Anal. Chem. lQ92, 64, 1379-1382
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Birefringent Single-Arm Fiber Optic Enthalpimeter for Catalytic Reaction Monitoring Raymond E. Dessy Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060
Eric W. Richmond’ Merck Manufacturing Division, Route 340 South, Elkton, Virginia 22827
A generlc thermal sensor Is dercrlbed whlch effectively dlrcrlmlnates agalnd slgnak produced by Isotroplc pressure fluctuatlons such a8 those occurring In flow Injectlon systems. Prlnclpks found In analytlcal chemlstry and fiber optlc Interferometry were comblned In the development of thk devlce. A catalytlc system of palladlum, platlnum, and hydrogen was chosento probe and characterlrethe sensor response. Metal Is deposlied on the surface of a “blrefrlngent” optlcal flber whlch Is subsequently exposed to hydrogen gas. Several reactlons occur, changlng the system’s heat content and affecting the flber’s refractive Index proflle. DMerence8 In the phase characterlstlcs of the propagatlng polarized llght are measured, COrr@8pondlngto concentratlons of hydrogen In alr ranglng from 1.4% to 10%.
INTRODUCTION Presently, fiber optic sensors applicable to chemistry are designed basically as light pipes. Light is brought t o a process which is to be monitored, is affected, and is returned to the detection electronics by the same fiber or a return fiber. Some examples of these include fluorescent and absorbance spectra instruments.lS2 These types of instruments monitor light energy by using the optical fiber to deliver excitation energy and/or return emission energy to the detector. The fluorescence- or absorbance-producing chemistry is usually placed in proximity to the distal output end of the optical fiber. This type of optical fiber sensor is known as an ”extrinsic” fiber optic sensor. Extrinsic fiber optic sensors do not take advantage of the fact that an optical fiber is itself a mechanical and chemical system in equilibrium with the light propagating in the fiber. Very slight perturbations to the fiber or ita environment can have measurable effects on light transmission down the fiber. Optical fiber sensors whose transduction mechanism is based on disturbances to this equilibrium are known as “intrinsic” fiber optic sensors. Interferometers are an important class of intrinsic fiber optic devices. These phase-modulated instruments are among the most sensitive methods for measuring physical perturbations.3~4 Double-arm interferometers have been studied extensively, and work with this type of instrument using palladium and hydrogen has been published by Butler et aL576 A relatively (1)Abdel-latif, M. S.;Guilbault, G. G. Anal. Chem. 1988, 60, 26712674. (2) Dessy, R. E. Anal. Chem. 1989, 61, A361.
(3) Schuetz, L. S.; Cole, J. H.; Jarzynski, J.; Lagakos, N.; Bucaro, J. A. Appl. Opt. 1983,22,478. (4) Bucaro, L. A,; Hickman, T. R. Appl. Opt. 1979, 18, 938. (5) Butler, M. A. Appl. Phys. Lett. 1984, 45, 1007. (6) Butler, M. A,; Ginley, S. J. Appl. Phys. 1988, 64, 3706.
