Early stationary phases of gas-solid chromatography—Aluminum oxide adsorbent

Aluminum oxide has 5 crystal forms, and g-type is commonly used in gas chromatography, it has good thermal stability and mechanical strength, and its adsorption capacity varies greatly depending on its water content, so it needs the activation treatment before using. In the 1980s, the deceased chromatographic scholar Ju Yunfu conducted in-depth research on alumina adsorbents and reached the following conclusions:

(1) The specific surface area of g-alumina microspheres can be controlled by changing the heat treatment temperature. The alumina microspheres undergo phase transformation at 350 ℃ and completely transform into g-alumina at 420 ℃.

(2)The acidity on the surface of g-alumina microspheres, mainly Lewis acid, can be reduced by modification with coating fixative. The surface acidity of the modified g-alumina microspheres is lower than that of foreign alumina microspheres, this modification weakens the polarity of the stationary phase.

(3)The heat treatment temperature has a significant impact on the retention value of the components to be separated, if 0.3% apisone-L is used to modify g-alumina microspheres obtained after 4 hours of calcination at 500 ℃ to obtain a stationary phase, it can completely separate 15 hydrocarbon components of C1-C4 at a column temperature of 85 ℃(Ju Yunfu et al., Journal of Fuel Chemistry, 1983, 12 (1): 69-76)

But later studies have shown that using alkali metal halides to modify alumina can also achieve good results. A. Braithwaitel et al. from UK studied the surface treatment of alumina with alkali metal halides and obtained the following conclusions:

(1) The surface of unmodified alumina has Lewis acid activation points that can interact with p-electrons of unsaturated hydrocarbons, the retention time of unsaturated hydrocarbons is longer than that of saturated hydrocarbons, and the chromatographic peak of unsaturated hydrocarbons produces tailing. Modifying the surface of alumina with alkali metal halides will eliminate tailing, but it can also affect the separation and retention factors of saturated and unsaturated hydrocarbons.

(2) The modification of alumina must reduce the Lewis acid activation point in order to form a more uniform surface performance, assuming that the modification process on the surface of alumina is the combined action of alkali metal cations and anions，then the anions of the modifier have the ability to selectively block most of the Lewis acid activation points, these activation points can no longer interact with the analyte, but not all halide anions have this effect. The cation of the modifier can also affect the adsorption of alumina, mainly due to the decrease in cation volume of halides, which increases the separation degree of olefins/alkanes. The reason is obviously that the polarity on the surface or the increase in charge density of cations on the surface, or a combination of the two factors.

(3) Assuming that the modification of aluminum oxide surface by cations is due to its reduction in the adsorption characteristics of the adsorbent, thereby reducing the interaction force between the adsorbent and the adsorbate. The activity of the modified adsorbent can be controlled by the amount of modifier, but with only a small amount of modifier, the tailing of the chromatographic peak can be eliminated, then obtaining a symmetrical chromatographic peak. If the concentration of the modifier exceeds a critical value, the salt will precipitate, then it cannot block the activation point, and the concentration of the modifier is between 2-4%. (Chromatographia，1996，42(1/2)：77-82)