measurements of 80 ppb tetrachloroethylene as exemplary analyte have been achieved. I. INTRODUCTION. The demand for continuously operating remote ...
WATER MONITORING USING INFRARED FIBER OPTIC SENSORS B. Mizaikoff, M. Kraft and M. Jakusch Institute of Analytical Chemistry, Vienna University of Technology Getreidemarkt 9/151, A-1060 Wien, A U S T R I A Abstract - A portable sensor system for the continuous detection of chlorinated hydrocarbons in water based on the principle of evanescent wave spectroscopy has been developed. Using silver halide optical fibers and a flow cell remote measurements of 80 ppb tetrachloroethylene as exemplary analyte have been achieved.
I. INTRODUCTION The demand for continuously operating remote chemical sensor systems is steadily increasing due to the necessity of on-site and real-time monitoring of contaminants such as chlorinated hydrocarbons (CHCs) in air, water and soil. Such pollutants originating from industrial effluents and domestic waste are among the major contaminants in ground and drinking water posing serious threats to our world water resources. The determination of environmentally relevant compounds using standard analytical methods usually has to be carried out in a particularly equipped laboratory following complex sampling procedures. Therefore, such methods are considerably slow and expensive. Especially the increasing pollution of seawater demands for robust and accurate monitoring systems of the marine environment. Hence, the EU-project SOFIE - "Spectroscopy using optical fibers in the marine environment" (Project MAS3-CT97-0157) aims at the development of an exclusively optical measurement system for the continuous perception of selected pollutants such as chlorinated hydrocarbons, heavy metals, etc. [ 11. Currently applied analytical techniques for the determination of organic seawater pollutants are mainly involving discontinuous methods collecting and analyzing discrete samples. During the last decades chemical sensors have gained significant acceptance as flexible and accurate analytical systems for the evaluation of compound-selective signals produced by chemical reactions taking place at the interface between the modified sensor surface and the substrate. Such fiber optic chemical sensors (FOCS) are capable of performing measurements on site without sampling procedure. This should in principle be ideal for both, environmental and also industrial applications. The introduction of new fiber optic materials and enhanced optical sensing schemes has contributed to the fact that sensors are considered now as one of the most important methods in modem Analytical Chemistry besides separation techniques and spectroscopy.
0-7803-5045-6/98/$10.00 0 1998 IEEE
However, most of the FOCS described in literature operate in the UV or visible spectral range and are irreversible, slow or of limited selectivity [2, 31. More specific approaches for the in-situ determination of CHCs are based on light absorption in the near infrared (NIR) or mid infrared (MIR) spectral range. The sensing principle can be improved from irreversible and reagent consuming reactions to the fully reversible process of enriching the hydrophobic analytes in (as well hydrophobic) polymer layers adjacent to the optical fiber surface. Due to the weak, partially overlapping and not very distinctive absorption bands of these compounds in the NIR the sensitivity is limited and again the discrimination between structurally similar analytes is difficult [4]. In contrast, the MIR range provides more pronounced substance specific information and higher sensitivity, since the strong fundamental vibrational bands of the analyte molecules rather than the weak overtone bands in the NIR are detected. This leads to a promising sensor concept, the so-called 'physico-chemical mid-infrared fiber optic evanescent wave (MIR-FEWS) sensor', which was intensively investigated during the last years by our research group [5-131.
11. BASIC PRINCIPLE The presented sensor system takes advantage of the sensitivity and selectivity of fiber optic evanescent wave spectroscopy (FEWS). The optical fiber serves as both, waveguide and transducer by representing an elongated attenuated total reflection (ATR) element [ 141. The incident light and the reflected light interact at the interface between the optically denser fiber and the adjacent optically thinner medium characterized either by the ambient phase or the coating polymer. Subsequently, a standing wave emanates at the interface and propagates exponentially decaying into the surrounding matrix. IR-absorbing compounds that are present within the penetration depth of the so-called evanescent wave interact with the radiation, thus providing molecule-specific information about the ambient medium. The achievable penetration depth dp can be estimated as a rule of thumb in the order of magnitude of the wavelength and depends upon the incident angle, the wavelength of the utilized radiation and the refractive indices of the fiber nl and the ambient medium n2 (with nl > n2). Details can be found elsewhere [ 141.
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The fiber connections were established using modified ST-connectors, which are commonly used in telecommunications.