thick film of palladium (10pm) was deposited on the sensing arm of the interferometer. Exposure to hydrogen increases the palladium lattice volume and stretches the sensing fiber (axial and radial strain components). A phase shift is measured between the sensing and reference fibers corresponding to hydrogen content. Single-arm interferometers (used in this research) have only one light path carrying the two channels of information found in double-arm instruments. However, there is no isolated reference path in the single-arm configuration. Many different designs have been proposed and used to study onearm interferometers; these can be found in ref 7. In the early 1980s Eickhoff et al. investigated the use of specialty fibers known as bow-tie and elliptical core fibers.* These fibers are made with a refractive index profile along specific radial directions that are not equal and are known as “birefrigent” optical fibers (Figure 1). When polarized light is lauched at the correct angle into a birefringent fiber, the light is divided into two orthogonal eigenmodes. Anisotropies in the thermal and mechanical properties associated with the refractive index profile along the fiber cause the light energy in each orthogonal mode to propagate at different rates. Perturbations to this type of fiber affect the phase of each energy mode, changing the polarization of the resultant output beam. Heat will affect the refractive index of the glass waveguide core and is a possible mechanism for signals produced by the catalytic single-arm interferometer used in this research. Platinum and palladium were chosen as the catalyst system because of the extensive use and characterization of these metals in solid-state gas sensors.9 Palladium is one of the few metals in which hydrogen is soluble, forming an CY and j3 phase of palladium hydride.’O Platinum, when vapor deposited in thin films, is porous to hydrogen. Light energy being transmitted in an optical fiber is not an ignition source and makes an ideal configuration for detecting volatile processes. Other advantages include remote sensing capabilities, where the sensor is physically located long distances from the lauching and analyzing instrumentation. Fiber optic based sensor systems are also immune to electromagnetic interference which can plague electrical based sensors (60 cycle power line frequency and higher frequency radio energy). It was the intention of this current investigation to develop and characterize a simple, inexpensive, and generic analytic probe for the scientist based on intrinsic fiber optic detection. (7) Davis, C. M.; Carome, E. F.; Weik, M. H.; Ezekiel, S.;Einzig, E. Fiberoptic Sensor Technology Handbook; Optical Technologies, Inc.: 1986. _.
(8) Eickoff, W. Opt. Lett. 1981, 6, 204. (9) Mcaleer, J. F.; Moseley, P. T.;Bourke, J. 0.;Norris, W.; Stephan, R.Sew. Actuators 1985,8, 251. (10) Lewis, F. A. The Palladium Hydrogen System; Academic Press: London, 1967; p 4.
0003-2700/92/0364-1379$03.00/0 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
I
SILICA
Ny
REFRACTIM I M E X PROF I LE
SENS I NG
TICAL F I B E R Figure 4.
Flgure 1.
Cross section of bow-tie optical fiber. Rotatmaters
Gas I
i 1t e r s
Temperature lemperature data
1ine
1""
Air Head
I
H2
Gas c o n t r o l line Figure 2.
Basic instrument schematic. GAS I N
SIGNAL RECOVERY ELECTRONICS WOLLASTON
,-
I
Figure 3.
GAS OUT
'i
I LASER LAUNCH ' HEAD
I
COfiPONENTS
Sensor configuration.
Applications for this type of sensor can be found in flow injection analysis and specifically in biosensor technologies.
EXPERIMENTAL SECTION Schematics of the basic instrument layout and sensor configuration are shown in Figures 2 and 3. This instrument is designed in three major sections. Only light is coupled between separate areas. This gives freedom from unwanted perturbations such as mechanical coupling of vibration and allows for efficient alignment procedures. A plane polarized heliumineon laser tube (Spectra Physics) is mounted in a v-block to provide for rotation of the polarization plane. The output end of the laser tube is butted against a fiber optic launching stage (NRC Model FP-2). This stage houses a XlOmicroscopeobjective which focuses the laser onto the optical fiber. The bow-tie fiber's ability to act as a thermal transducer is based on its temperature-dependent refractive indices and its inherent birefrigence. During the fiber's manufacturing process, stresses are deliberately developed which account for the fiber's unusual refractive index profile." These fibers are sensitive to bends that may impose added stress. Therefore, the manner in which the fiber is mounted is crucial in developing consistent signals. (11)Birch, R. D.; Payne, D. N.; Varnham, M. P. Electron. Lett. 1982, 18, 1036.
Sensor head.
