Solid Waste: Assessment, Monitoring and Remediation Twardowska, Allen, Kettrup and Lacy (Editors) �9 2004 Published by Elsevier B.V. 485

IV.3

Remote monitors for in situ characterization of hazardous wastes

Tuan Vo-Dinh

IV.3.1. Introduction

The development of remote monitors for detection of trace quantities of toxic chemicals is critical to the achievement of environmentally viable and safe technologies. Problems pertaining to the identification of specific compounds at trace levels and the needs to perform in situ analysis of complex mixtures continue to present new analytical challenges. An important problem area in chemical sensing is the sensitive identification of trace compounds in complex hazardous waste samples. In order to detect minute amounts of a compound in a complex "real-life" sample, sensors must be able not only to differentiate compounds having different molecular sizes but also to identify specific substituents and/or derivative chemical groups attached to the basic structure. Contaminants in environmental samples frequently encompass a wide variety of chemical species. Due to the generally complex nature of hazardous wastes, several techniques are often required to provide unambiguous identification and accurate quantification of the trace contaminants. The various spectrochemical techniques investigated in the Oak Ridge National Laboratory for use in optical sensing and in trace organic analysis include synchronous luminescence (Vo-Dinh, 1978, 1982), room temperature phosphorescence (Vo-Dinh, 1984; Alak and Vo-Dinh, 1988), surface-enhanced Raman spectroscopy (Vo-Dinh et al., 1984; Moody et al., 1987; Vo-Dinh, 1989, 1995a,b), and fiberoptic laser antibody-based biosensor technology (Vo-Dinh et al., 1987, 1993, 2000). Heavy atoms and cyclodextrins were used to enhance phosphorescence analysis (Alak and Vo-Dinh, 1988) whereas antibodies were developed to improve the specificity and sensitivity of laser-based biochemical sensors (Vo-Dinh et al., 1987, 1993).

This chapter presents an overview of two complementary techniques developed in our laboratory, involving luminescence and Raman spectroscopies used for environmental detection. Molecular fluorescence is often employed for strongly fluorescing contami- nants, such as the polycyclic aromatic compounds (PAC), due to the speed and the inherent sensitivity of this technique for PAC. When synchronous fluorescence (Vo-Dinh, 1978, 1982) is used for PAC analysis, the amount of sample pre-treatment can often be reduced due to the greater selectivity of the technique relative to conventional fluorescence emission. An alternative detection method involves Raman and surface-enhanced Raman scattering (SERS) techniques (Vo-Dinh et al., 1984, 1996b; Moody et al., 1987; Vo-Dinh,

486 T. Vo-Dinh

1989, 1995a,b), which can be used to detect non-luminescing or weakly luminescing contaminants. The instrumental systems and environmental applications of the luminescence and Raman monitors are discussed in the following sections.

IV.3.2. Laser-based synchronous fluorescence monitors

IV.3.2.1. Synchronous luminescence method

We have developed a unique methodology for enhanced selectivity in luminescence analysis based on the idea of synchronous excitation (Vo-Dinh, 1978, 1982). The SL methodology provides a simple way to measure the luminescence signal and spectral fingerprints for rapid screening of complex chemical samples. Conventional luminescence spectroscopy uses either a fixed-wavelength excitation (he• to produce an emission spectrum or a fixed wavelength emission (hem) to record an excitation spectrum. With synchronous spectroscopy, the luminescence signal is recorded while both/~em and hex are simultaneously scanned. A constant wavelength interval AA is maintained between the excitation and the emission monochromators throughout the spectrum. As a result, the intensity of the synchronous signal I,~, can be written as a product of the two functions as follows:

lo-(/~ex), (hem) = kcEx(hex)EM(hex),

where

k = a constant, c = concentration of the analyte, Ex = excitation function, and EM -- emission function.

