Gradient in SFC

The density and therefore the solvating power of supercritical fluids varies according to the pressures to which they are submitted. As a consequence, a pressure gradient in SFC is equivalent to an elution gradient in HPLC, or a temperature gradient in GC. 

I 9 Modeling of Simulated Moving-bed Chromatography 9.4.2.6 Modeling of Pressure Gradient in SFC... 

Gradients in SFC are usually performed either with increasing pressure (increasing density) or with increasing temperature, or with increased concentration of polar modifiers (such as methanol). 

Pressure or density programming is the most popular of the gradient techniques in SFC. Density is the important parameter with respect to retention but pressure is the physical property which is directly monitored by SFC instruments. If enough experimental density-volume-temperature data are available for the mobile phase then a computer-based algorithm can be used to generate specific density programs. Such data are available for only a few mobile phases, such as carbon dioxide and the n-... 

Principles and Characteristics SFC-MS is a sensitive coupled technique that can be selective or universal it was first mentioned in 1978 [396]. Further developments are given in Table 7.36. It is used in an on-line mode with open cell gas-phase interfaces, where the mobile phase is decompressed to low pressures. SFC presents a number of features which allow for easier coupling with MS than other chromatographies. In practice, however, SFC-MS coupling did not turn out to be as easy as expected, a fact which can be ascribed to the problems met in the adiabatic expansion of the mobile phase and the effects of pressure gradients in the ion... 

A gradient that runs with 30-80% methanol or acetonitrile is not uncommon. This amount of modifier is generally not needed in supercritical fluid chromatography to affect the same separation. Typical modifier composition in SFC is 1.0-10% and would achieve higher Hildebrand Solubility Parameter adjustment overall than the broader gradients found in LC. 

Column pressure usually has little effect on enantioselectivity in SFC. However, pressure affects the density of the mobile phase and thus retention factor [44]. Therefore, similar to a modifier gradient, pressure or density programming can be used in fast separation of complex samples [106]. Later et al. [51] used density/temperature programming in capillary SFC. Berger and Deye [107] demonstrated that, in packed column SFC, the effect of modifier on retention was more significant than that of pressure. They also showed that the enhanced solvent strength of polar solvent-modified fluid was nof due fo an increase in densify, caused by fhe addition of fhe liquid phase modifier, buf mainly due fo fhe change in composition. 

As noted earlier, however, numerous variables have been identified as significant in SFC, including temperature the type of stationary phase the polarity, density (or pressure), and modifier content of the mobile phase and the corresponding gradients of temperature, density (pressure), and composition. Moreover, from chemometric principles it is clear that any procedure which does not consider all the significant variables simultaneously will seldom, if ever, locate the true set of optimum conditions. This point is illustrated in the section below. ... 

The pressure gradient in the column causes substantial density gradient, which in turn leads to a gradient in velocity. Since the mass transfer properties are a function of density, they change at every point along the column. Moreover, unlike HPLC and GC systems, where the Henry s coefficient, H , does not depend on the pressure, in SFC systems the retention characteristics are a function of pressure (density) [1]. Hence, the dynamics of an injected pulse is governed by several factors. This establishes the need to develop a... 

Gradient elution can be achieved in SFC by systematically changing the column pressure or the density of the supercritical fluid. 

A constant gradient, like 5%/min or 10%/min tends to push all the less retained solutes together and spends excessive time at the end effecting small changes in retention. A doubling gradient offers the best trade-off between speed and resolution in SFC. The modifier concentration starts at some low level like 1% and doubles every column hold-up time. 

FIGURE 29-2 Effect of pressure programming in SFC. Note the shorter time for the pressure-gradient chromatogram on the right compared with the constant pressure (isobaric) chromatogram on the left. (Courtesy ol Brownlee Labs. Santa Clara, CA.)... 

Manipulating adsorption strength is most readily achieved by variation of solvent composition (or column pressure in the case of SFC-SMB, see Section 5.2.6.2). However, there exist further options for implementing gradients. In particular, the use of salt and pH gradients is attractive in the context of bioseparations (see some of the... 

As the adsorption equilibrium in SFC is dependent on density, the application of a pressure gradient along the zones of the process can be used for a directed... 

A large number of gradients are possible in SFC. These include temperature, pressure and/or density, velocity, eluent composition and combinations of these (Klesper and Schmitz, 1987). There are thus many possibilities for optimizing performance. 

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