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Montano, L. A.; Ingle, J. D. Anal. Chem. 1979, 5 1 , 919-926. Veazey, R. L.; Nieman, T. A. Anal. Chern. 1979, 5 7 , 2092-2096. Veazey, R. L.; Nieman, T. A. J . Chromatogr. 1980, 200, 153-162. Steen, R. A.; Nieman, T. A. Anal. Chim. Acta 1983, 155, 123-129. Hinze, W. L. I n “Solution Chemistry of Surfactants”; Mittai, K. Ed.: Plenum: New York, 1979; Vol. 1, pp 79-127. (7) Spurlin. S.;Hinze, W.; Armstrong, D. W. Anal. Lett. 1977, 10, 997-1008. (8) Cline Love, L. J.; Skrilec, M.;Habarta, J. G. Anal. Chern. 1980, 52, 754-759. (9) Armstrong, D. W.; Hinze, W. L.; Bui, K. H.; Singh, H . N. Anal. Lett. 1981, 14, 1659-1667. (10) Weinberger, R.; Yarmchuk, P.; Cline-Love, L. J. Anal. Chem. 1982, 5 4 , 1552-1558. (11) Vil-Shanskii, V. A.; Epimakhov, Yu. K. Zh. Fiz. Khim. 1982, 56, 2577-2578. (12) Lasovsky, J.; Grambal, F. Acta Univ. Palacki. Olomuc. Fac. Rerum (2) (3) (4) (5) (6)
Nat. 1980, 6 1 / 6 5 , 57-61. Chem. Abstr. 1981, 94, 218980. (13) Paleos, C. M.; Vassiiopoulos, G.; Nikokavouras, J. J . Photochem. 1982, 78, 327-334. (14) Elworthy, P. H.; Mysels, K. J. J . Colloid Sci. 1986, 27, 331-341. (15) Hartley, G. S. J . Chem. SOC. 1938, 1968-1975. (16) Ray, A.; Nemethy, G. J . A m . Chem. SOC. 1971, 93, 6787-6793. (17) Hulsman, H. F. K . Ned. Akad. Wet., Proc., Ser. B : Phys. Sci. 1964, 67, 388-392. (18) Schick, M. J. J . Phys. Chem. 1964, 6 6 , 3585-3592.
RECEIVED for review April 15, 1983. Resubmitted December 19, 1983. Accepted March 1, 1984. This research was supported in part by the National Science Foundation (CHE81-08816).
Laser- Induced Fluorescence Spectrometry of Methylnaphthalene Derivatives Prepared in a Low-Temperature Aromatic Crystal Steven M. Thornberg and Jon R. Maple* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 Laser-excited Shpol’skii spectrometry (LESS) has been demonstrated to be a highly useful technique for the characterization of complex mixtures of structural isomers of polycyclic aromatic hydrocarbons (PAHs) (1,2) and of some PAH substitutional isomers (3,4). LESS involves the preparation of PAH samples in a low-temperature Shpol’skii matrix, which consists of n-alkanes with the same linear dimensions as the analyte molecules, prior to spectroscopic investigations (1-4). In this polycrystalline environment many PAHs have extremely narrow absorption bandwidths and a dye laser tuned to one of these bands can selectively induce fluorescence from a single component in complex samples. Although LESS has been spectacularly successful in characterizing complex mixtures of PAHs (1-4), substantial difficulties can be encountered for some PAH derivatives, such as derivatives of naphthalene or biphenyl, when the size of the solute and the n-alkane matrix molecules cannot be suitably matched. Even when the most appropriate Shpol’skii matrix is employed, fluorescence spectra of multicomponent mixtures of methylnaphthalenes cannot be adequately resolved, primarily because of inhomogeneous broadening (5). One possibility for minimizing spectral broadening is to eliminate the matrix. This has been explored by Hayes and Small in rotationally cooled laser induced fluorescence/gas chromatography (RC-LIF/ GC) experiments with naphthalene, 1-methylnaphthalene (1-MN), and %methylnaphthalene (2MN) (6, 7). At present results have not been presented for the dimethylnaphthalenes (DMNs) and the ultimate utility of RC-LIF/GC has not been established. In this paper we are presenting a different approach for minimizing spectral broadening. By preparation of the sample in a crystal consisting of an aromatic hydrocarbon with the same molecular dimensions as the analyte, the sample molecules should be incorporated into nearly identical sites within the crystalline lattice. Consequently, inhomogeneous broadening will be minimized and the high spectroscopic resolution that is associated with LESS should ensue. In order to demonstrate the analytical utility of a sampling medium consisting of a low-temperature aromatic crystal, methyl derivatives of naphth-lene are used as prototypes. Since durene has approximately the same molecular dimensions as the methylnaphthalenes (MNs), durene has been used as the crystalline host for all of the results presented here. Because a poly-
crystalline sampling medium has been employed, the results presented here can be viewed as an extension of both LESS and Wright’s technique (8) for the analysis of lanthanides in inorganic crystals.