111. INFRARED FIBERS In contrast to the solid crystal waveguide used in classical ATR-spectroscopy fiber optic evanescent wave spectroscopy utilizes an optical fiber as lightguide resulting in an increased number of internal reflections. Thus, higher light absorption and therefore an enhancement of the sensitivity is achieved. The introduction of a new generation of IR transparent fibers made from silver halide (AgBr/AgCl) represents a significant step forward in optical sensor technology. These polycrystalline fibers with a composition AgClo 3Br0 offer a utilizable transmission window in the MIR range from 4 to I8 pm [ 15, 161. Hence, the fingerprint region of CHCs above 8 pm (1200 cm-') where the strongest absorption bands are located becomes accessible for fiber optic sensor applications. Table 1 summarizes the most relevant mechanical and optical properties.
Sample outlet
x, y. z Positioner
1
Fiber - couplers
U
MCT-Detector
A
Sample inlet
AgX-Fiber
Table 1: Mechanical and optical properties of silver halide fibers
SILVER HALIDE FIBER Mechanical properties ____
~
~~
~~
Composition Melting point Diameter Minimal bending radius Tensile strength (25" C) Hardness (Knoop) Optical properties Refractive index Transmission range Attenuation losses (@ 10.6 pm) Practical numerical aperture
AgC 10.3Br0.7 412 "C 0.7 mm 9 mm 100 MPa 15 kg/mm2
Figure 1: Experimental set-up: Bruker Vector 22 FTIR-spectrometer with attached flow cell sensor system
2.21 4-18 pm 0.5-1 dB/m 0.5
Although silver halide fibers exploited in evanescent field sensors require long time protection measures against UV radiation (protective polymer jacket) and base metal contact according to their composition, their spectral range and mechanical flexibility enables a new field of applications for remote sensing devices.
For the continuous in-situ determination of chlorinated hydrocarbons in water the surface of the silver halide fiber serving as active sensor head inside the flow cell is coated with a 5 to 15 pm thick layer of a suitable polymer such as polyisobutylene (PIB) or ethylene/propylene copolymer (E/PCo), which have been investigated thoroughly by several groups [17, 181. The exclusion of water from the fiber surface as well as the enrichment of the lipophilic analytes in the polymer enables the detection of chlorinated hydrocarbon traces down to the low ppb level. Up to five different species could already be measured simultaneously within a few minutes, exploiting the good transparency of the silver halide fibers below 1000 cm-' [5].
IV. RESULTS The sensor set-up depicted in Figure 1 consists of four main modules: ( 1 ) FTIR-spectrometer (Bruker Vector 22) (2) Launching unit using an off-axis parabolic mirror (3) Flow cell with detachable silver halide sensor head (Figure 2) (4) Custom made fiber optic MCT-detector
Figure 2: Flow cell with detachable polyisobutylene coated silver halide fiber sensor head (polymer layer thickness: approx. 8 pm, active sensor length: 10 cm)
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The enrichment behavior of CHCs into a polymer membrane is demonstrated in Figure 3 for the exemplary analyte tetrachloroethylene (TeCE).
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In a final version the spectrometer itself will be scaled down significantly since the conventional sample chamber is not required. Hence, a portable sensor system for field measurements is provided. Figure 5 demonstrates that this sensor principle is not limited to CHCs but can also be applied to hydrocarbons such as toluene. Due to the inherent molecular specificity of the MIR range, especially the fingerprint region, discrimination of similar compounds such as toluene, benzene and xylene (“BTX-sensor”) in a mixture will be possible.
Figure 3: ATR-spectra of the enrichment of 5 ppm tetracloroethylene in a polyisobutylene layer (estimated LOD: 300 ppb)
The thickness of the polymer coatings used in such systems causes a considerable diffusion time of the analyte molecules for reaching the equilibrium (30-45 min). However, complete enrichment is not necessary for evaluation of the signal, since the slope of the response curve after few minutes of enrichment can be used as well. A regression line then fits these primary data. The resulting calibration curve is shown in Figure 4 with the figures of merit calculated from these calibration data concluded in Table 2 .
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4,50E-03
/.’
4 00E-03 3.50E-03
3 ODE-03
/’
2 50E-03
a
2.00E-03
/ .”
