Anal. Chem. 1084, 56,821-823
wavelength positions corresponding to the other two components are also seen in the excitation spectrum, with one wavelength each for 1,7-dihydroxynaphthaleneand 1-naphthol (Figure 2). The mixture second derivative RTF emission spectrum indicated the presence of 1,7-dihydroxynaphthalene, with three bands corresponding to that compound. However, one wavelength position corresponded to 1-naphthol,and there were no correlations for 2-naphthol. The mixture second derivative RTP emission spectrum is characteristic of the most intense RTP emitter of the three, 1-naphthol, with five bands corresponding to that compound. Also, three bands corresponding to 1,7-dihydroxynaphthalenewere indicated in the short wavelength region of the spectrum of Figure 2, but none was indicated for 2-naphthol. With this three-component mixture, the advantage of the second derivative technique for providing additional spectral information compared to conventional luminescence spectrometry is quite apparent because considerably more data for identification is available. In Figure 2, five of the mixture second derivative RTP excitation and six of the RTF emission bands corresponded to bands of two or three of the individual standards. This overlap of second derivative bands of the mixture with several of the standard bands is the result of very similar features in the zeroth derivative spectra of the individual components. Also, a few uncorrelated bands appear for the various mixture second derivative spectra (Figure 2). There can be several reasons for the appearance of uncorrelated bands in second derivative mixture spectra. The appearance of wings or combinations of wings can account for some of the uncorrelated bands. Griffiths et al. (11)have noted that the absence of a second derivative band can be caused by the overlap of a positive wing of one band with an adjacent negative band. Another possible cause for uncorrelated bands occurs with the combination of two or more overlapping zeroth derivative luminescence bands yielding a new zeroth derivative band which is between the positions of the original bands. The second derivative band would then indicate the position of this new band. Because of the appearance of a few uncorrelated bands, there is some ambiguity about the presence of only three components, but there is no ambiguity about the identification of the three components. Two other three-component mixtures containing 1naphthol/2-naphthol/4-phenylphenoland 2-naphthol/l,7dihydroxynaphthalene/4-phenylphenolwere also examined by the zeroth and second derivative luminescence technique. In these cases, each component was identified by the various derivative luminescence spectra. Several two-component mixtures were examined by the zeroth and second derivative luminescence technique, with 150 ng of each component adsorbed can filter paper. The two-component mixtures examined included l-naphthol/2naphthol, l-naphthol/4-phenylphenol,l-naphthol/l,7-dihydroxynaphthalene, 2-naphtholf4-phenylpheno1, 2naphthol/ 1,7-dihydroxynaphthalene,and 4-phenylphenolf 1,7-dihydroxynaphthalene. Two two-component mixtures containing 4-phenylphenol and 1,7-dihydroxynaphthalene,
021
where one component was in 10-fold excess (300 ng and 30 ng), were also examined. For all of the two-component mixtures examined, each component in the mixture showed characteristic second derivative bands which clearly indicated its presence. A four-component mixture containing 100 ng each of 1naphthol, 2-naphthol, 1,7-dihydroxynaphthalene,and 4phenylphenol adsorbed on filter paper was examined by the zeroth and second derivative technique. Several characteristic bands in the derivative spectra appeared for 1-naphthol, 2naphthol, and 1,7-dihydroxynaphthalene,which indicated their presence. The bands that appeared for 4-phenylphenol, however, overlapped other component bands in each case. The presence of 4-phenylphenol was therefore not directly indicated. This study has shown that by using RTP excitation and emission and RTF emission zeroth and second derivative spectra from compounds with very similar spectral characteristics, identification of several components in various mixtures can be accomplished. The additional spectral information provided by using both phosphorescence and fluorescence spectra facilitated the identification of mixtures. The advantages of solid surface luminescence analysis, namely, good sensitivity and selectivity, small sample size, and ease of sample handling, coupled with the second derivative technique, make it a valuable method for trace organic analysis of simple mixtures.
