Liquid phase analyte distribution between

Adsorbent nonpermeability is an important condition, since it essentially states that all processes occurs in the liquid phase. Since adsorption is related to the adsorbent surface, it is possible to consider the analyte distribution between the whole liquid phase and the surface. Using surface concentrations and the Gibbs concept of excess adsorption [20], it is possible to describe the adsorption from binary mixtures without the definition of adsorbed phase volume. 

Because of the assumption made above, the solution is limited to the linear region of analyte partitioning and adsorption isotherms. The analyte distribution between two liquid phases (eluent and adsorbed phase) at equilibrium could be described as follows ... 

In high-performance liquid chromatography (HPLC), as in many chromatographic techniques, separations result from the great number of repetitions of the analyte distribution between the mobile and stationary phases that are linked. 

The principle of solvent extraction—the distribution of chemical species between two immiscible liquid phases—has been applied to many areas of chemistry. A typical one is liquid partition chromatography, where the principle of solvent extraction provides the most efficient separation process available to organic chemistry today its huge application has become a field (and an industry ) of its own. The design of ion selective electrodes is another application of the solvent extraction principle it also has become an independent field. Both these applications are only briefly touched upon in the chapter of this book on analytical applications (Chapter 14), as we consider them outside the scope of... 

The liquid liquid partition chromatography (LLPQ method involves a stationary liquid phase that is more or less immobilized on a solid support, and a mobile liquid phase. The analyte is therefore distributed between the two liquid phases. In conventional LLPC systems, the stationary liquid phase is usually a polar solvent and the mobile liquid phase is an essentially water-immiscible organic solvent. On the other hand, in reversed-phase chromatography (RPQ, the stationary liquid is usually a hydrophobic... 

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. 

Volatilization of a chemical from the surface of a liquid is a partitioning process by which the chemical distributes itself between the liquid phase and the gas above it. Organic chemicals said to be volatile exhibit the greatest tendency to cross the liquid-gas interface. When compounds volatilize, the concentration of the organic analyte in the solution is reduced. Semivolatile and nonvolatile (or involatile) describe chemicals having, respectively, less of a tendency to escape the liquid they are dissolved in and pass into the atmosphere above the liquid. 

Vr(csi) is the analyte retention as a function of the eluent concentration, Vo is the total volume of the liquid phase in the column, y Cei) is the volume of adsorbed layer as a function of eluent composition, Kp(cei) is the distribution coefficient of the analyte between the eluent and adsorbed phase, S is the adsorbent surface area, and is the analyte Henry constant for its adsorption from pure organic eluent component (adsorbed layer) on the surface of the bonded phase. 

For an electronically neutral analyte that is partitioning between water and organic liquid phases, the equilibrium process can be described as the distribution... 

Extraction or separation of dissolved chemical component [X]A from liquid phase A is accomplished by bringing liquid solution of [X]B into contact with a second phase B that is totally immiscible. A distribution of the component between the immiscible phases occurs. After the analyte is distributed between the two phases, the extracting analyte is released and/or recovered from phase A for analysis. The theory of chemical equilibrium leads us to a reversible distribution coefficient as follows ... 

In hquid chromatography (LC), the separation of analytes is based on their distributions between a mobile liquid phase and a stationary phase. The efficiency of the separation depends on the chemical properties of the analytes, on the structure and pore size of the stationary phase, on the length and the... 

Liquid chromatography (LC) is a physical separation technique conducted in the liquid phase. A sample is separated into its constituent components (or analytes) by distributing between the mobile phase (a flowing liquid) and a stationary phase (sorbents packed inside a column). For example, the flowing liquid can be an organic solvent such as hexane and the stationary phase can be porous silica particles packed in a column. HPLC is a modem form of LC that uses small-particle columns through which the mobile phase is pumped at high pressure. 

