Anal. Chem. 1996, 68, 1599-1604
Development of a Room-Temperature Phosphorescence Fiber-Optic Sensor A. D. Campiglia,† J. P. Alarie, and T. Vo-Dinh*
Advanced Monitoring Development Group, Health Sciences Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830-6101
The design of a new fiber-optic sensor based on solidsurface room-temperature phosphorimetry is presented for the analyses of polycyclic aromatic hydrocarbons in water samples. Analytical figures of merit are given for several compounds of environmental importance. Limits of detection at the nanograms per milliliter level were estimated for pyrene, benzo[e]pyrene, benzo[ghi]perylene, 1,2:3,4-dibenzanthracene, coronene, and 2,3-benzofluorene. The linearity of response of the phosphorescence sensor was evaluated, showing a fairly linear behavior for quantitative analysis. Finally, the feasibility of monitoring polycyclic aromatic hydrocarbons in aqueous media was illustrated by identifying pyrene in a contaminated groundwater sample. We first report the development of a fiber-optic sensor using a room-temperature phosphorescence probe. The sensor is evaluated for the analysis of polycyclic aromatic hydrocarbons (PAHs) in water. The occurrence of PAHs in the aqueous environment has been extensively investigated over the past two decades.1,2 Analytical investigations have put special emphasis on developing sensitive techniques to detect trace levels of PAHs. Depending on the compound of interest and the water supply, the analytical approach should be sensitive enough to determine concentrations ranging from less than 1 ppt (pg/mL) in pure groundwater supplies to greater than 1 ppm (µg/mL) in heavily contaminated sewage.1,2 Traditional techniques for the analyses of PAHs in waters are usually time consuming and follow the general pattern of sample collection, transport, treatment, and instrumental detection.3,4 The sample treatment generally includes a cleanup procedure and a preconcentration step. Serious errors, however, can arise from handling samples with trace levels of pollutants. Contamination and losses by adsorption at the surfaces of containers are among the most common causes of inaccurate results for PAHs analyses.3 In a tentative effort to reduce handling and transport of samples, and therefore minimize inaccuracy, cost, and analysis time, recent studies have been focused on developing analytical † Permanent address: Departmento de Quimica, Universidade de Brasilia, DF CEP 70910-900, Brazil. (1) Grimer, G., Ed. Environmental Carcinogens: Polycyclic Aromatic Hydrocarbons; CRC Press: Boca Raton, FL, 1983. (2) Afghan, B. K., Chau, A. S. Y., Eds. Analysis of Trace Organics in the Aquatic Environment; CRC Press: Boca Raton, FL, 1990. (3) Futoma, J. D.; Smith, S. R.; Smith, E. T.; Tanaka, J. Polycyclic Aromatic Hydrocarbons in Water Systems; CRC Press: Boca Raton, FL, 1981. (4) Vo-Dinh, T. Chemical Analysis of Polycyclic Aromatic Compounds; Wiley: New York, 1990.
0003-2700/96/0368-1599$12.00/0
© 1996 American Chemical Society
techniques for field monitoring applications.5 A promising approach involves the detection of PAHs by laser-induced fluorometry using optical fibers.6-12 Limits of detection (LODs) in the nanomolar level were obtained for pyrene with a cyclodextrinbased sensor.6 β-Cyclodextrin (β-CD) was immobilized at the tip of an optical fiber, and pyrene’s fluorescence was excited with a He-Cd laser (325 nm). According to the authors,6 the selectivity of the device toward pyrene and other PAHs resulted from the hydrophobic nature of the β-CD tip, which did not form stable complexes with heterocyclic compounds usually present in contaminated waters. The selectivity of fluorescence sensors has also been improved by time-resolved fluorometry.7-12 