Anal. Chem. 1988, 60, 1291-1296
1291
Room-Temperature Luminescence Properties of Benzo[ f ]quinoline and Phenanthrene Adsorbed on a-Cyclodextrin-Sodium Chloride Mixtures J. M. Bello a n d R. J. Hurtubise* Chemistry Department, University of Wyoming, Laramie, Wyoming 82071
The room-temperature fluorescence (RTF) and room-temperature phosphorescence (RTP) intensities of benzo[f]quindlne (B[f ]a) and phenanthrene were obtained from several a-cyclodextrin-sodium chloride mixtures. The results showed the importance of the initial wet chemistry in the sample preparatlon procedures for the observatlon of iuminescence signal for compounds adsorbed on a-cyciodextrin-sodium chloride mixtures. The data indicated that in order to obtain the optimum RTP and RTF slgnals, the solvent used to adsorb the analytes had to be saturated with a-cyciodextrin. Also, the solid-surface fluorescence and phosphorescence quantum yield, triplet formation efficiency, and phosphorescence lifetime values were Obtained for B[f]Q and phenanthrene adsorbed on 0.05 % and 80 % a-cyciodextrinsodium chloride mixtures. From these data, rate constants for phosphorescence and for radiationless transition from the triplet state were obtained.
Several analytical applications of solid-surface room-temperature phosphorescence (RTP) have appeared in the literature (1,2). R T P has been used to characterize pharmaceutical and biologically important compounds (3,4),synfuels and polluand coal-related products ( 5 , 6 ) ,pesticides (7,8), tants in air samples (9, 10). Although several papers have been published on the analytical applications of RTP, a better understanding of the phosphor-substrate interactions responsible for the RTP phenomenon for compounds adsorbed on solid surface is still needed. Several mechanisms of interaction, however, have been proposed such as hydrogen bonding (11-13), “matrix packing” (14),and “matrix isolation” (15). In addition, Ramasamy and Hurtubise (16) obtained the solid-surface fluorescence and phosphorescence quantum yield values and phosphorescence lifetime values for p aminobenzoic acid (PABA) anion adsorbed on sodium acetate (NaOAc) over a wide temperature range. Their data supported a rigidly held mechanism for the R T P of the PABA anion on NaOAc. Also, Ramasamy and Hurtubise (17) determined several fundamental luminescence parameters for the PABA anion adsorbed on NaOAc mixtures. We reported previously that a 80% a-cyclodextrin-sodium chloride mixture can be used as a substrate for both solidsurface room-temperature fluorescence (RTF) and RTP (18, 19). In addition, we recently reported that inclusion complexes were formed for PABA, benzo[flquinoline (B[flQ), 4phenylphenol, and phenanthrene on the 80% a-cyclodextrin-sodium chloride mixture in the solid state (20). In this work, the R T P and RTF intensitites of BWQ and phenanthrene adsorbed on several a-cyclodextrin-sodium chloride mixtures were obtained, and important conclusions concerning the initial wet chemistry step in the sample preparation were made. Furthermore, the solid-surface luminescence quantum yields and phosphorescence lifetimes of the two compounds adsorbed on a-cyclodextrin-sodium chloride mixtures were determined. From the quantum yield 0003-2700/88/0360-1291$01.50/0
and lifetime data, several fundamental luminescence parameters were obtained. EXPERIMENTAL SECTION Apparatus. All RTP and RTF intensity measurements were obtained with a FLUOROLOG 2+2 spectrofluorometer (SPEX Industries,Edison, NJ), and data were processed through a SPEX Datamate computer interfaced with the spectrofluorometer. A cooled Hamamatsu R928 photomultiplier tube was employed. For RTF measurements a 450-W Xe lamp (Osram, Germany) was employed. RTP intensities were obtained by employing the SPEX 1934C phosphorimeter accessory with the spectrofluorometer. The phosphorimeter system used a programmable pulsed excitation source and selectable gating of the photomultiplier tube. Data for the quantum yield and lifetime determinations were obtained with a Farrand MK-2 spectrofluorometer. A Tektronix 5223 digitizing oscilloscope (Englewood, CO) with a 5B25N digitizer time base/amplifier connected to the MK-2 spectrofluorometer was used in obtaining phosphorescence lifetimes. The instrumental setup used for the quantum yield and the phosphorescence lifetime studies was described previously (16, 17). Reagents. Methanol (Photrex Grade, J. T. Baker) was used as received. The ethanol was distilled prior to use. 1-Propanol (Gold Label, Aldrich) and water (HPLC grade, Aldrich) were used as received. The a-cyclodextrin hydrate and the sodium chloride (Reagent grade) were washed with distilled ethanol prior to use. BMQ (Gold Label, Aldrich) was used as received. Phenanthrene was recrystallized from distilled ethanol. Details of the sodium salicylate standard and NaOAc, purification of nitrogen gas, cell compartment, sample holders, temperature monitoring, and cryogenic systems that were used for quantum yield and lifetime determinations were described earlier (16, 17). Procedures: Sample Preparation for RTP and RTF Intensity