Supercritical fluid chromatography diffusivity

Supercritical fluid chromatography (SFC) refers to the use of mobile phases at temperatures and pressures above the critical point (supercritical) or just below (sub-critical). SFC shows several features that can be advantageous for its application to large-scale separations [132-135]. One of the most interesting properties of this technique is the low viscosity of the solvents used that, combined with high diffusion coefficients for solutes, leads to a higher efficiency and a shorter analysis time than in HPLC. 

Supercritical fluid chromatography provides increased speed and resolution, relative to liquid chromatography, because of increased diffusion coefficients of solutes in supercritical fluids. (However, speed and resolution are slower than those of gas chromatography.) Unlike gases, supercritical fluids can dissolve nonvolatile solutes. When the pressure on the supercritical solution is released, the solvent turns to gas. leaving the solute in the gas phase for easy detection. Carbon dioxide is the supercritical fluid of choice for chromatography because it is compatible with flame ionization and ultraviolet detectors, it has a low critical temperature. and it is nontoxic. 

Supercritical fluid chromatography (SFC) is a GC method of analysis of compounds in systems where normal GC presents resolution difficulties (Lee and Markides, 1987). A supercritical fluid has properties at a critical temperature intermediate between a liquid and a gas. At and above this critical temperature, a gas cannot be compressed into a liquid, irrespective of the pressure, but it solvates solid matter as if it were a liquid. A supercritical fluid diffuses freely into and out of adsorbent pores with a minimum of resistance. A major advantage of SFC chromatography is its ability to effect separation of oligomers without derivatization. 

In addition to conventional liquid chromatography, supercritical fluid chromatography (SFC), using a supercritical fluid as mobile phase (mostly scf-C02), has attracted attention in the last decades [58, 164, 168, 169]. Supercritical fluids provide a favourable medium for the transport of solutes through a chromatographic column because they resemble a gas in terms of viscosity, a liquid in terms of density, and are intermediate between these two phases in terms of diffusivity. For some physieal properties of supercritical solvents, see Section 3.2. 

Supercritical fluid chromatography (SFC) combines the high diffusion coefficient of GC and the solubility properties of LC and this separation mode has also been coupled with atomic spectrometric detection. " ... 

Sub-/supercritical fluid chromatography is essentially NP chromatography with the added advantage that the lower viscosity and higher diffusivity of the mobile phase results in higher column efficiencies allowing for rapid resolutions. The columns employed are the same as those utilized in conventional NP chromatography. Carbon dioxide is the most commonly used nonpolar eluent but requires a more polar modifier such as an alcohol for the elution of polar solutes. The modifier increases the polarity of the mobile phase and... 

In the case of supercritical fluid chromatography, the eluent is a substance above its critical conditions, where the viscosity is dramatically decreased and the diffusion... 

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