Fiber optic sensors for environmental monitoring

Pergamon PII: SOO45-6535(96)00255-X

Chemosphere. Vol. 33, No. 6. pp. 1151-l 174. 1996 Published by Elsevler SCEIIC~ Ltd Printed in Great Britam W45-6535/96 $I S.OO+O.oO

FIBER OPTIC SENSORS FOR ENVIRONMENTAL MONITORING

*Kim R. Rogers’, Edward J. Poziomek’ TJ.S. Environmental Protection Agency National F.xposure Research Laboratory Las Vegas, NV 89193-3478 *Old Dominion University Department of Chemistry and Biochemistry Norfolk, VA 23529-0126

(Recetved in USA 5 February 1996; accepted 22 June 1996)

ABSTRACT Due to decades of neglect as well as ever-increasing industrial activity, environmental monitoring has become an important issue. Given the expense and time constraints associated with classical laboratory analysis, there exists a growing interest in cost-effective and real-time technologies suited to environmental monitoring applications. Recent advances in fiber optic technology and miniaturized optical instrumentation, as well as increasing environmental regulations, are driving an expanded interest in fiber optic sensors for environmental applications. This review of current literature will provide information concerning some of the recent advances in this area. Published by Elsevier Science Ltd INTRODUCTION

Monitoring the environment for the presence of compounds that may adversely affect human health and local ecosystems is a fundamental part of the regulation, enforcement, and remediation processes. Due to the scope and enormous expense associated with this task, there exists a clear need for field-portable, real-time, and cost-effective environmental monitoring technologies. Although no single field-monitoring technology can solve every environmental monitoring challenge, fiber optic sensors, because of their versatility and unique characteristics, show considerable potential for development in a number of field monitoring applications. The purpose of this paper is to outline some of the basic chemical and optical principles

used in fiber-optic (chemical) sensors and provide specific information on recent developments in fiber-optic chemical sensors for environmental applications. Additional sources of 1151

1152 information related to this subject include several excellent reviews on fiber optic chemical sensor technology (Seitz, 1988; Janata, 1994; Angel, et al., 1989; Steinberg et al., 1994; Steinberg et al., 1995; Wolfbeis, 1991; Camara et al., 1991), and a rationale for using fiber optic sensors for in situ and real-time monitoring at hazardous waste sites (Eccles et al., 1987; Klainer et al., 1993; Eastwood and Vo-Dinh, 1991).

FIRER OPTIC SENSORS Monitoring

EnvIrontnental

Enviromnental monitoring in the U.S. is primarily driven by legislation enacted by the Congress.

Examples include the Resource Conservation and Recovery Act (RCRA), the

Comprehensive

Environmental Response, Compensation and Liability Act of 1980

(CERCLA), a.k.a. Superfund, and its 1986 amendment, the Superfund Amendments and Reauthorization Act (SARA), as well as mandates that require exposure assessments for agricultural and industrial chemical releases into the environment. monitoring challenge is enormous.

The scope of this

For example, Superfund alone requires the identification,

evaluation, and cleanup of numerous sites placed on the National Priorities List (NPL).

An

additional complexity arises from the variety of pollutants involved in Superfund as well as other environmental monitoring scenarios.

Over 600 chemical compounds have been

identified at hazardous waste sites alone and there are thousands of unidentified pollutants (ATSDR, 198990).

Potential Applications for Fiber-Optic Sensors There are a variety of analytical methods currently used in support of assessment, monitoring, and cleanup of environmental pollutants.

The choice of a specific method depends

on such issues as analyte, matrix, decision made on the basis of the data, and regulatory acceptance.