Measurements. Several a-cyclodextrinsodium chloride mixtures over the range from 0.0005% to 80% a-cyclodextrinsodium chloride were prepared by mixing the appropriate amounts of a-cyclodextrin and sodium chloride with a mortar and pestle and then further mixing in a ball mill. The mixture8 were then stored in a desiccator prior to use. Samples for RTP and RTF intensity measurements were prepared as follows. A 0.20-mL aliquot of methanol was added to a test tube and followed by the addition of 5 pL of a methanol solution of the analyte (0.1 pg/mL). Next, a known amount of the a-cyclodextrin-sodium chloride mixture was added. The amounts of the various a-cyclodextrinsodium chloride mixtures used were 60 mg for the 80% mixture, 87.8 mg for the 20% mixture, and 120 mg for the 0.5% mixture. For the 0.1-0.0005% mixtures, 130 mg of a-cyclodextrin-sodium chloride mixture was used. The slurry solution containing the analyte and the matrix was then sonicated for a few seconds, and the sample was placed in an oven at 110 “Cand dried for 30 min. The sample was then transferred into a sample holder, placed in the sample compartment of the SPEX instrument, and degassed with nitrogen for 10 min. Then RTP and RTF intensities were measured with nitrogen gas continuously flowing. Sample Preparation for Quantum Yield Determinations at 23 OC. Samples for quantum yield determinations at room temperature were prepared in the following manner. A 150-pL aliquot of methanol was added into a test tube, and this was followed by the addition of a 5-pL methanol solution containing 1.2 pg of the compound. Next, 85 mg of 80% a-cyclodextrin0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
sodium chloride mixture (150 mg for the 0.5% and 0.05% a-cyclodextrin mixtures) was added to the test tube, and the mixture was sonicated for a few minutes. The sample was then placed in an oven at 100 "C and dried for 15 min. After drying, the powdered sample was ground in a ball mill for 10 min and then packed loosely into a triangular brass plate sample holder and placed in the oven to dry for another 5 min. Then the sample was placed in the sample compartment of the MK-2 instrument, the sample compartment was purged with nitrogen for 10 min, and then the measurements were made. Nitrogen gas was flowing continuously as the measurements were performed. The same procedure was followed for the blank. The sodium salicylate standard and NaOAc blank standard were prepared as described by Ramasamy et al. (21). Sample Preparation for Quantum Yield Determination at -180 OC and for Phosphorescence Lifetime Determinations. Powdered samples of analyte-cyclodextrin mixtures for quantum yield determinations at -180 "C were prepared as follows. Aliquots of 0.50 mL of methanol and 5 pL of a methanol solution of the analyte were added to a test tube. The amount of B[flQ and phenanthrene added was 3.6 pg. Next, 0.24 g of 80% a-cyclodextrin-sodium chloride mixture (0.45 g for the 0.05% mixture) was added to the test tube, and the solution was sonicated for a few minutes. The sample was then placed in an oven at 100 O C and dried for 30 min. After drying, the sample was ground in a ball mill for 10 min, and then 0.20 g of the 80% a-cyclodextrin-sodium chloride sample (0.40 g for the 0.05% sample) was packed loosely in the depression of a black Delrin sample holder and covered with a quartz plate. The sample holder containing the sample was again placed in the oven for 5 min and then transferred to a Dewar in the sample compartment of the MK-2 instrument and allowed to cool to room temperature in a flowing nitrogen atmosphere. Then by use of a very low flow rate of cold, dry nitrogen, the temperature of the sample was g r a d d y cooled to -120 "C in the span of 45 min. From -120 to -180 O C , the flow rate of the cooled nitrogen gas was increased gradually so that the temperature inside the Dewar was decreased at a rate of 20 OC/15 min. Once -180 "C was reached the temperature was held constant for another 30 min. The same procedure was followed for the phosphor blank, standard, and standard blank. For phosphorescence lifetime determinations, B[flQ and phenanthrene were adsorbed on the a-cyclodextrin-sodium chloride mixture by using the procedure described above for the sample preparation of RTP and RTF intensity measurements. The powdered sample containing the analyte was packed loosely in the depression of the Delrin sample holder used in the quantum yield determination, placed inside a Dewar in the MK-2 sample compartment, and then purged with nitrogen for 15 min. Finally, the phosphorescence decay curve at room temperature was recorded. The temperature of the sample was then cooled slowly to -180 "C and held at -180 "C for 30 min prior to recording the phosphorescence decay curve. A similar procedure was followed for the a-cyclodextrin-sodium chloride blank. Determination of Quantum Yields and Lifetimes. The procedures used to determine the quantum yields and lifetimes of BMQ and phenanthrene adsorbed on a-cyclodextrin-sodium chloride mixtures were the same as those described earlier (16, 17, 21).