The addition of an independent dimension such as time to the emission spectra of complex mixtures of PAHs reduced the overlapping of fluorescence bands. In all cases, fluorescence excitation was performed with a nitrogen laser, and LODs at the picograms per milliliter level were obtained for most compounds studied.7-12 Analysis of PAHs by phosphorimetry is a well-established method which has been described in detail elsewhere.13,14 In particular, solid-surface room-temperature phosphorimetry (SSRTP) has been shown to be a rapid, selective, and sensitive approach. Basically, the RTP procedure consists of enhancing PAH signals by using a heavy atom salt in the solid substrate. Among the heavy atom salts and substrates tested, thallium(I) nitrate and filter paper appear to be the best combination to obtain limits of detection at the nanogram and subnanogram levels.15-18 To the extent of our knowledge, this is the first time that an SSRTP fiber-optic sensor is reported. Previous studies involving fiber optics for phosphorescence measurements dealt with im(5) Klainer, S. M.; Thomas, J. R.; Francis, J. C. Sens. Actuators B 1993, 11, 81. (6) Alarie, J. P.; Vo-Dinh, T. Talanta 1991, 38, 529. (7) Chudyk, W. A.; Carrabba, M. M.; Kenny, J. E. Anal. Chem. 1985, 57, 1237. (8) Inmam, S. M.; Thibado, P.; Theriault, G. A.; Lieberman, S. H. Anal. Chim. Acta 1990, 239, 45. (9) Miuone, A.; Smith, B. W.; Winefordner, J. D. Talanta 1990, 37, 111. (10) Niessner, R.; Panne, U.; Schroder, H. Anal. Chim. Acta 1991, 255, 231. (11) Panne, U.; Niessner, R. Sens. Actuators B 1991, 13-14, 288. (12) Robbat, A.; Tyng-Yun, L.; Abraham, B. M. Anal. Chem. 1992, 64, 1477. (13) Vo-Dinh, T. Room Temperature Phosphorescence for Chemical Analysis; Wiley: New York, 1984. (14) Hurtubise, R. J. Phosphorimetry: Theory, Instrumentation, and Applications; VCH: New York, 1990. (15) Vo-Dinh, T.; Lue Yen, E.; Winefordner, J. D. Talanta 1977, 41, 2131. (16) Lue-Yen Bower, E.; Winefordner, J. D. Anal. Chim. Acta 1978, 102, 1. (17) Parker, R. T.; Freelander, R. S.; Schulman, E. M.; Dunlap, R. B. Anal. Chem. 1979, 51, 1921. (18) Su, S. Y.; Winefordner, J. D. Michrochem. J. 1982, 27, 151.
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Figure 1. Block diagram of the instrumental system. L1 is a convex lens; F1 and F2 are fiber-optics mounts; PMT is the photomultiplier tube; pre-amp is the amplifier.
proving the instrumentation for low-temperature phosphorimetry19 and optosensing with conventional spectrofluorometers based on selective chemical sensors.20-22 In this study, we demonstrate that the SSRTP sensor is suitable for environmental monitoring of PAHs in water samples. Limits of detection at the parts per billion (ng/mL) level were estimated for pyrene, 2,3-benzofluorene, benzo[e]pyrene, benzo[ghi]perylene, 1,2:3,4-dibenzanthracene, and coronene. The linearity of response of the proposed sensor was also evaluated, which showed linear dynamic ranges extended over ∼3 orders of magnitude. Finally, the feasibility of monitoring PAHs in contaminated sites was demonstrated by the identification of pyrene in a groundwater sample. EXPERIMENTAL SECTION Reagents. All the chemicals were analytical-reagent grade and were used without further purification. Whatman No. 40 filter paper was employed as a solid substrate, and distilled, deionized water was used throughout. Pyrene, benzo[ghi]perylene, coro(19) Carrol, M. K.; Hieftje, G. M. Appl. Spectrosc. 1992, 46, 126. (20) Garcia, R. P.; Liu, Y. M.; Diaz-Garcia, M. E.; Sanz-Mendel, A. Anal. Chem. 1991, 63, 1759. (21) Valencia-Gonzalez, M. J.; Liu, Y. M.; Diaz-Garcia, M. E.; Sanz-Mendel, A. Anal. Chim. Acta 1993, 283, 439. (22) Alava-Moreno, F.; Diaz-Garcia, M. E.; Sanz-Medel, A. Anal. Chim. Acta 1993, 281, 637.