EXPERIMENTAL SECTION Reagents. Durene and naphthalene (Aldrich) were sublimed prior to usage. All MNs were used as received and were obtained from Aldrich, except for 1,5-dimethylnaphthalene (l,B-DMN), which was purchased from Wiley Organics. The oil shale is an NBS Standard Reference Material (Lot No. 1580). Sample Preparation. All crystals were made by adding appropriate amounts of MN stock solution into a glass tube (0.6 cm X 6 cm) containing 0.1 g of durene. The tube was sealed and the sample was melted in an oil bath heated to 90 “C. Then the temperature was lowered to approximately 82 “C and the sample tube was moved to the wall of the beaker (which contained the oil bath). A slight temperature gradient, which was induced by squirting acetone on the beaker (at the point of contact between beaker and sample tube), was used to initiate crystallization. Vertical crystal fingers usually would start growing within about 5 s. The room temperature crystals were always cut along the “vertical” axis (relative to the glass tube). A slice of the original crystal was glued to the copper cold finger of a closed-cyclehelium refrigerator (Air Products, Model CSA-202)so that the crystal’s “vertical” axis would be perpendicular to the polarization of the laser beam. Initial cooling to 77 K was performed by immersing the finger in liquid nitrogen. The refrigerator was then turned on and the sample was maintained at 10 K for all experiments. Instrumentation. The excitation source for all experiments was an excimer laser (Lumonics TE-861T) pumped dye laser (Quanta Ray PDL-1E). The laser output was 1-2 mJ/pulse in normal operation with a temporal bandwidth of 10 ns fwhm. Dye laser output in the wavelength ranges 280-290, 297-305, and 310-330 nm was obtained with rhodamine 6G, Kiton Red, and DCM dyes, respectively,followed by frequency doubling with an angle-tuned KDP crystal. The dye laser induced fluorescence from the sample was dispersed by a 0.64-m grating monochromator (Instruments SA HR-640, equipped with a 1200 groove/mm holographic grating), which was set at a band-pass of about 0.1 nm. The fluorescence was detected by an RCA 8850 photomultiplier tube and a WG335 glass cutoff filter was used to minimize laser scattering interference. A gated integrator system was used for data collection. The signal from the photomultiplier tube was processed by an AD507KH (Analog Devices) operational amplifier in a current following configuration and then integrated by an Evans Associates
0003-2700/84/0356-1542$01.50/00 1984 American Chemical Society
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Table I. S, Origins for Methylnaphthalenes 0-0 wavelength, nm compound 316.7a naphthalene 319.5, 320.0 1-MN 2-MN 319.5, 320.5 1,3-DMN 321.1, 322.4 320.9 1,4-DMN 322.5a 1,5-DMN 321.1 2,3-DMN a 0-0 band was broad and this measurement is subject to uncertainty. Model 4130 gated integrator. In the normal mode of operation several pulses were integrated before the integrator output was digitized (by a Data Translation DT2781 Analog 1/0System) and the integrator reset, This signal averaging capability greatly reduced noise due to pulse to pulse fluctuations in the laser output. The number of pulses that were integrated prior to sampling was software controlled and usually was three or four. The digitized signal was stored on a floppy disk and plotted by an X-Y recorder (via a D/A port on the DT2781).
