Chromatographic Separation of Complex Mixtures

In chromatography, complex samples are introduced onto a column comprised of a solid support that is coated with a stationary phase (having a special chemical formulation) through which a mobile phase (a gas in GC and a liquid in LC) of variable composition passes. The premise is that the components of the injected mixture are adsorbed initially onto the stationary phase followed by a sequential release of analytes into the mobile phase as its properties are altered by temperature (in GC) or solvent composition (in LC). 

Because the vast majority of samples are complex mixtures, they generally require the separation of their components, by GC or LC, prior to their introduction into the ion source. GC is usually carried out on fused silica capillary columns. LC is available in two formats in conventional LC the flow rates are O.l-l.O ml/min, while nano-LC operates at sub pl/min flow rates. Capillary electrophoresis (CE) can be interfaced to mass spectrometers (similarly to LC). Thin-layer chromatography (TLC) is compatible with the newer surface ionization methods. 

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Eor products having relatively low specific activity, such as some compounds labeled with and which are synthesized on the scale of several millimoles, classical organic chemical separation methods may be utilized, including extraction, precipitation, and crystallization. Eor separation of complex mixtures and for products having high specific activity, such as those labeled with tritium, etc, chromatographic methods utilizing paper, thin... 

More effective separation of complex mixtures and those consisting of components with similar R values can be achieved by successive developments at 90° to each other. The sample is spotted at one corner and developed with the first solvent. After thorough drying, the plate is turned through 90° and developed with the second solvent. The two-way chromatogram (Figure 4.49) can be compared with a standard map obtained from chromatographing known mixtures. 

This is achieved by controlling and changing the flow-rate of both of the pumping systems used. Gradient elution is often necessary for satisfactory separations of complex mixtures. Commercial chromatographs which have gradient devices are summarized in Table 3.3. 

Some thirty years ago the perfumery profession was shaken by the commercialization of the gas chromatograph. In lectures, roundtable discussions, and private conversation hot debates centered around the question whether this analytical tool, by greatly simplifying the separation of complex mixtures of volatile materials, would make the perfumer redundant. 

Unsymmetrically substituted diene iron tricarbonyl complexes are formed as racemic mixtures of compounds having planar chirality. Optically pure diene iron tricarbonyl complexes can been prepared by classical chromatographic separation of diastereomeric mixtures of a-methylbenzylamine and ephedrine salts, and of semioxa-mazones and oxazohdines derivatives. Direct asyimnetric complexation to cychc dienes can be achieved 73% ee using iron pentacarbonyl in the presence of a chiral l-aza-1,3-butadiene. 

Countercurrent Chromatography Countercurrent chromatography (CCC) or centrifugal partition chromatography (CPC) has gained much attention in recent years for isolation of polyunsaturated fatty acids. This new liquid chromatographic technique uses liquid-liquid partition, countercurrent distribution of solute mixmre between two liquid phases, in the absence of a solid support, to perform separation of complex mixture of chemical substances (44, 45). 

Fig. 13-13. Chromatographic separation of complex phosphate mixtures. Comer of a two-dimensional paper chromatogram, showing the positions of the pentameta- through octameta-phosphate rings in relation to the positions of the well known ring and chain phosphates. The basic solvent traveled 23 cm in 24 h, whereas the acid solvent traveled 11.5 cm in 5.5 h.

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