For example, at Supertimd sites, different types of analytical data are required for

site characterization,

remediation or cleanup, and post-containment

Site characterization

or post-closure monitoring.

may require a combination of diagnostics and field-screening

tasks. Although some information concerning potential pollutants is usually available for these

sites, the analytical methods used for diignostics applications must be capable of identitying expected as well as unexpected pollutants. Methods that are well suited for this task include several state-of-the-art laboratory techniques such as gas chromatography (GC), gas chromatography-mass spectrometry @C-MS), and mass spectrometry (MS). After the diagnostics data have been analyzed, field-screening methods may he used to map the spatial distribution of specific pollutants (Friedman, 1993-94). These measurements consist of numerous discrete samples, which are non-repetitively analyzed for the presence or absence of a particular compound or compound class at a specific action level. Although fiber optic sensors may be developed for these types of measurements, they must compete with other methods that are well suited for this task. Examples include hmnunoassay test kits and miniaturized laboratory instrumental methods, many of which are already commercially available. &ce the site has been characterized, the analytical tasks associated with the remediation and closure typically require frequent and repetitive analysis for particular compounds of interest. For example, in many cases continuous monitoring may be required for fine-tuning of remediation procedures. Surveillance of the site for contamination with hazardous material outside the site perimeter is also an important consideration. In some cases; on-site real-time monitoring may be required to prevent off-site contamination of ground water, especially where flow patterns are quickly and dramatically altered as a result of remediition procedures such as soil excavation, treatment, and backfdling. Another application for which fiber optic sensors may prove valuable is for down-hole well monitoring. After ground-water monitoring wells have been established, monitoring must conthme (m some cases mandated by law) even though the contamination has been comai&

and samples

are consistently characterized through laboratory analysis as non-detects. In tltese cases. a sentme capability, which could continuously monitor for non-compliance analyte concentrations would be highly costeffective.

Similarly, for the analytical problem of

determining the extent and changing status of the nation’s ground and surface water, technologies that can provide cost-effective, continuous, and down-hole well-monitoring capabilities at key locations would also be extremely useful.

1154

Optical Sensorsand Spectrometers Fiber-optic chemical sensors are composed of an optical fiber interfaced on one end to a selective coating or immobilized indicator compound, and on the other end to an optical spectrometer.

Fiber-optic sensor configurations that have been recently reported are too

numerous to diagram in detail. These configurations, which include: end-of-fiber,

side-of-fiber,

however, fall into three main groups,

and porous or interrupted fiber configurations

(Figs.

lA-C). For end-of-fiber the sample.

sensors, the optical fiber acts as a conduit to carry light to and from

The modulation of intensity for a given range of wavelengths is dependent on the

absorbance or fluorescence of the analyte, indicator, or analyte-indicator complex.

(A) End of Fiber

The

(D) BifurcatedFiberSpectrometer

(B) Side of Fiber (Evanescent) Ykzszz

(E) Single FiberSpectrometer

Figure 1. Sensor Configurations. sensor types.

A) end-of-fiber, B) side-of-fiber, C) porous or interrupted fiber

Fiber Optic Spectrometers.

D) bifurcated fiber type, E) single fiber type.

1155

indicator compound can be trapped behind a membrane, in a polymer, or covalently immobilized to the end of the fiber. For absorbance measurements, the signal may be enhanced by placing the indicator between a reflector and the end of the fiber (Fig. 1A). Another recently introduced variation of the end-of-fiber chemical sensor involves the use of an optical imaging fiber bundle. This 350 @m-diameter fiber bundle contains 6000 optical fibers, which allow pattern resolution using enzyme or pH-indicator dyes incorporated into a polymer layer at the distal end of the fiber (Bronk et al., 1995). Side-of-fiber configurations typically rely on the use of the evanescent wave (Fig. 1B). This effect occurs when light is propagated down an optical waveguide. An electromagnetic wave is generated at the fiber surface and decays exponentially into the medium surroundmg the fiber. This evanescence zone, which is usually limited to less than 100 nm, can be used to detect the presence of optical indicators or changes in refractive index at the surface of the unclad fiber. In the case of fluorescent indicators, the method is very sensitive, primarily because only the chromophores which are bound to the fiber are detected. For refractiveindex-based sensors, the optical effect is generic, and selectivity must be imposed by the use of chemically selective membranes. For porous or interrupted fiber configurations, indicator chemistry is typically incorporated directly into the structure of the fiber (Fig. 1C). For several reasons, this configuration has the potential to be extremely versatile. For example, because of the large surface area provided by the porous fiber core, this method is particularly well suited for absorbance measurements. Further, because the porous regions are intrinsically coupled with the fiber (i.e., they are part of the fiber), measurements can be made at multiple locations along a single fiber. Sol-gel-glass methods can also be used to introduce a range of porosities and chemistries into a fiber or at the fiber surface to facilitate analyte detection using absorbance, fluorescence, or refractive index measurements (Lev et al., 1992; MacCraith et al., 1992). Fiber-optic spectrometers also show a great deal of variability primarily due to differences in proposed applications and particular indicator chemistries used. The most

