Simultaneous Laser-Induced Multiphoton Ionization and Fluorescence

Anal. Chem. 1998, 70, 4333-4338

Simultaneous Laser-Induced Multiphoton Ionization and Fluorescence for Analysis of Polycyclic Aromatic Hydrocarbons Takanori Inoue,†,‡ Vladimir V. Gridin,† Teiichiro Ogawa,‡ and Israel Schechter*,†

Department Chemistry, TechnionsIsrael Institute of Technology, Haifa 32000, Israel, and Department of Molecular Science and Technology, Kyushu University, Kasuga-shi, Fukuoka 816, Japan

Laser multiphoton ionization (LMPI) and laser-induced fluorescence (LIF) techniques were simultaneously applied to analysis of polycyclic aromatic hydrocarbons (PAH) in apolar solution. This combination of singlephoton (LIF) and multiphoton (LMPI) processes provides unique information which is shown to be sufficient for both identification and quantification of PAH molecules in simple mixtures. We suggest 3D calibration plots (concentration as a function of both LMPI and LIF signals) where each PAH compound is represented by a unique vector in this space. The projections of such vectors onto the LMPI-LIF plane are the basis of PAH analysis, where only two measurements (LMPI and LIF signals) are required for speciation and quantification of a single component. A geometrical algorithm for decomposition of simple PAH mixtures, is also addressed. Many polycyclic aromatic compounds are classified as carcinogenic or mutagenic; therefore, these molecules and their derivatives are of considerable environmental interest.1-3 They are produced in many industrial processes when incomplete fuel combustion takes place. The long-range global transport of polycyclic aromatic hydrocarbon (PAH) compounds, as well as local industrial process control, requires screening and real-time monitoring. Reliable and simple in their operational mode techniques, which offer remote or on-line chemical information, they are still of current analytical interest. There are several well-established analytical procedures for PAH analysis, most of them based on GC/MS (for the low-mass and volatile compounds) and HPLC/MS (for the higher molecular masses). These laboratory methods are accurate and can handle quite complicated mixtures. However, they are both expensive and time-consuming; therefore, new low-cost or on-line techniques are needed. †

TechnionsIsrael Institute of Technology. Kyushu University. (1) Organic Chemistry of the Atmosphere; Hansen, L. D., Eatough, D. J., Eds.; CRC Press: Boca Raton, FL, 1991. (2) Futoma, D. J.; Smith, S. R.; Smith, T. E.; Tanaka, J. Polycyclic Aromatic Hydrocarbons in Water Systems; CRC Press: Boca Raton, FL, 1981 (and references therein). (3) Air Pollution, The Automobile and Public Health; Watson, A. Y., Bates, R. R., Kennedy, D., Eds.; National Academy Press: Washington, DC, 1988; Part II. ‡