RESULTS AND DISCUSSION Qualitative Features of Methylnaphthalene Fluorescence Spectra. Three of the MNs exhibited a doublet structure in the fluorescence spectra. Consequently, two 0-0 origins have been listed in Table I for 1-MN, 2-MN, and 1,3-DMN. The doublet is due to MNs occupying two distinctively different sites in the durene lattice (Le., site splitting). For both 2-MN and 1,&DMN either of the two groups of fluorescence bands could be selectively excited by using 0-0 band excitation. A comparison of the fluorescence spectra obtained by excitation of each 0-0 line yielded nearly identical spectra which differed only by a shift in the peak wavelengths that corresponded to the energy difference between the two 0-0transitions. The 1-MN doublet structure could be resolved by selectively exciting the 320.0-nm 0-0 line. Only a partial resolution of the doublets could be achieved by exciting the 319.5-nm 0-0 band. Evidently, the broad phonon wing associated with the lower energy 0-0 absorption band overlapped the zero phonon line a t 319.5 nm. Except for 1,5-DMN7for which evidence of site splitting is also (though weakly) discernible, all the other MNs are apparently similar to naphthalene, which occupies only one of the two possible sites in a durene lattice (9). In almost all cases the bandwidth of the largest fluorescence peaks was monochromator limited at about 0.1 nm. Evidently, the bandwidths of MNs in durene are not strongly dependent on the substitutional positions for the methyl groups. Since the fluorescence bandwidths did not exhibit any dependence on the excitation wavelength, regardless of whether SI or S2 excitation was employed, it has been concluded that the extent of inhomogeneous broadening of the absorption band is less than 0.1 nm. Thus, it is reasonable to conclude that the 0-0 and low-energy vibronic absorption bands also have a bandwidth of 0.1 nm or less. Because the separation between 0-0 peaks is greater than the absorption bandwidths, it should be possible to selectively excite individual MNs in a complex isomeric mixture. To test the selectivity that can be achieved with dye laser excitation of MNs prepared in a low-temperature crystal, a seven-component mixture consisting of equimolar amounts of all the compounds listed in Table I was prepared in durene. Figure 1shows fluorescence spectra resulting from laser excitation of the seven-component mixture a t optimum excitation wavelengths for 2,3-DMN, 1,5-DMN, 1,4-DMN, and 1-MN. Figure l a show the result of excitation of the 0-0band of 2,3-DMN. In this spectrum the major 2,3-DMN peaks are well-defined, while 1,4-DMN and 1,B-DMN contribute the
340
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320 350 38 0 350 960 Figure 1. Laser-induced fluorescence spectra of a seven-component mixture wherein 0-0 band excitation of (a) 2,3-dimethylnaphthalene, (b) 1,5dimethylnaphthalene,and (d) 1-methylnaphthalene(A,, = 320.0
nm) has been employed. In (c) 1,4-dimethylnaphthalenehas been selectively excited with an excitation wavelength of 310.0 nm. The arrows indicate the major interferences (see text). minor peaks marked with an arrow. In Figure l b 175-DMN has been selectively excited by utilizing 0-0 band excitation. All of the peaks are due to 1,B-DMN and there is no trace of fluorescence from any of the other components. Finally, one other example of the utility of 0 4 excitation is given by Figure Id, wherein 1-MN has been selectively excited a t 320.0 nm. Only a few s m d 2,3-DMN fluorescence peaks can be observed (see arrows) and all the other peaks are due to 1-MN. These results indicate direct 0-0 band excitation is often the easiest and best way to selectively excite an individual component from a complex isomeric sample. However, 0-0 excitation will not work in situations where a vibronic absorption band from another component overlaps the 0-0 band of the analyte. This was the situation for 2-MN, 1,3-DMN7 and 1,CDMN in this mixture. As demonstrated in Figure IC, however, an excitation wavelength of 310.0 nm was found to be useful for selectively exciting 1,4-DMN in this mixture. Some peaks due to 1,5-DMN also appear and are marked. Thus, although 0-0 excitation is often quite satisfactory, shorter wavelength excitation may sometimes be necessary. Quantitative Fluorometry in Aromatic Crystals. Quantitative fluorometry in aromatic and n-alkane (i.e., Shpol’skii) polycrystalline sampling media can