Characterisation of Detectors

It should be pointed out that the gain mechanism in a PMT tube operates as a random process. The number of secondary electrons is different for each primary electron. The relative width of the distribution can be expected at least of the size of the standard deviation of a poissonian distribution, n, of the secondary emission coefficient, n, at the first dynode. Therefore the single-photon pulses obtained from a PMT have a considerable amplitude jitter, see Sect. 6.2.5, page 226. 

PMTs of the same type but with different cathodes may differ considerably in gain. The reason is that the cathode is formed inside the tube. During the manufacturing process the cathode material spills into the dynode system. This can even be intentional, because the cathode materials are efficient secondary electron emitters. 

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Several reviews [767,768] and books [51,757,767, 769] deal with SEC in relation to the molecular weight characterisation of synthetic polymers (see also Bibliography). Trends in the development of column technology, detectors and data handling for SEC have recently been discussed [770,771]. The field produces some 1200 papers per year. 

Principles and Characteristics Ion mobility spectrometry (IMS) is an instrumental technique for the detection and characterisation of organic compounds as vapours at atmospheric pressure. Modern analytical IMS was created at the end of the 1960s from studies on ion-molecule chemistry with mass spectrometers and from ionisation detectors for vapour monitoring. An ion mobility spectrometer (or plasma chromatograph in the original termininology) was first produced in 1970 [272],... 

Principles and Characteristics Traditional analytical approaches include off-line characterisation of isolated components, and the use of several chromatographic separations, each optimised for a specific spectroscopic detector. Neither LC-NMR nor LC-MS alone can always provide complete structure determinations. For example, MS may fail in assigning an unequivocal structure for positional isomers of substituents on an aromatic ring, whereas NMR is silent for structural moieties lacking NMR resonances. Often both techniques are needed. 

Other detection systems, such as conductivity detector or refractive index detection are generally applicable for the determination of common anionic surfactants [1]. However, they are less sensitive than other techniques and are used more often for the characterisation of pure surfactants, than for their determination at low concentrations. 

There are many GC detectors available although the flame ionisation detector remains the most widely used and the most widely applicable to quality control of pharmaceutical products. However, newer detectors such as the plasma emission detector for analysis of trace impurities or the GC-FTIR detector for the structural characterisation of components in mixtures are becoming increasingly important. 

As already stated, complete speciation involves the identification, determination and characterisation of both metals and ligand constituents of the species. Although metals and ligands are determined by applying two different types of detector systems, it is essential that the results should clearly show the relationship between the metal(s) and ligand(s) that directly interact. The presence of a metal and a ligand in a fraction is not sufficient evidence for the existence of direct interaction. Ideally, the detector should respond specifically to the presence of the whole species. However, very few species can be detected in this manner. 

Durbin, R. P. I. Characterisation of the flame photometric detector for gas chromatography. II. Thermodynamics of some metal (3-diketonates via gas-liquid chromatography. Dissertation Abstr. B 27, 710 B (1966). 

The performance of all HPLC detectors can be characterised by certain parameters such as sensitivity, noise, drift, limit of detection, linear and dynamic range, and detection volume. Other factors are more specific to individual types of detectors, and are discussed in their respective sections. 

Molecular weightjmass detectors include light scattering detectors and viscosity detectors. When SEC is used in the characterisation of polymer systems, its main aim will be the production of a molecular mass/weight distribution and where possible absolute molecular weights. Mass calibration is a complicated matter (section 9.3.5.1) in that calibration curves differ for different polymer types, and for many commercial polymers, direct molar mass calibration is not possible because of the lack of suitable, known molecular weight standards. 

Chromatographic techniques are very commonly used in the characterisation of natural waters (Aoyama (1996a, 1996b), Huber (unpublished), Shaw et al. (1994), Mori (1988), Becher et al. (1985), Amy et al. (1987)). However, most authors use different techniques, detectors, standards and eluents, making the comparison of results difficult. 

In these last few years, even if ERCP and MRCP still represent an important diagnostic step in diagnosis and characterisation of cystic neoplasm, MDCT has strongly contributed to the depiction and the evaluation of these tumours. Particularly, the introduction and the use of very small detector size and the possibility of acquiring data from quite isotropic voxels have permitted the improvement of the reformatted imaging quality. 

Various types of detector have been employed for the detection of polymers separated by liquid adsorption chromatography, including conductimetric detectors for polyoxyethylenes [115] and UV detectors for ethyl methacrylate-butyl methacrylate copolymers [116]. Hancock and Synovec [117] carried out a rapid characterisation of linear and star branched polymers, particularly PS by a gradient detection method. This method measures average molecular weight in methylene dichloride solutions of the polymers and gives more specific... 

Further advantages of FMC over alternative techniqnes include the ability to measure heats of interactions of polymers in solution with solids, i.e., filler surfaces characterisation of filler surfaces by IGC is limited to reversibly adsorbed volatile molecules. Also, HPLC detectors can be connected in series with the calorimeter and used to determine quantities adsorbed, and hence provide a measure of surface coverage. 

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