S0003-2700(98)00430-2 CCC: $15.00 Published on Web 09/12/1998

© 1998 American Chemical Society

Several physical principles can be applied for fast PAH analysis, including laser-induced fluorescence (LIF), laser multiphoton ionization (LMPI), and photoelectron emission. Briefly speaking, in LIF-based analytical techniques, a coherent photon flux is used for excitation of such complex multielectron systems as those found in PAH compounds.4-6 The excitation/emission spectra, as well as the time-resolved intensities, are applied for speciation and quantification of organic traces. LIF has been successfully coupled to numerous detection systems and analytical instruments in mass spectroscopy, chromatography, plasma ionization, laser breakdown spectroscopy, etc.7-15 A number of recent reports promoted such environmentally relevant LIF applications as tracing of polycyclic aromatic hydrocarbons and minority species in combustion environment.16-18 Pulsed lasers, optical fibers, and various waveguides have largely increased the applicability of this technique, especially when its time-resolved and remote control potentials are fully utilized.19-21 New low-cost PC plugged-in optical fiber spectrometers and solid(4) Letokhov, V. S. Laser Analytical Spectrochemistry; Adam Hilger: Bristol, PA, 1986. (5) Multiphoton Ionization, Proceedings of the 3rd International Conference, Iraklion, Crete, Greece, September 1984; Lambropoulos, P., Smith, S. J., Eds.; Springer-Verlag: Berlin, 1984. (6) Laser Applications to Chemical Analysis; Technical Digest Series 2; Optical Society of America: Washington, DC, 1990. (7) Haugen, G. R.; Lytle, F. E. Anal. Chem. 1981, 53, 1554. (8) Hughes, K. D.; Huber, D. M.; Lytle, F. E. Anal. Chem. 1989, 61, 1656. (9) Azimi, N. T.; Huber, D. M.; Whitaker, J. E.; Haugland, R. P.; Lytle, F. E. Appl. Spectrosc. 1990, 44, 400. (10) Fasset, J. D.; Travis, J. C. Spectrochim. Acta 1988, 43B, 1409. (11) Kim, H.-B.; Hayashi, M.; Nakatani, K.; Kitamura, N.; Sasaki, K.; Hotta, J.-I.; Mashuhara, H. Anal. Chem. 1996, 68, 409-414. (12) Diebold, G. J.; Zare, R. N. Science 1977, 196, 1439. (13) Diebold, G. J.; Karny, N.; Zare, R. N.; Seitz, L. M. J. Assoc. Anal. Chem. 1979, 62, 564. (14) Omeneto, N.; Winefordner, J. D. In Inductively Coupled Plasmas in Analytical Atomic Spectrometry; Montaser, A., Golightly, D. W., Eds.; VCH Publishers: New York, 1987; Chapter 9. (15) Cremers, D. A.; Barefield, J. E.; Koskelo, A. C. Appl. Spectrosc. 1995, 49, 857. (16) Niessner, R.; Robers, W.; Krupp, A. Fresenius J. Anal. Chem. 1991, 341, 207. (17) Meier, U.; Plath, I.; Kohse-Hoeinghaus, K. NATO ASI, Ser. E 1993, 224, 195. (18) Alden, M.; Bengtsson, P. E.; Georgiev, N.; Lofstrom, C.; Martinsson, L.; Neij, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 1643. (19) Norris, J. O. W. Analyst 1989, 114, 1359. (20) Fujiwara, K.; Ito, S. TrAC, Trends Anal. Chem. 1991, 10, 184. (21) Brown, R. S.; Brennan, J. D.; Krull, U. J. Microchimie 1994, 50, 337-350.