1600 Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
nene, benzo[e]pyrene, 1,2:3,4-dibenzanthracene (1,2:3,4-DBA), and 2,3-benzofluorene were obtained from Aldrich at the highest purity available. Thallium(I) acetate (TlOAc) was acquired from Sigma. Methanol was purchased from J. T. Baker Inc. Instrumentation. A schematic block diagram of the instrumental system is shown in Figure 1. Excitation of samples at 337 nm was performed by using a pulsed nitrogen laser (Laser Science VSL-337) with typical pulse energy of 120 mJ. The repetition rate was 20 Hz, and the pulse width was 3 ns. The laser beam was focused onto one end of a bifurcated optical fiber with an f/4 convex lens and directed to the surface of the solid substrate probe. The bifurcated optical fiber (Fiber Guide Ind., silica-silica glass, 150-cm length) consisted of a 600-µm central optical fiber surrounded by 18 optical fibers of 200-µm diameter (emission). The phosphorescence signal from the sample was transferred into a 10-cm-focal length monochromator (ISA Model DH-10). The output from the photomultiplier (Hamamatsu R928) was amplified by a laboratoryconstructed amplifier and fed into a gated boxcar averager (Standford Research Systems RS250). For all measurements, the delay and gate time were respectively 1 and 4 ms. The time constant of the boxcar was 1 s. A stepper motor was used to control the scanning of the monochromator. An analog-to-digital converter card (MetraByte DASH-16) was used for instrument control, timing, and data collection. The soft-
Figure 2. Schematic diagram of the SSRTP sensor.
ware employed to control the laser system was developed in our laboratory. The solid substrate was placed in the sample compartment of the RTP sensor (see Figure 2). Basically, the RTP sensor consisted of two cylindrical parts easily attached by concentric incised threads. The screw located in the stainless steel sheath kept the distance between the fiber optic and the surface of the solid substrate constant during all measurements. This parameter was optimized at the beginning of the experiments. A hole (0.3cm diameter) located at the distal end of the Teflon stud allowed the entrance of solution into the sample compartment of the sensor. A commercial spectrofluorometer (Perkin Elmer LS 50B) was used to record the excitation spectra of the studied compounds. Procedure. Filter paper was cut in 0.4-cm-diameter circles with a hole puncher. The circular substrates were immersed in a 0.1 M TlOAc solution (methanol/water, 50:50 v/v) for ∼1 min, dried under an infrared (IR) heat lamp for 5 min, and then stored in a desiccator to be used as solid substrate probe. All 10-3 M stock solutions of PAHs were prepared in methanol/ water (80:20 v/v). Working solutions were obtained by appropriate dilution with a methanol/water (80:20 v/v) solution. RTP measurements and spectra collection were performed after the substrates were dried for 5 min under an IR lamp. The drying step was carried out with the solid substrate in the sample compartment of the RTP sensor, since the hole in the distal end of the Teflon stud allowed direct irradiation of the reverse side of the filter paper. Although no effort was made to optimize the exact immersion time of the RTP sensor in the sample solution, we observed that a period of 30 s to 1 min was enough to obtain maximum RTP signal from the studied compounds.
RESULTS AND DISCUSSION Effect of Drying Time. The RTP emission from compounds adsorbed on solid substrates may be drastically reduced by interactions with other chemical species.13,21 One of the most efficient triplet quenchers is molecular oxygen.16,22,23 The degree of oxygen quenching is greatly enhanced by the presence of moisture in the matrix of the phosphor, mainly when paper is used as a solid substrate. Apparently, the presence of water molecules in the paper disrupts hydrogen bonds between phosphor and substrate, facilitating the transport of oxygen in the vicinity of the phosphor.24 In addition, water molecules might compete with the phosphor for binding sites on the solid support, thus decreasing the analyte-substrate adsorption process. This effect increases the probability of collisional and vibrational deactivation of the triplet state and reduces the intensity of the RTP signal.24 For maximum phosphorescence emission, therefore, it is recommended to dry the sample prior to detection. The duration of the drying step, however, will depend on several parameters, which include the physicochemical characteristics of the analyte and the substrate, the solvent, the drying device, and the temperature. The drying conditions for RTP detection of PAHs have been previously optimized.13 The usual procedure involves IR irradiation of the PAH solution spotted on the solid substrate. The phosphorescence emitted by the dry surface is then measured in the spectrofluorometer. For ethanolic solutions of PAHs, maximum signals have been observed after a 5-min drying time. In the case of the RTP sensor, the IR radiation was directed toward the reverse side of the substrate probed with the fiber optic. By this procedure, the analyte solution absorbed in the paper substrate and responsible for the emission of phosphorescence was not directly exposed to IR radiation. This modification could lead to longer drying times than those previously optimized.13 This possibility was evaluated by monitoring the phosphorescence emission of two PAHs as a function of drying time. The studies were carried out with pyrene and benzo[e]pyrene as model compounds, using 10-5 M analyte solutions in methanol/water (80:20 v/v), and a drying temperature of ∼80 °C was used during all the experiments. By removing the sensor from the IR radiation, RTP spectra and signal intensities were recorded at different intervals of time. Figure 3 shows the net RTP intensities of the model compounds as a function of drying time. The net signals plotted in the graphics were calculated from successive measurements of the blank and the analyte signals. Each value is the average of single measurements recorded from three paper substrates. It can be noted that the optimum drying time is slightly different for each compound. Pyrene emitted maximum intensity after a 5-min drying time, while benzo[e]pyrene showed maximum signal after an 8-min IR exposure. At 5 min, the RTP signal of benzo[e]pyrene was ∼76% of its maximum emission. In all cases, the spectral characteristics of the compounds were in agreement with those previously reported with conventional instrumentation.15-18 Figure 4 shows the emission spectrum of pyrene obtained with the RTP sensor. For both compounds, longer periods of IR irradiation caused drastic reductions in the intensities of the phosphorescence emissions. After a 20-min (23) Schulman, S. G. Molecular Luminescence Spectroscopy Methods and Applications: Part 1; Wiley-Interscience: New York, 1985. (24) Schulman, E. M.; Parker, R. T. J. Phys. Chem. 1977, 81, 1932. (25) Vo-Dinh, T.; Walden, G. L.; Winefordner, J. D. Anal. Chem. 1977, 49, 1126.