1.50E-03 1.00E-03
5 00E-04
0,00E+00 0
---
50
--
/
7
s,, = 22 ppb --*-
100
150
ZOO
250
300
350
400
450
c (TeCE) [VPPml
Figure 4: Calibration graph obtained with FEWS sensor system Table 2: Figures of merit
FLOW CELL SENSOR SYSTEM
Sensor head length Regression coefficient Sensitivity Limit of detection (LOD)
10 cm 0.994 8.9.10-6AUcm-’min“/ppb 80 ppb
As can be seen from these results the use of infrared fiber optic sensors for monitoring volatile organic compounds in aqueous solutions is highly feasible. By using possibly long fiber optic inpudoutput lines remote sensing systems e.g. for wastewater monitoring could be established.
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Y
Figure 5: ATR-spectra of the enrichment of 200 ppm toluene in a polyisobutylene (PIS) layer (estimated LOD: 30 ppm)
V. CONCLUSION AND OUTLOOK During this work a portable MIR-FEWS system has been developed utilizing the principle of ATR spectroscopy and MIR-transparent silver halide fibers for the continuous analysis of volatile organic compounds in general and particularly chlorinated hydrocarbons at environmentally relevant concentrations. The exemplary analyte tetrachloroethylene has been detected down to a concentration of 80 ppb. The development of a modular remote measuring system consisting of possibly long signal input/output waveguides and a sensor head located in a flow cell offers a rugged and flexible sensor configuration, which can easily be adapted to new analytes. With this system also new groups of analytes such as BTX will be accessible in the near future. By significant miniaturization of the FTIR-module and improvement of the polymer coating properties the application of this sensor system for field measurements and in the marine environment will be enabled. Concluding, this system presents a powerful and flexible analytical tool for qualitative and quantitative detection of organic pollutants in water.
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ACKNOWLEDGEMENT The authors acknowledge the Austrian National Bank for support of this work under project 5341.
REFERENCES
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[15] [16] [I71 [18]
M. Kraft, M. Jakusch and B. Mizaikoff, Proc.OCEANS 98, accepted for publication (1998) S.M. Angel, P.F Daley and T.J. Kulp, Proc.Electrochem.Soc, 8791484-9(1987) D.K Dandge et al. Technical report, FCI-Fiber Chem., Inc., Las Vegas ( 1990) J. Burck, J.P. Conzen and H.J. Ache, Fresenius J. AndChem., 342:349-400 (1992) R. Krska, K. Taga, R. Kellner, AppLSpec. 47, 1484 (1993) R. Krska, R. Kellner, U. Schiessl, M. Tacke and A. Katzir, Appl. Phys. Lett., 63(14), 1868-1870 (1993) R. Gobel, R. Krska, S. Neal, R. Kellner, Fres. J. Anal. Chem. 350, 514 (1994) R. Kellner, R. Gobel, R. Gtitz, B. Edl-Mizaikoff, M. Tacke, A. Katzir, Proc. SPIE - Int. Soc. Opt. Eng. 2508,212 (1995) B. Mizaikoff, R. Gobel, R. Krska, K. Taga, R. Kellner, M. Tacke and A. Katzir, Sens. Act. E, 29(1-3), 58-63 (1994) R. Gobel, R.W. Seitz, S.A. Tomellini, R. Krska and R. Kellner, Vib. Spec., 8:141 (1995) J. F. Kastner, M. Tacke, A. Katzir, B. Edl-Mizaikoff, R. Gbbel and R. Kellner, Sens. Act. B 38-39 (1997) M.Jakusch, B. Edl-Mizaikoff, R. Kellner, A. Katzir, Sens. Act. B 3839, 83-87 (1997) M.Jakusch, B. Mizaikoff, R. Kellner, Proc. SPIE Vol. 3105,283-288 (1997) N. J Harrick, Internal reflectance Spectroscopy, Interscience, New York (1967) N.Barkay, A. Levite, F. Moser, D. Kowal and A. Katzir, Proc. SPIE In!. Soc. Opt. Eng. 1048,9-15 (1989) N. Barkay, A. Levite, F. Moser and A. Katzir, J Appl. Phys., 64 (lo), 5256-5258 (1988) R. Gobel, R. .Krska, R. Kellner, R. Seitz and S.A. Tomellini, Appl. Spec., 48(6), 678 (1994) J.E. Walsh, B.D. MacCraith, M. Meany, J.G. Vos and F. Regan, Proc. SPIE - Int. Soc. Opt. Eng. 2508,233 (1 995)
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