Choosing the Proper Activated Alumina PLOT Column

The recent addition of Alumina sulfate and Alumina chloride columns to the Supelco family of GC capillary PLOT columns has prompted several customer inquiries on why we offer two alumina chemistries and how to chose between the two. We present the following information and applications to address these questions.

Solution

Activated alumina PLOT columns are prepared using submicron, granular activated alumina (aluminum oxide) particles possessing a significant amount of mesopores. The use of sodium sulfate or potassium chloride as a desiccant and deactivating agent ensures that the activity of the alumina surface is effective in eluting (distilling) the unsaturated hydrocarbons after the saturated hydrocarbons. A small amount of micropores allows for the separation of methane from the C2 hydrocarbons, however surface chemistry plays the most significant role for these PLOT columns. The polarity of the aluminum oxide provides a unique selectivity, eluting/distilling small, unsaturated hydrocarbons after larger (or similar molecular sized) saturated hydrocarbons. This elution pattern is augmented with the alumina sulfate PLOT column, where acetylene elutes after n-butane, and methyl acetylene elutes after n-pentane and 1,3-butadiene.

Use of the alumina chloride still provides these unsaturated/saturated elution profiles, but to a reduced degree. For example, acetylene elutes after propane and propylene, but before n-butane (n-butane/acetylene is reversed for the alumina sulfate PLOT). The alumina chloride PLOTs are also the best choice for the analyses of Freons^®. Examples of these elution patterns (i.e, the differences between the alumina sulfate and alumina chloride PLOTs) are illustrated in Figure A for 18 component analyses and Figures B for mineral oil gases.