Housing the sensing fiber is an aluminum block with a 0.5-mm (depth) X 1.0-mm (width) X 170-mm (length) channelmachined in each half of the sensor head (Figure 4). Approximately 4 cm of each channel end is filled flush with a high-temperature lubricant. Mounting the sensing fiber in this material allows physical stability in the output signal but does not cause significant pressure points or signal variations from fiber to fiber. Construction of a fiber-sensing element begins with measuring and preparing a 26-cm length of special optical "bow-tie" fiber manufactured by Newport Corp. (F-SPV-10). A razor blade is used to strip away approximately 7 cm of the outer plastic jacket at the middle portion of the fiber (this becomes the active sensing area). As the unjacketed fiber is only 125 pm in diameter and made of glass, great care must be taken with subsequent fiber and catalysts deposition procedures. Once initial stripping has been accomplished,vapor deposition of palladium onto the exposed glass portion of the optical fiber can be accomplished. A Denon Model-15 vacuum vapor deposition chamber was modified to hold the optical fiber over a tungsten wire basket containing 7 cm of 0.25-mm-diameter palladium wire (99.9% Aldrich no. 32,669-0). Half of the fiber is coated, the fiber is rotated 180°, and the remaining half is coated using another tungsten wire basket and palladium. Double coating was performed to develop a thicker film but yielded unsatisfactory results. A single coating was eventually found to be the optimum coating thickness (60 A, approximately 2 pg). Platinum deposition was performed in an Edwards sputter coater Model S150B. A 50-mm X 50-mm X 0.025-mm platinum foil (99.9 % Aldrich no. 26,724-4) was used as a target. Sputtering times were varied. The optimum time appeared to be 2 min per side at 40 mA of electrode current. This gave a coating of approximately 300 A of platinum. After the catalyst system has been deposited, the fiber is trimmed to the proper length and mounted in the sensor head as follows. Centering the optical fiber, it is pushed gently down into the sensor head channel, whose ends are filled with viscous lubricant. A flush surface is prepared with a spatula. Mating of the opposite half of the sensor head will not cause undue pressure points. Bolting of both halves completes the sensing fiber mounting procedure. When done correctly a small length of fiber should be visible at each end of the sensor head (Figure 4). Polarized light exiting from the sensing fiber is focused by a microscope objective onto a Wollaston prism, which splits the beam into its two orthogonal components. These two beams are directed at two photodiodes (Silicon Detector Corp. Model Sd290-12-22-241) mounted side by side. All three components (objective, Wollaston, and detectors) are mounted sequentially in a black Delrin tube. Because it is necessary to rotate the Wollaston around the z-axis (light path axis), a circular positioner (Newport Research Corp. Model RSA-2) was modified to accept the black detector tube. In order to assure correct alignment with the signal beam, the circular positioner was attached to a platform, allowing translation in the r and y directions in a plane perpendicular to the light beam path. Proper orientation of the plane polarized light launched into the sensing fiber is required to equally excite each birefringent axis. Initially, the plane of Wollaston diagonal and the laser polarization plane are aligned to provide equal detector signals. After the sensor is mounted in the light path, the laser and the
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
Wollaston are simultaneously rotated until each detector's output is once again'equal.12 At this point the fiber's birefringent axis can be located approximately 4 5 O to either side of the polarization plane. Recovery of the detector signalsis performed by a circuit whose final output is the difference between signal intensities from each detector divided by the sum of both intensities. Electronic division theoretically eliminates variations in the output signal caused by unstable input laser power. Digitization of the signal is performed by a 12-bit analog-to-digital converter and stored using a DEC LSI-11 running a Polyforth operating system and programmed in Polyforth. Further signal enhancement was performed by 9-pointunweighted moving window software filter, written in Basic. Bottled gases were prepurified and terminally filtered using desiccants, 5-A molecular sieves,and activated charcoal (Figure 2). Carrier gas flow (nitrogen or air) is controlled by a Matheson Model 603 rotameter. A DEC LSI-11 computer was used to control release of hydrogen gas via a two-port gas solenoid into a Matheson Model 602 rotameter for metering into the carrier gas stream. Gases are mixed in a metal tee after exiting from their respective rotameters. This mixture is brought to and exhausted from the sensor head by l/le-in.Tygon tubing attached to l/le-in. polypropylene elbow fittings inserted into holes drilled in the sensor head. Programs for computer control of the hydrogen release solenoid were written in Polyforth. All electronic and optical instrumentation, except for the laser, were thermally isolated by a wooden enclosure insulated with Styrofoam. Caution must be exercised when working with and exhausting mixtures of hydrogen in air. The lower explosive limit for this combination is approximately 4 % .
