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Trends in fiber optic sensor development
trendsin analyticalchemistry, vol. 6, no. 4,1987
a5
2 T. A. H. M. Janse and G. Kateman, Anal. Chim. Acta, 159 (1984) 181. 3 G. M. Birtwistle, 0. J. Dahl, B. Myhrhaug and K. Nygaard, Simula Begin, PetrocelWCharter, New York, 1973.
Nice van Buggenum, Jo Klaessens, Bernard Vandeginste and Gerrit Kateman are at the Laboratorium voor Analytische Chemie, Katholieke Universiteit Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands
trends
Trends in fiber optic sensor development Linda A. Saari Lexington, MA, U.S.A. Fiber optic sensors continue to attract considerable attention and have been widely applied to diverse analytes. The current state of the art will be reviewed with the major focus on extrinsic fiber optic chemical sensors. Fiber optic characteristics, instrumentation and configurations will be described. Representative applications will be discussed from the clinical, industrial, and environmental areas. Advantages, limitations, andfuture directions will be discussed.
Introduction The number of applications of fiber optic sensors to measure both chemical analytes and physical parameters has grown steadily over the past five years. Fiber optic based sensors have been developed due to the need for selective, reliable sensors, due to the problems inherent in traditional methods, and because of the need for continuous monitoring. Recent advances in fiber optic technology in the communications industry has been largely responsible for the substantial research effort being expended in the area of fiber optic sensors. These advances, in addition to advances in materials and immobilization TABLE I. Physical parameters mined by fiber optic sensors
and chemical analytes
methods, coupled with a large well known and everincreasing area of chemistry (i.e., optical methods) have made fiber optic sensors a reality. Fiber optic sensors have considerable potential for use in industrial, environmental, and medical applications. Physical parameters and chemical analytes for which fiber optic sensors have been developed are listed in Table I. Few of these fiber optic sensors are available commercially. Most of the chemical fiber optic sensors are still in the research and development stage. Fiber optic sensors can be classified into two types: intrinsic or extrinsic. The difference between the two is displayed in Fig. 1. In an intrinsic sensor the fiber optic functions as the active element. The parameter to be measured affects the fiber characteristics in some way so that the light transmitted by the fiber will change in response to the parameter. It is important in these sensors that the pa-
a)
A
deter-
b) Position Flow-rate Pressure Ionic strength PH Hematocrit ClNa+ N% Zn2+ Alkanes uoz+ Albumin
0165~9936/87/$03.00.
Vibration Temperature Retractive index Electric current
Fill level Magnetic field Ionizing radiation Mechanical strain
PC02
PO2
Glucose BrA13+ cu*+ Cd2+ S2Chlororganics p-Nitrophenyl phosphate
Oxygen saturation IBe*+ Mg2 Hydrogen peroxide Moisture Immunochemicals
I
S
>
I -__--_
_--
D
t
Fig. I. (a) An intrinsicfiber optic sensor is shown in which physical parameter or analyte, A, interacts directly with the fiber optic causing a change in the transmission of source radiation, S, detected at detector, D. (b) An extrinsic fiber optic sensor in which physical parameter or analyte, A, causes a change in transducer, T, which then affects the transmission of source radiation, S, through the fiber optic which is detected at detector, D.
@Elsevier Science PublishersB.V.
trends in analytical
86 rameter to be measured specifically alters the fiber optic properties. In an extrinsic sensor the fiber optic is passive and is used only as a conduit to transmit light to and from the sensing region. It is crucial in these sensors that the fiber optic characteristics do not change in response to the parameter to be measured.
Fiber optic characteristics Optical fibers have a number of properties which make them attractive in sensing applications. Fiber optics are simple, relatively inexpensive, easy to manufacture, flexible, corrosion resistant, easily multiplexed, and can be made very small and thin. Because fiber optics are resistant to adverse environments, have high data carrying capacity, and are not subject to electrical interference they can be used for remote sensing and in vivo applications. Optical fibers are fabricated from two different compositions of glass. The core of the fiber is made from glass with a relatively high index of refraction and is surrounded by cladding which is made from glass with a slightly lower refractive index. Light is propagated down the fiber by total internal reflection. Optical fibers can be broadly classified as single mode (single frequency) or multimode (multiple frequency). Most of the fiber optic sensor applications utilize multimode fibers as they are easier to couple and have a higher light carrying capacity. There have been advances in fiber materials and in manufacturing methods to produce low loss optical fibers.
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been used for many applications, but a single optical fiber can also be used in conjunction with a dichroic filter or a beam splitter. Detectors which have been used include photomultiplier tubes and photodiodes. A number of manufacturers now sell instrumentation specifically for use in fiber optic sensing applications. The components can be purchased singly or as a system and include the source, detector, fiber optics, wavelength selector, optics, and connectors. Spectrophotometers, fluorometers, Raman spectrometers, and other optical instrumentation may be used with fiber optics to do fiber optic sensing. The real advantage of using fiber optics in conjunction with expensive instrumentation such as this is that remote sensing using multiple sensors can be done with multiplexed optics. Thus, an instrument can be utilized concurrently for more than one application. Typical configurations for instrumentation for use in extrinsic sensing are shown in Fig. 2. The instrumentation for fiber optic sensors is not an area that requires much development, the crucial area is the development of selective transducers. Intrinsic sensors Continuing studies of fiber optic properties have lead to the development of intrinsic fiber optic sensors. In this case the optical fiber is both the transducer and the conduit used to transmit the light. Any
a)
S
D Instrumentation The basic instrumentation needed for fiber optic sensing applications includes a source, optical fibers, and a detector. The instrumentation can be simple or complex, inexpensive or costly, depending on the application. Sources which have been used include light emitting diodes, tungsten halogen lamps, xenon lamps, and lasers. A reason for using laser light is that the high source intensity may be desirable if remote sensing is done over long distances. In many cases, however, simple sources such as light emitting diodes are sufficient. If multiwavelength sources are used, some method of wavelength selection is needed. Typically, interference filters are used, as are monochromators. Some sort of focusing optical system is needed in most cases, especially when lasers are used in conjunction with single mode fibers. Bifurcated bundles or a pair of optical fibers have
b)
o)
S
(y--IO T
D
Fig. 2. Three configurations of extrinsic fiber optic sensors showing source, S, transducer, T, and detector, D. (a) A pair of optical fibers or a bifurcated bundle is used. (b) A single optical fiber or bundle is used with a dichroic filter or beam splitter, F. (c) A single optical fiber is used.
