Organic modifiers elution strength

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

The polarity index or the solubility parameter may be used as a measure of solvent strength, which would be a measure of polarity in those cases. For reversed phase HPLC, solvent strength parameters have been proposed for the four most common solvents used, i.e. water (Si = 0), methanol (Si = 2.6), acetonitrile (Si = 3.2) and THF (Si = 4.5). Using these values water makes no contribution to the eluting power of the mobile phase and the solvent strength is measured by the volume fraction of organic modifier. 

Lopez-Grio, S. Baeza-Baeza, J.J. Garcfa-Alvarez-Coque, M.C. Evaluation of the elution strength of organic modifier, and surfactant in micellar mobile phases. J. Liq. Chromatogr. Relat. Technol. 2001, 24, 2765-2783. 

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