Focus ANALYTICAL C H E M I S T R Y , Seitz pre-
dicted that the use of multiwavelength and temporal information could allow fiber-optic sensors to determine two or more analytes simultaneously. Multiwavelength measurements could also be used to monitor reagent-phase stability or to relate analyte concentrations to intensity ratios at two wavelengths. And if the sensor involves luminescence, time resolution can also be used. Comparison of fiber-optic sensors with microelectrodes and CHEMFETs Recently there has been a great deal of interest in sensors of all types, and other approaches to biosensing, including development of microelec-
The newest trend in fiber-optic sensor development is that of affinity sensors, which combine fluorescence detection with competitivebinding reactions. trodes and chemically sensitive field effect transistors (CHEMFETs), are being pursued by bioanalytical researchers. Although electrodes offer small size, low cost, and sensitivity to a variety of analytes, their performance has been disappointing because of various stability problems. But there are indications that this may be changing. "Because of the new technologies and ideas that have come out in terms of all types of electroactive polymers that can be used both as coatings for the electrodes and as membrane materials," says Peterson, "you can now do things with electrodes that you couldn't do just a few years ago. It has become a very interesting field." CHEMFETs, which combine an electrode with a solid-state charge indicator, are attractive for in vivo chemical monitoring because of their small size and versatility, but although they have gotten a great deal of publicity, they have not worked out satisfactorily so far. There is, however, still a great deal of interest in CHEMFET development. of proven value in most applications, Peterson believes that fiber-optic devices can be as small as electrosensors and that they offer several advantages
for medical applications. They are safe (because no electrical connection with the body is involved), the optical leads are flexible, and materials suitable for long-term implantation, such as plastic, can be used. Fiber-optic biosensors are also easily made, and some are even sufficiently simple in design to be disposable. And because the measurement is equilibrium based rather than diffusion rate dependent, as is the case with some electrodes, fiberoptic sensors exhibit particular advantages in long-term stability and simplification of calibration. But optical sensors are also subject to some limitations relative to microelectrodes. Because the reagent and the analyte are in different phases, there is necessarily a mass transfer step before constant response is reached, and this leads to relatively slow response times for fiber-optic sensors. According to Peterson, the response time of a fiber-optic sensor is related to the design of the sensor; a response time of 30 s is typical for 90% response. To some extent fiber optics is in direct competition with microelectrodes and CHEMFETs, but, says Peterson,
"Each has its own unique advantages, and all three are important because they offer the capability of on-line, in situ analysis." Because optical techniques are the workhorse of the analytical lab (it has been estimated that 80% of all laboratory measurements are done by optical methods), researchers developing fiber-optic sensors have a broad base of known analytical reagent systems from which to derive new sensor designs. "Reversible indicators are available for much of what is of interest in analytical measurements," says Peterson, "and their adaptation t o fiber optics is limited only by one's ingenuity." M.D.W. References (1) Peterson, J. I. Proceedings of the Symposium on Biosensors; Sponsored by the IEEE Engineering in Medicine and Biology Society and the National Science Foundation, Los Angeles, Calif., Sept. 15-17, 1984; pp. 35-39. (2) Coleman, J. T.; Eastham, J. F.; Sepaniak, M. J. Anal. Chem. 1984, 56, 2246-49. (3) Arnold, M. A. Anal. Chem. 1985, 57, 565-66. (4) Schultz, J. S.; Mansoure, S.; Goldstein, I. J. Diabetes Care 1982, 5, 245-53.
New Monochromator Attracts Attention of IR Spectroscopists "There's a long dry spell between new technologies in some fields of instrumentation, and this is something quite radically new—a whole new technology on how to do spectroscopic measurements in the field of infrared [IR] spectroscopy. However, it does have a number of limitations and, in my opinion, it's probably not likely to replace gratings and interferometers for general spectroscopic use." The instrument engineer quoted above (he requested anonymity) was referring to a device called an acoustooptic tunable filter, or AOTF (photo). The AOTF was introduced commercially at this year's Pittsburgh Conference by the Westinghouse Electric Corporation Combustion Control Division. The device drew a significant amount of interest at this year's Pittsburgh Conference exhibit, and, despite some assertions to the contrary, the question as to whether or not the AOTF will one day be used in general spectroscopic applications is still very much an open one.
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Essentially, the AOTF is a new type of monochromator. Historically, four types of devices—filter wheels, prisms, diffraction gratings, and interferometers—have been used to separate IR wavelengths spatially or temporally for molecular spectroscopy. Westinghouse is promoting its AOTF as a possible fifth major type of spectroscopic analyzer. At its Pittsburgh
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How to minimize pressure drop in preparative LC. The high flow rates used in modern preparative LC can cause excessive pressure drops. For example, at 300 mL/min a standard Rheodyne analytical valve can cause a pressure drop of 150 psi. By substituting a Rheodyne valve with larger flow passages, one designed for preparative work, this can be reduced to 17 psi. Rheodyne's Tech Note #7 contains formulas and graphs that enable you to calculate pressure drop in valves and tubes. One handy graph plots pressure drop vs. flow rate for Rheodyne valves. The data is supported by theoretical calculations (considering both laminar and turbulent flow) and verified by experimental results.
Send for Tech Note #7 Please address Rheodyne, Inc., PO. Box 996, Cotati, California 94928 U.S.A. Phone (707) 664-9050.
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Conference booth, the company was showing two AOTF-based devices for specific applications—a flue gas analyzer and a fiber-optic-coupled AOTF for process control in hazardous locations. But Westinghouse was making no secret about the fact that its interest lay not only in selling complete instruments, but also in selling AOTF devices to other instrument companies. The AOTF device is made of thallium arsenic selenide, a single-crystal biréfringent optical material patented by Westinghouse. In operation, a highfrequency (MHz-range) acoustic wave from a transducer is propagated across the optical path of the filter. This sound wave modulates the refractive index of the crystal and creates a sort of diffraction grating. A solid-state device with no moving parts, the AOTF can change wavelengths rapidly—in a matter of microseconds, according to Westinghouse— and can be easily controlled by a microprocessor to perform "smart" spectroscopy. The filter is available in four versions—high (