Solvent extraction involves the distribution of a solute between two iimnis-cible liquid phases. This technique is extremely useful for very rapid and clean separations of both organic and inorganic substances. In this chapter, we discuss the distribution of substances between two phases and how this can be used to form analytical separations. The solvent extraction of metal ions into organic solvents is described. 

According to International Union of Pure and Applied Chemistry (lUPAC)," chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary while the other moves in a definite direction. HPLC is then defined as an analytical separation technique used to detect and quantitate analytes of interest in more or less complex mixtures and matrices that uses elevated pressures to force the liquid through the bed of the stationary phase. " The bed is most often located in a column. Since almost four decades of its inception, HPLC technique has advanced and there is enormous literature available on this technique. 

Any chromatographic process relates to the selective distribution of an analyte between the mobile and the stationary phase of a given chromatographic system. In liquid chromatography the solvent, with a volume V in the interparticle space, moving along the column at a certain velocity, is the mobile phase and the porous adsorbent, having a pore volume Vp, is the stationary phase. The distribution coefficient equals the ratio of the concentrations of the analyte in the stationary and the mobile phases. In classical SEC a distribution of the analyte between the interparticle volume and the accessible pore volume takes place and the retention volume Vr is determined by... 

Sorptive extraction techniques are based on the distribution equilibria between the sample matrix and a non-miscible liquid phase. Matrices are mostly aqueous and the non-miscible phase is often coated onto a solid support. Analytes are extracted from the matrix into the non-miscible extracting phase. Unlike adsorption techniques, where the analytes are bound to active sites on the surface, the total volume of extraction phase is important. Extraction of analytes depends on the partitioning coefficient of solutes between the phases (Ridgway et al., 2007). Two extraction techniques are commonly employed solid phase microextraction (SPME) and stir-bar sorptive extraction (SBSE). 

Capillary electrochromatography (CEC) is a hybrid of CE and liquid chromatography. The separation is carried out in capillaries that are either fully packed with octadecyT(ODS)-silica particles or partially packed having an open segment. The separation of the analytes is based on the difference in the magnitude of the distribution between the mobile and stationary phase. The flow of mobile phases is generated... 

Liquid-liquid extraction is an equilibrium process between two immiscible phases, described by a equilibrium constant, usually called the distribution constant or partition coefficient. The distribution constant, K, is defined as the ratio of the concentration of the substance in the two phases at equilibrium K = Cu/Cl, where Cu is the concentration of analyte in the less dense (upper) phase and Cl its concentration in the more dense (lower) phase). The distribution or partition ratio, G, defined as the ratio of the solute masses in the two phases (G = mi]/mi, where ntu is the mass of analyte in the less dense phase and Wl its mass in the more dense phase), is often more useful. The distribution constant and distribution ratio are related through the phase ratio, X by G = KV. The phase ratio is the volume ratio of the liquid phases (V = volume of the less dense phase/volume of the more dense phase). 

In most practical cases the analyte will be distributed between the sample and vapor phases. Calibration involves preparing standards in a matrix identical to that of the sample with known concentrations of the analyte. The headspaces of these standards are then analyzed under identical conditions to the sample and thereby a calibration graph can be constructed. This procedure is practical for liquids and gives eqn [10] ... 

Because of their therapeutic advantages, the above-discussed oral antidiabetic drugs are more and more frequently used in therapy and are extensively examined by analytical procedures. Especially, the methods for pharmacokinetic studies and for quantification of potential impurities in bulk drug substances are continually being developed. Most favored, in terms of a number of publications, are the HPLC techniques, although TLC methods are also represented. In HPLC, a RP technique with Ci8 columns is more prevalent than the alternative phases. Isolation steps are almost evenly distributed between conventional liquid—liquid extraction and SPE procedures. The UV detection can serve as a reliable tool for determination of most of these antidiabetics. However, new prospects are represented by the MS/MS detection. Sensitive and automated HPLC techniques with column switching are also more and more frequently applied. 

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