Molecule-sensitive detector

Another way to monitor the expected changes in the metal electronic structure is to look at the adsorbed molecules, which are sensitive in their properties to the changes in the electronic structure of surface metal atoms. Such a molecule is CO and the frequency of the CO stretch vibrations ( v(CO)) is a sensitive detector of the direct- and back-donation upon adsorption of CO. It has been reported, that v(CO) decreases for the VIII group metal by alloying of Pd with Ag (22), Ni with Cu (23), but also when mixing Ni with Co (24). This has been first explained (25) as an indication for an increased backdonation due to an assumed electron shift Cu Pt,... 

Size exclusion chromatography (SEC) separates molecules of a polymer sample on the basis of hydrodynamic volume. When the chromatograph is equipped only with a concentration-sensitive detector, i.e. conventional SEC, a molecular weight distribution (MWD) can be obtained from the chromatogram only through use of a calibration function relating molecular weight and elution volume V (2). 

Detectors are composed of a sensor and associated electronics. Design and performance of any detector depends heavily on the column and chromatographic system with which it is associated. Because of the complexity of many mixtures analysed and the limitation in regard to resolution, despite the use of high-resolution capillary columns and multicolumn systems, specific detectors are frequently necessary to gain selectivity and simplify the separation system. Many detectors have been developed with sensitivities toward specific elements or certain functional groups in molecules. Those detectors that exhibit the highest sensitivity are often very specific in response, e.g. the electron capture detector in GC or the fluorescence detector in LC. Because... 

A laser beam highly focused by a microscope into a solution of fluorescent molecules defines the open illuminated sample volume in a typical FCS experiment. The microscope collects the fluorescence emitted by the molecules in the small illuminated region and transmits it to a sensitive detector such as a photomultiplier or an avalanche photodiode. The detected intensity fluctuates as molecules diffuse into or out of the illuminated volume or as the molecules within the volume undergo chemical reactions that enhance or diminish their fluorescence (Fig. 1). The measured fluorescence at time t,F(t), is proportional to the number of molecules in the illuminated volume weighted by the... 

GC detectors can be grouped into concentration-sensitive detectors and mass-sensitive detectors. The signal from a concentration-sensitive detector is related to the concentration of solute in the detector, which does not usually destroy the sample. Mass-sensitive detectors usually destroy the sample, and the signal is related to the rate at which solute molecules enter the detector. The response of a mass-sensitive detector is unaffected by make-up gas, while that of a concentration-sensitive detector will lower with make-up gas. A summary of some important characteristics of the GC detectors specifically used in drug residue analysis is presented in Table 23.1. 

Altitude Response. Pressure response is an issue that needs to be addressed for every instrument deployed on an aircraft. First, it must be decided how chemical abundances are to be reported. If standard practice is followed and they are reported as mixing ratios, then it must be determined whether the instrument is fundamentally a mass- or a concentration-depen-dent sensor, because this definition determines the first-order means by which instrument response is converted to mixing ratios as a function of pressure. In this context, a mass-sensitive detector is a device with an output signal that is a function of the mass flow of analyte molecules a concentration-sensitive detector is one in which the response is proportional to the absolute concentration, that is, molecules per cubic centimeter. 

The need for more complicated considerations in conversion of instrument response to mixing ratios usually arises when instruments that are based on mass-sensitive detectors are used. Common reasons are either that the mass flow is not held constant or that the process whereby the flow of analyte molecules is converted to an electrical signal changes as the pressure changes. These effects are illustrated by a discussion of the pressure response of two instruments commonly used to measure atmospheric trace gases, both based on detection schemes that are inherently mass sensitive. 

Analytical methods used to identify monomers have improved significantly from those that quantify whole classes of compounds, such as amino acids, peptides, proteins, and primary amines (Undefriend et al., 1972) or carbohydrate-like compounds (Johnson and Sieburth, 1977) to ones that are molecule-specific (Table II). Most of these methods are based on combining chromatography techniques that can separate complex mixtures of molecules with highly sensitive detectors that can approach the nanomolar or picomolar range. Monomers are usually present at low concentrations, so... 

The determination of organic selenium compounds is done preferably by GC coupled to element-or molecule-specific detectors, such as GC-AED or molecular mass spectrometric detection (GC-MS).240 In this case, ICP-MS detection does not yield the improvement in sensitivity otherwise seen, which is due to spectral interferences. Dietz et al.241 have compared the analytical figures of merit of three detector systems for GC (AED, atomic fluorescence spectroscopy (AFS), and ICP-MS), arriving at the conclusion that GC-AED is the most sensitive and most practical... 

By using either a continuous or pulsed source of radiation and by measuring the amount of radiation absorbed by the reaction products, it is possible to determine product state distributions. The source of radiation can either be monochromatic (resonance lamp or laser) or broad-band (flash lamp or arc lamp) used in conjunction with a form of monochromator at the detector. The amount of absorption is monitored by an appropriate photosensitive or energy-sensitive detector. Particular care must be taken in the case of resonance lamps to avoid self-reversal of the output of the source, as this will complicate the quantitative analysis of product densities [17]. Similarly, laser sources must not be operated at such high output powers that the transitions involved become saturated, as this also complicates the analysis. Absorption measurements can be used for a wide range of reaction products, both ground and excited states of atoms, radicals and molecules [9,17, 22]. 

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