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state lasers enable construction of compact and portable LIF setups and field applications.22,23 While low light intensities (usually in the UV range) are needed for LIF measurements, high photon fluxes are required for LMPI, where several photons have to be simultaneously absorbed by the molecule. However, such photon fluxes (in the order of ∼1029 photons cm-2 s-1) are easily accessible using solid-state pulsed lasers. Thus, the LMPI-based techniques are now ready for numerous analytical applications. In particular, some of the recently reported approaches are related to LMPI-based mass spectrometry of airborne particulates,24-28 to environmental samples,29-33 and to direct PAH analysis in air by LMPI-induced mirror charges.34-38 The LMPI method has been extensively investigated4-5,39-41 and shown to provide extremely sensitive detection systems. Extensive research efforts have been devoted to merge LMPI with low-cost conductance measuring techniques operating at ambient pressure conditions. Actually, the first LMPI arrangement for liquid-phase studies of polyatomic molecules in dilute solutions was reported by Cristophorou et al.42,43 in 1980-1981. Considerable investigative efforts, related to environmental applications, have been recently devoted to studying LMPI-based fastconductance techniques for tracing polycyclic aromatic hydrocarbon molecules in polar and nonpolar liquids,44-50 particulate substrates,29,30 and aerosols.31,51 (22) Bulatov, V.; Gridin, V. V.; Polyak F.; Schechter, I. Anal. Chim. Acta 1997, 343, 93-99. (23) Kadosh, M.; Gridin, V. V.; Litani-Barzilai, I.; Horowitz B.; Schechter, I. Instrum. Sci. Technol. 1998, 26 (4), 1-9. (24) Dale, M. J.; Jones, A. C.; Pollard, S. J. T.; Langridge-Smith, P. R. R. Analyst 1994, 119, 571. (25) Fei, X.; Wei, G.; Murray, K. K. Anal. Chem. 1996, 68, 1143. (26) Alimpiev, S. S.; Belov, M. E.; Mlinsky, V. V.; Nikiforov, S. S. Analyst 1994, 119, 579. (27) Gittins, C. M.; Castaldi, M. J.; Senkan, S. M.; Rohlfing, E. A. Anal. Chem. 1997, 69, 286. (28) Weickhardt, C.; Boesl, U.; Schlag, E. W. Anal. Chem. 1994, 66, 1062. (29) Gridin, V. V.; Korol, A.; Bulatov, V.; Schechter, I. Anal. Chem. 1996, 68, 3359. (30) Gridin, V. V.; Bulatov, V.; Korol, A.; Schechter, I. Anal. Chem. 1997, 69, 478. (31) Gridin, V. V.; Litani-Barzilai, I.; Kadosh, M.; Schechter, I. Anal. Chem. 1997, 69, 2098. (32) Gridin, V. V.; Bulatov, V.; Korol, A.; Schechter, I. Instrum. Sci. Technol. 1997, 25, 321-333. (33) Gridin, V. V.; Schechter, I., Analysis of Environmental Aerosols by Multiphoton Ionization. In Analytical Chemistry of Aerosols; Spurny, K. R., Ed.; CRC Press: Boca Raton, FL, in press. (34) Schechter, I.; Schro ¨der, H.; Kompa, K. L. Chem. Phys. Lett. 1992, 194, 128134. (35) Schechter, I.; Schro¨der, H.; Kompa, K. L. Anal. Chem. 1992, 64, 27872796. (36) Schechter, I.; Schro¨der, H.; Kompa, K. L. Anal. Chem. 1993, 65, 19281931. (37) Schechter, I.; Schro¨der, H.; Kompa, K. L. Proc. SPIEsInt. Soc. Opt. Eng. 1994, 2092, 186-195. (38) Schechter, I. Proc. SPIEsInt. Soc. Opt. Eng. 1994, 2366, 21-31. (39) Rettner, C. T.; Brophy, J. H. Chem. Phys. 1981, 56, 53-61. (40) Boesl, U. J. Phys. Chem. 1991, 95, 2949. (41) Anderson, S. L. In State-Selected and State-to-State Ion-Molecule Reaction Dynamics, Part I: Experiment; Ng, C.-Y., Baer, M., Eds.; Advances in Chemical Physics Series LXXXII; Wiley & Sons: New York, 1992; pp 177212. (42) Siomos, K.; Christophorou, L. G. Chem. Phys. Lett. 1980, 72, 43-48. (43) Siomos, K.; Kourouklis, G.; Christophorou, L. G. Chem. Phys. Lett. 1981, 80, 504-511. (44) Yamada, S. Anal. Chem. 1991, 63, 1894. (45) Inoue, T.; Masuda, K.; Nakashima, K.; Ogawa, T. Anal. Chem. 1994, 66, 1012.