1156

frequently reported fiber-optic spectrometers, however, can be grouped into two classes (Figs. lD, E): the bifurcated fiber or fiber bundle, and single-fiber systems. Bifurcated systems are inherently simple but show certain limitations (Fig. 1D). For example, only the volume at the end of the sensor for which both excitation and emission cones overlap is measured. Consequently, sampling efficiency is limited. Also. this system is limited to end-of-fiber configurations. Single-fiber instruments, by contrast, typically use a simple and more efftcient sampling probe but require a more complex spectrometer format (Fig. 1E). Because the excitation and emission beams are propagated through the same fiber, the frequencies must be resolved. This is typically done using a dichroic beamsplitter in combination with band-pass filters. When a laser is used as an excitation source, a parabolic mirror with a small hole drilled through it is typically used as a beamsplitter, allowing highly columnated laser light to enter the fiber. The emitted light returning from the optical fiber can then be efftciently collected and focused onto the detector. Fiber optic spectrometers are typically used to measure the absorbance or fluorescence of a single analyte, analyte tracer-complex, or product of an analyte using a continuous intensity light source. Although these methods have proven useful in detecting a variety of analytes, the observed optical signal, particularly in the case of fluorescence, is susceptible to photo bleaching of the probe or interferences from a sample matrix which is colored, turbid, or fluorescent. A number of innovative approaches have been applied to fiber optic chemical sensors to solve some of these problems. For example, the use of multi-wavelength ratiometric methods and time-resolved techniques has, in some cases, reduced the need for frequent recalibration (Lakowicz, et al., 1993).

Sensor Chemistry

Current research and development of fiber-optic sensors for environmental applications can be categorized under several different molecular association effects (Table 1). Mechanisms used to detect analytes include: 1) reversible binding of the analyte to an indicator or sensor surface, resulting in an observed change in the indicator or sensor (e.g., polymer

Refractive Index (Yan et al., 1995; Spichiger et al.. 1993; Arman et al., 1993; Klainer et al., 1993; Ronot et al., 1993; Cohn et al., 1994) Refractive Index (Ewing et al., 1993) Signal hrtensity of Reflected Light (McCurley and Seitx, 1991) Fluorescence Quenching (Sharma and Fasihi, 1993; Sharma and Fasihi, 1994) Absorbance (B-d

Absorbance (Shahriari et al., 1993) Absorbance (Freeman et al., 1990) Absorbance (Jian and Seitx, 1991)

Sorption to Polymer Membrane Sorption to Epoxy Coating Polymer Swelling Dye Indicator Dye indicator Dye Indicator Metalloprotein Indicator Dye Indicator

Volatile Organics

Dense Non-aqueous Phase Liquids (DNAPLs)

Alcohols

Chloroform

Benzene. Toluene, Xylene (BTX)

Benzene

cyanide

2,4,6-Trinitrotoluene 2,CDitrotomene 1.3.5Trinitrotriaxine

Absorbance (Milanovich et al, 1991; Wells et al., 1993)

Dye Complex Dye Complex

ffaOH

Trichloroethylene (TCE) Chloroform

(Berman and Burgess, 1990)

Abwrbawe

cbfmkdPridpk

prladpk

(Cxolk et al., 1993; Spichiger et al., 1993)

A=Me

CHEMICAL REACTION

Absorhaw

Absorbance (Riechert et al., 1991; Riechert et al ., 1991a; Ozawa et al., 1991)

Dye Indicator

Ammonium Ion

Dye lmhcator

Fluorescence (Gary and Jorgensen, 1991; Bronk and Walt, 1994; Badii et al., 1995)