Purification of a-Cyclodextrin. A sample of a-cyclodextrin was recrystallized twice from a 60% 1-propanol-water solution and once from water (22). The recrystallized a-cyclodextrin was used in the experiment comparing the spectral characteristics of a recrystallized a-cyclodextrin and an ethanol-washed a-cyclodextrin.
RESULTS AND DISCUSSION RTP a n d RTF Intensities B[f]Q and Phenanthrene Adsorbed on Various a-Cyolodextrin-Sodium Chloride Mixtures. The R T P and RTF intensities of B M Q and phenanthrene adsorbed on various a-cyclodextrin-sodium chloride mixtures ranging from 0.0005% to 80% were obtained. Figure 1 shows plots of log relative intensity as a function of log of the ratio of the total millimoles of a-cyclodextrin in the matrix to the millimoles of analyte for BMQ and phenanthrene. The total millimoles of a-cyclodextrin is
I
I
0
I
2
3
4
5
L O G ( T O T A L mrnole8 C D / m m o l 8 8 A N A L V T E I
I 0
I
2
3
4
5
L O G l T O T A L mrnol88 C D l m r n o l 8 8 A N A L Y T E )
Figure 1. Graph of log relative intensity versus log (total millimoles of a-cyclodextrin (CD)/milllmolesof analyte): (a) phenanthrene;(b) B [ f ] Q . The arrows in the figures represent the a-cyclodextrinlanalyte ratio at which the absorbing solution becomes saturated with a-cyclodextrin.
the sum of the millimoles of dissolved a-cyclodextrin and the millimoles of undissolved a-cyclodextrin. In Figure 1, it can be seen that the general shapes of the corresponding RTP and RTF intensity curves for B[flQ and phenanthrene are approximately the same. Furthermore, it is shown that the RTP intensity is much more sensitive to the amount of a-cyclodextrin present in the matrix than is RTF intensity. In the RTP intensity curves three distinct regions are evident. Below a a-cyclodextrin-analyte millimole ratio of one [log (total millimoles of a-cyclodextrin/millimoles of analyte), -0.74 to -0.431 the RTP intensity increases very little. The RTP intensity increases dramatically beyond the ratio of one, and then the R T P intensity curve levels off approximately a t a ratio of 170 [log (total millimoles of a-cyclodextrin/millimoles of analyte), 2-23]. It can be seen from these results that the a-cyclodextrin has to be present in a large excess to obtain the optimum luminescence signal in this substrate. In order to determine the importance of the initial wet chemistry for the a-cyclodextrin matrix, the solubility of a-cyclodextrin was determined. It was found experimentally that the solubility of a-cyclodextrin in NaC1-saturated methanol was 0.82 f 0.06 mg/mL. Incidentally, in the 0.0005-80% a-cyclodextrin-sodium chloride mixtures, the methanol was saturated with NaCI. Thus, since the volume of the methanol (0.2 mL) used to adsorb the compounds was g of known, a simple calculation showed that 1.64 X a-cyclodextrin would yield a saturated solution. This corresponds to a a-cyclodextrin/analyte millimole ratio of 43. The log of this ratio (1.63) lies approximately at the break of the
ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
Table I. RTP and RTF Quantum Yields for B[f]Q and Phenanthrene Adsorbed on a-Cyclodextrin-Sodium Chloride Mixtures at 23 "C and at -180 "Capb 23 "C compound
bf
Table 11. Phosphorscence Lifetimes of B[f]Q and Phenanthrene Adsorbed on a-Cyclodextrin-Sodium Chloride Mixtures"
-180 "C 4P
bf
80% a-CD
bP
~ ~ ( "c), 23 9
80% a-Cyclodextrin phenanthrene 0.29 f 0.01 0.13 f 0.02 0.29 f 0.03 0.26 f 0.04 BVIQ 0.41 f 0.01 0.09 f 0.02 0.40 f 0.03 0.22 f 0.04
0.5% a-Cyclodextrin phenanthrene 0.30 f 0.05 0.10 f 0.03 BVIQ 0.38 f 0.05 0.10 f 0.03 0.05% a-Cyclodextrin phenanthrene 0.16 f 0.03 0.04 f 0.01 0.26 f 0.04 0.11 f 0.02 BVIQ 0.22 f 0.03 0.05 f 0.01 0.36 f 0.04 0.12 f 0.02
"Results are the average of at least two determinations. b95% Confidence level based on pooled standard deviation values. rising portion of the RTP intensity curves of the two compounds as indicated by the arrows in Figure 1. Therefore, it can be seen that with a a-cyclodextrin-sodium mixture, matrix packing of the a-cyclodextrin-sodium chloride mixture in the dry state is not very important beyond a saturated solution of a-cyclodextrin for enhancing the RTP signal (17). The above results and the fact that the model compounds formed inclusion complexes with the a-cyclodextrin in the solid acyclodextrin-sodium chloride matrix (21) indicate that formation of an inclusion complex in the methanol solvent prior to solvent evaporation is important for the observation of RTP. The importance of a saturated solution of a-cyclodextrin is to drive the inclusion complex formation reaction in solution to maximum product yield. Luminescence Quantum Yields and Phosphorescence Lifetimes of B[f]Q and Phenanthrene Adsorbed on aCyclodextrin-Sodium Chloride Mixtures. The phosphorescence and fluorescence quantum yields and phosphorescence lifetimes of BWQ and phenanthrene adsorbed on 0.05% and 80% a-cyclodextrin-sodium chloride mixtures were obtained at 23 "C and at -180 "C. In addition, the RTP and RTF quantum yield values for the compounds adsorbed on a 0.5% a-cyclodextrin-sodium chloride mixture were also obtained. In reference