For a single molecular species the observed intensity is simplified (often to a single peak), and the bandwidth is narrower than for the conventional emission spectrum. Figure IV.3.1A shows the fluorescence excitation and emission spectra of a strong fluorescent dye, fluorene. In Figure IV.3.1B the synchronous signal of the same sample is given; in this example a 3-nm interval (AA) between/~em and hex was used. Note the broad structure of both conventional spectra and the narrow peak of the synchronous signal. This feature can significantly reduce spectral overlap in multicomponent mixtures. Correlation of the signal wavelength position with the structure of the compounds becomes easier. For example, the spectrum of a higher ring-number cyclic compound occurs generally at a longer wavelength than the spectrum of a lower ring-number compound. With conventional spectroscopy, this basic rule cannot often be utilized advantageously due to severe spectral overlap. By confining each individual spectrum to a narrow and definite spectral band, the synchronous method offers the possibility of identifying specific compounds or a group of compounds in a mixture (Vo-Dinh, 1982).

The exceptional quality of synchronous luminescence spectrometry can be visualized by using an analogy. The excellent suitability of chromatographic techniques for the analysis of multicomponent mixtures resides in the fact that each component of the mixture provides a simplified signal only (usually one separate "peak"). Synchronous

Remote monitors for in situ characterization of hazardous wastes 487

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luminescence excitation can be regarded as a unique attempt to apply this property of chromatographic methods to spectroscopy without requiring separation of the com- ponents. This can be best demonstrated by a synchronous fluorescence spectrum of a mixture consisting of naphthalene, phenanthrene, anthracene, perylene, and tetracene (Fig. IV.3.2). Each compound gives essentially one signal only and apart from the order of "peaks" of a chromatogram, this mixture would not look much different from a conventional chromatogram.

IV.3.2.2. Instrumental systems

A remote fiberoptic monitor using the laser-based synchronous luminescence (LSL) technique has been developed for detection of PAC contaminants (Vo-Dinh et al., 1996b). The salient features of the device are described here. The source is a small

488 T. Vo-Dinh

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nitrogen-pumped dye laser (Laser Science VSL-337/VSL-DYE). The entire laser system weighs 7 lb, and can be powered by a battery pack. The dye-laser is extremely simple, consisting of a single oscillator cavity with a grating and an output mirror. Cuvettes containing different dye solutions can be rapidly inserted into the cavity, usually with no re-alignment needed. Typical pulse energies are 5 -10 IxJ/pulse with the dyes used in these experiments; the repetition rate of the laser was 15 Hz, and the pulse width was 3.0 ns.

Remote monitors for in situ characterization of hazardous wastes 489

Fluorescence from samples was collected at 90 ~ with anfl l lens and focused with an f/3.5 lens into a 10-cm focal-length monochromator (ISA model H-10). The output from the photomultiplier (Hamamatsu R928) was amplified (Stanford Research Systems SR445) and input into a gated boxcar averager (Stanford Research Systems SR250). The time constant of the boxcar was 0.2 s. Stepper motors replace the DC motors used previously to control the scanning of both the laser and the monochromator. An analog-to-digital converter (ADC) card (MetraByte DASH-16F) was used for instrument control, timing, and data collection. The software used to control the LSL instrument was developed in-house.

The system was designed to allow several different laser dyes to be used in a single scan (a "multi-dye scan"), thereby extending the wavelength range of the scan. When the laser wavelength reached the edge of the lasing region of one dye, the control program paused to allow a manual insertion of a new dye solution before continuing the scan. The process of dye exchange was rapid; only a few seconds elapsed before scanning was resumed. Due to the simplicity of the dye-laser used, no realignment was necessary when switching between dyes. The wavelength regions scanned by different laser dyes in a multi-dye scan need not be continuous, and can use different wavelength separation (AA) values. In the multi-dye scanning mode, the same AA values are used for all laser dyes, and the different dyes "overlap" at the extremes of their scanning ranges.

The LSL monitor could employ standard cuvette sample holders as well as fiberoptic probes for remote sensing. When the fiber probe was used, the cuvette holder was replaced with a fiat mirror with a small hole in its center. The end of the 3-m fiber probe was mounted behind the mirror so that the laser light passed through the hole in the mirror and onto the fiber's proximal end (Fig. IV.3.3). The probe consisted of a single fiber, which was used to transmit laser light to the sample and to collect the fluorescence. The fluorescence emitted from the fiber was reflected by the mirror toward the monochromator's collection optics. Only a small fraction (5%) of fluorescence was lost through the hole in the mirror. The position of the lenses was adjusted to optimize for the best laser-to-fiber coupling efficiency (--~ 85%) and fluorescence collection efficiency.