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parameter which alters an optical fiber’s properties so as to alter its transmission characteristics can potentially be measured by an intrinsic fiber optic sensor. Some of the fiber properties which can be used include the fiber refractive index, fiber absorption characteristics, fiber fluorescence characteristics or combinations of these’. Sensors to measure temperature, pressure, and ionizing radiation will be described in principle. A simple example of an intrinsic sensor to measure temperature is based on the change in refractive index of the fiber with temperature. Since the ability of an optical fiber to transmit light depends on the refractive index of the fiber, changes in transmission intensity can be related to temperature. Temperature sensitive cladding can be used as part of a fiber optic temperature sensor. As the temperature of the cladding changes, its refractive index changes and the light transmission varies. Commercial fiber optic temperature sensors are extrinsic and are based on temperature induced changes in fluorescence of materials coupled with fiber optics’. Pressure can potentially be measured by an intrinsic fiber optic sensor in conjunction with a microbending type of apparatus shown in Fig. 3. When force is applied to the fiber, the transmission characteristics are altered and can be related to the amount of force applied to the fiber. The variation can be related to pressure. Pressure changes can also be measured extrinsically by using displacement of a mirror or a light sensitive optical coating’. Ionizing radiation can be detected by its effect on the transmission characteristics of fiber optics. The effect is well pronounced when specially doped fibers are used. The fiber can be doped with material that undergoes radiation induced luminescence’. As more is learned about fiber optic properties and advances are made in their manufacture the amount of interest and research into the devel-
S
I
>
______ __ ___ -3 D
Fig. 3. A pressure or force sensor is shown in which the force, A, cazues an apparatus to bend the optical fiber which affecti transmission of source radiation, S, detected at detector, D.
opment of intrinsic fiber optic sensors should intensify. Specific doping of optical fibers to obtain a response to a desired analyte will probably be an area of future interest. Extrinsic sensors Two types of extrinsic fiber optic sensors have been developed. One sensor is the direct type where the optical fiber is used without a transducer. In this first type fiber optics are used to directly quantitate analytes which have inherent optical properties such as absorbance or luminescence. This is the simplest use of fiber optics as part of an extrinsic sensor. When fiber optics are used in this way the optical path must be held constant, requiring that a cuvette device be fabricated on the fiber optic. Any of the configurations in Fig. 2 can be used with the transducer replaced by a cuvette device. Quantitating analytes in this mode is very limited for a number of reasons. Direct absorbance is relatively non-selective and requires that the sample be treated. Thus, continuous monitoring of complex samples using absorbance is subject to interference from other absorbing substances. Direct luminescence, e.g., fluorescence, is less subject to interference, but there are fewer analytes that naturally luminescence. One example of this type of sensor is reported for in vivo monitoring. Fiber optics are used to successfully monitor the antitumor drug deoxorubicin in interstitial fluids of laboratory mice by laser-induced fluorescence3. A single 200-,_umdiameter quartz optical fiber is used in conjunction with a dichroic filter, laser source and photomultiplier tube detector. A clever method is used to sample the fluid and keep the optical path constant. In this method the protective coating and the cladding are removed from the end of the optical fiber and the fiber end is inserted into a capillary tube which is then inserted into a modified 20 gauge needle. To make a measurement the optical fiber is moved upward and sample is drawn into the capillary tube. The sample is then expelled from the capillary tube by pushing the fiber down. Reproducible volumes as small as 200 nl can be sampled and the detection limit for the drug is reported to be about 10e7 M. A recent example appears in the literature describing an absorbance based direct sensor for remote determination of Cu” in electroplating baths4. The sensor quantitates Ct.?+ by measuring its infrared absorbance using a 820-nm gallium arsenide light-emitting diode as the light source in conjunction with two 200~pm silica optical fibers, a red sensitive photomultiplier tube as a detector, and a specially fabricated absorption cell. The device is reported to be able to measure the Ct.? concentration
trendsin analyticalchemistry, vol. 6, no. 4,1987
88 from 50 to 500 mM with standard deviations of less than 1%. The prototype system is reported as being evaluated in an industrial setting. A second, indirect type of sensor uses a transducer that changes optical properties in response to the measured parameter or analyte. This type of sensor holds the greatest potential in fiber optic sensing to determine chemical analytes and has been the subject of recent review articles5-7. The most active area of research is in developing sensitive, selective transducers to be used in this type of sensor. These transducers can be based on any substance which changes optical properties in response to the analyte. These properties which can be exploited in fiber optic sensing include absorbance, reflectance, fluorescence, phosphorescence, chemiluminescence and bioluminescence. The majority of the work to data has been in the areas of absorbance, reflectance, and fluorescence. Requirements for these transducers (sometimes referred to as ‘optrodes’ or ‘optodes’7) follow. First, the substance making up the transducer must be selective for the analyte or parameter of interest. Selectivity is obtained by using