Dye Indicator

H+/OH-

pb*+ CdZ+ E,,r+

Absorbance (Baylor and O’Rourke, 1991; Tabacco et al.. 1991: Bocci et al., 1991; Jones et al., 1991; Edmonds et al., 1988)

Dye Indicator

H+/OH

and Walt. 1991)

Optical Principle

Chemical Principle

Myte

REVERSIBLE INDICATOR

Table 1. Fiber Optic (Chemical) Sensors for Environmental Pollutants

(Lumpp et al., 1992) (Berman and Burgess, (Lerchi et al.. 1992) (Ewing et al.. 1994)

Absorbance Absorbance Absorbance

Dye Complex Dye Complex

Cyanogen Chloride

Nitrate

Chlorine Metal Complex

(Jawad and Alder,

Absorbance

Fluorescence/Evanescence Fluorescence

Fluorescence/Evanescence Fluorescence

Immunochemical lmmunochemical Immunochemical Immunochemical Enzyme Inhibition

lmazethapyr

Parathion (insecticide)

Benzo(a)pyrene

Trinitrotoluene

Organophosphates

Benzene, Toluene, Xylene, and Derivatives

Cieneticallv

Eneineered Bacteria

Genetically Engineered Bacteria

1995) Luminescence 1Tencione and Belfort

Luminescence (Kobatake et al.,

Luminescence (Lee et al., 1992)

loxynil

Fluorescence (Thompson and Jones, 1993)

Enzyme Cofactor Genetically Engineered Bacteria

Zn2+

Memron, Isoproturon, Propanil,

Chemihuninescence (Hlavay and Guilbault, 1994)

Enzyme/Luminol

Sulfite

1995)

Absorbance (Andres and Narayanaswami,

Enzyme Inhibition

1991; Tretmak et al., 1993)

and Kramer,

et al., 1995) (Hobel et al., 1992; Guilbault

(Shriver-Lake

1965; Rogers et al.,

(Vo-Dinh et al., 1987; Vo-Dinh et al., 1991)

(Anis et al., 1992)

(Anis et al., 1993)

Hg, Ag, Cu, Zn, Pb, Cr, Co

(pesticides)

(herbicide)

Florescence/Evanescence

Immunochemical

Atrazine (herbicide)

(Oroszlan et al., 1993; Bier et al., 1992; Brecht et al., 1995)

Fluorescence

Chemical

Analyte

Optical principle

Dye Metal Complex

cuz+

Principle

Absorbance

Dye Metal Complex

Zn*+, Cd2*, Ga”

JNTEBACTIONS

(Klimant and Otto, 1992)

Fluorescence

BIOCHEMICAL

(Kurauchi et al., 1992)

Absorbance

Dye Metal Complex

Cd’+, Pb2+, Hg’+

1990)

lonophore-Dye

Pb”

(Cl,)

1991)

(Novak et al., 1990)

Absorbance

Chloride Dye Complex

Methanesulfonyl

Dye Complex

Table 1 continued

Laser-Induced Fluorescence (Alarie and Vo-Dinh, 1991) Synchronous Scanning Fluorometry

Raman Spectroscopy (Bilodeau et al., 1994; Selph et al., 1992; Be110and VoDinh. 1990; Carrabba et al., 1992; Farquharson et al., 1992; Vickers et al., lo=) Absorbance (Stanley et al., 1994)

NA NA NA

NA

Pyrene

PAHs, PCBs

Chlorinated Hydrocarbons

Nitrate

IR Absorbance (Zhengfang et al., 1995; Conzen et al., 1994; Glatkowski et al.. 1992; Buerck et al., 1992; Krska et al., 1992; Degrandpre and Burgess, 19%; Krska et al., 1993; Heo et al.. 1991; Krug et al., 1991; Khmder et al.. 1994) UV Absorbance

NA

Benzene, Heavy Metals

1 Laser-Induced Emission (Wisbrun et al., 1992)

Absorption to Polysiloxane or Polymer Membrane

Organics

Zn, Cr. Pb, Cu. Ni, Cd

Laser-Induced Fluorescence (Niessner et al., 1991; Taylor et al., 1991; St. Getmain et al., 1991)

