Chromatography distribution

Why discuss distribution coefficients Most everyone is familiar with the demonstration of iodine distributed between an organic and an aqueous layer. However, distribution equilibria are at the heart of many separation processes from liquid-liquid extractions to virtually every type of chromatography in which the distribution of the solute between the mobile phase and the stationary phase determines the effectiveness of the separation. In the practice of analytical chromatography, distribution coefficients are often called partition coefficients but the concept is identical, only the names have changed. The temperature dependence of a distribution coefficient is at the heart of temperature programming in gas-liquid chromatography (GC), and analyses of the temperature behavior depend on the heats of solution of the distributed solutes. Indeed, GC measurements have been used to measure heats of solution. 

In a similar study, Hama and Handa (1980) fractionated DOM from three Japanese lakes of different trophic status using gel permeation chromatography. Distribution patterns of DOC, carbohydrates, amino acids, proteins, and organic acids were determined. Bimodal distribution patterns of DOC... 

Ion-exchange chromatography involves more variables than other forms of chromatography. Distribution coefficients and selectivities are functions of pH, solute charge and radius, resin porosity, ionic strength and type of buffer, type of solvent and temperature. The number of experimental variables makes lEC a very versatile technique but a difficult one because of the effort needed to optimise a separation. 

The movement of the solute, relative to the phase system, can be understood by considering the distribution system. If the micelle electrolyte system is treated as a chromatography distribution system with the micelles stationary, and the mobile phase moving at an electrosmotic velocity of (u), then from chromatography theory, the velocity of the solute relative to the micelles (v) will be given by ... 

Determination of Ethyl and Methyl Alcohol in the Blood by Means of Gas Chromatography. Distribution of Ethyl Alcohol in the Human Fetus J. Pharm. Belg. 16(9-10) 334-341 (1964) CA 61 6157a... 

Although gas chromatography can give the concentration of each component in a petroleum gas or gasoline sample, the same cannot be said for heavier cuts and one has to be satisfied with analyses by chemical family, by carbon atom distribution, or by representing the sample as a whole by an average molecule. 

In gas chromatography (GC) the sample, which may be a gas or liquid, is injected into a stream of an inert gaseous mobile phase (often called the carrier gas). The sample is carried through a packed or capillary column where the sample s components separate based on their ability to distribute themselves between the mobile and stationary phases. A schematic diagram of a typical gas chromatograph is shown in Figure 12.16. 

Thus far all the separations we have considered involve a mobile phase and a stationary phase. Separation of a complex mixture of analytes occurs because each analyte has a different ability to partition between the two phases. An analyte whose distribution ratio favors the stationary phase is retained on the column for a longer time, thereby eluting with a longer retention time. Although the methods described in the preceding sections involve different types of stationary and mobile phases, all are forms of chromatography. 

Distribution Coefficients. Gel-permeation stationary-phase chromatography normally exhibits symmetrical (Gaussian) peaks because the partitioning of the solute between mobile and stationary phases is linear. Criteria more sophisticated than those represented in Figure 8 are seldom used (34). 

Salt Effects. The definition of a capacity factor k in hydrophobic interaction chromatography is analogous to the distribution coefficient, in gel permeation chromatography ... 

Before polyacrylamides are sold, the amount of residual acrylamide is determined. In one method, the monomer is extracted from the polymer and the acrylamide content is determined by hplc (153). A second method is based on analysis by cationic exchange chromatography (154). For dry products the particle si2e distribution can be quickly determined by use of a shaker and a series of test sieves. Batches with small particles can present a dust ha2ard. The percentage of insoluble material is determined in both dry and emulsion products. 

Fig. 3. Molecular weight distribution curves as determined by gel-permeation chromatography. A represents i9f2v (9-phthahc resins B, highest molecular...

Size-exclusion chromatography (sec) easily and rapidly gives the complete molecular weight distribution and any desired average (6). Thus, it has become the technique of choice for determining molecular weights despite its relatively high initial cost. 

---