Abstract

A relatively new and rapidly developing technique called supercritical fluid chromatography (SFC) offers a means of rapid hydrocarbon type analysis of petroleum fuels and liquids. SFC is similar to instrumental liquid chromatography except the mobile phase is a high pressure liquid or fluid near or above its critical temperature. A modified HPLC instrument is now commercially available which uses carbon dioxide (critical temp.: 31°C) as the mobile phase. The ultraviolet detector normally used with this instrument can be replaced with a modified, commercially available flame ionization detector (FID) in order to detect saturated hydrocarbons.

Conventional HPLC columns are used, and the eluting power of the mobile phase is controlled by varying its density, a function of pressure and temperature. A silica column separates saturates from aromatics, while silver nitrate-impregnated silica separates saturates from olefins. A reversed phase column (C₁₈-bonded), operated under low density conditions, will separate by boiling point. The use of a switching valve allows these columns to be combined to obtain more information in a single run. For example, jet fuels may be analyzed for carbon number distribution of the saturates and ring number distribution of the aromatics in under five minutes. Middle distillate cracked stocks may be analyzed for saturates, olefins and aromatics by ring number in seven minutes. Weight percent is obtained without response factors. The method agrees with existing standard methods of analysis, and offers greater precision, flexibility, and speed of analysis.