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Actually, LIF and LMPI are simple laser-based methods, capable of ambient condition operation, which provide complementary analytical information on chemical systems. At a single wavelength excitation, the LMPI often offers LOD in the sub-ppb range, although, with only limited speciation ability (based on ionization thresholds). The more detailed fluorescence spectrum (together with multivariate calibration methods) can be used for resolution of simple mixtures. In turn, the LMPI process can also be induced at a resonant mode, thus favoring some compounds in a mixture. Moreover, nonresonant ionization, performed via short-lived virtual state, can provide information when singlephoton absorption is not possible and no fluorescence is observed. Thus, simultaneous LMPI and LIF measurements may be useful, as already reported by Ishibashi et al. for several aromatic molecules in the gas phase.52 It was shown that each molecule has characteristic photon ionization and fluorescence signals. In the present paper, we report an experimental setup for combined LMPI and LIF methods in the liquid phase and suggest a multidimensional data presentation, which can be used for speciation and quantification of PAH compounds. The proposed setup can be used for on-line analysis and for monitoring of trace PAH compounds in the liquid phase. EXPERIMENTAL SECTION Setup. The experimental apparatus constructed for simultaneous LMPI-LIF analysis is shown in Figure 1. Excitation at 355 and 266 nm was obtained by the third and forth harmonic generations of a Nd/YAG laser (Brilliant B, Quantel, France). The fundamental pulse energy was 900 mJ in 5 ns, operated at 10 Hz. The sample exposure power was attenuated by mesh filters in the range of 10 µJ-2 mJ/pulse. The laser beam was softly focused (using a lens of f ) 40 cm) between two small stainless steel electrodes, 3 mm apart. One of the electrodes was connected to a high-voltage power supply (PS350, Stanford Research Systems) and the other one to a current amplifier (428, Keithley). The LMPI signals were measured and recorded with a digital storage oscilloscope (TDS220, Tektronix). All data were transferred to a PC (Pentium 166-MHz processor) for numerical analysis. Remote LIF readouts were obtained in a 90-deg (laser beam to detector) geometry by means of optical fibers and using the 100-ms exposure mode of the spectrometer equipped with a linear CCD detector (SD2000, Ocean Optics). Each LIF spectrum was averaged over 10 laser shots. Chemicals. Pyrene, anthracene, perylene, and naphthalene (guaranteed analytical grade, Sigma), were used without further purification. Spectroscopic grade hexane (Fluka) was used as solvent for PAH compounds. Nitrogen gas (99,999%) was circulated (15 mL min-1) in some of the experiments. To prevent concentration changes due to photodecomposition, the PAH solution was circulated (2 mL min-1) through a large reservoir. (46) Ogawa, T.; Kise, K.; Yasuda, T.; Kawazumi, H.; Yamada, S. Anal. Chem. 1992, 64, 1217. (47) Li, Y. Q.; Inoue, T.; Ogawa, T. Anal. Sci. 1996, 12, 691. (48) Ogawa, T.; Sumi, S.; Inoue, T. Instrum. Sci. Technol. 1995, 23, 311. (49) Chen, H.; Inoue, T.; Ogawa, T. Anal. Chem. 1994, 66, 4150. (50) Ogawa, T.; Sumi, S.; Inoue, T. Anal. Sci. 1996, 12, 455. (51) Zhan, Q.; Voumard, P.; Zenobi, R. Rapid Commun. Mass Spectrom. 1995, 9, 119. (52) Lin, C. H.; Hozumi, M.; Imasaka, T.; Ishibashi, N. Analyst 1991, 116, 1037.

Figure 3. Photoionization current, recorded for both pure hexane and 3 × 10-6 M pyrene in hexane at 266 and 355 nm, as a function of laser beam power. Lines of slope 2 and 3 are shown, indicating two- and three-photon processes, respectively.

Figure 1. Schematics of the experimental setup for simultaneous LMPI and LIF measurements of PAH in solutions.

Figure 2. Typical photoionization current profiles, I ) I(t), obtained for 3 × 10-6 M pyrene in hexane at 266 and 355 nm, using a laser power of 1 mJ/pulse.

Conditions. Experiments were conducted under usual laboratory conditions, i.e., room temperature (20 ( 2 °C), 60-80% humidity and ambient air pressure (750 ( 5 Torr). RESULTS AND DISCUSSION Typical LMPI signals produced at two laser wavelengths are shown in Figure 2. The time-resolved current signal is composed of two parts: a fast initial rise and a following slow “tail”. The former is due to the formation of cations and anions and is related to several charge recombination processes. The broad “tail” is due to arrival of low-mobility carriers and is governed by inhomogeneous space charge effects.46,47,53,54 This sort of timeresolved LMPI signal is well-known and has already been studied and characterized.45-50 The effect of oxygen contents upon LIF (53) Nakashima, K.; Kise, M.; Ogawa, T. Chem. Lett. 1992, 837-838. (54) Voigtman, E.; Winefordner, J. D. Anal. Chem. 1982, 54, 1834.

