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Anal. Chem. 1989, 6 1 , 612-615
neering for the loan of a spectrum analyzer and Royce Winge for helpful discussions during the course of this work.
LITERATURE CITED (1) Smith, R. G.; Brooker, E. J.; Douglas, D. J.; Quan, E. S. K.; Rosenblatt, G. J . Geochem. Explor. 1984, 21, 385. (2) Gregoire, D. C. Anal. Chem. 1987, 5 9 , 2479. (3) Longerich, H. P.; Fryer. B. J.; Strong, D. F.; Kantipuly, C. J. Spectrochim. Acta, Part B 1987, 4 2 8 , 75. (4) Date, A. R.; Cheung, Y. Y.; Stuart, M. E. Spectrochim. Acta, Part B 1987, 428, 3. (5) Gray, A. L. Analyst 1985, 710. 551. (6) Serfass, R. E.; Thompson. J. J.; Houk, R. S.Anal. Chim. Acta 1988, 188, 73. (7) Janghorbani, M.; Ting, B. T. G.; Fomon, S. J. Am. J . Hematol. 1988. 21, 277. (8) Dean, J. D.; Massey, R.; Ebdon, L. J. J . Anal. Atomic Spectrom. 1987, 2 , 369. (9) McLaren, J. W.; Beauchemin, D.; Berman, S. S. Anal. Chem. 1987, 5 9 , 610. (IO) Hall, G. E. M.;Park, C. J.; Pelchat, J. C. J . Anal. Atomic Spectrom. 1987, 2, 189. (11) Houk, R. S.; Thompson, J. J. Mass Spectrom. Rev. 1988, 7 , 425. (12) Russ, G. P., 111; Bazan, J. M. Spectrochim. Acta, Part B 1987, 428, 49. (13) Longerich, H. P.; Fryer, B. J.; Strong, D. F. Spectrochim. Acta, Part B 1987, 428, 39. (14) Ting, 8. T. G.; Janghorbani, M. Spectrochim. Acta, Part B 1987, 42B, 21. (15) Waiden, G. L.; Bower, J. N.; Nikdei, S.; Bolton, D. L.; Winefordner, J. D. Spectrochim. Acta, Part B 1980, 3 5 8 , 535. (16) Belchamber, R. M.;Horlick, G. Spectrochim. Acta. Part B 1982, 3 7 8 , 17.
(17) Davies. J.; Snook, R. D. J . Anal. At. Spectrom. 1986, 1 , 195. (18) Davies, J.; Snook, R. D. J . Anal. At. Spectrom. 1987, 2 , 27. (19) Winge. R. K.; Eckels, D. E.; DeKalb, E. L.; Fassel, V. A. J . Anal. A t . Spectrom. 1988, 3 , 849. (20) Van Dyke, M. An Album of Fluid Motion; The Parabolic Press: Stanford, CA, 1982. (21) Huang. L.-Q.; Jiang, S.J.; Houk, R. S. Anal. Chem. 1987, 5 9 , 2316. (22) Scott. R. H.; Fassel, V. A.; Kniseley, R. N.; Nixon, D. E. Anal. Chem. 1974, 4 6 , 75. (23) Houk. R. S.; Fassel. V. A.; LaFreniere, B. R. Appl. Spectrosc. 1986, 4 0 , 94. (24) Olivares, J. A.; Houk, R. S. Appl. Spectrosc. 1985, 3 9 , 1070. (25) Olivares, J. A.; Houk, R. S. Anal. Chem. 1985, 5 7 , 2674. (26) Crain, J. S.; Houk, R. S.; Smith, F. G. Spectrochim. Acta, Part B 1988, 438, 1355. (27) Houk, R. S.; Lim, H. B. Anal. Chem. 1986, 5 8 , 3244. (28) Horlick, G.; Tan, S. H.; Vaughan, M. A,; Rose, C. A. Spectrochim. Acta, Part B 1985, 408, 1555. (29) Vaughan, M. A.; Horlick, G.; Tan, S. H. J . Anal. At. Spectrom. 1987, 2, 765. (30) Koirtyohann, S. R.; Jones, J. S.; Yates, D. A. Anal. Chem. 1980, 5 2 , 1965. (31) Gray, A. L.; Houk, R. S.; Williams, J. G. J . Anal. At. Spectrom. 1987, 2 , 13. (32) Douglas, D. J. Can. J . Spectrosc., in press
RECEIVED for review September 7, 1988. Accepted December 14, 1988. The Ames Laboratory is operated for the U S . Department of Energy by Iowa State University under Contract No. W-7405-ENG-82. This work was supported by the Office of Basic Energy Sciences.