NA

Cresols, Carbaryl, Carbofuran, Benzene. Anthracene, Phenol, BTX. PAHs

I

Laser-Induced Fluorescence (Bratton et al., 1993; Cespedes et al.. 1993; Aptlz et al.. 1992; DNY et al., 1993)

Not Applicable (NA)

Diesel Fuel, Jet Fuel, Gasoline

I 1 NA I

Atomic Emission (Anheier et al., 1993)

He Plasma Excitation of Cl

Volatile Chlorinated Compounds

(JZastwwd et al., 1993)

opticpl principle

ChemicaI principle

Annlyte

SPECTROSCOPY

Table 1 continued

1160

attached to the optical fiber); 2) irreversible chemical reactions between the analyte and indicator resulting in the formation of chromophores or fluorophores; 3) binding of the analyte to an antibody, receptor, enzyme, or microbe resulting in changes in observed fluorescence or luminescence; as well as 4) direct spectroscopic examination of the analyte which is free in solution or has been accumulated at the sensor surface. Most reported reversible chemical sensors depend on the interaction of a chromophore with the analyte of interest, resulting in a change in fluorescence or absorbance properties of the indicator. sensors.

Within this class, there is a high interest in development of fiber-optic pH

Examples include the use of pH-sensitive dyes such as: bromocresol blue, green, or

purple, poly@henylquinoline),

congo red, phenol red, or fluorescein (Tabacco et al., 1991;

Baylor and O’Rourke, 1991; Bocci et al., 1991; Carey and Jorgensen,

1991; Jones et al.,

1991; Bronk and Walt, 1994). Though not apparent from Table 1, much of this research involves immobilization of the indicator molecules on particular materials.

Reported

substrates include: porous glass (Tabacco et al., 1991; Bocci et al., 1991)

porous polymers

(Tabacco et al., 1991) linear-chain rigid-rod polymers (Carey and Jorgensen, polybenzimidazole

1991),

(Baylor and O’Rourke, 1991), Fotmvar (Baylor and O’Rourke, 1991)

cellulose acetate (Jones et al., 1991). quartz powder (Cocci et al., 1991), and poly(viny1 chloride) (Lerchi et al., 1992). Immobilization substrates for indicators of analytes other than protons included: poly(viny1 alcohol) with oxirane groups (Reichert et al., 1991), dimethyl silicone polymer (Barnard and Walt, 1991). cellulose triacetate (Jian and Seitz, 1991), and XAD-7 resin (Jawad and Alder, 1991). Mechanisms other than indicator dyes include the observation of optical changes associated with alcohol-dependent

polymer swelling (McCurley

and Seitz, 1991) or refractive index changes resulting from absorption of the analyte to a surface (Yan, 1995; Ewing et al., 1993) or into a porous fiber (Shabriari et al., 1993). Chemical sensors based on irreversible chemical reactions rely primarily on the formation of a chromophore-analyte

complex which changes the absorbance or fluorescence

characteristics

Examples of irreversible reactions used in fiber-optic

of the chromophore.

chemical sensors include those used for the detection of NaOH (Berman and Burgess, 1990) trichloroethylene

or chloroform (Milanovich et al., 1991; Wells et al., 1993), methanesulfonyl

1161

chloride

(Novak

al., l!W2),

et al., MO), cyanogen chloride (Jawad and Alder, Ml),

chlorine (Berman and

nitrate (Lumpp et

Burgess, 1990), and transition or heavy metals (Lerchi et al.,

1992; Spichiger et al., 1993; Rlimant and Otto, 1!492;Ewing et al., 1994). For detection of heavy metals, specificity may be improved through the use-of membranes and ionophores in combination with chromophores (Spichiger et al., 1993). Although sensitivity may be increased by using irreversible reactions as opposed to reversible interactions, there appears to be less interest in this area because of the logistics involved in maintaining the sensor system by supplying fresh reagent or replacing the probes. Fiberoptic sensors based on biochemical interactions (i.e., biosensors) typically use a biological sensing element (enzyme, receptor, antibody, or microbe) with an optical transducer (Rogers and Lin, 1992). A variety of innovative optical signal transduction mechanisms have been reported for fiber optic biosensors in general (Camara et al., 1991) and those designed for potential environmental applications in particular. These optical transduction mechanisms include: absorbance, fluorescence, total internal reflectance fluorescence (evanescence), chetiuminescence,

