Costs, multiple detectors

A broad range of developments, generally described as laboratory-on-a-chip technologies, have been described and are being explored in several laboratories.15 A variety of lab-on-a-chip combinations can be used to multiplex a variety of separations with multiple detectors. Techniques such as molecular imprinting offer considerable promise for reducing the cost of these devices while maintain-... 

If multiple detectors are to be used in FIA, then the gradient must be left intact or complete and reproducible mixing must occur. Ideally all detectors must see a similar dispersed sample plug. The first detector will observe a reproducible concentration gradient. If that detector creates irreproducible turbulence, then subsequent detectors will not observe reproducible sample zones, which results in imprecision. This requirement complicates multiple-cell systems. The simplest approach is to split the stream into enough individual streams to feed all the detectors. This is more costly in terms of components, but avoids significant development time in sensor cell design. 

A third, usually more expensive, system has multiple detectors to count multiple samples simultaneously. Configurations available commercially are modular with groups of four detectors arranged over a slide-out drawer. Modules can be added for 8,16, or even 64 detectors. This counting system is useful for low-level samples that require long counting time (i.e., 24-96 h). A high sample load can justify the cost. 

Unlike the other systems, this system employs removable cassettes, which can be used in the bucky tray of a standard mammography unit While there are capital cost savings to this approach, it does require that phosphor plates be manually transported to the reader for processing. Because there are multiple detector plates, flat-fidd correction is normally not performed for the plates, but only for the plate reader. In principle, correction for nonuniformity of the individual plates could be done, but this would require precise registration within the reader and would be time-consuming. 

Photodiodes are used as detectors in many automated systems either as individual components or in multiples as an array. Photomultiplier tubes are required in many immunoassay systems to provide adequate sensitivity and fast detector response times for fluorescent and chemiluminescent measurements. Several approaches have been used for the electrooptical integration and packaging in different analyzers. A logarithmic amplifier or microprocessor and/or computer software converts transmittance to absorbance. Low-cost analog-to-digital converters with conversion times... 

Hyphenation refers to the online combination of a separation technique (mainly GC or LC) and a spectroscopic detection method that provides structural information for the analytes concerned. LC-MS and GC-MS are the most popular hyphenated techniques in use today, thanks to the incredible shrinking of mass spectrometers. However, MS does not provide the same information as IR or NMR. Hypernation (one higher than hyphenation) is the multiple hyphenation which may combine, for example, UV detection with PDA, NMR, FTIR, and MS as displayed in Figure 4.22, the last one is a destmctive detector. At the present time most analysts are using a single detector but it can be guessed that hypemation will be used more and more as size and cost of the instruments decrease. 

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