an analyte-permeable membrane to separate the reagent from the sample, e.g. an oxygen-permeable membrane with an oxygen-quenchable dye, or by using a reagent that selectively binds with the analyte, e.g., acid-base changes of a pH indicator or formation of fluorescent or colored metal chelates. Dynamic quenching of a fluorescent indicator without a membrane can be fairly specific for certain analytes, e.g., fluorescence quenching of acridinium and quinolinium indicators by halides7. The dynamic range of a reagent depends on the binding constant or, in the case of dynamic fluorescence quenching, on the quenching constant7. The dynamic range is limited to two to four orders of magnitude on either side of the binding constant for the reagent. This point was shown in a study of immobilized calcein to determine transition metal ions’. A second requirement for a transducer, in the ideal case, is that the substance should respond reversibly to the analyte. Metal chelators, pH sensitive indicators, and fluorescent substances which are quenched are all reversible. A number of applications of fiber optic ‘sensors’ are reported in which the transducer acts irreversibly. These are not true sensors, but have found some useful applications. A third requirement for a transducer is that it must be attached or immobilized on the fiber optic in some way. Immobilization of sensing reagents used as transducers has been accomplished by a number of methods. The three basic types of immobilization
which have been used are covalent bonding (either on a solid support or directly to the optical fiber), electrostatic attraction (ion exchange) or physical adsorption. Any of the immobilized reagents could be used in conjunction with an analyte-permeable membrane. There is also the specific case where a reagent solution is ‘immobilized’ on a fiber optic by a gas-permeable membrane, e.g., a pC02 sensor using dissolved phenol red behind a silicone membrane. Covalent bonding was used in a pH sensor based on fluoresceinamine bound to either controlled pore glass or to cellulose via cyanuric chloride6. Immobilization by ion exchange was employed in a pH sensor in which a sulfonated pH sensitive dye was immobilized on anion-exchange membrane’. Physical adsorption of a hydrophobic dye on a polystyrene support was used in a sensor for pOZ (ref. 5). It should be noted that immobilized reagents very often have very different properties than the free reagent. Binding constants, susceptibility to photodecomposition, spectral properties, pH sensitivity, and ionic strength sensitivity all may dramatically change depending upon immobilization. Applications Most of the applications of fiber optic sensors in the clinical area have been in the determination of blood gases @CO, andp0,) and pH. A fluorescence based system to measure these analytes in an extracorporeal loop is commercially available and performs well for trend monitoring”. A whole blood oximetry system has also been described to determine while blood oxygen saturation and hematocrit by measuring backscattered light”.
fiber
optic fill
solution
ionophore membrane
$1
I:11 Tl Na
+
Fig. 4. A fiber optic sensor is shown for determination of Naf using a sodium selective transducer. Na+ passes through a dialysis membrane and interacts with an ionophore selective for sodium causing ANS in the filling solution to ion pair with the Na+-ionophore complex and become fluorescent. In the absence of Na+, the ANS is bound to copper(H) polyethyleneimine in the filling solution and is non-fluorescent.
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A sensor to determine sodium _^ has been described based on ion pair extraction”. The components of this sensor are shown in Fig. 4. The response is based on the formation of a cationic complex between sodium ion and the ionophore which then ion pairs with 8-anilino-1-naphthalene sulfonic acid (ANS) rendering the ANS fluorescent. In the absence of sodium, the ANS ion pairs with copper(I1) polyethyleneimine forming a non-fluorescent complex. This sensor can be used to determine sodium ion in the range 20-200 mM, takes l-3 minutes to give a 90% response, and has potential for use in the clinical area. There is also work being done in using immunochemical, enzymatic and other specific reactions to determine clinical analytes. A sensor to determinepnitrophenyl phosphate by detection of the photometrically detectable product, p-nitrophenoxide was recently reported14. A recent environmental application is described in which a fiber optic sensor is used to monitor chlororganics in groundwater15. This sensor is based on the change in absorbance of basic pyridine when exposed to various chlororganics. A membrane which allows volatile chlororganics to pass through provides added selectivity and protection from water. This sensor was tested in wells known to contain chloroform. The sensor acts as an integrating device and not a true equilibrium based sensor. A preliminary study of a reversible fiber optic based sensor to determine sulfide has been described using reflectance changes of immobilized indicators including 2,6-dichlorophenolindo phenol16.
Advantages and limitations Many of the advantages of fiber optics sensors in relation to electrochemical sensors are due to the characteristics of fiber optics. Fiber optic sensors can be made very small, are resistant to harsh environments, and are not subject to electrical interference. There is no need for a reference electrode when making fiber optic sensor measurements. Fiber optic sensors may be made to withstand sterilization. Because the transducer in a fiber optic sensor can be distinct from the optical fiber, the transducer can be low cost and disposable. The potential exists for ‘precalibrated’ sensors. The concept of precalibrated sensors is attractive for both in vivo and in vitro clinical applications because of simplicity. The technical hurdles which need to be overcome to successfully make precalibrated sensors include stability of transducer materials and well defined, reproducible manufacturing methods.