to the discussion in the last section, the solvent used to adsorb the analyte for the 0.05% mixture was not saturated with a-cyclodextrin, while with the 80% and 0.5% mixtures, these matrices yielded a saturated solution of a-cyclodextrin. Table I shows the phosphorescence and fluorescence quantum yields of the compounds adsorbed on 80%, 0.5%, and 0.05% a-cyclodextrin-sodium chloride mixtures obtained at 23 "C and of the compounds adsorbed on 80% and 0.05% mixtures at -180 "C. It can be seen in this table that the coresponding RTP and RTF quantum yields of a given compound are approximately the same in the 0.5% and 80% a-cyclodextrin-sodium chloride mixtures at room temperature. On the other hand, the corresponding RTP and RTF quantum yields of BWQ or phenanthrene decrease considerably when adsorbed on the 0.05% a-cyclodextrinsodium chloride mixtures. The difference in the RTP and R T F quantum yields of the compounds in the three a-cyclodextrin mixtures is related to the different amount of a-cyclodextrin initially dissolved in the methanol solvent. As mentioned above, the 0.05%mixture did not yield a saturated a-cyclodextrin solution, while saturated a-cyclodextrin solutions were obtained with the 0.5% and 80% mixtures. Thus, in the 0.05% mixture, fewer a-cyclodextrin molecules could interact with the analyte, and the analyte would not be included as effectively in this mixture as in the other two acyclodextrin mixtures. Extraction of the unbound analyte on the 0.05% mixture was performed with ethyl ether (20))and
1293
T,(-180 "C), s
0.05% (u-CD ~ ~ ( "c), 2 3 S
T,(-180 "C),
phenanthrene 1.84 f 0.11 3.11 f 0.14 1.94 f 0.10 2.96 f 0.16 B[flQ 0.87 f 0.04 2.48 f 0.16 0.95 f 0.05 2.41 f 0.02 Reproducibility based on the 95% confidence level. the percentages of BMQ and phenanthrene extracted were 9% and 2%, respectively. In addition, it was reported recently that in the 80% mixture only small fractions of B[flQ and phenanthrene (2% and 0.7%, respectively) were extracted (20). Thus, for the solid 0.05% mixture, both included and unincluded molecules would appear in the matrix, while for the 80% mixture, mostly included molecules would appear in the matrix. The phosphorescence and fluorescence quantum yields of the compounds adsorbed on 80% and 0.05% mixtures were obtained at -180 "C,and the results are also shown in Table I. It is evident in Table I that the fluorescence quantum yields of phenanthrene adsorbed on the 80% and 0.05% a-cyclodextrin mixtures are approximately the same at low temperature. The same is true for B[flQ. However, it can also be seen that the phosphorescence quantum yields for both compounds in the 80% mixture are still much higher than the values in the 0.05% mixture at -180 "C. As mentioned above, the smaller phosphorescence yield for the 0.05% mixture is due to included and unincluded molecules on the surface. A further comparison of the quantum yield values for the model compounds adsorbed on the 80% a-cyclodextrin mixtures at room temperature and at -180 "C shows that the fluorescence quantum yields in the 80% mixture remain relatively constant with temperature (Table I) which indicats that experimental conditions are not as important for fluorescence as for phosphorescence for this mixture. However, the 0.05% mixture shows an increase in fluorescence quantum yield at -180 "C for both compounds. As indicated in Table I, the phosphorescence quantum yields for BWQ and phenanthrene in the 0.05% and 80% mixtures show an increase at -180 "C relative to their room-temperature values. The phosphorescence lifetimes of BMQ and phenanthrene on 0.05% and 80% a-cyclodextrin-sodium chloride mixtures at 23 "C and at -180 "C were also obtained, and Table I1 lists the phosphorescence lifetimes for the two compounds. At room temperature the phosphorescence lifetimes of a given compound are essentially the same in both the 0.05% and 80% a-cyclodextrin-sodium chloride mixtures. The same trend is observed for the lifetimes obtained at -180 "C. However, a comparison of the phosphorescence lifetimes at the two temperatures shows that the lifetimes in both solid surfaces at -180 "C are higher than the phosphorescence lifetimes at room temperature for a given compound. Calculation of Fundamental Luminescence Parameters. Ramasamy and Hurtubise (16, 17) gave a detailed discussion on the calculation of the rate constant for phosphorescence (kJ,the rate constant for radiationless transition from the triplet state (k,,J, and the calculation of triplet formation efficiency (dt). By use of the equations described by Ramasamy and Hurtubise, the k,, k,, and dt values were also calculated for BMQ and phenanthrene adsorbed on acyclodextrin-sodium chloride mixtures. Because the samples were dried and all measurements were performed on a dry nitrogen gas atmosphere, it was assumed in this work that the rate constants for bimolecular quenching such as for oxygen and water were negligible. In addition, it was also assumed
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988
Table 111. & and k, Values for B[f]Q and Phenanthrene Adsorbed on a-Cyclodextrin-Sodium Chloride Mixtures Obtained at 23 "C and at -180 "C"