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490 T. Vo-Dinh

IV.3.2.3. Application: characterization of PAC pollutants

PACs are present in many environmental samples, such as petroleum products, hazardous wastes, coal, tar, creosote, etc. (Vo-Dinh et al., 1987, 1996b). We have demonstrated the improved selectivity of synchronous fuorescence over conventional fluorescence by determining fluoranthene in a contaminated soil sample without previous separation (Stevenson and Vo-Dinh, 1993; Vo-Dinh et al., 1996b). The sample consisted of a reference matrix containing 20 PAC at the ppm level. The conventional emission spectra of fluoranthene and the soil sample extract recorded at the maximum excitation wavelength of the target compound (Aex = 297 nm) indicated the improved selectivity of the SL method. Two important features of the synchronous technique are noteworthy in complex mixtures such as hazardous wastes: (i) the broadness of synchronous emission bands are considerably narrower than conventional bands; and (ii) usually only one peak per compound is obtained. As a consequence, a lower probability of spectral interference should be expected from various constituents in the sample.

A further demonstration of the usefulness of the LSL method involved analysis of multicomponent mixtures using the fiberoptic probe (Vo-Dinh et al., 1996b). In this analysis, the mixture consisted of 1.8 ppb of benzo[a]pyrene (BaP) and 10 ppb perylene in water. A AA -- 25 nm was used to achieve optimal BaP fluorescence; if a smaller AA were used perylene would exhibit only one peak. A boxcar gate delay of 5 ns was used to avoid most of the laser scatter from the ends of the fiber. Three laser dyes were required to scan the range between 385 and 510 nm. The dye cuvettes were rapidly changed after each section of the scan. No dye-laser realignment was required, but the alignment of the laser beam and optical fiber coupling lens was checked after each dye exchange. The current availability of wide-range tuning lasers equipped with optical parametric oscillators will make the operation of LSL instruments simpler. The examples discussed here demonstrate the usefulness of laser-based luminescence monitors for remote sensing of PAC in hazardous wastes.

IV.3.3. Raman and SERS monitors

IV.3.3.1. Raman and surface-enhanced Raman methods

Vibrational spectroscopies are important techniques for chemical and biological analysis due to the wealth of information on molecular structures, surface processes, and interface reactions that can be extracted from experimental data. Raman spectroscopy has recently enjoyed a renewed interest in many fields due to the observations of enormous Raman enhancement for molecules adsorbed on special metallic surfaces (Jeanmaire and Van Duyne, 1977; Chang and Furtak, 1982). This increase in Raman signal, originally attributed to a high surface density produced by the roughening of the electrode surfaces, was later identified as a direct result of a surface enhancement process (Jeanmaire and Van Duyne, 1977), hence the term surface-enhanced Raman scattering (SERS) effect. The observed Raman scattering signals for the adsorbed molecules were found to be more than a million times (106) larger than those expected from gas phase molecules or from non-adsorbed compounds.

Remote monitors for in situ characterization of hazardous wastes 491

The experimental observations related to SERS, and the origin of the enormous Raman enhancement are believed to be the result of several mechanisms. There are at least two major types of enhancement mechanisms that contribute to the SERS effect: (a) an electromagnetic effect associated with large local fields caused by electromagnetic resonances occurring near metal surface structures, and (b) a chemical effect involving a scattering process associated with chemical interactions between the molecule and the metal surface. These enormous enhancement factors, which help compensate for the normally weak Raman scattering process, open new horizons to the Raman technique for trace analysis (Vo-Dinh, 1989, 1995a,b; Vo-Dinh and Stokes, 2002).

