Portable instruments, advantages

Chemiluminescent assays are conventionally monitored using photomultiplier-based instruments. However, portable instruments are becoming available that use as photodetectors silicon photodiodes, charge-coupled devices, and instant photographic or x-ray film (58, 63-68). Spacially resolved, quantitative light measurements are pcirticularly advantageous for assays based on membranes or microtiter plates. 

The application of the ASV technique to the analysis of seawater has been reviewed (6-9). Its advantages over other trace-metal methods are several (1) sea salts do not interfere (2) a high degree of sensitivity and selectivity can be achieved through electrolytic preconcentration, thus the analysis can be carried out directly in seawater, and few or no reagents need to be added (3) the measurement is easily automated (4) relatively inexpensive, rugged, and portable instrumentation can be built for field use (5) more than one metal may be measured at one time with a sensitivity approaching parts per trillion. 

A more recent, and very promising, development is due to Cohen and James, who adapted a position-sensitive proportional counter to stress measurement. This counter has the great advantage that it can measure the 29 position of a diffracted beam without a 29 movement of the counter (Sec. 7-5). Preliminary work was done with the counter mounted on a standard diffractometer [16.10]. Later a portable instrument was made, in which the counter and a miniature, air-... 

Rb, Mo, Ag. Ba, Tb). In this way a number of excitation lines can be produced. The advantages of these radioactive sources lie in their simplicity and small size as well as their independence ofelectrical power. Thus they are suited to portable instruments. Spectra obtained from fluorescent emission show much better resolution than primary X-ray spectra as they lack the continuum background, figure 8.27. Consequently sensitivity and precision are rather better for fluorescence methods than they are for electron excitation techniques. ... 

Although a large level of success has been achieved with fluorescent-labeled DNA microarrays, these methods are difficult to implement in portable instrumentation, so that their use is limited to specialized laboratories. Electrical detection of biomolecular interactions is highly desirable due to its suitability to low-cost portable sensors that can be used in the field by non-specialized personnel. The use of label-free techniques has the added advantages of reducing costs and avoiding the need for sample pre-treatment. 

The absence of an external light source in BRET assays is also advantageous in the development of a more portable instrument. 

Information presented in Table 1.16 allows one to evaluate advantages of one or another gas sensor for portable applications. High sensitivity, selectivity, and robusmess are desirable characteristics of any chemical sensor technology in order to meet the specifications of the sensing application. Supplemental to these basic requirements, however, portable instruments also require sensor technolo-... 

When relatively short capillary columns are operated at unusually high flowrates, fast separations are obtained, and peaks are significantly narrower than with conventional GC. To take full advantage of the resolving power available with HSGC, fast detectors and data systems are required. Most laboratory HSGC has been done with flame ionization detection (FID), and most on-site monitoring with portable instruments has been done with thermal conductivity detection (TCD) or FID. 

As we have stated earlier, the use of ME-based-polymers has several advantages with respect to glass such as cost and microfabrication simplicity. In this way, an interesting work dealing with a high-performance polymer polyether ether ketone (PEEK) microchip as a novel application in food environments has been reported in connection with a portable instrument [44]. In this work, CCD was also used as the detection principle. 

Gamma-ray sources can also be used without the need for an elaborate power supply, allowing use in small portable instruments. The sources include a radioactive isotope which supply gamma rays with characteristic energy and can supply an X-ray or gamma-ray beam with the flux of 10 photons s sr . The main advantages of radioisotope excitation over X-ray tube excitation are the monoenergetic character of radioisotope-emitted X-rays, and that it is an inexpensive technique that is easily commercially available. 

A number of FET-based biosensors have been developed to study biomolecular interactions, which are the key drivers of biological responses in in vitro or in vivo systems. Among the many different biosensing systems, the FET-type biosensor is one of the most attractive electrical biosensors due to its advantages of sensitive measurements, portable instrumentation, easy operation with a small sample requirements, low cost... 

Radioisotope excitation (RIXRF) A few radioisotope sources (with acceptable half-life) emit radiation in the X-ray region, such as Am-241 (59.5 keV), Cd-109 (Ag-K lines, 22 and 25 keV) and Fe-55 (Mn lines, 5.9 keV), and can be used for excitation. The advantage of radioisotope sources is their independence of a generator and power supply, which makes their use interesting for portable instruments, in field, on-line and even extraterrestrial applications. The disadvantage is the low photon flux in comparison to tube excitation. 

Instrument Designs for Infrared Absorption The simplest instrument for IR absorption spectroscopy is a filter photometer similar to that shown in Figure 10.24 for UV/Vis absorption. These instruments have the advantage of portability and typically are used as dedicated analyzers for gases such as ITCN and CO. 

The advantages of this type of instrument are (a) that only one instrument is required for all necessary measurements, and (b) that it is portable and thus particularly suitable for held work. 

The advantages of controlled-potential techniques include high sensitivity, selectivity towards electroactive species, a wide linear range, portable and low-cost instrumentation, speciation capability, and a wide range of electrodes that allow assays of unusual environments. Several properties of these techniques are summarized in Table 1-1. Extremely low (nanomolar) detection limits can be achieved with very small sample volumes (5-20 pi), thus allowing the determination of analyte amounts of 10 13 to 10 15 mol on a routine basis. Improved selectivity may be achieved via the coupling of controlled-potential schemes with chromatographic or optical procedures. 

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