Supercritical fluid mobile phases characteristics

On-line SFE-SFC modes present several distinct advantages that are beyond reach of either technique when used separately (Table 7.13). An obvious advantage of SFE is that it is an ideal way to introduce a sample into an SFC system. Because the injection-solvent is the same as the mobile phase, in this respect the criteria for a successful coupling of different techniques are fulfilled [94], i.e. the output characteristics from the first instrument and the input characteristics of the second instrument are compatible. Supercritical fluid techniques can separate high-MW compounds are significantly faster than classical Soxhlet extractions and require less heat and solvent. SFE-SFC techniques are versatile,... 

The first chiral separation using pSFC was published by Caude and co-workers in 1985 [3]. pSFC resembles HPLC. Selectivity in a chromatographic system stems from different interactions of the components of a mixture with the mobile phase and the stationary phase. Characteristics and choice of the stationary phase are described in the method development section. In pSFC, the composition of the mobile phase, especially for chiral separations, is almost always more important than its density for controlling retention and selectivity. Chiral separations are often carried out at T < T-using liquid-modified carbon dioxide. However, a high linear velocity and a low pressure drop typically associated with supercritical fluids are retained with near-critical liquids. Adjusting pressure and temperature can control the density of the subcritical/supercritical mobile phase. Binary or ternary mobile phases are commonly used. Modifiers, such as alcohols, and additives, such as adds and bases, extend the polarity range available to the practitioner. 

Supercritical fluid chromatography is a very important chromatographic technique still underestimated and underutilized. It presents characteristics similar to both GC and HPLC, although having its own characteristics. Whereas the column temperature control is the way to achieve a good separation in GC and the solvating power of the mobile phase is controlling factor in HPLC, in SFC the density of the fluid is the major factor to be optimized. Both packed (LC-like) and capillary (GC-like) columns have been used in this technique, which has found applications in practically all areas in which GC or HPLC has shown to be the selected separation technique. 

The supercritical fluids exhibit gas-like viscosities, diffusivities, and liquid-like densities. These favorable transport properties lead to enhanced mass transfer, permeation, and wetting characteristics. The mass transfer limited multiphase reactions will benefit from reduction of a number of phases, as in the case of most oxidation, hydrogenation, or replacement of the more viscous liquid phase with a supercritical or a less viscous expanded liquid phase. The mobility combined with tunability results in effective maintenance of catalyst activity in heterogeneous catalysis. 

A number of experimental techniques have been described for the determination of isotherms based on frontal analysis, frontal analysis by characteristic point, elution by characteristic point, and perturbation methods [12,21,27,169,176-179]. Most authors report single-component isotherm results. Multiple-component isotherm data are more complicated because all components are simultaneously in competition for the sorption sites on the stationary phase. The retention time and peak shapes of any solute is dependent on the concentration and properties of all other solutes in the mixture [12,170,180]. For multicomponent mobile phases in liquid and supercritical fluid chromatography this includes each component of the mobile phase. 

Today, most applications are based on packed columns at relatively low temperatures with binary and ternary mobile phases. It has become clear that the characteristics of interest are not limited to supercritical conditions. Most packed column applications actually use conditions just below the critical temperature, but well above the critical pressure of the mixed fluids, i.e., subcritical (in other words, liquid ). Unlike many observers expectations, important characteristics such as diffusion coefficients and viscosity are virtually identical whether the conditions are just supercritical or just subcritical. 

Loading test experiment was performed on a methylcarbamate CF6 CSP with truns-l-amino-2-indanol in the polar organic mode. As shown in Fig. 15, 3.37 mg of the primary amine can be baseline separated in 20 min on an analytical column (250x4.6 mm). Furthermore, cyclofructan-type CSPs operates best in organic solvents and supercritical fluid solvents as well, which makes the sample recovery much easier than with aqueous mobile phases. These characteristics make the cyclofructan-type CSPs viable candidates for the preparative separations of primary amines. In addition, high loading of N-blocked amino acids were also reported on aromatic-derivatized CF6 CSPs [39]. 

In addition to the stated characteristics, SEC does not require particular chromatographic supports. On the contrary, most CSPs, particularly those used in normal-phase conditions, can be used indistinctively in HPLC and SEC, with the only constraint being the need to have available the appropriate equipment. With respect to the chromatographic performance of such CSPs, although analysis time use is shorter and efficacy is higher, there are not remarkable/predictable changes in enantioselectivity for CSPs used with conventional liquids or supercritical fluids as a mobile phase. ... 

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