Читать книгу Chemical Analysis - Francis Rouessac - Страница 79

For packed columns, impregnation or deposit techniques can lead to many different stationary phases from a large selection of low‐volatility organic compounds. On the other hand, for capillary columns, manufacturing constraints require a much more limited selection of compounds. The current phases correspond in principle to three families: polysiloxanes, polyethylene glycols, and ionic liquids. Each category can have many structural variants. For the study of optically active compounds, specific phases are used.

Each of these phases can be used between a minimum temperature, below which concentration equilibria occur too slowly, and a maximum temperature, above which degradation of the polymer occurs. The high limit depends on the film thickness and the nature of the polymer.

Squalane is used as a reference phase, since it is the only one that is perfectly defined. On the McReynolds scale, squalane has a polarity of zero (Section 2.10.3). This saturated hydrocarbon (C₃₀H₆₂) is derived from squalene, a natural terpene extracted from shark’s liver. On this stationary phase, which can be used between 20 and 120°C (using either deposition or impregnation), the compounds are eluted in increasing order of their boiling points (retention time being inversely proportional to vapour pressure). Various grafted phases based upon polyalkylsiloxanes are also almost apolar.