ACKNOWLEDGMENT Financial support for this project was provided by the Department of Energy, Division of Basic Energy Sciences, Contract No. DE-AC02-80ER10624. Registry No. 1-Naphthol, 90-15-3; 2-naphthol, 135-19-3; 1,7-dihydroxynaphthalene,575-38-2; 4-phenylphenol, 92-69-3. LITERATURE CITED (1) Hurtubise, R. J. "Solld Surface Luminescence Analysis: Theory, Instrumentatlon, Applications"; Marcel Dekker: New York, 1981. (2) Parker, R. T.; Freedlander. R. S.; Dunlap, R. B. Anal. Chim. Acta 1980, 120, 1. ( 3 ) Parker, R. T.; Freedlander, R. S.; Dunlap, R. B. Anal. Chim. Acta 1980, 719, 189. (4) Ward, J. L.; Walden, G. L.; Wlnefordner, J. D. Talanta 1981, 2 8 , 201. (5) Vo-Dinh, T.; Winefordner, J. D. Appl. Specfrosc. Rev. 1977, 13, 261. (6) Ford, C. D.; Hurtublse, R. J. Anal. Left. 1080, 13 (A6), 485. ( 7 ) Dalterio, R. A.; Hurtublse, R. J. Anal. Chem., in press. (8) Dalterio, R. A.; Hurtubise, R. J. Anal. Chem. 1982, 5 4 , 224. (9) Vo-Dinh, T.; Gammage, R. B. Anal. Chim. Acta 1979, 707, 281. (IO) Talsky, G.; Mayring, L.; Kreuzer, H. Angew. Chem., I n t . Ed. Engl. 1978, 17, 785. (11) Griffiths, T. R.; King, K.; St. A. Hubbard, H. V.; Schwing-Welll, M. J.; Meullemeestre, J. Anal. Chim. Acta 1982, 743, 163.
R. A. Dalterio R. J. Hurtubise* Chemistry Department The University of Wyoming Laramie, Wyoming 82071
RECEIVED for review October 13,1983. Accepted December 23, 1983.
Photoacoustic Spectroscopy of Matrix-Isolated Polycyclic Aromatic Compounds Sir: Polycyclic aromatic compounds (PACs) are pervasive in our environment. Many are known or suspected carcinogens, or precursors to carcinogenic metabolites ( I ) . The principal anthropogenic source of environmental PACs is the
incomplete combustion of organic matter, primarily fossil fuels (2). The detection and quantification of these toxic compounds in environmental matrices have become high-priority analytical problems. Because many PACs fluoresce with high
0003-2700/84/0356-0821$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
quantum yield, fluorescence spectroscopy has become a popular and sensitive method for their determination ( I ) . However, there are many PACs that are only weakly fluorescent or nonfluorescent. For those, the photophysics of the lowest singlet excited state (SI)is dominated by nonradiative depopulation. The n-heterocycles with (n,?r*)SI states serve as examples of this behavior (3). Because the creation of photoacoustic signals also depends on nonradiative release of electronic excitation energy, it is conceivable that photoacoustic spectroscopy (PAS)could be a particularly advantageous technique for the determination of nonluminescent PACs. In this paper we demonstrate the feasibility of measuring dye laser-excited photoacoustic spectra of PACs isolated in low-temperature matrices.
EXPERIMENTAL SECTION Quinoline and 2,6-dimethylquinoline, with a specified purity of 98%, were obtained from Aldrich Chemical Co. (Milwaukee, WI) and used without further purification. Nitrogen gas (99.997%) was obtained locally. Xenon gas (99.9%) was purchased from Air Products (Allentown, PA). Rhodamine 6G (Eastman Kodak, Rochester, NY) and Kiton Red 620 (Exciton Chemical Co., Dayton, OH) laser dyes were employed in a flashlamppumped dye laser to cover the wavelength ranges 305-317 nm and 315-328 nm, respectively. The equipment and procedure for preparing matrix-isolated (MI) PAC samples have been described elsewhere (4,5). Because the PACs we have studied are relatively volatile at room temperature, they were introduced to the vacuum deposition chamber of the cryostat using the side arm tube delivery arrangement described by Hembree and co-workers (5); its temperature was controlled with liquid nitrogen or dry ice/solvent baths. Our matrices were deposited in a flowing liquid helium cryostat (Air Products, Allentown, PA; Model LT-3-10 “Heli-Tran”). The design of this refrigerator provided for transfer of boiling liquid helium from a storage Dewar via a dual-jacketed liquid helium transfer line to a massive cold finger block. A sapphire substrate for codeposition of the matrix gas and the PAC sample was mounted in an oxygen-free copper clamp in good thermal contact with the cold finger. The cryostat was capable of cooling this sample window to 4.2 K; however, it was usually operated at 5-20 K. Temperature measurements were made with a digital temperature indicator/controller (Air Products, Model ADP-E) using a chrome1 vs. gold-0.07 atom % iron thermocouple. The photoacoustic (PA) detector is similar to one described earlier (6) used to record PA spectra of neodymium hydroxide samples at 10 K. A two-piece, oxygen-freecopper clamp assembled with nylon screws compressed a 1.6 mm thick, 9 mm diameter lead zirconate titanate (PZT)disk (EdoWestern Corp., Salt Lake City, UT; Type EC-65) against the sapphire sample substrate (1.6 mm thick). Short 28 gauge wires connected the copper clamp halves (i.e., the signal pick-up electrodes) to electrical feedthrough in the cryostat window port. Both copper clamp halves were coated with a plasma-sputtered aluminum film to reduce the PA background signal produced by