Optical sensors may be developed for determining parameters and analytes for which no suitable electrochemical sensor exists. The limitations of fiber optic sensors in relation to electrochemically based sensors include the need to exclude ambient light and photodecomposition of transducer materials. Source modulation is one alternative to the exclusion of ambient light. Photodecomposition of the transducer material which may occur can be corrected by using a ratio of wavelengths which is insensitive to photodecomposition as was done in a pH sensor’. Because mass transfer is needed in many cases fiber optic sensors generally have longer response times than electrochemically based sensors. Although fiber optic sensors can be made very small, there are a number of factors which determine how small a transducer can be made. As the transducer is made smaller the signal-to-noise ratio becomes smaller and for a sensor without an analyte permeable membrane, non-specific adsorption of optically absorbing or luminescing substances can interfere with the measurement.
Future directions Fiber optic sensors are still in early stages of development. Because of the number of advantages of fiber optic sensors, the development of new fiber optic sensors is sure to continue. New reagents and immobilization methods will increase the range and number of analytes which can be measured. The future should include work directed toward development of new selective transducers, precalibrated sensors, and work on miniaturization and reproducibility of manufacturing methods. There should also be work on the problems of photodecomposition and response time. Additional composite sensors like the enzyme based fiber optic sensor-l3 should also increase in number. Different types of intrinsic fiber optic sensors should also attract attention. Future fiber optic sensors will take advantage of the properties of uew fiber optic materials and will use the fiber optic as more than just a conduit to carry light to and from the transducer.
References R. P. Main, Sensor Review, July (1985) 133. T. B. Hirschfeld, U.S. Pat., 4,599,901(1986). M. J. Sepaniak, B. J. Tromberg and J. F. Eastham, Clin. Chem., 29 (1983) 1678. J. E. Freeman, A. G. Childers, A. W. Steele and G. M. Hieftje, Anal. Chim. Acta, 177 (1985) 121. J. I. Peterson and G. G. Vurek, Science, 224 (1984) 123. W. R. Seitz, Anal. Chem., 56 (1984) 16A.
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7 0. S. Wolfbeis, Trends Anal. Chem., 4 (1985) 184. 8 L. A. Saari and W. R. Seitz, Anal. Chem., 56 (1984) 810. 9 Z. Zhujun and W. R. Seitz, Anal. Chim. Acta, 160 (1984) 47. 10 J. L. Gehrich, D. W. Lubbers, N. Opitz, D. R. Hansmann, W. W. Miller, J. K. Tusa and M. Yafuso, IEEE Trans. Biomed. Eng., BME-33 (1986) 117. 11 J. M. Schmitt, F. G. Milm and J. D. Meindl, Ann. Biomed.
Eng., 14 (1986) 35. 12 Z. Zhujun, J. L. Mullin and W. R. Seitz, Anal. Chim. Acta, 184 (1986) 251.
13 M. A. Arnold, Anal. Chem., 57 (1985) 565.
14 F. P. Milanovich, Environ. Sci. Technol., 20 (1986) 441. 15 R. Narayanaswamy and F. Sevilla, Analyst (London), (1986) 1085.
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Linda A. Saari received her B.S. in Chemistry from the Uriiversity of Massachusetts, Amherst, MA, U.S.A., in 1974. She received her Ph.D. in Analytical Chemistry from the University of New Hampshire, Durham, NH, U.S.A., in 1983. She is presently a Staff Scientist at Instrumentation Laboratory, 113 Hartwell Avenue, Lexington, MA 02173, U.S.A.
Overpressured layer chromatographic methods in the study of the formaldehyde cycle in biological systems E. Tyihdk Budapest,Hungary The basic principles and potentiul of overpressured layer chromatography as a new instrumentalized version of planar liquid chromatography are presented. The technique is illustrated for the determination of formaldehyde and its natural precursors. Future development trends are considered.
Introduction Column and planar liquid chromatographic techniques are supplementary to each other and have developed in constant mutual interaction. Hence, it is not surprising that the intensive development of high-performance column liquid chromatography (HPLC)‘?* was complemented by a fundamental renewal of planar liquid chromatography. In classical thin-layer chromatography (TLC)3S4 the solvent mixture migrates through the sorbent layer by capillary forces. A vapour phase of solvent lies above the sorbent layer. This layer, which is not yet wetted by the mobile phase, is in contact with the vapour of the solvent, which it progressively adsorbs. This reduces its porosity and increases the apparent speed of the mobile phase, which does not penetrate all of the channels inside the porous sorbent layer. Modern methods of column liquid chromatography employ constant flow-rates, which have not hitherto been used in TLC. However, the greatly increased developing time on a fine-particle size sorbent layer [high-performance thin-layer chromatographic (HPTLC) chromatoplates5’6] has made it 0165-9936/87/$03.00.
necessary to employ forced flow. Other factors such as the problems of optimization of the flow-rate and reproducibility of the development process have also played a role. The employment of forced flow has been achieved by the development of overpressured layer chromatography (OPLC), using a pressurized separation chamber with a special edge-impregnated chromatoplate and an overpressure for the introduction of the solvent7”. The essential feature of this chamber system is that the sorbent layer is completely covered with a flexible covering membrane under external pressure and the vapour phase above the sorbent layer is thus virtually eliminated. By adjusting the eluent by means of a pump, it is possible to separate substances on thin or thick sorbent layers of both small and big scale with optional development distances and directions’. OPLC and other liquid chromatographic techniques Comparison of different classical and modern planar liquid chromatographic techniques shows that in one group of these techniques, the solvent mixture migrates through the sorbent layer b capillar (e.g. paper chromatography 70; TLC Y; forces HPTLC’). The other group of planar liquid chromatographic techni ues [e.g. centrifugal layer chromatography (CLC)9 r, high-speed TLC (HSTLC) using electroosmosisr*, OPLC “1 is characterized by application of a forced solvent flow, which is already in common use. In HPLC there is a characteristic relationship between the average theoretical plate height, w, and OElsevier Science Publishers B.V.
a5
2 T. A. H. M. Janse and G. Kateman, Anal. Chim. Acta, 159 (1984) 181. 3 G. M. Birtwistle, 0. J. Dahl, B. Myhrhaug and K. Nygaard, Simula Begin, PetrocelWCharter, New York, 1973.
