Supercritical fluid chromatography viscosity

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 (SFC) is an intermediate chromatographic technique between GC and HPLC. It depends upon the fact that when a fluid becomes supercritical (both the temperature and pressure are at or above its critical point) it develops some of the solvating properties of a liquid whilst retaining the low viscosity of a gas. Hence, mass transfer (essential to efficient chromatography) is more akin to that of GC than HPLC, but many compounds can be chromatographed at temperatures much lower than what would be required by GC, so some thermally labile compounds are amenable to SFC where they would degrade under GC conditions [28]. 

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. 

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... 

Both packed columns and open tubular columns are used in supercritical fluid chromatography. Packed columns can provide more theoretical plates and can handle larger sample volumes than open tubular columns. Because of the low viscosity of supercritical media, columns can be much longer than those used in liquid chromatography, and column lengths of 10 to 20 m with inside diameters of 50 or 100 p,m are common. For difficult separations, columns 60 m in length or longer have... 

Supercritical fluid chromatography is actually a poor name for this technique because the fluids are often (usually) not supercritical. The distinguishing characteristics of interest are that the fluid acts as a polar solvent while retaining many of the positive attributes of a gas (high dilfusivity, low viscosity) (26). 

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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