scattered ultraviolet laser light. An indium gasket placed between the sapphire substrate and the copper clamp and another gasket at the threaded connection of the clamp with the cold finger ensured adequate thermal contact. Nonaq stopcock grease (Fisher Scientific, Fair Lawn, NJ) was used at all acoustic interfaces to maximize acoustic coupling. An overall block diagram of the pulsed laser PAS instrumentation is shown in Figure 1. A flashlamp-pumped dye laser (Chromatix, Sunnyvale, CA; Model CMX-4) was used as the excitation source for the PAS measurements. The temporal width of the light pulses was -1 ws (fwhm) and the nominal spectral bandwidth was 3 cm-l. Production of ultraviolet output was accomplished by intracavity second harmonic generation with angle phase-matched ammonium dihydrogen phosphate crystals. Typical pulse energies were 0.02-0.2 mJ in the 290-345 nm range. The laser was operated at a repetition rate of 5 or 10 Hz. The optical beam emerging from the laser was passed through a Corning 7-54 ultraviolet band-pass optical filter in order to remove the residual visible component of the beam. A quartz plate beam splitter was used to deflect 5-10% of the laser beam to a photodiode detector/preamplifier (Molectron Corp., Sun-
FLOWING LIQUID HELIUM
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Figure 1. Block diagram of the instrumentation for laser-excited PAS.
nyvale, CA; Model 141) to correct the PA spectra for variation in the laser pulse energy as a function of wavenumber position in the dye tuning curve (vida infra). A quartz lens was used to focus and position the laser beam at the sample. The signal generated by the PZT disk for each laser pulse was amplified by a wide band preamplifier (EG&GPrinceton Applied Research, Princeton, NJ; Model 115)with a gain of 20 or 200. This preamplifier was placed as close to the piezoelectric transducer as possible (25 cm) in order to minimize attenuation of the signal due to capacitive cable loading. The resulting P A signal was enhanced with a filter amplifier (Tektronix Inc., Beaverton, OR; Model 7A22) plug-in of a storage oscilloscope (Tektronix, Inc., Model 7633). The fdter amplifier was used in the band-pass mode with upper and lower corner frequencies of 1MHz and 10 kHz, respectively,and a roll-off of -6 dB/octave. The filter amplifier provided pass-band gains of 10 to 50. The amplified and filtered signal was then processed with a boxcar averager (EG&G Princeton Applied Research; Model 162/164). The PA signal wave form produced by the PZT disk for each laser pulse was a complex, damped bipolar oscillation, similar to that reported by Pate1 and Tam (7). Judging from the oscillation periods observed, the frequencies present appeared to be in the 100-500 kHz range. The gate of the boxcar averager (0.5 119) was set to one of the predominant, early excursions, usually 10-100 ys following the laser pulse. The actual time delay varied from sample to sample. Twenty pulses were averaged to give 4 s or 2 s time constants for laser repetition rates of 5 Hz or 10 Hz, respectively. The boxcar averager output was used as the numerator input to a ratiometer (EG&GPrinceton Applied Research; Model 118). The reference photodiode (vida supra) signal was processed by a pulsed photometer (MolectronCorp.;Model LP20) that averaged 30 pulses and provided a dc output proportional to the average pulse energy. This signal served as the denominator input to the ratiometer. For plotting spectra, the ratiometer output (i.e., PA signal/laser pulse energy) was used to drive the Y axis of an analog X-Y recorder. A digital-to-analog converter was used in conjunction with the laser wavenumber scanner output to drive the recorder X axis. Electronic trigger pulses, synchronous with the laser firing, were used to trigger the filter amplifier-oscilloscope, boxcar averager, and pulsed photometer. Conventional absorption spectra were recorded with a Cary 14 absorption spectrophotometer (Varian, Palo Alto, CA), with a spectral bandwidth of 0.1-0.2 nm. All absorption spectra of room-temperature liquid solutions were obtained by using a 1-cm path length fused quartz cuvette. The birefringent filter tuning element of the CMX-4 laser results in scans that are linear in energy (cm-I). The laser PA spectra shown here are superimposed on a wavelength scale (nm) for comparison with the conventional absorption spectra. This yields approximately 0.3 nm errors for the PA spectra at the
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984 I
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Figure 2. Spectra of quinoline: (A) MI-PA spectrum of 5 pg of quinoline in N2 at 10 K; (6)MI-absorption spectrum of the same sample; (C) roomtemperature solution absorption spectrum (1.5 X lo4 M in heptane).
extremes of the scan range. In addition, the conventional absorption spectra are plotted on an absorbance (i.e., logarithmic) scale; the PA response is linear in the fraction of the incident light absorbed. The line widths (full width at half fraction absorbed) reported here for the Gary 14 data have been corrected for this scale difference.