Nice van Buggenum, Jo Klaessens, Bernard Vandeginste and Gerrit Kateman are at the Laboratorium voor Analytische Chemie, Katholieke Universiteit Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands
trends
Trends in fiber optic sensor development Linda A. Saari Lexington, MA, U.S.A. Fiber optic sensors continue to attract considerable attention and have been widely applied to diverse analytes. The current state of the art will be reviewed with the major focus on extrinsic fiber optic chemical sensors. Fiber optic characteristics, instrumentation and configurations will be described. Representative applications will be discussed from the clinical, industrial, and environmental areas. Advantages, limitations, andfuture directions will be discussed.
Introduction The number of applications of fiber optic sensors to measure both chemical analytes and physical parameters has grown steadily over the past five years. Fiber optic based sensors have been developed due to the need for selective, reliable sensors, due to the problems inherent in traditional methods, and because of the need for continuous monitoring. Recent advances in fiber optic technology in the communications industry has been largely responsible for the substantial research effort being expended in the area of fiber optic sensors. These advances, in addition to advances in materials and immobilization TABLE I. Physical parameters mined by fiber optic sensors
and chemical analytes
methods, coupled with a large well known and everincreasing area of chemistry (i.e., optical methods) have made fiber optic sensors a reality. Fiber optic sensors have considerable potential for use in industrial, environmental, and medical applications. Physical parameters and chemical analytes for which fiber optic sensors have been developed are listed in Table I. Few of these fiber optic sensors are available commercially. Most of the chemical fiber optic sensors are still in the research and development stage. Fiber optic sensors can be classified into two types: intrinsic or extrinsic. The difference between the two is displayed in Fig. 1. In an intrinsic sensor the fiber optic functions as the active element. The parameter to be measured affects the fiber characteristics in some way so that the light transmitted by the fiber will change in response to the parameter. It is important in these sensors that the pa-
a)
A
deter-
b) Position Flow-rate Pressure Ionic strength PH Hematocrit ClNa+ N% Zn2+ Alkanes uoz+ Albumin
0165~9936/87/$03.00.
Vibration Temperature Retractive index Electric current
Fill level Magnetic field Ionizing radiation Mechanical strain
PC02
PO2
Glucose BrA13+ cu*+ Cd2+ S2Chlororganics p-Nitrophenyl phosphate
Oxygen saturation IBe*+ Mg2 Hydrogen peroxide Moisture Immunochemicals
I
S
>
I -__--_
_--
D
t
Fig. I. (a) An intrinsicfiber optic sensor is shown in which physical parameter or analyte, A, interacts directly with the fiber optic causing a change in the transmission of source radiation, S, detected at detector, D. (b) An extrinsic fiber optic sensor in which physical parameter or analyte, A, causes a change in transducer, T, which then affects the transmission of source radiation, S, through the fiber optic which is detected at detector, D.
@Elsevier Science PublishersB.V.
trends in analytical
86 rameter to be measured specifically alters the fiber optic properties. In an extrinsic sensor the fiber optic is passive and is used only as a conduit to transmit light to and from the sensing region. It is crucial in these sensors that the fiber optic characteristics do not change in response to the parameter to be measured.
Fiber optic characteristics Optical fibers have a number of properties which make them attractive in sensing applications. Fiber optics are simple, relatively inexpensive, easy to manufacture, flexible, corrosion resistant, easily multiplexed, and can be made very small and thin. Because fiber optics are resistant to adverse environments, have high data carrying capacity, and are not subject to electrical interference they can be used for remote sensing and in vivo applications. Optical fibers are fabricated from two different compositions of glass. The core of the fiber is made from glass with a relatively high index of refraction and is surrounded by cladding which is made from glass with a slightly lower refractive index. Light is propagated down the fiber by total internal reflection. Optical fibers can be broadly classified as single mode (single frequency) or multimode (multiple frequency). Most of the fiber optic sensor applications utilize multimode fibers as they are easier to couple and have a higher light carrying capacity. There have been advances in fiber materials and in manufacturing methods to produce low loss optical fibers.
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vol. 6, RO. 4,1987
been used for many applications, but a single optical fiber can also be used in conjunction with a dichroic filter or a beam splitter. Detectors which have been used include photomultiplier tubes and photodiodes. A number of manufacturers now sell instrumentation specifically for use in fiber optic sensing applications. The components can be purchased singly or as a system and include the source, detector, fiber optics, wavelength selector, optics, and connectors. Spectrophotometers, fluorometers, Raman spectrometers, and other optical instrumentation may be used with fiber optics to do fiber optic sensing. The real advantage of using fiber optics in conjunction with expensive instrumentation such as this is that remote sensing using multiple sensors can be done with multiplexed optics. Thus, an instrument can be utilized concurrently for more than one application. Typical configurations for instrumentation for use in extrinsic sensing are shown in Fig. 2. The instrumentation for fiber optic sensors is not an area that requires much development, the crucial area is the development of selective transducers. Intrinsic sensors Continuing studies of fiber optic properties have lead to the development of intrinsic fiber optic sensors. In this case the optical fiber is both the transducer and the conduit used to transmit the light. Any
a)
S
D Instrumentation The basic instrumentation needed for fiber optic sensing applications includes a source, optical fibers, and a detector. The instrumentation can be simple or complex, inexpensive or costly, depending on the application. Sources which have been used include light emitting diodes, tungsten halogen lamps, xenon lamps, and lasers. A reason for using laser light is that the high source intensity may be desirable if remote sensing is done over long distances. In many cases, however, simple sources such as light emitting diodes are sufficient. If multiwavelength sources are used, some method of wavelength selection is needed. Typically, interference filters are used, as are monochromators. Some sort of focusing optical system is needed in most cases, especially when lasers are used in conjunction with single mode fibers. Bifurcated bundles or a pair of optical fibers have
b)
o)
S
(y--IO T
D
Fig. 2. Three configurations of extrinsic fiber optic sensors showing source, S, transducer, T, and detector, D. (a) A pair of optical fibers or a bifurcated bundle is used. (b) A single optical fiber or bundle is used with a dichroic filter or beam splitter, F. (c) A single optical fiber is used.