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Table 1. Reproducibility of the Solid-Surface Room-Temperature Phosphorescencea of PAHs Measured with the Fiber-Optic Sensor
Figure 3. Phosphorescence intensity as a function of drying time. Net RTP signals of 10-6 M solutions of pyrene and benzo[e]pyrene prepared in methanol/water (80:20 v/v); 0.1 M TlOAc was used to enhance phosphorescence emission.
compoundb
concnc (mM)
emission wavelengthsd (nm)
RSDe (%)
pyrene benzo[e]pyrene benzo[ghi]perylene
0.5 0.5 7.8
600, 654 543, 581 630, 687
6.4 10.3 5.9
a Whatman No. 40 filter paper was used as a solid substrate; 0.1 M TlOAc was employed as a phosphorescence enhancer. b Analyte solutions prepared in methanol/water (80:20 v/v). c Analyte concentration given a phosphorescence signal approximately equal to 3× the background signal. d Maximum emission wavelengths. The peak with maximum intensity is shown in italics. Wavelength variation for instrumental response is 5 nm. e Relative standard deviation; see text for definition.
Table 2. Solid-Surface Room-Temperature Phosphorescencea Limits of Detection of Some PAHs Obtained with a Fiber-Optic Sensor
compoundb
measurement wavelengths (nm)
pyrene benzo[ghi]perylene benzo[e]pyrene
337/600 337/630 337/543
LODsd in µM ng/mL 0.03 0.1 0.02
6 39 5
a Whatman No. 40 filter paper was used as a solid substrate; 0.1 M TlOAc was employed as a phosphorescence enhancer. b All analyte solutions were prepared in methanol/water (80:20 v/v). c Excitation/ emission wavelength. The emission wavelengths correspond to the maximum emission peaks of the compounds. d LODs estimated as the concentration of analyte that yields a net RTP signal 3× the standard deviation of the background.
Figure 4. Laser-induced RTP emission spectrum of a 10-6 M solution of pyrene obtained with a fiber-optic sensor; 0.1 M TlOAc was used to enhance phosphorescence emission. Excitation wavelength, 337 nm.
