The Stationary Phase

Many of the disparate materials employed as 'liquid phases' on the earliest packed columns soon proved unsatisfactory for open-tubular columns. It was eventually realized that higher temperature stabilities in open-tubular columns required highly viscous phases whose viscosity endured at the operating temperatures. With this realization, the polysiloxanes attracted interest, because they could be cross-linked to produce a semisolid gum-like phase; the term 'stationary phase' soon replaced 'liquid phase' in common usage. Columns coated with these materials exhibited higher temperature tolerances and longer lifetimes. This is another area in which a multitude of individual investigators have contributed to progress.

As discussed earlier, packed columns are typically limited to lengths that can generate 10 000 to 15 000 theoretical plates because of their higher pressure drops. With this restriction on N, the separation factor, a, becomes more critical to the packed column (eqn [2]), and a is best controlled through the choice of stationary phase. While a values do respond to changes in temperature, the direction of any given response can be predicted only for solutes whose functionalities are similar. For example, in a series of methyl ketones or a series of paraffin hydrocarbons, a values vary inversely with temperature. But if the column temperature is decreased for a sample containing a mixture of methyl ketones and paraffin hydrocarbons, one can predict only that the a of any two hydrocarbons (and that of any two ketones) will increase. The a of a mixed pair - hydrocarbon and ketone - can increase or decrease as the temperature is lowered, and that effect will be reversed if the temperature is raised. In short, the effect of temperature on a cannot be predicted for two solutes of different functionality. Hence for any given group of mixed solutes, a values are controlled primarily through experience and observations on the probability of interactions between given solutes and a given stationary phase.

The limitation on N and the resultant increased reliance on a made stationary phase selectivity of major import to the packed column, and has been largely responsible for the proliferation of some 300 different stationary phases for packed columns. Because much higher numbers of theoretical plates are attainable in open-tubular columns, solute separation factors - a - were initially less critical to the user of open-tubular columns. However, the inexorable increase in the demands placed upon the analytical chemist ultimately necessitated the development of high-N columns coated with high-a phases.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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