Multi-functional detectors

SPECIAL DETECTOR TECHNIQUES Multi-functional Detectors... 

Multi-functional detectors monitor the column eluent by the measurement of more than one physical or chemical property simultaneously, employing a single sensing cell. To date, three bifunctional detectors and one trifunctional detector have been described. The three bifunctional detectors have combined UV absorption and fluorescent detection, UV absorption and electrical conductivity detection and UV absorption and refractive index detection. The latter uniquely combines a bulk property detector with a solute property detector producing, at least in theory, the nearest approach to a universal detector. The trifunctional detector incorporates UV absorption, electrical conductivity and fluorescence functions. Multi-functional detection provides detector versatility and a means of confirmir solute identity. Such detectors have to be designed, so that the performance specifications are not seriously compromised, and the cell and eluent conduits do not contribute significantly to peak dispersion. 

Only general purpose equipment used in FI systems is described in this chapter, together with its proper usage, particularly in separation and preconcentration applications. Included are solution delivery (propulsion) devices, injection and multi-functional valves, transport and mixing systems, and the most frequently used detectors. Devices used for specific separation purposes are described in the corresponding chapters on individual separation techniques. 

The DPE (digital), together with the interfaces to spacecraft, resides within a dedicated box that is isolated from the detector. The tasks to be performed by this multi-functional micro-computer controlled system include event selection and validation, background rejection, and collection and formatting of scientific data. It also provides the interface to the spacecraft, command and control interface, detector control, the collection of housekeeping data, power conversion and conditioning, and an overall experiment watch-dog . 

In its most elementary version as depicted in Figure 1, a miniaturized analysis system has been used for the measurement of ion concentrations with ion sensitive field effect transistors (ISFET). The system comprises two pumps and two multi-ISFET detector cells, one of which acts as a reference. Preliminary experiments have been performed with a functional model of the system where the individual elements were connected with lengths of silicone rubber tubing. Measurement of potassium concentrations using an ISFET with a valinomycin / PVC membrane in a glass flow through cell showed the feasibility of the setup [8]. 

Recent advances have greatly decreased the difficulties raised by these cautionary points. In particular, on-line viscosimetry and multi-angle lightscattering make it possible to determine the molecular weight and viscosity of samples as a function of elution volume. With such detectors, effects of solute-column packing interactions become unimportant, since the properties of narrow fractions can be measured. These detectors will be discussed in greater detail below. 

Fig. 3 GPC setup for functional group profiling. RI refractive index detector Fluorescence fluorescence detector MALLS multi-angle laser light scattering detector...

The multielement function of the plasma-based techniques has been a source of challenges in the AAS field. This has resulted in the fast sequential technique, which is a simple way to mimic the multi-element function. However, it only works for FAAS applications. Moreover, simultaneous multielement ET-AAS systems for analysis have also been placed on the market, although there are some spectral limitations. It also has the drawback of using the same time and temperature programme for all elements. In the future, further developments in the multielement technique can be envisaged which will resort to continuum sources as well as CCD and other multiwavelength detectors. 

The second major environmental application of FFF has been the use of an element-specific detector, usually in series with a UV detector, to provide elemental composition data along with the PSD. Graphite-furnace atomic absorption spectrometry has been used off-line on fractions collected from the FFF run. However, the multi-element detection, low detection limits and capability to function as an online detector have made inductively coupled plasma mass spectrometry (ICP-MS) the ideal detector for FFE85-86 The sample introduction system of the ICP-MS is able to efficiently transport micron-sized particles into the high-temperature plasma,... 

With development of solid-state detectors, relative x-ray intensities, such as, KP/Ka ratios, have been measured and compiled in the tabulated or graphical forms [2-4]. However, these values are still considered as an atomic property and compared with the theoretical calculations for free atoms [5]. This is because for calculation of x-ray emission rates in molecules it is necessary to perform multi-center integration for molecular wave function. Such calculations are tedious and require a lot of computation time and large memory capacity. 

The energy dispersive. spectrometer (Fig. 7.12) comprises a semi-conductor detector (diode) that collects the entire X-ray spectrum and transmits it to a multi-channci analyser which classes the various X-ray spectrum lines as a function of their energy. 

Electrochemical detection is very sensitive for the compounds that can be oxidized or reduced at low-voltage potentials. Therefore, it could also be applied in the HPLC analysis of phenolic acids that are present in natural samples at very low concentrations. With the recent advances in electrochemical detection, multi-electrode array detection is becoming a powerful tool for detecting phenolic acids and flavonoids in a wide range of samples. The multi-channel coulometric detection system may serve as a highly sensitive way for the overall characterization of antioxidants the coulometric efficiency of each element of the array allows a complete voltametiic resolution of analytes as a function of their reaction (redox) potential. Some peaks may be resolved by the detector, even if they are unresolved when they leave the HPLC column. ... 

The instrumentation used to measure y-rays is generally a semiconductor detector (Nal(Tl) crystal) associated to a multi-channel analyser to cover energies over the range from about 60 keV to 3.0 MeV. The functioning is similar to that of liquid scintillators. 

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