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parameter which alters an optical fiber’s properties so as to alter its transmission characteristics can potentially be measured by an intrinsic fiber optic sensor. Some of the fiber properties which can be used include the fiber refractive index, fiber absorption characteristics, fiber fluorescence characteristics or combinations of these’. Sensors to measure temperature, pressure, and ionizing radiation will be described in principle. A simple example of an intrinsic sensor to measure temperature is based on the change in refractive index of the fiber with temperature. Since the ability of an optical fiber to transmit light depends on the refractive index of the fiber, changes in transmission intensity can be related to temperature. Temperature sensitive cladding can be used as part of a fiber optic temperature sensor. As the temperature of the cladding changes, its refractive index changes and the light transmission varies. Commercial fiber optic temperature sensors are extrinsic and are based on temperature induced changes in fluorescence of materials coupled with fiber optics’. Pressure can potentially be measured by an intrinsic fiber optic sensor in conjunction with a microbending type of apparatus shown in Fig. 3. When force is applied to the fiber, the transmission characteristics are altered and can be related to the amount of force applied to the fiber. The variation can be related to pressure. Pressure changes can also be measured extrinsically by using displacement of a mirror or a light sensitive optical coating’. Ionizing radiation can be detected by its effect on the transmission characteristics of fiber optics. The effect is well pronounced when specially doped fibers are used. The fiber can be doped with material that undergoes radiation induced luminescence’. As more is learned about fiber optic properties and advances are made in their manufacture the amount of interest and research into the devel-
S
I
>
______ __ ___ -3 D
Fig. 3. A pressure or force sensor is shown in which the force, A, cazues an apparatus to bend the optical fiber which affecti transmission of source radiation, S, detected at detector, D.
opment of intrinsic fiber optic sensors should intensify. Specific doping of optical fibers to obtain a response to a desired analyte will probably be an area of future interest. Extrinsic sensors Two types of extrinsic fiber optic sensors have been developed. One sensor is the direct type where the optical fiber is used without a transducer. In this first type fiber optics are used to directly quantitate analytes which have inherent optical properties such as absorbance or luminescence. This is the simplest use of fiber optics as part of an extrinsic sensor. When fiber optics are used in this way the optical path must be held constant, requiring that a cuvette device be fabricated on the fiber optic. Any of the configurations in Fig. 2 can be used with the transducer replaced by a cuvette device. Quantitating analytes in this mode is very limited for a number of reasons. Direct absorbance is relatively non-selective and requires that the sample be treated. Thus, continuous monitoring of complex samples using absorbance is subject to interference from other absorbing substances. Direct luminescence, e.g., fluorescence, is less subject to interference, but there are fewer analytes that naturally luminescence. One example of this type of sensor is reported for in vivo monitoring. Fiber optics are used to successfully monitor the antitumor drug deoxorubicin in interstitial fluids of laboratory mice by laser-induced fluorescence3. A single 200-,_umdiameter quartz optical fiber is used in conjunction with a dichroic filter, laser source and photomultiplier tube detector. A clever method is used to sample the fluid and keep the optical path constant. In this method the protective coating and the cladding are removed from the end of the optical fiber and the fiber end is inserted into a capillary tube which is then inserted into a modified 20 gauge needle. To make a measurement the optical fiber is moved upward and sample is drawn into the capillary tube. The sample is then expelled from the capillary tube by pushing the fiber down. Reproducible volumes as small as 200 nl can be sampled and the detection limit for the drug is reported to be about 10e7 M. A recent example appears in the literature describing an absorbance based direct sensor for remote determination of Cu” in electroplating baths4. The sensor quantitates Ct.?+ by measuring its infrared absorbance using a 820-nm gallium arsenide light-emitting diode as the light source in conjunction with two 200~pm silica optical fibers, a red sensitive photomultiplier tube as a detector, and a specially fabricated absorption cell. The device is reported to be able to measure the Ct.? concentration
trendsin analyticalchemistry, vol. 6, no. 4,1987
88 from 50 to 500 mM with standard deviations of less than 1%. The prototype system is reported as being evaluated in an industrial setting. A second, indirect type of sensor uses a transducer that changes optical properties in response to the measured parameter or analyte. This type of sensor holds the greatest potential in fiber optic sensing to determine chemical analytes and has been the subject of recent review articles5-7. The most active area of research is in developing sensitive, selective transducers to be used in this type of sensor. These transducers can be based on any substance which changes optical properties in response to the analyte. These properties which can be exploited in fiber optic sensing include absorbance, reflectance, fluorescence, phosphorescence, chemiluminescence and bioluminescence. The majority of the work to data has been in the areas of absorbance, reflectance, and fluorescence. Requirements for these transducers (sometimes referred to as ‘optrodes’ or ‘optodes’7) follow. First, the substance making up the transducer must be selective for the analyte or parameter of interest. Selectivity is obtained by using an analyte-permeable membrane to separate the reagent from the sample, e.g. an oxygen-permeable membrane with an oxygen-quenchable dye, or by using a reagent that selectively binds with the analyte, e.g., acid-base changes of a pH indicator or formation of fluorescent or colored metal chelates. Dynamic quenching of a fluorescent indicator without a membrane can be fairly specific for certain analytes, e.g., fluorescence quenching of acridinium and quinolinium indicators by halides7. The dynamic range of a reagent depends on the binding constant or, in the case of dynamic fluorescence quenching, on the quenching constant7. The dynamic range is limited to two to four orders of magnitude on either side of the binding constant for the reagent. This point was shown in a study of immobilized calcein to determine transition metal ions’. A second requirement for a transducer, in the ideal case, is that the substance should respond reversibly to the analyte. Metal chelators, pH sensitive indicators, and fluorescent substances which are quenched are all reversible. A number of applications of fiber optic ‘sensors’ are reported in which the transducer acts irreversibly. These are not true sensors, but have found some useful applications. A third requirement for a transducer is that it must be attached or immobilized on the fiber optic in some way. Immobilization of sensing reagents used as transducers has been accomplished by a number of methods. The three basic types of immobilization