IV.3.3.2. Raman and SERS monitors and probes

For laboratory analyses, various Raman spectrometers are now commercially available. A portable SERS field spectrometer (Gamma-Metrics, Inc., San Diego, CA) was available for measurements (Bello et al., 1990). We have used both commercial and laboratory- designed systems for our studies. An instrument consisted of a Spex Model 1403 double monochromator with a Spex Datamate DM1 control and data acquisition system. The detection employed the photon counting technique accomplished using a cooled RCA C31034-02 photomultiplier tube. Excitation was provided by a Spectra Physics Model 166 argon ion laser, a Coherent Radiation Model Innova 90K krypton ion laser, or a Liconix Model 4240PS helium-cadmium laser.

The second system was based on a Jobin-Yvon/ISA Ramanor 2000M double-grating monochromator. The data acquisition system was an LSI 11/23 minicomputer purchased from Data Translation Corporation and a DSD/880 Winchester/floppy disk drive. Photon counting was accomplished using a cooled RCA C31034-02 photomultiplier tube. The excitation was provided by a Spectra Physics Model 171 argon ion laser. Scanning electron microscope (SEM) photographs are obtained with an ISI DS-130 scanning electron microscope.

The substrate of the SERS probes preparation involved two steps. The first step was the deposition of microbodies (such as polystyrene latex spheres, fumed silica, titanium oxide, and aluminum oxide particles) on glass plates. This deposition was accomplished by placing a glass slide on a spin-coating device. A few drops of the microparticle/water solution were placed on the glass slide, which was then immediately spun at 2000 rpm for 20 s. Spinning has been found necessary to preclude clumping of the microparticles on the glass surface. The microparticles adhered to the glass providing a uniform coverage. The second step was the coating of the microparticle-covered glass slide with silver (75-100 nm thickness). The glass slide was placed inside a vacuum evaporator. The pressure was less than 5 X 10 - 6 Torr. The rate of silver deposition was controlled at approximately 1.5- 2.0 nm/s. The rate and thickness of silver deposition was measured using a Kronos Model QM-311 quartz crystal thickness monitor. Data from the quartz crystal thickness monitor exhibited a standard deviation of 10%. Following silver evaporation, 2 - 4 ml of sample solution was spotted on the glass plate substrate. The Raman spectrum was then scanned over the region of interest. For solution measurements, 1 ml of the sample solution was pipetted into a standard quartz cell. The SERS substrate was then inserted directly into the cell and the SERS spectrum was recorded. For in situ measurements, the substrates were mounted on a fiberoptic probe and inserted into liquid samples for spectral recording.

492 T. Vo-Dinh

The substrate can serve as a probe to collect analyte compounds adsorbed onto its SERS-active surface. In general, microparticles of oxides (alumina, titania, silica) have been used on glass plates, which provide simple, and inexpensive practical supports (Vo-Dinh, 1989, 1995a,b). The size of the surface microstructure can be easily controlled by simply selecting the appropriate microparticle sizes. In most of our studies, silver was used as the coating metal for SERS substrates. Previous research indicates that the type of metal on the surfaces is an important factor affecting the SERS effect. Silver exhibits the strongest enhancement followed by copper and gold. The development of SERS as an analytical technique is relatively recent and many experimental factors require careful optimization in order to obtain the maximum signal enhancement. One of the major difficulties in the development of the SERS technique for analytical applications is the development of surfaces or media that have an easily controlled protrusion size and reproducible structure. In a previous work (Moody et al., 1987) we have shown that the SERS effect depends upon several factors including excitation wavelength, microparticle size, and silver coating thickness. Using 364 nm diameter microspheres, we measured the SERS signal intensity using different excitation frequencies. In this previous study, we investigated this excitation dependence effect for a variety of sphere sizes and silver coating thickness combinations (Moody et al., 1987). Optical fiber-based probes have been developed and used for remote in situ measurements (Bello et al., 1990; Alarie et al., 1992; Vo-Dinh, 1995a,b). With these remote systems, the excitation fiber was placed at the back (glass) side of the SERS substrate while the collection fiber was placed at the front (metal) side of the substrate.