RESULTS AND DISCUSSIONS CharacterizingSensor Response. Signal development can arise from two possible transduction mechanisms. The first is a change in phase and polarity based upon stresses developed in the palladium layer upon exposure to hydrogen. These stresses are translated to the optical fiber causing straininduced changes in the refractive index profile. Theoretical strain effects have been calculated for different dimensions of palladium layer t h i ~ k n e s s . ~Strain-induced changes in refractive index due to a 60-Acoating of palladium (exposed to 6.7% H2) are approximately 2 orders of magnitude less than the refractive index change due to the 1-deg temperature rise encountered in this research.12J3 Although mounting and coating procedures were performed with care, differences in coating thickness and mounting angles were inevitable, leading to a possible source of stress. Of the 17 fibers constructed and tested, all but 1 fiber responded with a similar shape, response time, and amplitude. A second source of the signal developed by the bow-tie fiber can be attributed to its temperature-dependent refractive index profile. Each orthogonal lightwave component travels through regions of the fiber whose refractive index responds differently to heat, changing the phase of that component, and the resultant output polarity. Heat can be generated from several reactions taking place on and in the surface of the sensing element:14J5 (12) Richmond, E. W. Ph.D. Thesis, Virginia Polytechnical Institute and State University, 1990. (13) Gottlib, M.;Brandt, G. B.; Butler, J. ISA Trans. 1980,19, 55. (14) Lundstrom, I.; Shivaraman, S.; Svensson, C.; Lundkvist, L. Appl. Phys. Lett. 1975,26, 55. (15) Rosen, B.;Dayan, V. H.; Proffit, R. L. Hydrogen Leak and Fire Detection, U S . Government Printing Office: Washington, D.C., 1975; p 19.
H,(gas)
OH,
-
-
+ HA 2HA
4Pd
+ H,
2HA
-
("A"implies adsorbed state) (1)
H,O(gas)
H,(gas)
2Pd,H
1381
AH = 270 kJ/mol
AH = 470 kJ/mol
(4)
(5)
(ratio H / P d is normally C1) (6)
Heat from reaction 6 which has a AH of 40 kJ/mol is believed to be a major contributor to the signal shape and amplitude shown by the optical fiber catalytic sensing element. This conclusion is based on the following experimental observations: (1) A signal is developed from the sensor in air and in oxygen-free nitrogen carrier gases. Without oxygen, water formation cannot take place (reactions 1-4). (2) Hydrogen desorption (reaction 5) is exothermic, but very (3) If reactions 1-5, which can take place on the surface of the platinum, were responsible for heat evolution, the palladium layer would not be required. Experiments with only a platinum sensing layer provided no useful signal.12 After hydrogen is removed from the air carrier stream, reactions 2-5 play a large role in the sensor's return to baseline. As less hydrogen is available to diffuse into the platinum and palladium from the sample gas stream, oxygen begins to cover the platinum surface. Oxygen in turn reacts with hydrogen atoms diffusing back out of the palladium to the platinum surface forming water. In the flowing gas streram water desorption causes a cooling effect and the sensor's return to baseline. Hydrogen Sensor Calibration. There are many hydrogen sensor configurations, each with its own advantages and limitations.15 Table I and Figure 5 show the statistics and response curves for 1of 17 sensing elements developed in this research. There are two ways to change the response of this instrument. First, adjusting the flow of the hydrogen rotameter will vary the ratio of hydrogen to carrier gas. The second is to program the computer to allow the hydrogen solenoid to introduce gas into the carrier stream for longer or shorter periods of time. It should be noted that the two lower concentrations (Figure 5 ) were measured with the hydrogen solenoid adjusted to allow the sample gas to flow for a longer time than the higher concentration. This procedure produced better response a t lower hydrogen to air ratios. Concentrations greater than 10% seemed to saturate the fiber chemistry and will quickly degrade the sensor response. Much