4t
kln
4t
80% a-Cyclodextrin phenanthrene 0.71 f 0.01 0.44 f 0.06 0.71 f 0.03 0.20 B[flQ 0.59 f 0.01 0.97 f 0.06 0.60 f 0.03 0.25
krn
a
0.05% a-cyclodextrin 23 "C -180 "C
phenanthrene 0.10 f 0.05 f 0.07
0.05% a-Cyclodextrin phenanthrene 0.38 f 0.16 0.46 i 0.06 0.74 f 0.04 0.29 f 0.05
WflQ
80% a-cyclodextrin 23 "C -180 "C
-180 "C
23 " C
compound
Table IV. k, Values for Phenanthrene and B[f]Q Adsorbed on a-Cyclodextrin-Sodium Chloride Mixtures Obtained at 23 "C and -180 "C"
0.78 f 0.03 0.98 f 0.05 0.64 f 0.04 0.33 f 0.02
Error based on propagation of error equations.
that the a-cyclodextrin matrix was essentially free of luminescence impurities. This assumption was supported by the data from the comparison of the corrected luminescence emission spectra of the compounds adsorbed on recrystallized a-cyclodextrin and ethanol-washed a-cyclodextrin samples. The ethanol-washed a-cyclodextrin was the sample used in determining the various luminescence parameters. The corrected R T P and RTF emission spectra of B[flQ and phenanthrene adsorbed on the 80% a-cyclcdextrin-sodium chloride mixtures of the recrystallized and washed a-cyclodextrin samples were essentially the same in shape, band intensities, and emission areas. Therefore, these results imply that little or no quenching impurities were present in the washed acyclodextrin sample. Luminescence Parameters for B[P]Q and Phenanthrene Adsorbed on a-Cyclodextrin-Sodium Chloride Mixtures. Table I11 lists the q5t values at room temperature and at -180 "C for BMQ and phenanthrene adsorbed on 80% and on 0.05% a-cyclodextrin-sodium chloride mixtures. It can be seen in Table I11 that the q5t value for phenanthrene is considerably higher on the 80% mixture than on the 0.05% mixture at room temperature. However, the & of B[flQ a t 23 "C shows a decrease on the 80% a-cyclodextrin mixture compared to the 0.05% mixture. The difference in 4t values in the two a-cyclodextrin mixtures a t 23 "C for the two compounds will be discussed later. At -180 "C, the values were approximately the same for a given compound on both matrices. In addition, the & values for a given compound at low temperature were approximately the same as that for the compounds adsorbed at room temperature on the 80% mixtures. Table I11 also lists the k , values of BMQ and phenanthrene on 0.05% and 80% a-cyclodextrin-sodium chloride mixtures at 23 "C and at -180 "C. At room temperature, the k , values of the compounds are relatively large in both substrates, and in addition, the k , values a t room temperature for a given compound are approximately the same in the two mixtures. The large k , values a t 23 "C indicate that the nonradiative transition from the triplet state makes an important contribution to the loss of phosphorescence emission from compounds adsorbed on a-cyclodextrin-sodium chloride mixtures. Furthermore, k , is considered as a measure of the rigidly held mechanism of R T P (14). Thus, the similar values of k , for the compounds on the two substrates suggest that the rigidity of the matrix is approximately the same in the two a-cyclodextrin mixtures. However, this is somewhat of an oversimplification because other factors are involved such as the magnitude of the +t and k , values, which also determine the final phosphorescence quantum yield values. In the 0.05% mixture, it was indicated above that included and unincluded molecules would be present on this matrix. However, it is most likely that only the included molecules would contribute to the RTP on the 0.05% matrix. Evidence for this is that the RTP quantum yield from the 0.05% mixture (Table I) is lower
WflQ
f 0.02 0.12 f 0.02 0.05 f 0.02 0.05 f 0.01 0.18 f 0.04 0.15 f 0.03 0.07 f 0.01 0.08 i 0.01
Error based on propagation of error equations. than that from the 80% mixture and that the spectral features of compounds adsorbed on a-cyclodextrin-sodium chloride mixtures were sharper relative to substrates like filter paper (20). Further evidence that the unincluded molecules do not contribute significantly to the RTP involves the following: (a) no RTP was observed from the model compounds when adsorbed on a glucose-sodium chloride mixture or on NaCl; (b) the R T P signal observed from compounds adsorbed on a a-cyclodextrin-sodium chloride mixture is a t least 2-fold greater than that of RTP for the compounds adsorbed on a hexamaltaose-sodium chloride mixture. Hexamaltaose is the linear analogue of a-cyclodextrin. Table I11 also indicates that at -180 "C the 12, values of the compounds are considerably lower for the a-cyclodextrin mixtures, although to some extent k , still contributes to the loss of phosphorescence emission. Obviously, k , is a function of temperature, and the more rigid matrix a t -180 "C results in lower k,. At -180 "C, the k , values for a given compound are approximately the same in the two a-cyclodextrin mixtures. The k , values of B[flQ and phenanthrene on a-cyclodextrin-sodium chloride mixtures a t the two temperatures are shown in Table IV. By comparison of the k , values of the compounds adsorbed on 0.05% and 