Stationary-phase viscosity

The 1983 work of the Dorsey research group established the use of 3% v/v 1-propanol in micellar phases to reduce the MLC efficiency problem [3]. It is now confirmed that the addition of alcohol to micellar phases (i) increases the rate of the solute mass-transfer between the micelles and the aqueous phase by increasing the solute micelle exit rate constant, (ii) increases the solute mass transfer kinetics between the stationary phase and the aqueous phase by decreasing the stationary phase viscosity and the amount of adsorbed surfactant. The problem of alcohol addition to micellar phases is that kinetics enhancements cannot be dissociated from thermodynamics changes. The efficiencies increase and the retention times decrease. A hybrid alcohol-micelle mobile phase has necessarily a higher solvent strength than a purely aqueous phase [34]. It was shown that alcohols were changing the micelles and the stationary phase in a comparable manner [26] as noted on the Pws and parallel variations in Table 6.4. 

HPLC columns are packed with the stationary phase, which retains the sample molecules. Retention of compounds depends on not only various factors predominant on molecular properties but also particle size, pore size, homogeneity of the stationary phase, viscosity and polarity of the mobile phase, etc. These effects are summarized in the van Deemter equation [14] (Equation (6), analogous to the Golay equation used in GC), which describes peak broadening in LC ... 

Together with this solvent effect, another effect, called phase soaking, occurs in the retention gap technique if a large volume of solvent vapour has saturated the carrier gas, the properties of the stationary phase can be altered by swelling (thicker apparent film), a change in the viscosity or changed polarity. The consequence is that the column shows an increased retention power, which can be used to better retain the most volatile components. 

The high diffusivity and low viscosity of sub- and supercritical fluids make them particularly attractive eluents for enantiomeric separations. Mourier et al. first exploited sub- and supercritical eluents for the separation of phosphine oxides on a brush-type chiral stationary phase [28]. They compared analysis time and resolution per unit time for separations performed by LC and SFC. Although selectivity (a) was comparable in LC and SFC for the compounds studied, resolution was consistently... 

In general, retention decreases as the modifier concentration increases because the modifier competes with the analytes for sites on the stationary phase. The effect on retention of changes in modifier concentration seems to be more pronounced for CSPs than for achiral stationary phases in SFC, and peak shapes are apt to degrade rapidly at low modifier concentrations [12]. Efficiency tends to decrease as the modifier concentration increases because analyte diffusion is slowed by the increased viscosity of the eluent [39]. 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

Mass transfer (the C term), which involves collisions and interactions between molecules, applies differently to both packed and capillary columns. Packed columns are mostly filled with stationary phase so liquid phase diffusion dominates. The mass transfer is minimized by using a small mass of low-viscosity liquid phase. Capillary columns are mostly filled with mobile phase, so mass transfer is important in both the gas and liquid phases. A small mass of low-viscosity liquid phase combined with a low-molecular weight carrier gas will minimize this term. 

It is apparent from early observations [93] that there are at least two different effects exerted by temperature on chromatographic separations. One effect is the influence on the viscosity and on the diffusion coefficient of the solute raising the temperature reduces the viscosity of the mobile phase and also increases the diffusion coefficient of the solute in both the mobile and the stationary phase. This is largely a kinetic effect, which improves the mobile phase mass transfer, and thus the chromatographic efficiency (N). The other completely different temperature effect is the influence on the selectivity factor (a), which usually decreases, as the temperature is increased (thermodynamic effect). This occurs because the partition coefficients and therefore, the Gibbs free energy difference (AG°) of the transfer of the analyte between the stationary and the mobile phase vary with temperature. 

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