and LMPI signals was investigated by purging clean nitrogen gas for long periods. However, no significant deviations from the results obtained at ambient conditions were found. (The fluorescence was not affected significantly, since the oxygen concentration in hexane can only partially be reduced by purging 99.999% pure nitrogen. The ionization current would not be affected by oxygen in a nonresonant multiphoton process.) The actual orders of the LMPI processes involved are revealed by their laser power dependence, as shown in Figure 3. At 266 and 355 nm, the photoionization current was quadratically proportional to the laser power for all aromatic molecules studied, indicating a two-photon ionization (only pyrene is shown in this figure). The current due to the ionization of hexane (the solvent) was quadratically proportional to the laser power at 266 nm and cubically at 355 nm, indicating, respectively, a two- and threephoton absorption in each case. At 266 nm, both hexane (the solvent) and the PAH compounds examined are ionized in a two-photon process. Nevertheless, the signal-to-background ratio was still high (S/B ) 80 at 10-6 M) due to the fact that at this wavelength two photons provide a significant energy excess above the ionization threshold of many PAH compounds (unlike hexane). At 355 nm, the solvent requires three photons for ionization while the PAH compounds are ionized by two photons only. Thus, at this wavelength, there is a clear discrimination between the PAH-originated signal and the background one. It should be noted that the signal of the analyte is substantially decreased at 355 nm, although the slope of the logarithmic current-energy plot indicates a two-photon process. This can be attributed to the distance from a resonant wavelength and to the smaller energy excess, which directly affects the charge separation efficiency. The ionization currents and the fluorescence intensities at the two laser wavelengths clearly characterize some of the studied PAH molecules: Naphthalene has negligible absorbance at 355 nm; thus, both the photoionization and fluorescence signals are very weak. Nevertheless, measurable signals are observed at 266 nm. On the other hand, pyrene shows significant photocurrent when excited at 355 nm, but with very weak fluorescence. It is interesting that in this case the LMPI signals can be detected, while the fluorescence is still negligible. The LMPI provides a better sensitivity when the light absorption coefficient is too weak. Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

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Figure 5. Concentration dependence of LMPI and LIF signals for pyrene in hexane solution at 266 nm (laser power of 1 mJ/pulse). The background signals were subtracted.

Figure 4. LIF spectra of several PAH compounds excited at 266 and 355 nm.

The above results cannot be used for a systematic comparison between the sensitivity of the two methods. Generally, the LIF technique has the potential to detect extremely low concentrations, when no monochromator is used. In our experiments a monochromator was applied, which provides additional analytical information. Such information was not obtained by the integrated current measurements of the MPI method. Moreover, the two techniques are based of different detectors (a photodiode array and a current amplifier) and in our experiments they were not the best available today. Also, a comparison of the sensitivity of these techniques involves optimization of the excitation wavelength, which requires application of a tunable source. The LIF spectra of the studied PAH compounds were measured simultaneously with LMPI in the range 300-600 nm, and the highest peak was recorded for quantification purposes (the 396-nm peak was used for naphthalene). The measured LIF spectra of perylene, anthracene, pyrene, and naphthalene are shown in Figure 4 and were found to fit the literature figures.55 The concentration dependence of both LIF and LMPI signals, over a wide concentration range, is shown in Figure 5. Clearly, a linear dependence is observed for over three decades. The 95% confidence interval based detection limit of pyrene at 266 nm was 1.7 × 10-8 M, which is slightly higher than previously reported results.46 The detection limit calculated from the LMPI data was lower that that from the LIF, indicating again the higher sensitivity of the former technique. Of course, these findings depend on the specific instrumentation used, since much better results are known for LIF analysis (LOD of 10-11 M for conventional (55) Spectral Atlas of Polycyclic Aromatic Compounds; Karcher, W., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988.

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fluorometry and 10-14 M for CW-excited LIF). In our experiment, the detection limits are higher, since we used a hand-held spectrometer, coupled to the measurement by optical fibers. The main power of the proposed combination of LMPI and LIF measurements is revealed by the 3D data presentation. Such combined calibration plots, of both LMPI and LIF intensities as a function of concentration, are shown in Figure 6 for the two laser wavelengths used. The linear regression vector corresponding to each PAH compound forms a unique direction (in the 3D space) characterizing the compound. These findings indicate that a set of just two data points (the LMPI and the LIF signals) are adequate for both speciation and quantification, in monocomponent systems: Each set of such two measurements is represented by a single point on the X-Y plane of these figures (the plane of LMPI and LIF signals). Once this point is drawn, a perpendicular extension in the Z (concentration) direction points to the corresponding vector (thus identifying the molecule) and the concentration (the corresponding Z value, where the perpendicular line crosses the compound vector). The results of Figure 6 indicate a correlation between the LIF and the MPI signals, which reflects the fact that the spectral profiles of these processes are similar (as long as resonant MPI is concerned). However, Figure 6 proves that ratio of the sensitivity factors (calibration plot slopes) can be used for compound characterization. As an example of the benefit of this presentation, consider the LIF signals of anthracene and perylene, which are almost identical. However, their LMPI response is different; thus, the final 3D vector changes and makes speciation and quantification possible. Actually, the resolution power is described by the angle between the projection of two such compound-characteristic vectors (refer to Figure 6). As this geometrical angle is larger, the better the corresponding compounds can be correctly resolved and analyzed. These angles originate from the very nature of the LIF and the LMPI processes: The photoionization and fluorescence intensities are described in terms of the laser power (P) and concentration (C).

