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  • Anal. Ch" 1992, 64, 2775-2782

    Acousto-Optic Tunable Filter as a Polychromator and Its Application in Multidimensional Fluorescence Spectrometry Chieu D. Tran' and Ricardo J. Furlan Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233


    Acowtwptlc tunable fllter (AOTF) Is an electronlcally driven dlspenlve devlce which operates on the prlnclple of scowto- optk lnteractlon In an ankotroplc medlum. Incldent whlte llght wlll be diffracted by the AOTF Into a speclflc wavelength when a speclflc rf 18 applled to It. The diffracted llght needs not be a monochromatic Ilght. Muitlwavelength llght can be diffracted from the AOTF when several rf dgnals are dmuttaneowly applled Into the fliter. Compared to conven- tlonal polychromatotr, advantages of thk electronlc AOTF polychromator Include its ablllty to lndlvldually amplitude- modulate each wavelength of the diffracted multiwavelength llght at different frequency. Thk I8 accomplished by lndC vldually and dnuroldally modulating each applled rf dgnal at the dedred frequency. Thlr feature makes lt posdble to develop a novel AOTF-based multklhnensional fluorlmeter In whlch the sample was dmuitanwourly exclted by two different wavelengths (514.5 and 488.0 nm) whose amplltudes were dnuroldally modulated at two different frequencies (100 and 86 Hz). Multicomponent samples, e.g., mixtures of rhodamlne 6Q and rhodamlne B, were successfully analyzed wlng thk novel fluorhetor and the developed data analydr.

    INTRODUCTION Fluorescence technique has been demonstrated to be a

    sensitive method for trace characterization. Since realtime samples are generally present in multicomponent form, their analyses usually require measurements of the fluorescence spectra at different excitation wavelengths. This time- consuming process was alleviated with the use of the videofluorimeter.1-5 In this instrument, the monochromators (excitation and emission) are replaced with polychromators. By appropriately configurating the two polychromators and by using a two-dimensional detector such as silicone-inten- sified target vidicon, emission spectra of the sample produced by the multiwavelength excitation can be simultaneously measured.1-5 The two-dimensional spectra obtained are then analyzedwith the linear algebratechniques.6J The technique has proven to be a very powerful method for the determination of multicomponent trace chemical species. However, in addition to the relatively low sensitivity, the videofluorimeters normally require elaborate data analysis. Novel two-dimen- sional detectors such as charge-coupled devices (CCDe) and charge-injection devices (CIDs) may provide improvement in the sensitivity but also undesiredly increase the By

    (1) Warner, I. M.; Callis, J. B.; Davidson, E. R.; Gauyeman, M.;

    (2) Warner, I. M.; Patonay, G.; Thomas, M. P. Anal. Chem. 1985,57, (3) Ndou, T. T.; Warner, I. M. Chem. Rev. 1991,91,493. (4) Warner, I. M.; McGown, L. B. Advances in Multidimemsional

    (5 ) Vo-Dinh, T. Room Temperature Phosphorimetry for Chemical

    (6) Warner, I. M.; Callii, J. B.; Christian, G. D.; Davidson, E. R. Anal.

    (7) Roesi, T. M.; Warner, I. M. Appl. Spectrosc. 1984, 38, 422.

    Christian, G. D. Anal. Lett. 1975,8, 665.


    Luminescence; JAI Press, Inc.: Greenwich, CT, 1991.

    Analysis; John Wiley: New York, 1986.

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    use of the recently developed acousto-optic tunable filter, a novel and relatively inexpensive multidimensional fluorimeter which has a high sensitivity and simple data analysis can be developed.

    Acousto-optic tunable filter (AOTF) is an electronic dispersive device.&17 It is based on the acousto-optic inter- action in an anisotropic medium. The filter is generally constructed from such birefringent crystal as TeO2 onto which an array of piezoelectric transducers are bonded. These transducers wil l transform the applied rf into acoustic wave.8-17 The propagating acoustic waves produce a periodic moving grating which will diffract portions of an incident beam. Due to the conservation of momentum, only a very narrow band of optical frequencies can be diffracted. Therefore, the spectral bandpass of the filter can be tuned over large optical regions by simply changing the frequency of the applied rf.+17 The AOTF is thus similar to the diffraction grating, but in the former, the grating constant, which in this case is the frequency of the acoustic wave, can be electronically changed. Furthermore, since the scanning speed of the AOTF is controlled by the transit time of an acoustic wave across an optical beam which is on the order of few microseconds, the tuning speed of the filter can be as fast as a few micro- seconds.%-17 Taken together, compared to other dispersive devices, the AOTF has such advantages as (1) compact, all solid state, rugged, and contains no moving parts; (2) wide angular field; (3) high throughput (diffraction efficiencies of the filter are generally greater than 85%); (4) wide tuning range (from UV through visible to IR); (5) high spectral resolution (bandwidth of light transmitted by the filter is about 2-6 A); (6) rapid scanning ability (order of few microseconds); (7) light with adjustable intensity; (8) high- speed random or sequential wavelength accese; and (9) imaging capability.611 The AOTF has been called "the new gener- ation" monochromator and has offered unique means to develop scientific instruments for fundamental research as well as industrial, medical, and aerospace applications. In fact, advantages of the AOTF have been exploited recently to develop a wide range of instruments which include the fast scanned multiwavelength thermal lens spectrophotometer,12 the UV, visible and IR absorption, and circular dichroism spectrophotometers,13-'5 the astronomical photometer,lO the multigas analyzer,lO and the fluorescence microscope.17