Ultraviolet and Infrared Laser Excited Two-Color Multiphoton Ionization for Determination of Molecules in Solution Sunao Yamada Laboratory of Chemistry, College of General Education, Kyushu University, Ropponmatsu, Fukuoka 810, Japan
A novel two-color multiphoton ionization technique with ultraviolet (UV) and infrared ( I R ) laser excltatlon has been applied to highly sensttlve detection of molecules in solution. The simultaneous excitatlon by 10-ns laser pulses at 355 and 1064 nm substantially enhanced the photocurrent signal of an analyte as compared with that when the laser pulse at 355 nm acted alone. Some basic factors affecting the photocurrent enhancement have been Investigated. The additional excitation by the I R pulse Improved the detection sensitivity of aromatic molecules and vitamins by 3-6-fold as compared with the case of the UV pulse alone. The lowest detection limit ( S I N = 3) was 2.4 pg/mL for 9,lO-dlmethylanthracene. The results suggest that a three-photon process (two UV photons and one I R photon) most likely occurs in this study, and that the I R pulse acts through a gemlnate electron rather than through an excited neutral molecule.
The laser multiphoton ionization technique based on photocurrent measurement has proven to be powerful for the detection of various types of organic molecules in solution (1-8). The detection sensitivity depended on laser power ( 8 ) , excitation wavelength (7, 8), and molar absorptivity of an analyte ( 4 , 6-8); these interdependent factors should be optimized for successful application of this technique. 0003-2700/89/0361-0612$01.50/0
Another important factor influencing the detection sensitivity is the polarity of solvents (3, 6). In solution, photoionization of molecules produces geminate cation-electron pairs, and they either recombine geminately or escape into free ions and electrons that induce photocurrent. The escape probability (P(r,E,T))of an ion (electron) from the geminate pairs is given first order by (9) P ( r , E , T ) = e x.p ( - r c/r)(l + erJ3/2hT) (1) where e is the electron charge, E the electric field, k the Boltzmann constant, T the temperature, r the initial separation distance of the ion and the electron of the geminate pair, and r, ( = e 2 / t k T ,where t is the dielectric constant of the solvent) the Onsager radius. Most of the geminate pairs recombine very quickly (10-13) and the quantum yield of free charge carries is extremely low (at most a few percent) in nonpolar solvents (9); a lower escape probability must give a smaller photocurrent signal. However, nonpolar solvents give extremely low leakage currents ( 3 ) and high electron mobility ( 1 4 ) as compared with polar solvents. Accordingly, nonpolar solvents show higher detection sensitivity than polar solvents (3, 6). Also, there still must be considerable room for improvement of the detection sensitivity in this technique. Enhancement of the escape probability P(r,E,T) seems to be an important key for improving the detection sensitivity in nonpolar solvents. A shorter excitation wavelength (higher photon energy) tends to give a higher escape probability be1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
cause of the larger separation distance of the ion and the electron of the geminate pair (15), but it also increases unwanted photodecomposition (photodissociation) of parent molecules and gives a larger blank signal (8);thus it is impractical. In recent multiphoton ionization of aromatic molecules in nonpolar alkane solvents, the cooperative action of both ultraviolet (UV) and visible (vis) nanosecond pulses at relatively low temperatures (16, 17) or of both UV and infrared (IR) picosecond pulses (10-13) enhanced the photocurrent signal compared to that produced by a UV pulse acting alone; the result is attributed to an increase in the quantum yield of charge carriers due to photoexcitation of geminate electrons by a vis or IR pulse. Even the simultaneous action of both UV and IR nanosecond pulses at room temperature enhanced the photocurrent signal and the detection sensitivity (18). In this paper, some basic factors affecting the photocurrent signal and the photoionization mechanism have been investigated in the two-color multiphoton ionization with simultaneous excitation by 10-ns UV and IR pulses. In addition, this technique has been applied to the detection of several aromatic molecules and vitamins.