80% a-cyclodextrin mixtures, it can be seen that the corresponding k , values of B[flQ and phenanthrene are somewhat higher in the 80% mixture than in the 0.05% mixture at both temperatures. However, k , is approximately constant with temperature in a particular matrix for the compounds. In general, k , is dependent upon molecular structure and is affected slightly by the nature of the molecular environment (23). The difference in k , in the two matrices at room temperature for these compounds suggests that there is a difference in the molecular environment in the two a-cyclodextrin mixtures which would cause some minor structural changes in the phosphors in the solid matrix. Percentages of Radiative and Nonradiative Transitions. By use of the dt, &, and 4, values in Table I and 111, the percentages of radiative and nonradiative transitions were calculated for B[flQ and phenanthrene adsorbed on 0.05% and 80 % cu-cyclodextrin-sodium chloride mixtures as discussed by Ramasamy and Hurtubise (17). For example, for phenanthrene adsorbed on the 0.05% mixture at 23 "C, it was determined that & = 0.16, c#Jt = 0.38, and 4, = 0.04. Therefore, 16% of the absorbed photons were converted to fluorescence, 46% were involved in nonradiative transition from the singlet state to the ground state, and 38% were involved in the singlet to triplet transition (intersystem crossing). From the triplet state 4% of the transitions were converted to phophorescence and 34% were converted to nonradiative transitions from the triplet state to the ground state. Figures 2 and 3 show the energy diagrams for phenanthrene and B[flQ, respectively, adsorbed on 0.05% and 80% a-cyclodextrin-sodium chloride mixtures at 23 "C and at -180 "C. It is clear in Figure 2 that for phenanthrene the major loss of energy from the 0.05% mixture at room temperature is from internal conversion from the singlet excited state to the ground state. For the 80% mixture there is no loss of energy by internal conversion. One condition that favors internal conversion is if the potential
ANALYTICAL CHEMISTRY, VOL. 60, NO. 13, JULY 1, 1988 S!
-.ISC
1295
38%
ISC 5 9 %
'
E
4 I 96 L 0 %
R
9 Yo T
5 0 %l
SO
(b) 23°C
SO
(c)
63% ( c ) -I80'C
SO
IF
SIC
-180aC
I S C 71%
IP
."I to% l2i IC
P
2%
SO
Flgure 2. Energy diagrams for phenanthrene adsorbed on a-cyciodextrin-sodium chloride mixtures: (a) 0.05% mixture at 23 OC, (b) 80% mixture at 23 OC, (c) 0.05% mixture at -180 OC, (d) 80% mixture at -180 OC; So,ground state; Si, singlet state; Ti, triplet state; F, fluorescence; IC, internal conversion; ISC, intersystem crossing; P, phosphorescence.
energy curves for the excited singlet state and ground state undergo a surface crossing at a nuclear geometry available to the molecule as it undergoes low-energy vibrations (17,24). It was reported previously that phenanthrene was included in the a-cyclodextrin cavity in the 80% a-cyclodextrin-NaC1 matrix (20). In addition, it was discussed above that the amount of a-cyclodextrin present in the matrix affects how strongly the analyte would interact with the matrix. For example, in the 80% mixture, where excess a-cyclodextrin was present in the matrix, it was shown that most of the analyte was included in the a-cyclodextrin cavity (20). On the other hand, much less a-cyclodextrin was present in the 0.05% matrix; therefore, both included and unincluded molecules would be present on the surface. Thus,in the 0.05% mixture, phenanthrene would probably be more accessible to low-energy vibrations than in the 80% mixture. Figure 2 also shows that the increase in fluorescence and intersystem crossing for phenanthrene on the 80% mixture at 23 "C is primarily due to no energy loss by nonradiative transition from the singlet excited state. Furthermore, although phenanthrene gave a higher RTP yield in the 80% mixture, the 80% a-cyclodextrin sample also gave a relatively large percentage of nonradiative loss of energy from the triplet state. BMQ,on the other hand, shows different results. It was reported previously that the nitrogen-containing ring of this compound was inside the a-cyclodextrin cavity in a a-cyclodextrin-sodium chloride mixture, where the nitrogen was shielded from protonation by the hydroxyl groups at the rims of the a-cyclodextrin in the excited state (20). As suggested by the data in Figure 3, both the 0.05% and 80% mixtures protect the B M Q molecule very well from internal conversion from the singlet excited state to the ground state at 23 "C. The 0% internal conversion for B M Q in the two mixtures indicates that the BV]Q was interacting strongly with the a-cyclodextrin in these surfaces. It is also evident in Figure 3 that the intersystem crossing is greater for this compound on the 0.05% mixture compared to the 80% mixture at room
ISC 38% ( d ) -180°C
Flgure 3. Energy diagrams for B[f]Q adsorbed on a-cyciodextrinsodium chloride mixtures: (a) 0.05% mixture at 23 OC, (b) 80% mixture at 23 OC, (c) 0.05% mixture at -180 OC, (d) 80% mixture at -180 O C ; So, ground state; S,,singlet state; T,, triplet state; F, fluorescence; IC, internal conversion; ISC, intersystem crossing; P, phosphorescence.
