50/00 1985 American Chemical Society
