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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
0
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The results shown in Figures 1-6 demonstrate that FD interpolation of electrochemical data can be performed with precision, and to the point of allowing generation of a continuous analog readout, the ultimate interpolation. Ease of identification of peak potentials, widths, and magnitudes are greatly assisted. When F F T digital filtering and FD interpolation are combined, rather substantial data enhancement can be realized, allowing one to recover from a relatively noisy and/or sparse data array quite satisfactory electrochemical response parameter values. The concept of reducing sampled data density to conserve computer memory and/or measurement time, and recovering the lost resolution via FD interpolation is supported by the above results.
ACKNOWLEDGMENT We are indebted to Richard Schwall for developing the polynomial modification software. LITERATURE CITED
Figure 6. Application of FD interpolation on admittance spectral data. System and applied as in Figure 2. Measured: peak admittance vs. u1/2 . Notation: as in Figure 4
interpolation when a distressingly noisy admittance polarogram is presented. The peak potential (-0.545 V) and full-width at half-height (49 mV) obtained from the filtered, interpolated data are in excellent agreement with the values obtained from the more reliable raw data of Figure 2 and Table I. Figures 4 and 5 illustrate typical interpolation results for a cot 4 polarogram and a dc cyclic voltammogram, respectively. Figure 6 depicts an attempt to apply the FD interpolation procedure to admittance spectra data, where the sampled data array is not equally spaced along the abscissa, as the F F T algorithm assumes. Because of this, interpolated data point separations along the abscissa are nonuniform. Nevertheless, this not-strictly-valid procedure yields satisfactory results, including an only slightly flawed continuous spectral response (Figure 6B).
P. R. Griffiths, Appl. Spectrosc., 29, 11 (1975). G. Horlick and W.K. Yuen, Anal. Chem., 48. 1643 (1976). D. E. Smith, Anal. Chem., 48, 221A (1976). R. J. Schwall, A. M. Bond, R. J. Loyd, J. G. Larsen, and D. E. Smith, Anal. Chem., 49, 1797 (1977). A. M. Bond, R. J. Schwall, and D. E. Smith, J Electroanal. Chem., 8 5 , 231 (1977). R. J. O’Halloran, J. C. Schaar, and D. E. Smith, Anal. Chem., 50, 1073 (1978). S. C. Creason, R. J. Loyd. and D. E. Smith, Ana/. Chem., 44, 1159 (1972). J. W.Hayes, D. E. Glover, D. E. Smith, and M. W.Overton. Anal. Chem., 45, 277(1973). G. Horlick, Anal. Chem., 44, 943 (1972). R. de Levie, S. Sarangapani, P. Czekaj, and G. Benke, Anal. Chem., 50. 110 (1978). M L Forman, Appl O p t , 16, 2801 (1977) A M Bond, R J O’Halloran I R u m , and D E Smith, Anal Chem , 48, 872 (1976)
Roger J. O‘Halloran Donald E. Smith* Department of Chemistry Northwestern University Evanston, Illinois 60201 RECEIVED for review March 20, 1978. Accepted May 9, 1978. Work supported by the National Science Foundation (Grant NO. CHE7S-15462).