and bioluminescence (see Table 1). Notable among these fiber optic

sensors for environmental applications is the biosensor reported by Shriver-Lake et al. (1995) which has recently been demonstrated in field trials for detection of TNT in groundwater and soil extracts (contacts listed in Rogers, 1995). Because of their selectivity and high affinity toward certain environmental pollutants, a variety of biological macromolecules which have been reported for use in environmental biosensors (Rogers and Poziomek, 1993) may prove to be useful candidates as sensing elements for fiber-optic biosensors. For environmental applications, these biological recognition elements include: antibodies, enzymes, DNA, and genetically engineered bacteria. These biosensors may be considered as reversible or irreversible, depending on the relative aftIn@ of the antibody or enzyme for the analyte or inhibitor. In some cases, the biosensor surface may be regenerated by disruption of the enzyme-inhibitor complex (Rogers et al., Ml),

antibody-antigen bindii

using changes in buffer pH (Shriver-Lake et al., 1995),

or by removal of the antibody-antigen complex from the sensor surface using proteolytic enzymes (Bier et al., 1992; Brecht et al., 1995). In other cases, the sensor may operate

1162

reversibly and continuously (Anis et al., 1993; Lee et al., 1992). Although fiber-optic biosensors demonstrate some advantages over chemical sensors in terms of selectivity, they also show certain stability limitations, especially for environmental applications. include a less-than-optimal

These

shelf life and operational lifetimes, in addition to the fact that

biosensors are not typically as well suited as chemical sensors for detection of volatile organic compounds and heavy metals (two of the largest classes of chemical pollutants).

Nevertheless,

several strategies are being pursued in development of biosensors for these compound classes (Andres and Narayanaswamy,

1995; Kobatake et al., 1995; Tescione and Belfort, 1993).

Another mechanism used in conjunction with fiber-optic sensors to monitor environmental pollutants is direct spectroscopy.

Direct spectroscopic examination of analytes

may be accomplished through the measurement of unique optical properties of these compounds either in solution or adsorbed onto an organic polymer or biological recognition element.

Examples include: the use of laser-induced fluorescence (Bratton et al., 1993;

Cespedes et al., 1993; Apitz et al., 1992; Druy et al., 1993; Alarie and Vo-Dinh, 1991), synchronous scanning fluorometry (Eastwood et al., 1993), UV-absorbance

spectroscopy

(Haas et al., 1991; S&lager et al., 1991; Barber et al., 1995, Stanley et al., 1994) induced atomic emission spectrometry (Anheier et al., 1993) I&absorbance

plasma-

spectroscopy

(Zhengfang et al., 1995; Conzen et al., 1994; Heo et al., 1991) and surface-enhanced

Baman

spectroscopy (Bilodeau et al., 1994; Vickers and Mann, 1992; Farquharson and Simpson, 1992, Angel et al., 1989). Spectroscopic properties of the analyte may be used to both identify and quantitate compounds in solution.

The use of spectroscopy,

especially laser-induced fluorescence,

measure environmental analytes is an attractive approach to many investigators.

to

For example,

the measurement of lead content in dry paint has been recently reported using laser-induced breakdown spectroscopy (Marquardt et al., 1996). Lead (down to 0.014%) could be detected in latex paint even through over layers of non-lead-containing

paint. In another example, in

situ field screening of petroleum hydrocarbon contamination (lOO-10,000 ppm) in soil to depths of 50 m has been demonstrated with laser-induced fluorescence transmitted through optical fibers in conjunction with a truck-mounted cone-penetrometer

(Lieberman et al., 1991;

1163

Cespedes et al., 1993). A second-generation field-transportable laser-fluorescence instrument has been developed to analyze ground water (3- and 4component mixtures in the ppb-ppm range) at the end of optical fibers (Taylor et al., 1991). A tunable laser-fluorescence transportable instrument has been developed to analyze ground water at the ppb level (St. Germain and Gillespie, 1991). A field-portable UV instrument has also been demonstrated for the analysis of benzene in ground water at up to 50-m depths (Haas et al., 1991).