be expected to be affected by similar problems, such as strong laser scattering, polarized emission from localized points in the crystal, and an inhomogeneous distribution of analyte (10). These latter two problems were minimized by defocusing the laser beam with a lens so that essentially the whole sample was illuminated. In order to avoid excitation of delocalized electronic states of the lattice (Le., excitons) and the concomitant problems caused by energy transfer to the analyte, lattice impurities, empty lattice sites, and dislocation lines (11-14), it is highly desirable to employ excitation energies below the first excited electronic state of the lattice. For a durene lattice the excitation wavelength should be above 280 nm (15). Exciton creation is not a problem with Shpol’skii matrices since the lowest excited electronic state of n-alkane’s is inaccessible with light sources normally employed for fluorometry. In order to determine the linear dynamic range of fluorescence from MNs prepared in an aromatic crystal, 30 crystals containing 2,3-DMN with concentrations ranging from 1 to 10000 ppm were prepared from stock solutions. These
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340 360 Laser-induced fluorescence spectra of a shale oil in a durene lattice, using (a) nonselective excitation (285.0 nm) and (b) 1methylnaphthalene 0-0 band (320.0 nm) excitation. In (a)the naphthalene (V)and 2,3-dimethylnaphthalene (V)peaks have been identified, while in (b) the major peaks which are not due to 1-methylnaphthalene have been marked (V). Figure 2.
samples also contained 1,5-DMN, which was used as an internal standard. The amount of internal standard in each crystal was chosen so that the fluorescence intensity from 1,5-DMN and 2,3-DMN differed by less than an order of magnitude. An excitation wavelength of 285.0 nm was employed and the 0-0 band peak heights of 1,5-DMN and 2,3DMN were determined from the average of three measurements per crystal. The 0-0 band peak height ratio (2,3DMN/1,5-DMN) was determined and multiplied by the concentration of internal standard in the crystal. A plot of this computed value vs. 2,3-DMN concentration was linear from 1 to 1000 ppm of 2,3-DMN, indicating that exciton excitation was negligible with 285.0-nm excitation. A leastsquares fit of the linear portion of the curve yielded a correlation coefficient of 0.980 and an average relative deviation from the least-squares line of 29.2%. These last two numbers represent the measurement precision and are comparable to results obtained by RC-LIF/GC (6) and by fluorometric determinations with a Shpol'skii matrix (10). The detection limit for 2,3-DMN fluorescence has been determined and is about 0.1 ppm when an excitation wavelength of either 285.0 or 321.1 nm is employed. Similar detection limits were obtained with RC-LIF/GC (6). Potential for Real Sample Analysis. A shale oil sample was prepared by injecting 3 pL of an undiluted shale oil into 0.1 g of durene prior to the crystallization procedure. A fluorescence spectrum of the low-temperature sample is shown in Figure 2a. This spectrum was obtained with nonselective laser excitation at 285.0 nm and is similar in appearance to the spectrum that would be expected with lamp excitation. The most prominent peaks are due to naphthalene, while some 2,3-DMN peaks have also been identified. The fingerprint quality of this spectrum is evident and suggests the potential usefulness of a durene host as a sampling medium for shale oil or oil spill identification. Laser excitation is entirely unnecessary for this purpose. Another interesting feature of the spectrum in Figure 2a is the flat base line a t short and long wavelengths. Evidently, durene displays a distinct selectivity toward naphthalene derivatives, whose predominate fluorescence peaks are always in the wavelength range of 315-350 nm. Consequently, durene would not be expected to be a useful sampling medium for PAHs in general. Presumably, MN molecules (and possibly other naphthalene derivatives) replace a durene molecule in the lattice without causing a significant amount of disruption of the local lattice structure, while larger PAHs cannot be readily incorporated into holes in the lattice and the resulting repulsive forces between solute and matrix cause a significant amount of broadening of the absorption and emission peaks and a concomitant reduction in peak fluorescence intensities.