IV.3.3.3. Application: fiberoptic remote SERS sensing

The development of SERS-active substrates that allow direct measurements in liquid samples is critical for in situ analysis. SERS has been observed using different solid substrates such as metal electrodes, metal islands, films, glass or cellulose coated with silver-covered microparticles. However, with the exception of metal electrodes and colloidal solutions, most of the SERS studies performed with solid substrates to date have been performed in the dry state. Recently, we have developed the technique of measuring SERS in solution using probes covered with silver-coated substrates mounted in fiberoptic sensors.

Figure IV.3.4 shows a schematic diagram of a prototype fiberoptic remote SERS monitor. A preliminary version of a SERS fiberoptic probe has been developed and described previously (Bello et al., 1990; Alarie et al., 1992; Vo-Dinh, 1995a,b; Stokes and Vo-Dinh, 2000; Vo-Dinh and Stokes, 2002). Only the salient features are described here. A single optical fiber was used to transmit the laser excitation into the SERS probe, and a second fiber was used to collect the scattered radiation from the sample. The laser beam transmitted through a bandpass filter was focused into one end of the excitation fiber with the use of a microscope objective lens. This end of the excitation fiber was held by a fiberoptic holder. The terminus end of the excitation fiber was positioned close to the SERS substrate in order to contain the laser beam to a very small spot on the substrate. New SERS-active substrates based on nanoparticles of silver in sol-gel have been developed for use in probes (Volkan et al., 1999).

Remote monitors for in situ characterization of hazardous wastes 493

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The SERS probe was prepared with a glass backing (microscope slide, 1 mm thick) so that the excitation and collection fibers could be positioned either head on, with the fibers positioned on opposite sides of the SERS substrate, or side-by-side, with the two fibers on the same side of the substrate. The terminus end of the collection fiber was positioned next to the entrance slit of a spectrometer. Since the flnumber of the fiber and that of the spectrometer were different, it was necessary to focus the input radiation from the collection fiber with lenses. An f l l lens was used to collect and collimate the output beam from the collection fiber. A second lens with an f/number matching that of the spectrometer (f17) was then used to focus the collected SERS signal into the slit of the spectrometer equipped with a red-enhanced intensified charge-coupled device (ICCD)

494 T. Vo-Dinh

from Princeton Instruments, Inc. The length of excitation and collection fibers used was 1-20 m, with minor alteration in the SERS signal. Figure IV.3.5 shows a SERS spectrum of 1-aminobenzoic acid recorded from only 9 ms to 10 s using the fiberoptic remote SERS sensor. The results indicated that the combination of the ICCD sensitivity and the SERS probe effectiveness allowed rapid in situ chemical sensing (Alarie et al., 1992). The usefulness of the SERS technique in chemical analysis of a wide variety of species (listed in Table IV.3.1) has been demonstrated (Vo-Dinh, 1989).

IV.3.4. Multispectral imaging and sensing systems

Acousto-optic tunable filters (AOTF) are a relatively new technology used to isolate one or more wavelengths of light. They operate as a tunable optical band pass filter. In contrast to a grating monochromator, an AOTF offers the advantage of having no moving parts and can be scanned at very high rates (millisecond time scale) without the possibility of error due to gear backlash or other mechanical problems. Since AOTFs with high spatial resolution (typically 100 lines/mm) and large optical apertures are available, they can be applied for spectral imaging applications (Hayden et al., 1987; Chao et al., 1990; Cheng et al., 1993).

We have developed several remote spectral imaging systems combining a two- dimensional CCD detector, an AOTF device, and optical imaging fiberoptic probe (IFP) technology (Moreau et al., 1996a,b; Vo-Dinh et al., 1996a). These devices can have useful applications in remote sensing and imaging of hazardous waste samples. The spectral imaging concept combining conventional imaging and spectroscopy is illustrated in Figure IV.3.6.

IV.3.4.1. Operating principle of AOTFs

AOTF devices consist of a piezoelectric transducer bonded to a birefringent crystal. The transducer is excited by a radio frequency (rf) (50-200 MHz) and generates acoustic waves in a birefringent crystal. Those waves establish a periodic modulation of the index of refraction via the elasto-optic effect (Moreau et al., 1996b). Under proper conditions, the AOTF will diffract part of the incident light within a narrow frequency range. This is the basis of an electronically tuned optical filter using the Bragg diffraction of light by periodic modulations in the index of refraction in the crystal established by the acoustic waves. Only light that enters the crystal such that its angle to the normal of the face of the crystal is within a certain range can be diffracted by the Bragg grating. This range is called the acceptance angle of the AOTF. The percentage of light diffracted is the diffraction efficiency of the device. This parameter greatly depends on the incidence angle, the wavelength selected, and the power of the rf signal.