SMPI ) B0,MPI + S0,MPICP2

(1)

SLIF ) B0,LIF + S0,LIFCP

(2)

where B0,MPI and B0,LIF are the corresponding background signals.

Figure 7. Schematic 3D presentation of LMPI and LIF signals from a binary PAH mixture and definition of the decomposition scheme which results in quantification of both components.

Concerning the analysis of PAH mixtures, it should be realized that the LMPI and the LIF signals are a summation of the individual signals of all components. The resolution of such mixtures involves vector decomposition methods, which are well established in linear algebra. Studying the performance of such methods and the influence of the noise level is beyond the scope of this paper; however, we present here only the simple treatment for a binary system of components A and B. Then

SMPI,obsd ) SMPI,A + SMPI,B SLIF,obsd ) SLIF,A + SLIF,B Figure 6. 3D plots for LMPI and LIF signals as a function of concentrations, obtained at two laser wavelengths. These plots enable both speciation and quantification of PAH compounds in certain systems.

S0,MPI and S0,LIF are proportional to the ionization cross section and to the fluorescence quantum yield, respectively. These quantities include instrumental functions such as the electrode distance and the applied voltage in the LMPI measurements and geometrical characteristics and detector response in the LIF analysis. However, under constant experimental conditions, S0,MPI and S0,LIF can characterize the PAH compound, and the ratio of these quantities actually defines the abovementioned angle between the projection of the corresponding vectors. Since both speciation and quantification are carried out using the vectors of Figure 6, error analysis is, in principle, a simple linear algebraic procedure. Nevertheless, the common formula for error estimation in linear calibration analysis is not applicable to 3D data. A generalization of this formula seems possible, but it beyond the scope of this paper. The speciation error can also be estimated using chemometric algorithms and can be based on the square of the geometric distance between the compound vectors and a line perpendicular to the MPI-LIF surface, passing through the point corresponding to the measured responses.

(3)

where SMPI,A and SMPI,B are the signals of species A and B, respectively. The signal decomposition of a binary mixture is schematically illustrated in Figure 7. Vectors A and B represent the concentration dependence of the LMPI and LIF of the two compounds. A point in the X-Y plane corresponds to a set of LMPI and LIF measurements. The plane vector pointing to this point (through the origin) has to be decomposed along the X-Y projections of the vectors A and B (designated as line A and line B in the figure). This decomposition results in two new points along line A and line B. Vertical extensions (in the Z direction) at these two points indicate the required concentrations of the two compounds (at the point where the vertical lines cross the compound vectors). CONCLUSIONS An analytical method based on simultaneous LMPI and LIF measurements was established and demonstrated for monocomponent PAH analysis in hexane solutions. It was pointed out that the combined signals provide sufficient information for both speciation and quantification of some PAH compounds. A geometrical presentation of the signals, where both LMPI and LIF are plotted against concentration, provided linear and unique calibrations, along 3D vectors. It was shown that binary systems can be resolved as well, and it was suggested that further mixture Analytical Chemistry, Vol. 70, No. 20, October 15, 1998

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resolution may be studied by application of known algebraic techniques. The scope of this article was the presentation of the principles and their exemplification. It is clear that further investigation of the errors involved and of the expected matrix effects is needed. ACKNOWLEDGMENT This research was supported, in part, by the James-Frank Program for laser-matter interaction, by the Technion-VPR fund,

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by the Israel Ministry of the Environment and by the Israel Science Foundation founded by the Israel Academy of Sciences & Humanities. T.I. is grateful for a Technion-Israel Institute of Technology postdoctoral fellowship.

Received for review April 22, 1998. 1998. AC980430W

Accepted July 9,