NEEDS AND FUTURE DIRECTIONS There are a number of issues which must be considered to facilitate widespread acce-ptance of fiber-optic sensors for environmental applications. These issues fall into two broad areas, practical requirements and technical barriers. Of the practical requirements, one of the most important considerations is that fiber optic sensors must be available for testing and use in “real-world” scenarios. This means that these devices must be commercially available. Issues that must be addressed in the commercialization process include the following: .

substantial cost-benefits must be realized by the user;

.

method must be simple and user friendly;

.

manufacturing costs must be reasonable;

.

user data quality objectives must be met;

.

there must be some form of regulatory acceptance for most applications; and

.

there must be a sufftcient market for the intended application. Use of these sensors for a particular application must afford a substantial benefit to the

user. In many cases, marginal cost-benefits will not be sufficient to compete with a variety of other emerging field analytical techniques or to overcome the tendency of users to continue with familiar methods. To give this benefit to the user while maintaining a reasonable profit margin, the technology must be simple enough for the equipment to be manufactured at a competitive cost. This applies to both the initial instrumentation investment as well as the cost per analysis. Next, fiber-optic sensors must facilitate meeting the overall data quality objectives of the user. Within the context of hazardous waste site evaluations, several issues

1164

must be considered.

Site characterization modeling studies suggest tbat (within limitations) the

number of samples analyzed has a greater effect on reducing uncertainty than on reducing analysis error (Englund et al., 1992). Consequently, a significant reduction of the cost per analysis and timeliness of monitoring data can potentially increase the overall quality of the site characterization. Efforts are currently being made to define requirements for field screening applications.

For example, several rapid field screening-methods

have been adopted, and a

number are currently under consideration for inclusion in SW-g46 (a collection of analytical methods promulgated by the U.S. EPA through the Office of Solid Waste) (Lest&

1993).

Data requirements for field monitoring methods and particularly continuous monitoring methods, however, have not been addressed in the same detail as those used for field-screening methods. In addition to efforts of the Office of Solid Waste, the EPA’s Environmental Monitoring Management Council (EMMC) Workgroup plans to provide guidance concerning a performance-based

methods paradigm.

More specifically current objectives of the EMMC

Workgroup include: providing written guidelines for demonstration of the performance capabilities of a new method or technology; determining the minimum set of performance criteria necessary to characterize environmental measurement needs, and serve as meaningful targets to the technology developer; and providing guidance on how to implement a performance-based,

rather than proscriptive approach for regulatory monitoring and other

environmental measurement (Williams, 1996). Field demonstration of analytical technologies such as fiber optic methods is also a critical step in the approval and widespread use and acceptance of these techniques.

Several

mechanisms exist for access to Superfund sites as well as guidance for the design and execution of field validation studies.

These studies can be conducted through EPA Regional

Offices and the Consortium for Site Characterization Technology. Finally, there must be a sufficiently large market for a company to yield a profit after an investment of, in some cases, millions of dollars.

The environmental monitoring market is

relatively large and diverse in terms of the number of potential analyses, the range of chemical