drying time, pyrene and benzo[e]pyrene signals decreased to 57% and 54% of their maximum values, respectively. At present, we do not have experimental data to explain such behavior. However, some plausible possibilities include evaporation of the compounds from the surface of the paper and/or photodecomposition of the analytes to form products with no RTP emission in the wavelength region monitored (between 450 and 700 nm). Reproducibility of Measurements. Table 1 shows the precision of measurements obtained with the RTP sensor. The relative standard deviations (RSDs) were calculated from the formula RSD ) [(sA+B)2 + sB2]1/2[100/IA-B]. Six measurements of the analyte signals and respective blanks were performed to obtain the standard deviation of the analyte and blank intensities (sA+B), the standard deviation of the blank (sB), and the average of the net analyte signal (IA-B). All measurements were taken at the maximum emission wavelengths of the compounds, after a 5-min drying time. The RSD values estimated with the RTP sensor were within the range usually obtained with conventional instrumentation.13,14 The poorer precision observed for benzo[e]pyrene is partially due to the higher variation of the blank signal at the maximum emission wavelength of the compound. The sB at 543 nm was considerably higher than the value observed in the wavelength regions of pyrene and benzo[ghi]perylene’s phosphorescence emission. In addition, the RTP signal of benzo[e]pyrene 1602 Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
showed higher variations than those observed for the other two compounds. Probably, better precision could have been obtained if the measurements had been taken at the optimum drying time for benzo[e]pyrene (8 min). Analytical Figures of Merit. Table 2 shows the LODs obtained with the RTP sensor for pyrene, benzo[e]pyrene, and benzo[ghi]perylene. The values were estimated with analyte solutions giving phosphorescence signals 10-20× the background signal, which was evaluated with 16 determinations of the blank. Although pyrene and benzo[e]pyrene emit phosphorescence in wavelength regions of higher background signal than benzo[ghi]perylene, their LODs were about 1 order of magnitude lower. The observed difference is probably the result of the excitation wavelength employed (337 nm). Pyrene and benzo[e]pyrene have their maximum excitation wavelengths at 340 and 335 nm, respectively. On the other hand, benzo[ghi]perylene is excited for maximum emission at 385 nm, and only its residual absorption is overlapped by the excitation line of the nitrogen laser. The excitation characteristics of the compounds were obtained in a commercial spectrofluorometer under the same experimental conditions as those used for the RTP sensor (data not shown). The linearity of response of the proposed sensor was also evaluated by using coronene, 1,2:3,4-DBA, and 2,3-benzofluorene as model compounds. Table 3 summarizes some analytical figures of merit for these three compounds. Each phosphorescence intensity plotted on the calibration graph was the average of three replicates. The studied compounds showed calibration curves with linear dynamic ranges (LDRs) extended over ∼3 orders of
Table 3. Analytical Figures of Merita of Some PAHs Obtained with an RTP Sensor
compoundb
emission wavelengthc (nm)
LDR(×102)
correl coeff
slope, log-loge
in µM
coronene 1,2:3,4-DBA 2,3-benzofluorene
530, 550 565, 606 505, 550
8.80 17.10 11.36
0.9984 0.9996 0.9972
1.15 0.94 0.76
0.05 0.2 0.3
LODf in ng/mL 15 56 6
a Whatman No. 40 filter paper was used as a solid substrate; 0.1 M TlOAc was employed as a phosphorescence enhancer. b All analyte solutions were prepared in methanol/water (80:20 v/v). c Maximum emission wavelengths. The peak with maximum intensity is shown in italics. d Linear dynamic range estimated by dividing the upper linear concentration by the limit of detection. e Calculated from the plot log Ip versus log concentration. f Limit of detection estimated as the concentration of analyte that yields a net RTP signal 3× the standard deviation of the background.
Figure 5. Laser-induced RTP emission spectrum obtained with the fiber-optic sensor of a contaminated groundwater sample (A) and a groundwater sample spiked with pyrene (B); 0.1 M TlOAc was used to enhance phosphorescence emission. Excitation wavelength, 337 nm.
magnitude. The slopes of the log-log plots, associated to the correlation coefficients of the calibration graphs, indicate a fairly linear relationship between phosphorescence intensity and analyte concentration. Within the LDR, the precision of measurements was ∼10% (n ) 3). Although there is a considerable difference between the excitation maximum of coronene (310 nm) and the excitation wavelength employed, this compound showed a LOD of the same order of magnitude as those estimated for pyrene and benzo[e]pyrene. 1,2:3,4-DBA and 2,3-benzofluorene, whose maximum excitation wavelengths are respectively located at 300 and 267 nm, showed LODs 1 order of magnitude higher than that for coronene. In all cases, however, the estimated values were in the nanograms per milliliter level of concentration, and therefore within the concentration range required for the analysis of PAHs in water samples. Identification of Pyrene in a Groundwater Sample. The feasibility of the RTP sensor for monitoring PAHs in aqueous environments was evaluated by using a groundwater sample contaminated with pyrene. Previous work involving fluorescence measurements with a selective cyclodextrin-based sensor showed the presence of the compound in the water sample.6 The sample collection was simulated by immersing the sensor in a sample vial containing ∼2 mL of groundwater. After a 5-min drying time, no phosphorescence emission from pyrene was detected at 600 nm. It was observed that the solid substrate was still humid. The presence of water in the sample matrix probably caused quenching of the analyte signal. The drying time was then increased to 10 min of IR irradiation. Figure 5A shows the RTP emission
