Detector choice

Detection in CC may be visually for coloured compounds. Different methods can be used to monitor colourless compounds (collecting fractions addition of an inorganic phosphor to the column adsorbent). The detector choice is quite limited, with UV, RI and molecular fluorescence (F) emission being the most popular. A fluorescent column adsorbent is extremely... 

Limited detector choice (need for extremely sensitive detection systems)... 

The preparation of a coated special fibre from a suitable material in a proper structure and the source and detector choice are not the final list of problems to be solved in the framework of sensor design. From Figure 3 it... 

Since the compound(s) you are analyzing are sufficiently volatile to be separated by gas chromatography, I am assuming that you will use a GC for your separations. Here are the detector choices we have to consider a = Mass spectrometer (general purpose)... 

This is obvious in cases where known compounds fail to elute. However, these restrictions may be subtle, and potentially more costly, when the thermal stability and volatility of the sample components is unknown. HPLC is restricted by detector limitations -- detector choice is a guessing game when the analytes are unknown. SFC is superior to GC and HPLC for the first screening of many new samples. For this purpose SFC provides ... 

The use of a hybrid approach increases flexibility as to detector choice at the expense of packing density and problems associated with inputting the photosignal into the CCD. The photocurrent is first integrated and then periodically dumped into the CCD, which causes a sampling of the signal in the... 

When entering the field of liquid chromatography, the scientist is always faced with the problem of detector selection. The subject of detector choice will be dealt with later in this book but at this point, it should again be emphasized, that there is no ideal LC detector. Consequently, the practicing liquid chromatographer needs to have at least two, if not more, different types of detector available, or the full versatility of the technique will not be realized. It is therefore recommended that one of the detectors available should be a bulk property detector, which should probably be the refractive index detector. This would be a particularly appropriate detector if preparative or semi-preparative chromatography is likely to be required. If the separation and quantitative analysis of ionic materials are contemplated, then the refractive index detector might be replaced... 

Detectors Detector choice is crucial in a RAIRS experiment. The sensitivity of a detector is characterized by the noise equivalent power (NEP WHz / ), defined as that amount of radiant power which must fall on the detector to give a root mean square (rms) electrical signal equal to the rms value of the detector noise. The lower the NEP, the more sensitive is the detector. Generally, pho-toconductive or photovoltaic detectors are utilized for RAIRS experiments, such as liquid-nitrogen-cooled InSb, PbSnTe, and PbSe with a typical spectral range of 5000—1400cm and NEP values of WHz / and the widely... 

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

Radiation exits the monochromator and passes to the detector. As shown in Figure 10.12, a polychromatic source of radiation at the entrance slit is converted at the exit slit to a monochromatic source of finite effective bandwidth. The choice of... 

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

Thermal Conductivity Detector One of the earliest gas chromatography detectors, which is still widely used, is based on the mobile phase s thermal conductivity (Figure 12.21). As the mobile phase exits the column, it passes over a tungsten-rhenium wire filament. The filament s electrical resistance depends on its temperature, which, in turn, depends on the thermal conductivity of the mobile phase. Because of its high thermal conductivity, helium is the mobile phase of choice when using a thermal conductivity detector (TCD). 

The most popular device for fluoride analysis is the ion-selective electrode (see Electro analytical techniques). Analysis usiag the electrode is rapid and this is especially useful for dilute solutions and water analysis. Because the electrode responds only to free fluoride ion, care must be taken to convert complexed fluoride ions to free fluoride to obtain the total fluoride value (8). The fluoride electrode also can be used as an end poiat detector ia titration of fluoride usiag lanthanum nitrate [10099-59-9]. Often volumetric analysis by titration with thorium nitrate [13823-29-5] or lanthanum nitrate is the method of choice. The fluoride is preferably steam distilled from perchloric or sulfuric acid to prevent iaterference (9,10). Fusion with a sodium carbonate—sodium hydroxide mixture or sodium maybe required if the samples are covalent or iasoluble. 

Spectroscopy. Infrared spectroscopy (48) permits stmctural definition, eg, it resolves the 2,2 - from the 2,4 -methylene units in novolak resins. However, the broad bands and severely overlapping peaks present problems. For uncured resins, nmr rather than ir spectroscopy has become the technique of choice for microstmctural information. However, Fourier transform infrared (ftir) gives useful information on curing phenoHcs (49). Nevertheless, ir spectroscopy continues to be used as one of the detectors in the analysis of phenoHcs by gpc. 

The choice of a detector for a specific appHcation should be made in order to minimize the cooling requirements and the magnitude of the background radiation noise therefore, in detector selection the cutoff wavelength should be only slightly greater than that required by the appHcation. If the... 

Interferometry is difficult in the uv because of much greater demands on optical alignment and mechanical stabiUty imposed by the shorter wavelength of the radiation (148). In principle any fts interferometer can be operated in the uv when the proper choice of source, beam spHtter, and detector is made, but in practice good performance at wavelengths much shorter than the visible has proved difficult to obtain. Some manufacturers have claimed operating limits of 185 nm, and Fourier transform laboratory instmments have reached 140 nm (145). 

The most widely used method of analysis for methyl chloride is gas chromatography. A capillary column medium that does a very good job in separating most chlorinated hydrocarbons is methyl siUcone or methyl (5% phenyl) siUcone. The detector of choice is a flame ionisation detector. Typical molar response factors for the chlorinated methanes are methyl chloride, 2.05 methylene chloride, 2.2 chloroform, 2.8 carbon tetrachloride, 3.1, where methane is defined as having a molar response factor of 2.00. Most two-carbon chlorinated hydrocarbons have a molar response factor of about 1.0 on the same basis. 

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