Why is TLC not a form of partition chromatography and paper chromatography not a form of absorption chromatography?
TLC is primarily an adsorption chromatography technique where compounds separate based on their affinity for the solid stationary phase (e.g., silica gel). In contrast, paper chromatography is a partition chromatography method where separation occurs due to compounds' differential solubility between the aqueous stationary phase (water held by cellulose fibers), and the organic mobile phase.
because it relies on adsorption of analytes onto a solid stationary phase (e.g., silica gel) rather than partitioning between two liquid phases.
I disagree. TLC is also partition chromatography. Silica adsorbs solvent as a thin layer and so acts as liquid-liquid chromatography (partition chromatography) as well. A properly run TLC plate provides a retention factor that can be converted to column volumes- TLC is used to predict retention in column chromatography (1). This means the mechanism of chromatography is the same for both columns and TLC plates. Partition chromatography on silica has been known for a long time (2) and is used to explain aqueous normal phase (HILIC) on silica gel. Thin layer chromatography plates need to be saturated with the vapor in the chamber to give reproducible results (1) that match the column chromatography, so the adsorption of the solvents is much more subtle. With a few exceptions, such as hydrocarbon compounds, compounds won't elute on a silica column unless the surface is saturated with the strong solvent (which causes elution down the column). Reference 3 below shows this effect with hexane/ethyl acetate (3) where the compound only elutes after the column is saturated with hexane- like water, the ethyl acetate forms a layer on the silica. Adsorption does play a role, as the silica pH and surface structure does play a role. However, the adsorbed solvent causes partitioning!
1) Let Us Teach Proper Thin Layer Chromatography Technique! | Journal of Chemical Education (acs.org)
2) biochemj00969-0073.pdf (nih.gov)
3) Don’t start your normal phase gradients from 0% B! (teledynepharma.com)
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