Fluorescence Line Narrowing Spectrometry in Organic Glasses Containing Parts-per-Billion Levels of Polycyclic Aromatic Hydrocarbons Sir: The fact that carcinogenic and mutagenic properties of polynuclear aromatic hydrocarbons (PAHs) can be strongly dependent on isomeric structure ( I ) has prompted us to develop new laser based methodologies characterized by resolution sufficient to distinguish between structural isomers. In addition to very high selectivity, the requirements that our techniques be quantitative, sensitive (51ppb), nondestructive, and rapid have limited the scope of our investigations. Given the critical dependence of the electronic structure of polyatomic molecules on nuclear geometry and the sub. stantial fluorescence quantum efficiencies of PAHs ( 2 ) , fluorescence based methods for PAH measurements seem very attractive. However, the aforementioned selectivity requirement presents real difficulties and precludes, for example, liquid solution or conventional gas phase fluorescence spectrometry as viable starting points. In the latter case, overlapping rotational structure produceq broad rovibronic 0003-2700/78/0350-1394$01 OO/O
bands while in the former case, solute-solvent interactions also afford broad bandwidths (FWHM -200 cm-’). (All bandwidths stated below are “full-width half maximum” or FWHM.) Although it appears that the problem of overlapping rotational structure can be eliminated (3),we wish to report here on a solid state fluorescence based technique which we believe satisfies all the requirements delineated above. Before describing it, we note that it has been known for over 20 years that the low temperature electronic absorption and luminescence spectra of PAHs imbedded in crystalline matrices can be sharp (55 cm-’) ( 4 ) . This degree of sharpness satisfies the selectivity requirement. For any matrix, selectivity can also be enhanced temporally ( 5 ) since PAH fluorescence lifetimes are known to vary by several orders of magnitude (2). Possible matrices include host PAH crystals, Shpol’skii (n-paraffin) solvents, and, to a lesser extent, “inert” gases like 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AlJGUST 1978
Ar. T h e first and last matrices do not seem very attractive, partly because of lengthy (-several hours) sample preparation time. Quantitative measurements on and characterization of PAHs in Shpol’skii matrices are being pursued (6). It should be noted that mismatch of the P A H and n-paraffin lengths leads to broad fluorescence bandwidths when excitation is provided with broad band or high energy sources. T h e multiplet structure (due to energetically inequivalent impurity sites) characteristic of Shpol’skii matrices poses another problem since the multiplet fluorescence pattern depends on the sample cooling rate (7). In our opinion, these problems are soluble but a t the expense of sample analysis time (for example, resolution of a complex mixture of PAHs would require the use of several different Shpol’skii matrices). Fluorescence line narrowing spectroscopy (FLNS) of PAHs in organic glasses is the alternative solid state approach t o be described here (8). Although low temperatures (-4 K) electronic molecular absorption spectra in glasses are severely broadened (FWHM of vibronic bands -200 cm-’) due to site inhomogeniety, narrow laser line excitation near (vide infra) the absorption origin of the fluorescent state affords sharp lined fluorescence spectra. This effect in glasses was first observed by Personov and co-workers (9). The same effect in mixed molecular crystals was observed using incoherent monochromatic excitation as early as 1967 (10). FLNS is simply a manifestation of the fact that only impurity sites whose excitation profiles overlap with the laser frequency profile are able to fluoresce. It is most pronounced a t low temperature (-4 K) where impurity site interconversion rates are not competitive with fluorescence. Frequently, the reported vibronic linewidths in line narrowed spectra are laser limited, Le., the homogeneous fluorescence linewidth is narrower than the laser linewidth. Organic glasses were the media of choice for our FLNS studies for the following reasons: PAH solubility and concentration gradient problems are minimized, high optical quality which minimizes laser scattering, broad