    In all applications to date the AOTF is used as a fast scanned electronic grating. That is at any given time, only a single

    (8) Chang, I. C. Opt. Eng. 1981,20, 824. (9) Sivanayayam, a.; Findley, D. Appl. Opt. 1984, 23, 4601. (10) Bates, B.; Halliwell, D. R.; McNoble, S.; Li, Y.; Catney, M. Appl.

    Opt . 1987,26,4783. (11) Gottlieb, M.; Feichtner, J. D.; Conroy, J. SPIE International

    Optical Computing Conference 1980,232, 33. (12) Tran, C. D.; Simianu, V. Anal. Chem. 1992,64,1419. (13) Shipp, W. S.; Biggins, J.; Wade, C. W. Rev. Sci. Zmtrum. 1976,

    47, 565. (14) Kradjel, C. Fresenius' J. Anal. Chem. 1991, 339, 65. (15) Hatano, M.; Nozawa, T.; Murakami, T.; Yamamoto, T.; Shigehiaa,

    M.; Kimura, S.; Takakuwa, T.; Sakayanagi, N.; Yano, T.; Watanabe, A. Rev. Sci. Instrum. 1981, 52, 1311.

    (16) Nelson, R. L. ISA Tram. 1986,25, 31. (17) Spring, K. R.; Smith, P. D. J. Microsc. 1987, 147, 265.

    0003-2700/92/0364-2775$03.00/0 0 1992 American Chemical Society

  • 2776 ANALYTICAL CHEMISTRY, VOL. 64, NO. 22, NOVEMBER 15, 1992

    rf is applied into the filter thereby providing a diffracted beam with only a single wavelength. It is, however, possible to simultaneously apply several rf signals into the filter. This multifrequency application will enable the AOTF to simul- taneously diffract more than one wavelength, thereby serving as a polychromator. Optically, this electronic polychromator is relatively easier to use than the conventional grating polychromator because in the former, the diffracted beam is not deviated by the chromaticity of the light, i.e., light with different wavelengths are diffracted from the filter at the same angle. The AOTF is thus particularly suited for the multidimensional fluorimeter. Furthermore, the diffracted light can be made to amplitude modulate at any specific frequency by simply amplitude modulating the applied rf at that same AM frequency. Thus, by AM modulating each applied rf at different AM frequency, each wavelength of the multiwavelength beam diffracted from the AOTF will be AM modulated at a different frequency. When this multiwave- length beam is used to excite the sample, the fluorescence signal will be modulated at those AM frequencies. Therefore, the data analysis will be much simpler and faster than those used in the existing multidimensional fluorimeter. However, in spite of its potentials, AOTF has not been used as a polychromator, and different AM of the AOTF has not been performed. Such consideration prompted us to investigate these possibilities, to implement the AOTF as an electronic polychromator, and to use it to develop a novel, all solid state, fast scanned multidimensional fluorimeter whose data anal- ysis is relatively faster and simpler. Preliminary results on the instrumentation development and its utilization to characterize multicomponent samples will be reported in this paper.

    EXPERIMENTAL SECTION Experiments were designed to investigate the possibility of

    simultaneously applying many rf signals into the AOTF, to amplitude modulate each rf signal with a different sine wave, and to characterize the light diffracted by the filter for each case. The schematic diagram for the instrument is shown in Figure 1A. A Spectra-Physics argon ion laser (Model 165) operating in the multiline mode was used as the light source. The acousto- optic tunable filter used in this work was the same as previously. Specifically it is a noncollinear type, fabricated from a single crystal of paratellurite (TE02) (Matsushita Electronic Compo- nents Co., Ltd., Osaka, Japan, Model EFL-F2O) and has a clear aperture of 3.0 mm X 5.0 mm. The filter resolution which is defined as the full width at half maximum, AX, is given by8

    X2 AX = 21An sin