WESCAN INSTRUMENTS


WESCAN INSTRUMENTSpubs.acs.org/doi/pdf/10.1021/ac00146a744ingto develop a chemical system that will enable us to use the...

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FOCUS SULFITE ANALYSES 232 ppm Sulfite (as S 0 2 )

TOTAL SULFITE* IN WHITE WINE \ Column: Cat. No. 26&051 Eluant5mMH 2 SOA Flow: 0.85 ml/minute I Sample: White Wine, 1)250 Detector: WESCAN SULFITE ANALYZER Potential: 0.6V Range: 100nAFS

petition between fluorophor-labeled and unlabeled antigen (analyte) for a limited number of receptor binding sites. An example of this competitive type of assay is a technique developed by S. M. Angel of Lawrence Livermore National Laboratory in which a competition is set up between untagged antigen (analyte) and tagged antigen for binding sites on the surface of an optical fiber. Displacement of the tagged antigen results in a decrease in the intensity of the fluorescence signal. According to Angel, detection limits as low as 10~12 M have been accomplished with this technique. In a similar effort by Michael J. Sepaniak, Tuan Vo-Dinh, and colleagues at the University of Tennessee and Oak Ridge National Laboratory (3), rabbit immunoglobulin-G (IgG) is covalently immobilized on the distal sensing tip of a quartz optical fiber. The sensor is then exposed to fluorescein isothiocyanate-labeled and unlabeled antirabbit IgG. As expected, the sensor response to fluorescence emission at the optical fiber sensing tip is once again inversely proportional to the amount of unlabeled antigen (anti-IgG) in the sample. According to Seitz, one important goal in future research on fiber-optic sensors involves the development of reversible indicators for ionic species such as K + , Na + , and Cl~. "We're trying to develop a chemical system that will enable us to use the kinds of ionophores found in ion-selective electrodes for optical instead of electrochemical readout," Seitz explains. Piezoelectric devices

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with the WESCAN Sulfite Analyzer • Excellent correlation with Monier-Williams • Detects to sub-ppm levels • Less than 12 minutes per sample • Automated systems available WESCAN INSTRUMENTS 2051 Waukegan Road Deerfield, IL 60015 PHONE: 312-392-2670

Piezoelectric devices are sensitive mass-to-frequency transducers that originated in the electrical engineering community and are now of interest for potential analytical chemistry applications. There are two types of piezoelectric sensors: bulk-wave devices and surface acoustic-wave (SAW) devices. SAWs are capable of fundamental oscillations at higher frequencies than are bulk-wave devices, and some people believe that this makes SAW devices capable of lower detection limits, although this is still a matter of some controversy. A piezoelectric device is sensitive to changes in the mass, density, or viscosity of samples in contact with its active surface. The most straightforward application of piezoelectric sensors involves the microweighing of substances deposited on the active surface of the device in a nonspecific manner. At the next level of sophistication, piezoelectric devices are used for surface reaction sensing (e.g., the real-

CIRCLE 173 ON READER SERVICE CARD 1162 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987

time monitoring of coating processes or of chemical reactions involving covalent bonding). In addition, they are used to measure the viscosity of liquids in contact with the sensor surface, and they are applicable to the monitoring of electrochemical reactions based on their sensitivity to mass changes associated with redox processes. Potential analytical applications depend on efforts to enhance this capability for simple mass detection with tech-

If you want to make a sensor you can s e l l . . . , you have to think in terms of a great deal of information going far beyond what one or two or even an array of several sensors will give you. % tt niques to improve selectivity for analytes of interest. "If you can place an antibody or other specific material on a surface, resulting in a selective addition, a wide variety of commercial applications are possible," says Glenn Bastiaans of Integrated Chemical Sensors Co. (Newton, Mass.). "The most immediate market would be for clinical assays, especially in doctors' offices or other locations where you would prefer to avoid a large, sophisticated instrument. Home health care applications may also be important down the line." Bastiaans and his colleagues have already developed SAW devices for the determination of antihuman IgG (using a surface coating containing human IgG), and human chorionic gonadotropin (HCG, the hormone that serves as an indicator of pregnancy). These two devices are now in the proof-of-principle stage of commercialization. Work also is proceeding in a number of other laboratories to develop specific surface coatings t h a t will enhance piezoelectric selectivity while simultaneously achieving device stability, rapid response time, and high sensitivity. Although there have been significant successes, such as the IgG and HCG sensors, this effort is proving to be quite a challenge. "I hope that people will not be led down the primrose path, thinking that all of this is very simple and trivial," explains Hank Wohltjen of Microsensor Systems, Inc. (Fairfax, Va.). "It's really not. There are some