Analyte distribution coefficient

Applying this function into the mass-balance equation (2-33) and performing the same conversions [Eqs. (2-34)-(2-39)], the final equation for the analyte retention in binary eluent is obtained. In expression (2-67) the analyte distribution coefficient (Kp) is dependent on the eluent composition. The volume of the acetonitrile adsorbed phase is dependent on the acetonitrile adsorption isotherm, which could be measured separately. The actual volume of the acetonitrile adsorbed layer at any concentration of acetonitrile in the mobile phase could be calculated from equation (2-52) by multiplication of the total adsorbed amount of acetonitrile on its molar volume. Thus, the volume of the adsorbed acetonitrile phase (Vj) can be expressed as a function of the acetonitrile concentration in the mobile phase (V, (Cei)). Substituting these in equation (2-67) and using it as an analyte distribution function for the solution of mass balance equation, we obtain... 

The degree of analyte capture or retention within the sorbent layer is determined by the uptake affinity or analyte distribution coefficient, D ... 

To be absolutely correct the terms in., the brackets should be activities and not concentrations, also, two of the terms involve stationary phase activities that cannot be easily evaluated. The weight distribution coefficient for the analyte A" is designated as and is given by ... 

It is known that the RPLC retention parameters are often strongly correlated to the analyte s distribution coefficient in organic solvent/ water. Generally, the relationship between liquid/liquid (LL) distribution and RPLC retention are of the form of the dimensionless Collander-type equations, e.g., see Eq. (15.21)... 

What weight of analyte is found in 50.0 mL of an extracting solvent when the distribution coefficient is 231, and the weight of analyte found in 75.0 mL of the original solvent after extraction is 0.00723 g ... 

How many moles of analyte are extracted if 50 mL of extracting solvent is brought into contact with 50 mL of original solvent, the concentration of analyte in the original solvent is 0.060 M, and the distribution coefficient is 238 ... 

In an extraction experiment, it is found that 0.0376 g of an analyte are extracted into 50 mL of solvent from 150 mL of a water sample. If there was orginally 0.192 g of analyte in this volume of the water sample, what is the distribution coefficient What percent of analyte was extracted ... 

The distribution coefficient for a given extraction experiment is 527. If 0.037 g of analyte are found in 75 mL of the extracting solvent after extraction and 100 mL of original solvent was used, how many grams of analyte were in this volume of original solvent before extraction ... 

As in number 15 above, the distribution coefficient is very large and the calculation shows, after rounding according to significant figure rules, that virtually all of the analyte is extracted, 0.037 g. 

These two quantities are the same, except that the distribution ratio takes into account all dissolved forms of the analyte, while the distribution coefficient takes into account only one form. If only one form exists, then the two are identical. 

Application In Analytical And Inorganic Chemistry Knowledge about distribution coefficients is used in analytical chemistry to determine the feasibility of quantitative separation by precipitation. Therefore, D and X are also called separation factors. In order to precipitate 99.8% or more of the primary substance, X must be 3.2 x 10 or smaller. For larger values of X more than one precipitation step is necessary, and the number of steps can be calculated when X is known. 

The extent to which an analyte can penetrate the pore system is governed by its distribution coefficient Kn, which is related to its elution volume VR by ... 

The ISE life-time is closely connected with the drift and is at least one year for good electrodes. With some systems, e.g. enzyme electrodes (see chapter 8), the life-time is only a few weeks. It follows from the results of Oesch and Simon [119] that the life-time of electrodes based on ionophores in solventpolymeric membranes depends on the kinetics of dissolution of the ionophore and the plasticiser in the analyte. If both the ionophore and the membrane solvent have distribution coefficients between water and the membrane greater than 10 , then the ISE life-time is at least one year. 

The retention factor is related to the distribution coefficient K of the analyte between the micellar phase and the aqueous phase, according to the following equation ... 

One other parameter characterizing the ion exchange is the concentration- [8] or weight-based [9] distribution coefficient K = CJC, where and represent the concentrations of analytes in the stationary and mobile phases, respectively. 

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. 

By using experimentally obtained data for 1 mM salicylic acid, a plot of reciprocal analytical signal versus reciprocal a, yielded a linear relationship for the pH range 1.65-3.01. This result supported the solvent extraction model. The corresponding estimate of capacity ratio and distribution coefficient using this treatment was 8.5. 

Distribution coefficients. The amount of a component in a specified amount of stationary phase, or in an amount of stationary phase of specified surface area, divided by the analytical concentration in the mobile phase. 

The value of A ex is, of course, fixed for any given cation-extracting agent-solvent system. Analytically important distribution coefficients, D, can be varied by changing the concentration of ligand relative to that of the metal and more significantly by adjusting the pH for 50% extraction D = 1 and equation (21) reduces to... 

Porous MIP membranes are much closer to our topic. These are similar to SPE disks which are commercially available and commonly used in analytical laboratories. Such disks allow the collection of low concentration analytes from large volumes of sample. This type of problem is often encountered in environmental analysis. The disks have high flow permeability and show fast adsorption kinetics so that large sample volumes can be extracted with them in a short time. One may consider these disks as extremely short and wide chromatographic or SPE columns. The purpose may be mainly the concentration of the analyte(s) but separation may also be achieved between substances differing rather much in their distribution coefficients. 

They are often plasticized PVC matrices, which occlude an ionophore as the key selective element, a chromoionophore or a fluoroionophore as the chemical-optical transducer and, sometimes, ionic additives to maintain electroneutrality. Such optodes follow ion-exchange mechanisms between the membrane and the aqueous solution and the analytical response originates from the ratio of the concentration of ions in the solution or from their product (Fig. 3). Moreover, selectivity is ruled by the ion distribution coefficients between both phases and by the formation constants of complexes within the membrane. 

Figure 13.3. Appearance of non-size-exclusion effects on SEC-elution curves of polyelectrolytes and other charged analytes including low-molecular-weight organic acids. Kd is the distribution coefficient and Ve, V0, and Vt are elution volume of the analyte, column void volume, and total column volume, respectively.

The sampling and analytical conditions are described in detail in Section 8.2.1. For estimation of a medium water influence on the sediments the mean values of the water components over the investigation period of two years were used as the first (independent) data matrix the metal distribution coefficients were taken as the dependent data matrix. For description of deposition-remobilization effects a heavy metal distribution coefficient, DCh was defined as ... 

Analytes distribute themselves between aqueous and organic layers according to the Nernst distribution law, where the distribution coefficient, Kq. is equal to the analyte ratio in each phase at equilibrium. 

The analyte distributes itself between the two immiscible liquids according to the relative solubility in each solvent [1,38,44,45]. To determine the effect of the distribution coefficient on an extraction, consider the following example. 

Repeated extractions may be required to recover the analyte sufficiently from the aqueous phase. Neutral compounds can have substantial values of Ko. However, organic compounds that form hydrogen bonds with water, are partially soluble in water, or are ionogenic (weak acid or bases) may have lower distribution coefficients and/or pH-dependent distribution coefficients. Additionally, the sample matrix itself (i.e., blood, urine, or wastewater) may contain impurities that shift the value of the distribution coefficient relative to that observed in purified water. 

The net amount of analyte extracted depends on the value of the distribution coefficient. 

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