RESULTS AND DISCUSSION Spectra of quinoline are presented in Figure 2. The MI-PA spectrum of quinoline (5 pg isolated in N2 a t 10 K) is shown in Figure 2A. The MI-absorption spectrum of the same quinoline sample matrix is shown in Figure 2B. For comparison, the room-temperature absorption spectrum of quinoline (1.5 X M in heptane) is shown in Figure 2C. The MI-PA and MI-absorption spectra have identical bandwidths (185 cm-l) and exhibit more than a 3-fold resolution enhancement relative to the room-temperature solution spectrum (690 cm-I). The peak wavelength position of both the PA and absorption spectra of the MI sample is blue shifted by 2.5 nm (250 cm-l) from the room-temperature spectrum maximum. Although the wavelength maximum and bandwidth are approximately the same in the MI-absorption and MI-PA spectra, the base line of the PA spectrum slopes upward in the blue direction relative to the base line of the MI-absorption spectrum. This sloping base line is attributed to an instrumental artifact resulting from slowly varying dc offsets, emphasized by the spectral normalization process of the ratiometer. The MI-PA spectrum of quinoline was also measured in Ar and Xe and exhibited comparable bandwidth; however, a blue shift in the wavelength maximum was not observed for the Xe matrix. The MI-PA spectrum of 2,6-dimethylquinoline (1pg in Xe at 10 K) in the 315-327-nm region is shown in Figure 3A; the corresponding room-temperature solution ( lo4 M in hexane) absorption spectrum is shown in Figure 3B. No significant blue shift in the wavelength maximum was observed. The blue shift (relative to room-temperature solution spectra) observed for spectra of PACs in Ar or N2 was found to be characteristically absent for PAC spectra in Xe matrices. A 2-fold bandwidth reduction is observed for 2,6-dimethylquinoline: 290 cm-l in the MI-PA spectrum compared with 670 cm-l in the solution spectrum. Figure 3A indicates that the detection
320
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Figure 3. Spectra of 2,6-dimethylqulnoline: (A) MI-PA spectrum of 1 pg of 2,6-dimethylquinoline in Xe at 10 K; (B) room-temperature M in hexane). solution absorption spectrum
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limit (SIN 2) for this compound by the MI-PA technique is approximately 500 ng.
CONCLUSION These spectra of quinoline and dimethylquinoline are the first reported photoacoustic spectra of polycyclic aromatic compounds isolated in vapor-deposited, low-temperature matrices. The present limit of detection is ca. 0.5-1 pg for these particular compounds. We are continuing to explore the analytical utility of photoacoustic spectroscopy for the determination of these and other polycyclic aromatic compounds isolated in low-temperature rare gas matrices. Registry No. N2, 7727-37-9;Xe, 7440-63-3;quinoline, 91-22-5; 2,6-dimethylquinoline, 877-43-0. LITERATURE CITED (1) Lee, M. L.;Novotny, M. V.; Bartie, K. D."Analytical Chemistry of Polycyclic Aromatic Compounds"; Academic Press: New York, 1981. (2) Guerin, M. R. "Polycyciic Hydrocarbons and Cancer"; Geiboin, H. V., T'so, P. 0.P., Eds.; Academic Press: New York, 1978; pp 3-42. (3) Turro, N. J. "Modern Molecular Photochemistry";Benjamin/Cummings: Menlo Park, CA, 1978. (4) Wehry, E. L.;Mamantov, G. Anal. Chem. 1979, 5 1 , 643A-656A. (5) Hembree, D. M.; Hinton, E. R.,Jr.; Kemmerer, R. R.; Mamantov, G.; Wehry, E. L. Appl. Spectrosc. 1979, 33, 477-480. (6) Shaw, R. W.; Howell, H. E. Appl. Opt. 1982, 27, 100-103. (7) Patel, C. K. N.; Tam, A. C. Rev. Mod. Phys. 1981, 53, 517-550.
' Present address:
The Procter & Gamble Company, Cincinnati, OH.
H. E. Howell' Gleb Mamantov E. L. Wehry Department of Chemistry University of Tennessee Knoxville, Tennessee 37996
R. W. Shaw* Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831 RECEIVED for review October 17, 1983. Accepted December 21,1983. This research wm sponsored by the Office of Energy Research, U S . Department of Energy, under Contract W7405-eng-26 with the Union Carbide Corporation and by the National Science Foundation (Grant CHE-8025282). H.E.H. acknowledges receipt of an Oak Ridge Associated Universities Fellowship.