EXPERIMENTAL SECTION The UV (355 nm) and IR (1064 nm) two-color laser photoionization detection system is almost identical with the previous one (18). A Nd:YAG laser (Quantel YG 580A, pulse duration 10 ns) was operated at a repetition rate of 5 Hz. Both UV and IR laser beams (diameter 3.5 mm) were combined collinearly by a dichroic mirror. They were focused by a quartz lens (focal length 6 cm) and irradiated the solution simultaneously. The initial current signal was converted to voltage by a homemade current-to-voltage converter (los or lo' V/A) and was amplified by an amplifier (NF P61). Its output was monitored by an oscilloscope (Iwatsu SS-6122),and the peak intensity of the output was averaged by a boxcar integrator (NF BX-530A). The boxcar output was recorded by a strip chart recorder (RDK R-02). The electrode spacing and the bias voltage were 0.2 cm and -(0.5-2.5) kV, respectively. Vitamin K2 was a gift from Eisai Co., Ltd. n-Heptane (liquid chromatographic grade, Kishida Chemicals) and other chemicals (reagent grade) were used as received. Sample solutions were M). All freshly prepared from stock solutions (lo-' to measurements were carried out at 20 f 2 "C. The observed signal of a sample solution (i,) consists of a photocurrent signal (i,) and a blank signal (ib). The blank signal was negligibly small, and i, = i, when the concentration of an analyte (except vitamins) was higher than -lo-' M. Calibration curves and detection limits were obtained as described previously (7, 8).
Flgure 1. Typical photocurrent signals generated by singlecolor (PC(355))and two-color (PC(355+ 1064))photoionization for anthracene (1 X lo-' M) in n-heptane. Laser pulse energies are 1.2 mJ at 355 nm and 30 mJ at 1064 nm. Arrows denoted by t and 1 show on and off positions of the laser pulse at 355 andlor 1064 nm. I
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RESULTS AND DISCUSSION The simultaneous action of UV (355 nm) and IR (1064 nm) pulses (two-color) generated a larger photocurrent signal than that observed when the UV pulse acted alone (single-color); a typical result for anthracene (1 X lo4 M) is shown in Figure 1. Neither the solvent nor the sample solution showed appreciable photocurrent signals when the IR laser acted alone, Electric Field. The photocurrent signal from two-color photoionization, PC(355 + 1064), and that from single-color photoionization, PC(355), increased roughly linearly with increasing electric field. The increase in the photocurrent signal induced from the additional excitation by the IR pulse, APC = PC(355 + 1064) - PC(355), was also larger for a higher electric field. However, its relative increase compared with that of PC(355), APC/PC(355), was independent of the electric field. This indicates that APC is closely related to the efficiency of photoexcitation by the UV pulse and the electric field basically does not affect the detection sensitivity. Laser Power. Figure 2 shows the effect of the UV pulse energy on the photocurrent signal of anthracene (1 x lo4 M) in n-heptane a t a constant energy of the IR pulse (30 mJ).
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Figure 2. Effects of the UV pulse energy on PC(355) (O),PC(355 1064) (0),and APClPC(355) (0)for anthracene (1 X lo-' M) in n-heptane at a constant energy of the I R pulse (25 mJ).
The slope of logarithmic plots of PC(355) versus the UV pulse energy was roughly 2 in the lower energy region (