lower concentrations can be detected, but accurate mixing and maintenance of low-level calibration gases requires sophisticated controllers for volume and flow. Mass spectrometry was used to verify calibration gas concentrations. Pressure Insensitivity. Any sensor using a carrier gas or liquid must be insensitive to pressure fluctuations caused by the introduction of sample into the carrier stream. In an experiment to test for pressure sensitivity, gas was initially allowed to flow over the fiber as normal. A plastic tee was placed at the exhaust. One end of the tee is open, the other is connected to a 5-lb (psig) pressure gauge. Figure 6 shows the sensor response when the open end of the tee is totally blocked, allowing the pressure in the sensor head to rise between 1 and 2 lb. The pressure is then released and
ANALYTICAL CHEMISTRY, VOL. 64, NO. 13, JULY 1, 1992
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Table I. Sensor Statistics % Hz in air
peak height (V)
X 5 = 1.97
6.7
I
SD = 0.16 RSD = 7.9% X 5 = 0.86 SD = 0.05 RSD = 6.2% X5 = 0.62 SD = 0.02 RSD = 4.0
2.8 1.4
$-
1.2
2
p
2
0.0 -0.4
>
IN A I R
0
260
I30
z
0 ' 1
PRESSURE ON I
-50
1
0
f
I
I
0
130
260
iPRESSURE ON
IPRESSURE
iESSWE
OFF
PRESSURE OFF
LSECl
CONCLUSIONS
PRESSURE ON
L
/
of hydrogen. These results shows that the sensor responds quickly and with sufficient sensitivity to the presence of explosive concentrations of hydrogen gas. Temperature Calibration. In order to estimate the fiber optic sensor's thermal response, a solid-state temperature device (Analog Digital 590) was mounted in the sensor head. Both temperature and fiber optic output data were recorded, revealing a sensitivity to temperature of 2.6 rad/(deg/m) of fiber.l3 This response compares favorably with several other novel thermal sensor configurations found in ref 17.
Figure 5. Average response curves (average of 5).
lJl
-
Flgure 7. Sensor response time.
TIME I S E C l
-
0.5 SEC PULSE
TIME
0
> c
A
d
OFF
I
I
130
260
T I N ISEC I
Figure 8. Sensor pressure response.
the sequence repeated several times. The signal produced by these relatively harsh pressure changes is approximately 20 mV, which is about 1% of the normal response to 6.77% hydrogen in air (2 V). A remarkable insensitivity to isotropic pressure changes is demonstrated by the birefringent singlearm interferometer. ResponseTime of the Sensor. A figure of merit for many transducers is their response time. At a flow rate of approximately 1 L/min, the time required to completely flush the volume of the sensor head is in the millisecond range. Calculations based on diffusion constants for hydrogen through palladium support the observed response times.16 Hydrogen was introduced by a computer-controlled solenoid. Figure 7 displays the sensor response to a 2.5-and 0.5-spulse
The single-arm interferometer has been demonstrated to be an effective thermal sensor in a catalytic gas-flow injection scheme. Because light traveling in an optical fiber is not an ignition source, this instrument has the potential for safely detecting species of interest in explosive environments. It was not the intention of this research to investigate catalytic chemistries or to develop a hydrogen sensor; however, interesting phenomena were observed. In particular, there were radical differences in sensor response caused by using different palladium deposition techniques. Further study of the structural effect of the palladium and platinum layers on the kinetics, thermodynamics, and overall sensor output will be required. Development of mathematical models describing sensorresponse curves for different types of optical media, e.g. multimode fibers, nonbirefringent fibers, and slab waveguides, is continuing. These studies may help determine which optical parameters provide the optimum environment for data acquisition.
RECEIVED for review November
14, 1991. Accepted March
23, 1992.
Registry No. Hydrogen, 1333-74-0. (16) Simmons, J. W.; Flanagan, T.B.J. Phys. Chem. 1965,69, 10. (17) Dessy, R.; Arney, L.; Burgess, L.; Richmond, E. Biosensor Technology, Fundamentals and Applicatiom; Marcel Dekker, Inc.: New York, 1990;p 251.
