Thin Layer Chromatography - Cellulose
Cellulose is basically another support that has a similar function to kieselguhr but, in this case, is organic in origin. Cellulose is contained in most vegetable matter in conjunction with a wide variety of other substances. In its purist form, it occurs in those vegetable fibers that are used in the manufacture of textiles (e.g. cotton). The molecule of cellulose is made up of filliform macromolecules consisting of D-glucopyranose units coupled b-glycosidically at positions 1 and 4 by oxygen atoms. A section of a cellulose chain is shown as follows.
There are two kinds of cellulose commonly used in TLC, native cellulose that has between 400 and 500 units per chain and micro-crystalline cellulose that is prepared by the partial hydrolysis of regenerated cellulose and has between 40 to 200 units per chain. Employing a polar solvent mixture (e.g. acetonitrile/water) the more polar component (water) is absorbed into the cellulose that functions as a liquid stationary phase. If the cellulose is acetylated or esterified, dispersive groups are introduced on to the cellulose support and more dispersive solvents can be absorbed that act as a stationary phase. Cellulose is not commonly employed in general TLC separations but has been used for the separation of a number of amino acid mixtures and, in the form of ion exchange cellulose, for the separation of inorganic ions.
There are a number of polyamides that have been employed as stationary phases in TLC, (e.g. polyamide 6,6 (Nylon 6,6) polyhexamethylenediamine, polyamide 6, (Nylon 6), aminopolycapro-lactame and polyamide 11 (Nylon 11) and polyaminoundecanoic acid. Polyamides can be produced. by polycondensing a dicarboxylic acid with a diamine and they can be represented by the following formula.
(if x = 4 then the product is Nylon 6,6, if x = 8 then the product is Nylon 6,10 etc.).
The polyamide material is interesting from a chromatographic point of view in that it can exhibit dispersive interactions with its aliphatic chains, polar characteristics due to interactions with the CO and the amino groups and even offer ion exchange capabilities if appropriately buffered. Nevertheless, the polar characteristics of the polyamides are the most commonly exploited and such materials have been used successfully for the separation of phenols, sulfonic acids, anilines, carbohydrates and a number of substances of biotechnological interest (e.g. nucleotides and nucleosides).