1165

properties of these compounds, and the matrices they corttamina~. This becomes a significant challenge for fiber-optic sensors many of which show a limited flexibility in the analytes they detect and potential range of applications. Because the market sire and development are still being defined by improving technologies and changing regulatory requirements, these market issues can become significant obstacles for small-to medium-sized companies considering the major investment required, or a disincentive for large companies looking for expansive and well-defined markets. Some potential markets include: monitoring of underground storage tanks and petroleum products delivery systems; ground-water and surface-water monitoring; monitoring at wastewater treatment plants; and continuous well monitoring in and around waste-storage facilities. A variety of research and development efforts are underway in the use of fiber-optic chemical sensor systems for environmental applications; however, there are a number of technical barriers that must be overcome. The design and immobilization of reversible indicator materials which meet sensitivity and selectivity requirements is not trivial. There are no guidelines for practitioners in choosing indicator systems. Furthermore, indicator probes for fiber optic use are, in many cases, not commercially available. Research and development on recognition coatings for fiber optic sensors needs to be expanded. Although a variety of compound classes such as inorganics, phenols, phenoxy acids, nitrosamines, and aromatic amides have been reported as environmental pollutants, most frequently encountered pollutants at hazardous waste sites include: chloroalkenes, chloroaliphatics, and low-molecular-weight aromatic compounds (ATSDR, 1989-1990). These compounds are not easily measured using fiber-optic chemical sensors because molecularassociation complexes of these compounds are generally weak, chemical reactions require vigorous conditions; these compounds are not easily measured or differentiated using UVvisible spectroscopy; and these compounds tend not to interact with biologically active recognition elements. One of the notable exceptions to the relative paucity of optical information yielded by these compounds, however, is in the mid- infrared spectral region. Virtually all organic compounds exhibit fundamental vibrational frequencies in this spectral region. This is

1166

particularly true for petroleum products for which a great deal of information can be extracted using FT-IR methods (Fodor et al., 1996). Although vibrational spectroscopy shows great potential for use in fiber optic sensors for environmental applications, technical problems such as energy attenuation characteristic of typically used optical fibers has limited the application of this technique.

Nevertheless,

advances in the use of chalcogenide fibers may allow the

future development of field analytical methods using mid-IR spectroscopy (MacLuarin et al., 1996). Because options in designing fiber-optic chemical sensors have been limited, new detection concepts, such as the use of compound-selective refractive index properties, are needed.

polymers that change in thickness or

In addition, further investigation needs to be focused

on the interaction of environmental pollutants with enzymes, antibodies, and microorganisms. New strategies such as multi-sensor fiber-optic arrays show particular promise for multianalyte analysis.

The use of neural net calculations in conjunction with optical sensor arrays

could be. used to identify compounds that respond semi-selectively within the array.

to several chemical coatings

This approach has been demonstrated by Bronk and Walt (1994b) who

measured pH using a four-sensor array immobilized on the end of a 350 pm imaging fiber bundle.

This approach has also been used with chemically active coatings to probe- the

chemical properties of Martian soils (Grunthaner et al., 1995). Application of these methods to the characterization

of pollutants could greatly expand the versatility of these devices for

environmental monitoring uses. Fiber-optic sensors have been reported to measure compounds from a number of environmentally characteristics

significant classes.

In many cases, however, design and operational

of these devices have been optimized under pristine laboratory conditions and

with insufftcient thought given to such issues as manufacturability, requirements of potential environmental applications.

potential user needs, and

Attention is needed in defining

requirements and practical limits for field monitoring using fiber-optic sensors.

Advances in

this area are necessary for laboratory prototypes to be compatible with field-monitoring scenarios using environmentally

significant matrices such as ground water, soils, sludges,

combined organic wastes, and vapors.

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Developers of fiber-optic chemical sensors need to be encouraged to evaluate their systems early in the research and development cycle. Applied research in the laboratory under field-simulated conditions can facilitate the identification of the most promising systems and speed commercialization. A lhnited amount of such evaluation is being done and needs to be expanded. Current reports on fiber-optic chemical sensors deal primarily with research aspects, thus reflecting the state of the technology. Much more research is needed before the technology advances to the point that off-the-shelf items are available to meet the many pollution monitoring needs. Based on the current state of the technology, it will be a number of years before a single fiber-optic system will be developed which can monitor complicated mixtures. Commercialization of fiber-optic chemical sensors for pollution monitoring has been slow. This is not surprising because of the number of technical challenges to be met and barriers to be overcome. It is judged that the potential advantages to be gained from applying fiber-optic systems to environmental applications will continue to attract the interest of researchers, developers, and users. Additional research and development needs to be encouraged. Notice: The U.S. Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), funded the work involved in preparing this article. It has been subject to the Agency’s peer review and has been approved for publication. The U.S. Government has the right to retain a non-exclusive, royalty-free license in and to any copyright covering this article.

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