Thin Layer Chromatography - Magnesia


The use of magnesia is relatively rare today but in the early days of TLC it was employed for the separation of the carotenoids and other plant materials. Magnesia is manufactured by heating magnesium hydroxide to about 350oC. If heated to 500oC or above, the retentive activity of magnesia starts to fall and at 1000oC it is rapidly transformed into inactive magnesium oxide. The chromatographic characteristics of magnesia are similar to those of silica gel but the material is basic in nature as opposed to silica gel which is acidic. The surface moieties of magnesia that are interactive are again hydroxyl groups consequently, the molecular forces involved in solute retention are largely polar in nature. Magnesia interacts with solvents in much the same way as silica gel.


Kieselguhr is a purified, thermally treated, diatomaceous earth that has been ground to a particle size of about 5-40 mm. Kieselguhr has very large pores and, as a consequence, has a relatively small surface area (1-5 m2/g). In TLC, it is used largely as a support for liquid stationary phases and not as an adsorbent or stationary phase per se. Some untreated kieselguhrs can show some slight adsorption properties that are normally removed by silanation. The use of kieselguhr as a support in TLC is afforded by the separation of the phenols in tobacco smoke condensates (9). The phenols are initially coupled with diazotized p-nitro aniline and extracted with ether. The ether solution is separated on a TLC coated with kieselguhr impregnated with formamide and the separation developed with a solvent mixture containing 30 parts by volume of benzene, 70 parts by volume of cyclohexane and 3 parts by volume of dipropylene glycol. This separation is accomplished by using a liquid-liquid distribution system as opposed to a liquid-solid system and such systems are rarely employed today. In fact, supports generally are not well-liked and adsorbent stationary phases such as silica and silica-based bonded phases are far more popular.