This situation is analogous to the Shpol'skii effect, which is characterized by narrow absorption and emission lines when the linear dimensions of PAH solute and n-alkane solvent are closely matched. Figure 2b demonstrates the potential utility of combining a durene sampling medium with dye laser excitation of fluorescence for the determination of the MN content of complex samples, such as the shale oil used here. I t can be seen that 1-MN has essentially been selectively excited with 0-0 band excitation at 320.0 nm. We have found that a large number of MNs in this sample can be selectively excited, and the results of our shale oil analysis will be presented elsewhere after obtaining a more comprehensive spectral library of naphthalene derivatives. Comments on the Analytical Utility of Crystalline Sampling Media. Although the potential utility of a durene crystalline host for the analysis of the MN content of real samples has been demonstrated, it has been suggested that durene is not likely to be useful for other PAHs. However, it is quite possible that other aromatic crystals will be found to be very useful for the analysis of other PAHs. It should be noted that LESS has been extremely successful in characterizing complex mixtures of PAH structural isomers, as well as isomeric derivatives of benzo[a]pyrene and benz[a]anthracene (1-4). Consequently, in these cases an n-alkane matrix is quite satisfactory and there is no apparent need for other sampling media, such as an aromatic crystal. However, in situations where spectral broadening inhibits the selectivity of laser excitation, an aromatic crystal offers an important alternative sampling medium. MNs are an example of a situation in which a suitable n-alkane matrix cannot be found, and other examples will occur whenever the linear dimensions of PAH and n-alkane cannot be closely matched. Thus, the use of aromatic crytals is complementary, rather than competitive, with the use of n-alkane sampling media. Although the Shpol'skii effect is almost always associated with n-alkane crystals, it appears to be part of a more general phenomenon which can be characterized by the appearance of quasi-linear absorption and emission whenever a PAH (or derivative) can be incorporated into a crystalline lattice without disrupting the local lattice structure, regardless of the type of crystal (16). Registry No. 1-MN, 90-12-0; 2-MN, 91-57-6; 1,3-DMN,57541-7; 1,4-DMN,571-58-4; 1,5-DMN,571-61-9; 2,3-DMN,581-40-8; durene, 95-93-2; naphthalene, 91-20-3.
LITERATURE CITED (1) Maple, J. R.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1980, 52, 920-924. (2) Yang, Y.; D'Silva, A. P.; Fassel, V. A.: Iles, M. Anal. Chem. 1980, 52. 1350-1351. (3) Yang, Y.; D'Silva, A. P.; Fassel, V. A. Anal. Chem. 1981, 53, 894-899. (4) Conrad, V. B.; Wehry, E. L. Appl. Specfrosc. 1983, 37,46-50. (5) Conrad, V. 8.Ph.D. Dissertatlon, University of Tennessee, 1983. (6) Hayes, J. M.; Small, G. J. Anal. Chem. 1982, 5 4 , 1202-1204. (7) Hayes, J. M.; Small, G. J. Anal. Chem. 1983, 55, 565A-574A. (8) Gustafson, F. J.; Wright, J. C. Anal. Chem. 1977, 4 9 , 1680-1689. (9) McClure, D. S.J. Chem. fhys. 1954, 22, 1668-1675. (IO) Rima, J.; Lamotte, M.; Juossott-Dubien, J. Anal. Chem. 1982, 5 4 , 1059-1064. (11) Wolf, H. C.; Benz, K. W. Pure Appl. Chem. 1971, 27, 439-456. (12) Braun, A.; Pflsterer, H.; Schmid, D. J. Lumh. 1878, 17, 15-28. (13) Avakian, P.; Merrifleld, R. E. Mol. Crysf. 1968, 5 , 37-77. (14) Schmidt, P. P. Mol. Crysf. 1968, 5 , 185-210. (15) Wolf, H. C. SolldStafe fhys. 1959, 9 , 1-81. (16) Hochstrasser. R. M.; Small, G. J. Chem. Commun. 1965, 87,87-89.
RECEIVED for review July 11, 1983. Resubmitted February 13, 1984. Accepted February 13,1984. Acknowledgment is made to the Sandia UNM Research Program and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research.
