Portable and on-line devices

Moreover, ion-selective electrodes can be manufactured in very robust probes and, provided they are protected from fouling and physical attrition, can often be used for continuous and on-line analyses as well as laboratory determinations. Their application is expected to increase substantially in the coming years as active electrode materials continue to be improved and instrumentation becomes increasingly automated, simplifying its use. 

One of the most ambitious trends in ion-selective electrode design is the development of ion-selective membranes combined with a metal oxide semiconductor field-effect transistor (MOSFET) such as TSFETs in a single package. Rapid advances are being made with these devices which would facilitate miniaturization. 

The criteria used to judge an analytical device for fieldwork and on-line monitoring are often quite different from those for a procedure to be employed in a laboratory with full facilities. Clearly, portable devices should be small, light, robust, and auxiliary equipment such as gas supplies should be avoided both the active components and the control electronics must be capable of miniaturization and not require frequent adjustment or calibration. On the other hand, the device need only have the selectivity, sensitivity and speed of response essential to the particular analysis. Thus, for example, the method need only discriminate against interferences likely to be found in the particular situation of the analysis. Electrochemical cells and their control circuits are much more able to meet these requirements than the other common analytical techniques (e.g. chromatography and spectroscopy) and their output is well-suited to continuous monitoring or as an input to a control loop or an automatic warning. 

It will be apparent from the previous section that ion-selective electrodes can be made suitable for analysis in the field and that, in circumstances where the overall composition of the medium is not too variable, they may also be used directly to monitor metal ion and/or anions in plant streams, effluents, water supplies and even rivers. In such applications of ion-selective electrodes, however, it is necessary to avoid electrical pick-up and ground loops by correct placement and shielding of the electrodes. In some applications, electrolyte preconditioning may be necessary and in many cases, the flow environment must also be controlled via suitable design of the ion-selective electrode-reference electrode cell. There are also many other situations where the capabilities of electrochemistry and the characteristics of an analysis can be matched to allow the manufacture of on-line or portable devices. The actual measurement may be the potential of an ion-selective electrode, the mass transport controlled current 

The fouling of sensors by, for example, scale, organic compounds or biofilms may cause serious problems in heavily contaminated samples. In certain cases, the sensor and its assembly are designed to incorporate a self-cleaning device, which may utilize mechanical jet flow or even ultrasonic methods. Mechanical 

---

Instruments built in this period can be divided in three groups desk instruments, compact portable instruments and on-line compatible devices. 

All analysts are familiar with the principles of potentiometry and potarography and indeed, most analytical laboratories will contain a pH meter and a polarograph. However, electrochemical methods arc, in general, not very important in modern analysis. In contrast, there arc spccifiG applications such as trace metal ion analysis in water and effluents and also some other aspects of environmental analysis for which electrochemical methods are particularly attractive. This is because (1) some methods, especially anodic stripping voltammetry, have a very high sensitivity for heavy-metal ions and the lowest detection limit of from 10 to mol dm is well below that of other available methods (2) electrochemical methods are well suited for modification to on-line and/or portable devices for analysis in the held. Whether the analysis is based on current, conductivity or the response of an ion-selective electrode, both the cell and the control electronics are readily miniaturized and operate on low power Hence, this chapter considers the principles of the electroanalytical methods important in environmental and on-line analysis, together with biochemical applications of electrochemical sensors. 

Historically, respirometers have been used for wastewater biodegradability evaluation. More recently [52], a mobile on-line respirometer was proposed and tested for monitoring the activated sludge inhibition due to industrial discharges in a sewer network. A derived portable device called a Baroxymeter [53], based on monitoring the respiration of a bacterial culture by pressure measurements and using respiration inhibition as a toxicity alert, was proposed for the rapid detection of the toxicity effect of some toxic substances. 

Immunosensors promise to become principal players in chemical, diagnostic, and environmental analyses by the latter 1990s. Given the practical limits of immunosensors (low ppb or ng/mL to mid-pptr or pg/mL) and their portability, the primary application is expected to be as rapid screening devices in noncentralized clinical laboratories, in intensive care facilities, and as bedside monitors, in physicians offices, and in environmental and industrial settings (49—52). Industrial applications for immunosensors will also include use as the basis for automated on-line or flow-injection analysis systems to analyze and control pharmaceutical, food, and chemical processing lines (53). Immunosensors are not expected to replace laboratory-based immunoassays, but to open up new applications for immunoassay-based technology. 

NAPLPS was designed specifically to meet the needs of videotext on slow speed telecommunication lines currently, it is the videotext standard in the United States and Canada. As a communication standard for the exchange of graphical and textual information, it is a useful device interface because it is compact, portable, and resolution independent. A decoder is needed on the terminal to receive and display NAPLPS. This can be a software emulator or it can be built into the hardware. 

On the other hand, even when field analysis is necessary in many steps of the food chain (farms, crops, etc.) and small devices with portable instmmentation are required, food processes in industries need in many cases automatization, especially when the number of samples is high (e.g., food quality). Moving the approaches from off- to at-, on-, and in-line analysis is always desirable, but all the possibilities have to be considered and all of them need innovative approaches for generating the ideal food analysis platform. In this section, some issues related to multianalyte determination and automatization are considered. 

Near-infrared instruments of the UV-VIS-NIR type have become commercially available about 1955 with applications for agricultural commodities. Instruments designed specifically for measuring NIR energy reflected from solids have been commercially available as from 1971 [214] the development of these devices was pioneered by Norris [215]. The first successful uses of modem MRS were in the 1100-2500 nm region. NIR instrumentation is now extremely varied from UV-VIS-NIR to FTIR instm-ments, NIR reflectance instruments, PAS technology, on-line and portable analysers. 

Practical FTIR solutions have been developed by paying attention to the fundamental design of the instrument. Moving an FTIR instrument out of the benign enviromnent of a laboratory to the more alien environment of either a process line or that of a portable device is not straightforward. A major emphasis on the instrument design in terms of both ruggedness and fundamental reliability of components is critical. Furthermore, issues such as enviromnental contamination, humidity, vibration and temperature are factors... 

The COMPU-RATE, developed by Faehr Electronic Timers, Inc., is a portable device using batteries, which provide about 120 hr of running time. Manual entries are required only for the element column and the top four lines of the form. This allows the analyst to concentrate on observing the work and the operator performance. Time can be in thousandths of a minute or one hundred-thousandths of an hour. The COMPU-RATE software system computes the mean and median element values, adjustment of mean times to normal time after inputting the performance rating, and allowed times in minutes and/or hours per piece and pieces per hour. Errors can be corrected through an edit function. 

---