Eluent components adsorption

The binary eluent adsorption equilibrium is considered to be not disturbed by the injection of a small amount of the analyte (essentially the third component in the system). In an isocratic mode at a fixed eluent composition, the organic adsorbed layer is a stationary phase for the analyte to partition into. The analyte can partition into the adsorbed layer followed by consequent adsorption on the surface of the reversed-phase adsorbent. The overall retention is a superposition of two consecutive processes. Since the eluent component adsorption could be measured independently and adsorbed layer volume could be represented as a function of the mobile phase composition, the analyte retention also could be expressed as a function of the eluent composition. 

Both radial and axial temperature gradients may appear. As shown is Section 16.3.5, adsorption of polymers depends on temperature. Given the temperature and pressure dependence of the preferential sorption of the mixed eluent components within column packing [146-149], one can expect also considerable changes in the column interactivity with the temperature and pressure variations that may result in a possible gradual departure from the critical conditions. 

The adsorption isotherms of the eluent components have been studied extensively [22,25,26,28,29]. However, significant controversy still exists with regard to the interpretation of where the accumulation of the molecules of organic eluent modifier actually occurs. Horvats and further development by Dill [30, 31] stated that organic molecules penetrate between the bonded ligands of the stationary phase, and thus this monomolecular layer of bonded ligands can be considered as a stationary phase. Significant drawback of this... 

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. 

Y. V. Kazakevich, R. LoBrutto, F. Chan, and T. Patel, Interpretation of the excess adsorption isotherms of organic eluent components on the surface of reversed-phase adsorbents Effect on the analyte retention J. Chromatogr. A 913 (2001), 75-87. 

Adsorption of macromolecules within HPLC columns can be affected also by pressure, which changes extent and sometimes even sense of the preferential sorption of the mixed eluent components on the colunm packing (see section 11.6.2). 

It is a well-known fact in classical TLC that the eluent components sorbed strongly by the sorbent sites can cause secondary fronts (Fp, Fy,...) (39) that are independent of F, . This effect can be found during adsorption as well as in reversed-phase development when the eluent consists of solvents of different strength. The effect of this chromatographic solvent demixing is stronger in fully off-line OPLC systems, owing to the total elimination of vapor space, than in chambers with small vapor spaces, e.g., in sandwich chambers. 

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

Millson [113] investigated components of sewage sludge and found elementary sulphur in the hydrocarbon fractions eluted from liquid adsorption columns. By using a solid adsorbent such as alumina, silica gel, or Florisil, and heptane as eluent, the sulphur could be separated from weakly adsorbed hydrocarbons, e.g. squalene or biphenyl, but not from more strongly adsorbed hydrocarbons such as phenyldodecane. 

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