In a non-collinear AOTF, the diffracted beam is separated from the undiffracted beam by a diffraction angle. The undiffracted beam exits the crystal at an angle equal to the incident light beam, while the diffracted beam exits the AOTF at a small angle with respect to the original beam. A detector can be placed at a distance so that the diffracted light can be monitored, while the undiffracted light does not irradiate the detector. In addition, when the incident beam is linearly polarized and aligned with the crystal axis, the polarization of the diffracted beam is rotated 90 ~ with respect to the undiffracted beam. This can provide

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Table IV.3.1. Some detection limits using SERS probe (after Vo-Dinh, 1995a,b).

Compounds Limit of detection (ppm) Comments a

p-Aminobenzoic acid 0.4 Fiberoptic sensor p-Diacetyl benzene 43 Fiberoptic sensor Terephthaldehyde 1 Fiberoptic sensor Terephthalic acid 3 Fiberoptic sensor p-Cresol 28 Fiberoptic sensor Benzoic acid 50 Fiberoptic sensor m-Nitrobenzoic acid 87 Fiberoptic sensor 3-Nitroaniline 36 Fiberoptic sensor p-Aminobenzoic acid 17 Benzoic acid 17 Benzo(a)pyrene-tetrol 8 Formothion 11 Pyrene 0.002 Terephthalic acid 15 Carbonphenothran 32 Chlorinated pesticide Bremophos 36 Chlorinated pesticide Methyl chloropyrifos 32 Chlorinated pesticide Dichloran 20 Chlorinated pesticide Linuron 25 Chlorinated pesticide Chlordane 40 Chlorinated pesticide 1-Hydroxychlorodene 35 Chlorinated pesticide Methylparathion < 26 Organophosphorus Fonofoxon < 25 Organophosphorus Chlorfenvinhos < 36 Organophosphorus Cyanox < 25 Organophosphorus Diazinon < 25 Organophosphorus Formothion < 26 Organophosphorus Dimethoate < 23 Organophosphorus Trichlofon < 26 Organophosphorus Benzo(a)pyrene 0.1 Carbazole 0.2 1-Aminopyrene 1.4 Benzoic acid 0.3

"The limits of detection are given for a complete sample spot although only about 1% of the spot is illuminated by the analyzing laser. The actual limits of detection are therefore 100 times less.

a second means to separate the diffracted and undiffracted beams. One polarizer is placed before the AOTF, and is aligned with the crystal. At the exit of the AOTF, a second polarizer is rotated 90 ~ with respect to the first. The undiffracted light is blocked by the crossed polarizers, while most of the diffracted beam escapes.

IV.3.4.2. Multispectral imaging and sensing systems

A prototype AOTF-based mult ispectral imaging instrument was developed for fluorescence measurements (Moreau et al., 1996a). The light emitted from the output

Remote monitors for in situ characterization of hazardous wastes

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end of the IFP was collected by an imaging lens, filtered by the AOTF, and then imaged onto a CCD. By changing the wavelength of the AOTF, a spectrum could be acquired as a series of images (one for each wavelength).

The TeO2 AOTF used in this work was purchased from Brimrose, Baltimore, MD (model TEAF 10-45-70-S). According to the manufacturer, the AOTF had an effective wavelength range of 450-700 nm (corresponding drive frequency 178-100 MHz). The spectral resolution given by the manufacturer was 20 nm at 633 nm. The diffraction efficiency was 70% at 633 nm. The optical aperture was 10 by 10 mm and the acceptance angle was greater than 30 ~ The drive power was 1.0-1.5 W.

