Separation techniques INDEX

Contents Introduction. - Experimental Techniques Production of Energetic Atoms. Radiochemical Separation Techniques. Special Physical Techniques. - Characteristics of Hot Atom Reactions Gas Phase Hot Atom Reactions. Liquid Phase Hot Atom Reactions. Solid Phase Hot Atom Reactions. - Applications of Hot Atom Chemistry and Related Topics Applications in Inorganic, Analytical and Geochemistry. Applications in Physical Chemistry. Applications in Biochemistry and Nuclear Medicine. Hot Atom Chemistry in Energy-Related Research. Current Topics Related to Hot Atom Chemistry and Future Scope. - Subject Index. 

Since the development of HPLC as a separation technique, considerable effort has been spent on the design and improvement of suitable detectors. The detector is perhaps the second-most important component of an HPLC system, after the column that performs the actual separation it would be pointless to perform any separation without some means of identifying the separated components. To this end, a number of analytical techniques have been employed to examine either samples taken from a fraction collector or the column effluent itself. Although many different physical principles have been examined for their potential as chromatography detectors, only four main types of detectors have obtained almost universal application, namely, ultraviolet (UV) absorbance, refractive index (RI), fluorescence, and conductivity detectors. Today, these detectors are used in about 80% of all separations. Newer varieties of detector such as the laser-induced fluorescence (LIE), electrochemical (EC), evaporative light scattering (ELS), and mass spectrometer (MS) detectors have been developed to meet the demands set by either specialized analyses or by miniaturization. 

Liquid chromatography (LC) has already been described and is an excellent separation technique for compounds that are nonvolatile, thermally unstable and relatively polar in nature. The usual detectors for LC are based on refractive index, conductivity, amperometry, light scattering, UV and fluorescence, all of which have been discussed in Section 3.2. However, sometimes it is desirable to have a more powerful detector attached to an LC instrument and, as such, the following combinations are possible LC-infrared spectrometry, LC-atomic spectrometry, LC-inductively coupled plasma-mass spectrometry, LC-mass spectrometry, LC-UV-mass spectrometry, LC-nuclear magnetic resonance and even LC-nuclear magnetic resonance-mass spectrometry. 

Numerous research papers and reviews on carbohydrate separations by CE have been written for the past several years. Researches have successfully addressed problems, such as tremendous diversity and complexity of this class of compounds, polar and neutral nature of most carbohydrates, their low ultraviolet (UV) extinction coefficients, and lack of functional groups. In the previous edition of this book, Olechno and Nolan [16] published a comprehensive overview of the CE separation techniques, attempted and developed for intact and derivatized carbohydrates, charged and neutral, as well as detection approaches by UV, indirect fluorescence, electrochemical (e.g., amperometric) detection, refractive index, and laser-induced fluorescence (LIE). A variety of buffer systems were... 

Individual sugars can also be measured by separation techniques such as Hquid chromatography (LC) and capillary electrophoresis (CE). Separations are usually ion-exchange based with refractive index detection. With this type of method it is also possible to determine simultaneously other wine constituents such as carboxylic acids, sugars, glycerol, and ethanol in wines. Different detection modes may be used in conjunction with postcolumn derivatization or alternatively the eluate may be directly analyzed by ETIR. 

Figure 9). With micrometer diameter tubes, such a refractive index detector can probe volumes approaching a few picoliters and result in femtogram detection limits. This feature makes this detection scheme very useful in small channel separation techniques, such as capillary and chip technology. 

Both Mjj and are useful values, and their ratio, MJM called the polydispersity index, provides a measure of the breadth of the molecular-weight distribution. When the MJM ratio is equal to one, all the polymer molecules in a sample are the same length, and the polymer is said to be monodisperse. No synthetic polymers are ever monodis-perse unless the individual molecules are carefully fractionated using time-consuming, rigorous separation techniques based on molecular size. On the other hand, natural polymers, such as polypeptides and DNA, that are formed using biological processes are monodisperse polymers. 

GC is applied in a variety of ways and is one of the most important separation techniques in this particular area. GC provides the retention time or retention index (RI) of an unknown substance that can be used for its identification. GC is routinely utilized to separate the analyte from endogenous interferences for more specific identification via mass spectrometry and can also be used to provide quantitative information about the drugs present. The following applications focus on the identification and quantification of drugs and volatiles in biological fluids by GC. 

Why do we need separation techniques As will be discussed in Sections 5 and 6, state-of-the-art mass analyzers and tandem mass spectrometry allow mass spectrometry to be a powerful tool for the analysis of complex mixtures. The coupling of classical separation techniques with mass spectrometry further improves the utility of these combined techniques for mixture analysis. Mass spectrometers are the most sensitive and structure-specific detectors for separation techniques that, in general, provide more detailed and reliable structural information on components of complex mixtures than other conventional detectors (such as flame ionization, UV, reflective index detectors, etc.). 

Hplc techniques are used to routinely separate and quantify less volatile compounds. The hplc columns used to affect this separation are selected based on the constituents of interest. They are typically reverse phase or anion exchange in nature. The constituents routinely assayed in this type of analysis are those high in molecular weight or low in volatility. Specific compounds of interest include wood sugars, vanillin, and tannin complexes. The most common types of hplc detectors employed in the analysis of distilled spirits are the refractive index detector and the ultraviolet detector. Additionally, the recent introduction of the photodiode array detector is making a significant impact in the analysis of distilled spirits. 

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