Partition coefficient mixed stationary phases

Window Method of Purnell. In 1975 Laub and Purnell44 suggested a largely empirical method for finding the optimum mixture of liquids that would give the best separation for a given sample containing at least three analytes. Their only assumption was a common one—that GC retentions as measured by partition coefficients are additive in proportion to the volume fractions of the liquids used to make the mixed stationary phase that is,... 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

The basic hydrodynamic equilibrium system (Fig. 1, right) uses a rotating coil which generates an Archimedean screw effect where all objects in different density present in the coil are driven toward one end, conventionally called the head. The mobile phase introduced through the head of the coil is mixed with the stationary phase to establish a hydrodynamic equilibrium, where a portion of the stationary phase is retained in each turn of the coil. This process continues until the mobile phase elutes from the tail of the coil. After the hydrodynamic equilibrium is established throughout the coil, the mobile phase displaces only the same phase, leaving the other phase stationary in the coil. Consequently, solutes introduced locally at the head of the coil is subjected to an efficient partition process between the two phases and separated according their partition coefficients. 

R is the gas constant, Bn is the second virial coefficient for the probe, B13 is the mixed virial coefficient of the solute vapor and carrier gas, V is the partial molar volume of 1 at infinite dilution, P is the total pressure, and K is the equilibrium partition coefficient, defined as the ratio of concentration of solute in the stationary phase, q, to that in the gas phase, c, that is, K s q/c. 

Typically, the two phases of the solvent system are mutually saturated by shaking in a separatory funnel. The chosen stationaiy phase is loaded into the coil in the absence of rotation. The sample is then injected as a solution in either one, or a mixture of both phases. Rotation is then started and the mobile phase is pumped into the coil. Some stationary phase will be displaced and the sample constituents will be separated and eluted in the order of their partition coefficients. Mass transfer of the solute is promoted by a train of dynamic mixing zones, generated by the planetary motion of the coil as it rotates on its own axis while traversing a planetary orbit."... 

Figure 2.13. Plot of the gas-liquid partition coefficients (Ka and Kb) on two stationary phases to estimate the gas-liquid partition coefficients for all solutes at any intermediate mixed-phase composition and the resulting window diagram plot of the separation factor (a) as a function of the volume fraction of the B stationary phase. (From ref. [205] Elsevier)...

The partition and activity coefficients are used for determining the excess thermodynamic functions of the mixing of chromatographed substances on stationary phase free... 

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