impurity absorption linewidths which facilitate PAH excitation, and their potential for accepting water. The latter property allows for direct analysis of contaminated water samples, vide infra. At the outset, we realized that the applicability of FLNS for PAH measurements would be limited primarily by the glass characteristics. After surveying several different glasses, the 1:l glycerol:HzO system was found to be almost ideal. Glass formation a t T 54.2 K is facile (15-min cool down time from room temperature with no glass cracking), the glass has a high water content, and the high glass quality is reproducible. These properties will be emphasized in what follows. A full description of the FLNS system will be given a t a later date. Briefly, it consists of a 3-L Pope Scientific double nested glass liquid H e Dewar with quartz optical windows (hold time -6 h), and a Control 553 U Ar-ion laser with UV output a t 363.8, 351.4, 351.1,335.8, 334.5, and 333.6 nm. Laser powers delivered a t the sample are typically 100, 30, 110, 20, 20, and 20 mW, respectively. Spatial separation of the laser lines is achieved with external prisms. Saturation effects a t these power levels are negligible. The detection electronics include a photon counting system, although the data presented here were obtained using conventional analog detection with a cooled EM1 9558 QB P M T . A McPherson Model 218 1/3-meter spectrometer was utilized, unless otherwise stated, a t a 0.1-nm resolution with slits collinear to the laser beam. Thin walled plastic culture tubes (1-cm 0.d.) are used for glass formation and are a t least 85% transmitting a t all excitation wavelengths. It is worth noting that molecular absorption spectra of species imbedded in organic glasses can be subject to the phenomenon known as laser induced nonphotochemical hole
1395
Table I. Comparison of the Line-Narrowed Fluorescence Spectrum of Pyrene in a 1:l Glycerol-Ethanol Glass at 4.2 K with the Fluorescence Spectra of Pyrene in Biphenyl and Fluorene at ca. 10 Ka fluoreneb biphenylb glassC analysis 26690 vs 26734 vs 26902 vs origin 408vs 408 ms 408, A, 410 s 459 m 456m 4 6 0 w 456, B,, 597 m 596s 5 9 2 w 596,A, 738 m 736s 7 3 4 m 736,B,, 805 ms 801s 805ms 801,A 820 m 816 m 2 x 408 1063s 1 0 6 8 w 1063,A, 1066 ms 1111 ms 1110 ms 1111, B,, 1116 ms 1208 mw 1210 vw 408 t 801 -- 1 1213 m 1243 mw 1240, A 1246 ms 1240,1246 s 1332 vw 1330 mw 596 t 756 1335 w 1408vs 1410s 1408,A 1409 vs 1471 w 1476 vw 408 + 1863 1476 m 1552 s 1550 mw 1552, A, 1555 ms a The position of the origin is given in cm- and is vacuum corrected for the mixed-crystal spectra. All other entries show differences from the origin. Lines of “medium” strength, or greater, have been taken from reference 1 5 as has the analysis above. Spectrum excited by h e , = 363.8 nm, 90 mW at the sample. burning (11). The hole burning does affect fluorescence intensities but only to a small extent and, fortunately, in a way that would not affect the analytical capabilities of FLNS in organic glasses (12). Glycerol was found to exhibit sufficiently low background fluorescence to permit its use without further purification. For example, we were unable to detect pyrene and anthracene in blank glycerol:HzO glasses. The greatest amount of fluorescence line narrowing is obtained when A,, (excitation wavelength) lies within the absorption origin of the fluorescent state, SI. As the excess vibrational energy in SI provided by A,, increases, the probability of exciting different vibrational sublevels belonging to different sites of a given impurity increases. Ultimately, when excitation is into a very spectrally congested region, the fluorescence line narrowing effect is essentially completely lost. This behavior is shown in Figure 1 for pyrene in a 1:l glycero1:ethanol glass. The prominent band near 372 nm in the upper spectrum is the principal fluorescence origin. T h e 363.8-nm laser line corresponds to excitation ca. 560 cm-’ above the zero-point level of SI. The origin’s linewidth is 0.16 nm (instrument limited). The fluorescence spectrum is quite detailed and can be compared with the very sharp fluorescence spectra of pyrene obtainable in mixed crystals at low temperature, see Table I. The weak but sharp band to lower energy of the principal origin (at 370.6 nm) is a