which have been used are covalent bonding (either on a solid support or directly to the optical fiber), electrostatic attraction (ion exchange) or physical adsorption. Any of the immobilized reagents could be used in conjunction with an analyte-permeable membrane. There is also the specific case where a reagent solution is ‘immobilized’ on a fiber optic by a gas-permeable membrane, e.g., a pC02 sensor using dissolved phenol red behind a silicone membrane. Covalent bonding was used in a pH sensor based on fluoresceinamine bound to either controlled pore glass or to cellulose via cyanuric chloride6. Immobilization by ion exchange was employed in a pH sensor in which a sulfonated pH sensitive dye was immobilized on anion-exchange membrane’. Physical adsorption of a hydrophobic dye on a polystyrene support was used in a sensor for pOZ (ref. 5). It should be noted that immobilized reagents very often have very different properties than the free reagent. Binding constants, susceptibility to photodecomposition, spectral properties, pH sensitivity, and ionic strength sensitivity all may dramatically change depending upon immobilization. Applications Most of the applications of fiber optic sensors in the clinical area have been in the determination of blood gases @CO, andp0,) and pH. A fluorescence based system to measure these analytes in an extracorporeal loop is commercially available and performs well for trend monitoring”. A whole blood oximetry system has also been described to determine while blood oxygen saturation and hematocrit by measuring backscattered light”.
fiber
optic fill
solution
ionophore membrane
$1
I:11 Tl Na
+
Fig. 4. A fiber optic sensor is shown for determination of Naf using a sodium selective transducer. Na+ passes through a dialysis membrane and interacts with an ionophore selective for sodium causing ANS in the filling solution to ion pair with the Na+-ionophore complex and become fluorescent. In the absence of Na+, the ANS is bound to copper(H) polyethyleneimine in the filling solution and is non-fluorescent.
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A sensor to determine sodium _^ has been described based on ion pair extraction”. The components of this sensor are shown in Fig. 4. The response is based on the formation of a cationic complex between sodium ion and the ionophore which then ion pairs with 8-anilino-1-naphthalene sulfonic acid (ANS) rendering the ANS fluorescent. In the absence of sodium, the ANS ion pairs with copper(I1) polyethyleneimine forming a non-fluorescent complex. This sensor can be used to determine sodium ion in the range 20-200 mM, takes l-3 minutes to give a 90% response, and has potential for use in the clinical area. There is also work being done in using immunochemical, enzymatic and other specific reactions to determine clinical analytes. A sensor to determinepnitrophenyl phosphate by detection of the photometrically detectable product, p-nitrophenoxide was recently reported14. A recent environmental application is described in which a fiber optic sensor is used to monitor chlororganics in groundwater15. This sensor is based on the change in absorbance of basic pyridine when exposed to various chlororganics. A membrane which allows volatile chlororganics to pass through provides added selectivity and protection from water. This sensor was tested in wells known to contain chloroform. The sensor acts as an integrating device and not a true equilibrium based sensor. A preliminary study of a reversible fiber optic based sensor to determine sulfide has been described using reflectance changes of immobilized indicators including 2,6-dichlorophenolindo phenol16.
Advantages and limitations Many of the advantages of fiber optics sensors in relation to electrochemical sensors are due to the characteristics of fiber optics. Fiber optic sensors can be made very small, are resistant to harsh environments, and are not subject to electrical interference. There is no need for a reference electrode when making fiber optic sensor measurements. Fiber optic sensors may be made to withstand sterilization. Because the transducer in a fiber optic sensor can be distinct from the optical fiber, the transducer can be low cost and disposable. The potential exists for ‘precalibrated’ sensors. The concept of precalibrated sensors is attractive for both in vivo and in vitro clinical applications because of simplicity. The technical hurdles which need to be overcome to successfully make precalibrated sensors include stability of transducer materials and well defined, reproducible manufacturing methods.
Optical sensors may be developed for determining parameters and analytes for which no suitable electrochemical sensor exists. The limitations of fiber optic sensors in relation to electrochemically based sensors include the need to exclude ambient light and photodecomposition of transducer materials. Source modulation is one alternative to the exclusion of ambient light. Photodecomposition of the transducer material which may occur can be corrected by using a ratio of wavelengths which is insensitive to photodecomposition as was done in a pH sensor’. Because mass transfer is needed in many cases fiber optic sensors generally have longer response times than electrochemically based sensors. Although fiber optic sensors can be made very small, there are a number of factors which determine how small a transducer can be made. As the transducer is made smaller the signal-to-noise ratio becomes smaller and for a sensor without an analyte permeable membrane, non-specific adsorption of optically absorbing or luminescing substances can interfere with the measurement.