The rf generator used (Brimrose-model AT) could apply 0-25 W of rf power and was controlled by a DOS-based computer using a 16-bit computer controller board supplied by Brimrose. Custom software was developed at Oak Ridge National Laboratory to control the AOTF, supporting various scanning modes and fixed-frequency operation.

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Remote monitors for in situ characterization of hazardous wastes 499

The CCD was a model ST-6 purchased from Santa Barbara Instrument Group, Santa Barbara, CA, based on a Texas Instruments TC241 CCD detector. The operating spectral range was 330-1100 nm. The detector was 8.63 x 6.53 mm and had a resolution of 750 X 242 pixels, with a two pixels horizontal binning giving an effective resolution of 375 x 242 pixels. Standard pixel size was 23 x 27 lxm. Dark current could be kept as low as 13 electron/pixel/s at - 20 ~ The detector was installed on a regulated thermo-electric Peltier effect based cooler. Anti-blooming protection was also included. The analog- to-digital resolution was 16 bits. A mechanical shutter was included in the optical head to facilitate taking dark frames. The CCD controller is based on the IBM 8088 microprocessor and ran at 8 kHz. The interface to the PC compatible computer was accomplished through a regular RS232 cable and baud rate was 115.2 kbaud.

The IFP was purchased from Schott Fiber Optics Inc., South Bridge, MA. It was a rigid image conduit for image transmission made of more than 400,000 individual 12 txm diameter fibers fused together (resolution is twice as much as the CCD resolution). Flat polished ends were 9.5 X 7.0 mm rectangles. Numerical aperture was 0.56 (acceptance half-angle > 30~ Individual fibers were made of glass translucent from 400 nm up to the IR region and transmittance was higher than 0.990/25 mm for the range 480-700 nm. An opaque encapsulation of the IFP provided physical protection as well as light shielding. The excitation source was a HeCd laser from Omnichrome, Chino, CA (Omnichrome- model 3074-6) with a >>8 mW output at 325 nm.

Figure IV.3.7 shows a schematic of an instrument designed for remote sensing using multiple probes. The details of the device are described previously (Moreau et al., 1996b). The device has multiple sensor probes that can be used to detect different analytes simultaneously. We have developed several devices that take advantage of recent advances in several technologies, including a two-dimensional CCD detector, imaging fiberoptic, and AOTFs. The integration of these technologies leads to versatile and powerful imaging systems that can remotely detect and analyze fluorescent objects. This imaging system could find useful applications in environmental monitoring areas where the detection of multiple components in complex media is required. The results demonstrate the potential of the AOTF technology to be used for remote imaging spectroscopy and simultaneous spectrum acquisition of different contaminants in hazardous waste samples. For environmental applications, a compact Raman monitor based on AOTF has recently been developed for field monitoring (Cullum et al., 2000).

IV.3.5. Conclusion

Luminescence, Raman, and SERS spectroscopies are spectrochemical techniques that have a number of important advantages to remote sensing of hazardous wastes. The examples shown in this work illustrate the different uses of these techniques for monitoring a wide variety of chemical species. Laser-based luminescence is well known for its high sensitivity for polyaromatic compounds. On the other hand, Raman spectroscopy can be used for weakly luminescing compounds, and can provide an analytical tool having figures of merit that complement luminescence. The Raman technique is well known for its high selectivity. With the advances of fiberoptic technology, the SERS technique, which can amplify the Raman signal by several orders

500 T. Vo-Dinh

of magnitude, can provide a remote sensing technique with the added merit of improved sensitivity due to the surface-enhanced effect. Advanced multisensor systems using phosphorescence detection (Campiglia and Vo-Dinh, 1996) and AOTFs (Moreau et al., 1996a,b; Vo-Dinh et al., 1996a) developed in our laboratory further extend the capabilities of remote sensing technologies.

Acknowledgements

This research was sponsored by the U.S. Department of Energy, managed by UT-Bastille under contract No. DE-AC05-00OR22725. The author also thanks G.G. Griffin, B.M. Column, D.L. Stokes, J. Mobley, D. Hueber, C.L. Stevenson, J.P. Alarie, A. Campiglia, F. Moreau, and V.A. Narayanan for their collaboration and assistance in this work.

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