secondary pyrene fluorescence origin due to different pyrene sites than those which contribute to the major origin. In the middle spectrum, the excess vibrational energy provided by the A,, = 351.1, 351.4 nm doublet is -1600 cm Note that the principal fluorescence origin exhibits a prominent doublet (splitting = 0.3 nm) due to excitation with the two different laser lines. The doublet component linewidths are 0.3 nm. Finally, in the lower spectrum, the average excess vibrational energy is -3000 cm-’ and no sharp structure is observed. The origin linewidth is 1.7 nm, and this width equals the fluorescence linewidths due to any one of the three laser lines (as verified by spectra obtained using each of these three laser lines individually). Behavior similar to that depicted in Figure 1 has been reported previously (8, 9). The spectra suggest the potential advantage to be gained by using a tunable dye laser to generate the FLNS of PAHs. Only then can one be assured of generating sharp line fluorescence spectra for all species. The additional advantage of tunahility is that it provides a
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
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degree of selectioe excitation which will greatly simplify the analysis of complex mixtures. When A, lies above the absorption origin of a constitutent molecule, that molecule will not be electronically excited and will not fluoresce. Thus in a mixture of two isomers, e.g., anthracene and phenanthrene, which are known to be difficult to resolve using conventional GC-MS techniques (13), the excitation wavelength may be chosen to excite anthracene while being too low in energy to excite phenanthrene, thereby resolving the mixture. With our existing laser, sharp line spectra in glycero1:water glasses have been obtained for many PAHs including pyrene, anthracene, phenanthrene, azulene, 9-methylanthracene, triphenylene, and chrysene. Attention has been focused on pyrene and anthracene since their absorption spectra are quite compatible with the 363.8-nm laser line. Representative spectra are shown in Figure 2. The upper spectrum is that of anthracene (as verified by vibrational analysis) with the principal fluorescence origin near 380 nm. The broader feature built on the origin and displaced 0.5 nm (35 cm-l) to lower energy is a phonon side band. Similar structure can be seen
on the intramolecular zero-phonon vibrational hands. While the matrix used for these studies is formally a glass, we observe structures in our spectra which are similar to ones observed in crystal spectra. We use the term “phonon” broadly for we cannot envision the effective delocalization of a vibrational mode throughout the glass matrix. The side bands may be intimately connected with the solvent cage-excited molecule interaction and, in fact, the relative magnitude of the phonon wing to the 0-0 line varies from one glass to another. The 363.8-nm excitation wavelength is -1100 cm-’ above the fluorescence origin and at least partially explains why the anthracene zero-phonon linewidths are somewhat broader than those of pyrene (lower spectrum). The lower spectrum agrees well with that in Figure 1 particularly since the glasses are different. The lower two spectra in Figure 2 were obtained a t a sensitivity 5 X greater than the upper one. We note that the middle spectrum corresponds to an equal mixture of anthracene and pyrene (100-ppb concentration). Comparison of the pyrene fluorescence intensities in the two lower spectra gave us the first indication that the glycerokwater glass quality
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
Table 11. Calibration Data pyrene peak concn, height, PPb c1 Aa
I 2 ppm A l v T H P A i E V E in G L Y C E R O L H2C 5 4 GL&SS 3638nm LGSEG EXC TATIOh
100 p p S P N T H % A C E N E I O 0 ppcl P Y R E h E MlXTUFIk n G-YLEQOL h 2 C 3 6 3 8 n r L A S E R EXCITATION
5
4
0.006 0.014 0.061
1 5 10
100 500 1000 3000
0.120
50 100 500 1000 3000
2.8
0.0024 0.010
0.021 0.098 0.170 1.0
1.8 3.9
least squares least squares correlation correlation coefficient = coefficient = 0.989 0.993 a The origin, zero phonon, line was measured (background corrected). Between two and five points were taken a t each concentration with an average precision, calculated as the average deviation, of 8%.