Future directions Fiber optic sensors are still in early stages of development. Because of the number of advantages of fiber optic sensors, the development of new fiber optic sensors is sure to continue. New reagents and immobilization methods will increase the range and number of analytes which can be measured. The future should include work directed toward development of new selective transducers, precalibrated sensors, and work on miniaturization and reproducibility of manufacturing methods. There should also be work on the problems of photodecomposition and response time. Additional composite sensors like the enzyme based fiber optic sensor-l3 should also increase in number. Different types of intrinsic fiber optic sensors should also attract attention. Future fiber optic sensors will take advantage of the properties of uew fiber optic materials and will use the fiber optic as more than just a conduit to carry light to and from the transducer.
References R. P. Main, Sensor Review, July (1985) 133. T. B. Hirschfeld, U.S. Pat., 4,599,901(1986). M. J. Sepaniak, B. J. Tromberg and J. F. Eastham, Clin. Chem., 29 (1983) 1678. J. E. Freeman, A. G. Childers, A. W. Steele and G. M. Hieftje, Anal. Chim. Acta, 177 (1985) 121. J. I. Peterson and G. G. Vurek, Science, 224 (1984) 123. W. R. Seitz, Anal. Chem., 56 (1984) 16A.
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7 0. S. Wolfbeis, Trends Anal. Chem., 4 (1985) 184. 8 L. A. Saari and W. R. Seitz, Anal. Chem., 56 (1984) 810. 9 Z. Zhujun and W. R. Seitz, Anal. Chim. Acta, 160 (1984) 47. 10 J. L. Gehrich, D. W. Lubbers, N. Opitz, D. R. Hansmann, W. W. Miller, J. K. Tusa and M. Yafuso, IEEE Trans. Biomed. Eng., BME-33 (1986) 117. 11 J. M. Schmitt, F. G. Milm and J. D. Meindl, Ann. Biomed.
Eng., 14 (1986) 35. 12 Z. Zhujun, J. L. Mullin and W. R. Seitz, Anal. Chim. Acta, 184 (1986) 251.
13 M. A. Arnold, Anal. Chem., 57 (1985) 565.
14 F. P. Milanovich, Environ. Sci. Technol., 20 (1986) 441. 15 R. Narayanaswamy and F. Sevilla, Analyst (London), (1986) 1085.
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Linda A. Saari received her B.S. in Chemistry from the Uriiversity of Massachusetts, Amherst, MA, U.S.A., in 1974. She received her Ph.D. in Analytical Chemistry from the University of New Hampshire, Durham, NH, U.S.A., in 1983. She is presently a Staff Scientist at Instrumentation Laboratory, 113 Hartwell Avenue, Lexington, MA 02173, U.S.A.
Overpressured layer chromatographic methods in the study of the formaldehyde cycle in biological systems E. Tyihdk Budapest,Hungary The basic principles and potentiul of overpressured layer chromatography as a new instrumentalized version of planar liquid chromatography are presented. The technique is illustrated for the determination of formaldehyde and its natural precursors. Future development trends are considered.
Introduction Column and planar liquid chromatographic techniques are supplementary to each other and have developed in constant mutual interaction. Hence, it is not surprising that the intensive development of high-performance column liquid chromatography (HPLC)‘?* was complemented by a fundamental renewal of planar liquid chromatography. In classical thin-layer chromatography (TLC)3S4 the solvent mixture migrates through the sorbent layer by capillary forces. A vapour phase of solvent lies above the sorbent layer. This layer, which is not yet wetted by the mobile phase, is in contact with the vapour of the solvent, which it progressively adsorbs. This reduces its porosity and increases the apparent speed of the mobile phase, which does not penetrate all of the channels inside the porous sorbent layer. Modern methods of column liquid chromatography employ constant flow-rates, which have not hitherto been used in TLC. However, the greatly increased developing time on a fine-particle size sorbent layer [high-performance thin-layer chromatographic (HPTLC) chromatoplates5’6] has made it 0165-9936/87/$03.00.
necessary to employ forced flow. Other factors such as the problems of optimization of the flow-rate and reproducibility of the development process have also played a role. The employment of forced flow has been achieved by the development of overpressured layer chromatography (OPLC), using a pressurized separation chamber with a special edge-impregnated chromatoplate and an overpressure for the introduction of the solvent7”. The essential feature of this chamber system is that the sorbent layer is completely covered with a flexible covering membrane under external pressure and the vapour phase above the sorbent layer is thus virtually eliminated. By adjusting the eluent by means of a pump, it is possible to separate substances on thin or thick sorbent layers of both small and big scale with optional development distances and directions’. OPLC and other liquid chromatographic techniques Comparison of different classical and modern planar liquid chromatographic techniques shows that in one group of these techniques, the solvent mixture migrates through the sorbent layer b capillar (e.g. paper chromatography 70; TLC Y; forces HPTLC’). The other group of planar liquid chromatographic techni ues [e.g. centrifugal layer chromatography (CLC)9 r, high-speed TLC (HSTLC) using electroosmosisr*, OPLC “1 is characterized by application of a forced solvent flow, which is already in common use. In HPLC there is a characteristic relationship between the average theoretical plate height, w, and OElsevier Science Publishers B.V.