temperature and -180 "C. The increase in intersystem crossing, however, does not lead to an increase in phosphorescence emission on the 0.05% mixture, but instead an increase in nonradiative loss of energy from the triplet state is observed. The higher percentage of nonradiative energy loss from the triplet state for this compound on the 0.05% mixture suggests that the molecule was not included as effectively in this mixture, which was discussed above. At -180 "C, similar observations can be seen for the two compounds (Figures 2 and 3). No internal conversion from the singlet state to the ground state is observed in the two a-cyclodextrin-sodium chloride mixtures indicating that the model compounds are held rigidly. Also, it can be seen from Figures 2 and 3 that the increase in phosphorescence emission at low temperature for compounds adsorbed on the 80% mixtures is a result mainly of a lower nonradiative energy loss from the triplet state in this substrate compared to the compounds on the 0.05% mixture. Registry No. BMQ, 85-02-9; a-CD, 10016-20-3; sodium chloride, 7647-14-5; phenanthrene, 85-01-8.
LITERATURE CITED (1) Hurtubise, R. J. Solid-Surface Luminescence Analysis; Marcel Dekker: New York, 1981. (2) Vo-Dlnh, T. Room Temperature Phosphorimetry for Chemical Analysis; Wlley: New York, 1984. (3) Von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1978, 48, 1784. (4) Wellons, S. L.; Paynter, R. A.; Wlnefordner, J. D. Spectrochim. Acta, Part A 1974, 30A, 2133. (5) Vo-Dlnh, T.; Gammage, R. B.; Martlnez, P. R. Anal. Chim. Acta 1980, 178, 313. (6) Vo-Dinh, T.; Martinez, P. R. Anal. Chim. Acta 1981, 125, 13. (7) Su, S. Y.; Asafu-Adjaye, E.;Ocak, S. Analyst (London) 1984, 109, 1019. (8) Vannelll, J. J.; Schulman, E. M. Anal. Chem. 1984. 56, 1030. (9) Vo-Dinh, T. Environ. Sci. Technol. 1985, 19, 997. (10) Vo-Dlnh, T.; Bruewer, T. J.; Colovos, G. C.;Wagner, T. J.; Jungers, R. H. Environ. Scl. Technol. 1984, 18. 477. (11) Schulman, E. M.; Parker, R. T. J. Phys. Chem. 1977, 8 7 , 1932. (12) Von Wandruszka, R. M. A.; Hurtublse, R. J. Anal. Chem. 1977, 4 9 , 2164.
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Dalterlo, R. A.; Hwtublse, R. J. Anal. Chem. 1084, 56, 336. Nidey. 0. J.; Seybold, P. G. Anal. Chem. 1978, 50, 1577. McAleese, D. L.; Dunlap, R. B. Anal. Chem. 1984, 56, 2244. Ramasamy. S. M.; Hurtubise, R. J. Anal. Chem. 1987, 59, 432. Rarnasamy, S. M.; Hurtubise, R. J. Anal. Chem. 1987, 5 9 , 2144. Bello, J. M.; Hurtubise, R. J. Anal. Len. 1988, 19, 775. Bello, J. M.; Hurtubise, R. J. Appl. Spectrosc. 1988. 40, 790. Bello, J. M.; Hurtubise, R. J. Anal. Chem. 1987, 59, 2395. Ramasamy, S. M.; Senthllnathan, V. P.; Hurtubise, R. J. Anal. Chem. 1986, 58, 612. (22) French, D.; Levlne, M. L.; Pazur, J. H.; Norberg, E. J. Am. Chem. SOC. 1949, 7 1 , 353.
(13) (14) (15) (16) (17) (18) (19) (20) (21)
(23) Hercules, D. M. Fluorescence and Phosphorescence Analysis ; Wlley: New York, 1966; p 26. (24) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1978; pp 176-183.