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1
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anthracene peak height, concn, PPb IJ AU
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1397
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Figure 2. Resolution of a mixture of t w o PAHs through fluorescence line narrowing spectrometry. All spectra were measured at 4.2 K and excited by A,, = 363.8 nm
might be sufficiently reproducible to permit quantitative measurements on PAHs without having to resort to internal standards. T h a t this is the case has been firmly established for both pyrene and anthracene. The data for concentrations ranging between 3 ppm and 1ppb are contained in Table 11. For both, the principal fluorescence origin peak height was used for calibration. The linearity between peak height and concentration expected in the low concentration regime is observed, cf. caption to Table 11. The following points deserve emphasis. (i) Data in Table I1 were obtained using mixtures of pyrene and anthracene with the concentration of the PAH of interest varying between 0.2 and 5.0 times the concentration of the other component. (ii) Data were collected over a 3-day period so t h a t more than three liquid helium Dewar preparations and liquid helium transfers were involved. (iii) Facile glass formation of glycerol:H20 permitted analysis of 15 samples during a 6-h period. In regard to the first point, one would not expect Forster energy transfer (14) for the concentration levels used in our studies to be operative and the linearity of our data is consistent with this assertion. We have prepared quite complex multicomponent PAH samples containing known amounts of pyrene and anthracene and have found their analysis to be unaffected by the other species. At the present time, real samples are being analyzed for PAHs such as phenanthrene, anthracene, chrysene, and pyrene.
Within the near future, the FLNS system will be expanded to include a tunable N,-pumped dye laser, a gated detection system, and a rapid scan intensified diode array spectrometer. In this way, the resolution (selectivity) will be significantly enhanced, both through selective excitation and temporal discrimination. The addition of a diode array spectrometer promises to ensure that a convenient analysis time for even complex multicomponent samples can be maintained. In summary, it is our belief that laser induced FLNS of fluorescing organic pollutants such as the PAHs in organic glasses will develop into a practical analytical tool possessing the attributes discussed earlier. The high selectivity (resolution of structural isomers) and ability to analyze contaminated water samples directly (without preseparation of the contaminants) are particularly noteworthy.
LITERATURE CITED (1) M. L. Lea, M. Novotny, and K. D. Bartle, Anal. Chem., 48, 405 (1976) and references therein. (2) J. B. Birks, "Photophysics of Aromatic Molecules", Wiiey-Interscience, London, 1970. (3) R. E. Smalley, L. Wharton, and D. Levy, J. Chem. Fhys., 64, 3266 (1976). (4) D. S.McCiure, J . Cbem. Phys., 22, 1668 (1954):E. V. Shpol'skii, A. A. Ii'ina, and L. A. Klimova, Dokl. Akad. Nauk SSSR,87, 935 (1952). (5) J. H. Richardson and M. E. Ando, Anal. Chem., 49, 955 (1977). (6) G. F. Kirkbright and C. G. delima. Analyst(London), 99, 338 (1974). (7) C. Pfister, Cbem. Pbys., 2, 181 (1973);E. V. Shpoi'skii and T. N. Bolotnikova, Pure Appl. Cbem., 37, 183 (1974). (6) J. H. Eberly, W. C. McCoigin, K. Kawaoka, and A. P. Marchetti, Nature (London), 251, 215 (1974). (9) R. I. Personov and B. M. Khariamov, Opt. Commun., 7, 417 (1973). (IO) G.J. Small, Ph.D. dissertation, University of Pennsylvania, Philadelphia, Pa., 1967. (11) B. M. Kharlamov, R . I. Personov, and L. A. Bykovskaya. Opt. Commun., 12, 191 (1974). (12)J. M. Hayes and G. J. Small, Cbem. Pbys., 27, 151 (1978). (13) H. Svec, Ames Laboratory-USDOE and Department of Chemistry, Iowa State University, Ames. Iowa, private communication. (14) R. C. Powell and Z.G. Soos, J . Lumln., 11, l(1975). (15) A. Bree and V. V. B. Vilkos, Spectrocblm. Acta, PartA, 27, 2333 (1971).
J. C. Brown M. C. Edelson G. J. Small*
-
Ames Laboratory-USDOE Chemistry Iowa State University Ames, Iowa 50011
and Department of
RECEIVED for review March 20, 1978. Accepted May 1, 1978. Work performed and supported by the Division of Biomedical and Environmental Research (Physical and Technological Programs) of the U.S. Department of Energy.