spectrum of the water sample after this drying time. The presence of pyrene can be clearly noted at 600 nm. A confirmation of the spectral assignment of the 600-nm band to pyrene’s phosphorescence emission was performed by spiking 2 mL of water sample with 5 µL of a 10-4 M standard solution of pyrene (methanol/ water, 80:20 v/v). The RTP emission spectrum of the spiked solution is shown in Figure 5B. The increment in intensity observed at the maximum peak confirms the assignment of the 600-nm band to the presence of pyrene in the water sample. Using standard calibration data, the concentration of pyrene in the groundwater sample was estimated as 10.9 ng/mL. A unique feature of the RTP sensor is the possibility of confirming the identity of target compounds by spotting their standard solutions in the solid substrate. A humid substrate impregnated with groundwater sample was spotted with 5 µL of a 5 × 10-6 M pyrene standard solution. As expected, an increase in the intensity of the 600-nm peak was observed. This characteristic is of valuable use to confirm results of analysis in the field. CONCLUSIONS The SSRTP sensor reported in this study has the ability to identify PAHs in water supplies at the nanograms per milliliter level of concentration. In cases where lower LODs are required, the level of detection can be improved by using paper substrate previously treated with ultraviolet irradiation.26 By reducing the phosphorescence emission of the paper substrate, the background reduction treatment has improved the SSRTP LODs for several classes of compounds by at least 1 order of magnitude.27-29 The linearity of response of the proposed sensor was satisfactory, which showed its potential for quantitative analysis of PAHs. From the selectivity point of view, fiber-optical sensing based on phosphorescence emission offers several advantages over fluorescence optosensing. The higher selectivity is based on the nature of the phosphorescence phenomenon, since a small number of fluorescence interferents are able to reach the first triplet state. In addition, the interference from short-lived scattering and possible fluorescence signals can be easily avoided by using appropriate delay times. This feature will be particularly useful in minimizing or even eliminating the spectral interference of humic materials, a problem often related to fluorescence optosensing of PAHs.30 By spotting standard solutions on the paper substrates previously imbibed with the sample, on-site evaluation of chemical interferents and drying time effects can (26) Campiglia, A. D.; de Lima, C. G. Anal. Chem. 1987, 59, 2822. (27) Aucelio, R. Q.; Campiglia, A. D. Mikrochim. Acta 1994, 117, 75. (28) Aucelio, R. Q.; Campiglia, A. D. Talanta 1994, 41, 2131. (29) De Ribamar, F.; Campiglia, A. D. Michrochem. J. 1995, 52, 101. (30) Panne, U.; Lewitzka, F.; Niessner, R. Analusis 1992, 20, 533.
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be performed for the qualitative and quantitative analyses of PAHs. However, additional applications to a diverse variety of contaminated samples are necessary to further investigate the accuracy of this procedure. In the analysis of water samples containing mixtures of several phosphorescent compounds, conventional phosphorimetric techniques can be employed to increase the selectivity of the RTP sensor toward the target compound. These include selective external heavy atom perturbation,31 second derivative,32 and synchronous excitation techniques.33 Finally, we believe that the RTP optosensing setup is a valuable alternative for field applications. For instance, a portable system can be developed using a fiber-optic probe that can be rapidly dipped into a remote sample (undergroundwater), removed, dried, and subjected to phosphorescence analysis within a few minutes under field conditions. The laser system is very simple to operate
and relatively small. The alignment required is minimal, and measurement of RTP signals at long-wavelength visible regions allows one to use low-cost optical fibers. By changing the paper substrate in the sample compartment, the RTP sensor can be used for an infinite number of samples, which represents an important advantage when compared to non-regenerated sensing devices.
(31) Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 51, 1915. (32) Vo-Dinh, T.; Gammage, R. B. Anal. Chim. Acta 1979, 107, 261. (33) Vo-Dinh, T.; Gammage, R. B. Anal. Chem. 1978, 50, 2054.
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ACKNOWLEDGMENT This work was sponsored by the Office of Health and Environmental Research, U.S. Department of Energy, under Contract DE-AC0S-84OR2100. A.D.C. thanks Conselho Nacional de Desenvolvimento Cientifico e Tecnologico-CNPq for financial support. Received for review October 16, 1995. Accepted February 21, 1996.X
X
Abstract published in Advance ACS Abstracts, April 1, 1996.