RECEIVED for review October 20, 1987. Accepted February 16, 1988. Financial support for this project was provided by the Department Of Division Of Basic Science% Grant DE-FG02-86ER13547.
Direct Uranium Trace Analysis in Plutonium Solutions by Time-Resolved Laser-Induced Spectrofluorometry Thierry Berthoud, Pierre Decambox, Barbara Kirsch, Patrick Mawhien,* and Christophe Moulin
CEA/IRDI/DERDCA/DCAEA/SEA-SEACC, Centre d’Etudes Nucl&aires, 92265 Fontenay-aux-Roses C&dex,France
Thls paper describes a new procedure for the dlrect determhretkn of uankwn h pbtonknsobUon6 urlng thre-readv.d
epectr-etIy.
manalytlcalmcr(hodl8batwduponthe
use of sulfuric add as a dHutlon m e d b and leads to a detection lbnft of U/Pu = 0.2 ppm. A formula taklng Into account both quenchtng and Inner-fllter effects Is presented. It perthe calculatlon of the detectlon llmH of uranium In complex aolutlonr, and could be used to obtaln a theoretlcal llmft of detectlon for slmilar applcatlons.
Laser-induced spectrofluorometry is one of the most sensitive techniques for trace analysis of uranium. Uranium fluorescence has been measured in several media such as sulfuric or phosphoric acid solution (I,2) or fused pellets after coprecipitation of the uranium with calcium fluoride (3). The high sensitivity of the measurements is obtained by temporal resolution which permits discrimhation against short lifetime fluorescers, such as organic materials (4). For very complex solutions, several problems can occur due to the strong quenching effect of various elements (5-7). In the fusion method, a preliminary extraction separates the uranium from most of the interfering substances. Nevertheless, many factors affect the fluorescence in the fused pellet method. The procedure requires strictly regulated conditions of flux composition, temperature, and heating time. From an analytical point of view, this method is unsatisfactory and difficult to use routinely, especially in the nuclear industry. For this reason, the most common method used in such cases is the measurement in liquid media, for which the quenching problem can be solved by two ways: First, the fluorescence measurement is achieved after a liquid-liquid extraction that separates uranium from quenching species. The uranium is then back extracted into molar orthophosphoric acid which is the medium giving the best fluorescence yield. This procedure has been successfully used by Young and Deason (8) with T B P as extractant. Second, the fluorescence measurement is achieved after simple dilution of the sample in orthophosphoric acid to such an extent that quenching by impurities no longer influences *Author to whom correspondence should be sent. 0003-2700/88/0360-1296$01.50/0
the analysis. This latter procedure has the advantage of being very quick and very simple to perform because it does not require any chemical preparation or extraction. For these reasons this procedure is generally preferred by laboratories of the nuclear industry for which uranium determination requires glovebox manipulations. Nevertheless, when quenching species are present at a high concentration, it is necessary to make a large dilution leading to a very low concentration of uranium and, in such cases, the fluorescence apparatus must be very sensitive to yield satisfactory results. This report describes a new procedure for the direct analysis of uranium in solutions containing plutonium with a homemade high-performance spectrofluorometer adapted for glovebox manipulations. Samples are diluted in sulfuric acid media. Both the time-resolved spectrum and the fluorescence lifetime are recorded and the inhibition coefficient of plutonium is obtained by the Stern-Volmer equation. From these results, the uranium detection limit is evaluated in the presence of plutonium. EXPERIMENTAL SECTION Apparatus. The experimental setup is shown schematically in Figure 1. A nitrogen laser (Model GBM SOPRA, Bois Colombes, France) operating at 337 nm and delivering about 1 mJ of energy in a 5-ns pulse with a repetition rate of 25 Hz was used as the excitation source. The beam was focused by a 10 cm focal length lens into a 4-mL quartz cell, located within the glovebox. The fluorescence is analyzed at right angles to the excitation beam by a monochromator (H10 vis, Jobin Yvon, Longjumeau, France). The fluorescence intensity is measured by a photomultiplier operating at a potential between 800 and 1200 V (Hamamatsu 9285,Hamamatsu, Japan). The pulsed photomultiplier signals were amplified by a laboratory-made amplifier and fed into an oscilloscope (Model Tektronix, Beaverton, OR) and a boxcar integrator (Model 162/165 PAR, Princeton, NJ). The boxcar was connected to a microcomputer (HP 86, Hewlett-Packard, Palo Alto, CA) to construct either the time-resolved fluorescence spectrum or the lifetime measurements for storage and display. The boxcar integrator was run by a data acquisition control unit (DI-AN Micro Systems, Ltd.,England). Reagents. Reagent grade chemicals and deionized distilled water were used throughout the procedure. Standard solutions of plutonium(IV) were prepared by suitable dilutions of reference stock solutions. For the evaluation of the uranium detection limit, a very pure solution (Sl) containing 3.02 g/L of plutonium in 4 0 1968 American Chemical Society
