Solvent programming, elution chromatography

Horizontal and descending TLC are other forms of chromatography which require special equipment. The advantages of these systems are not clear. They are usually more expensive than ascending equipment, but often have other features for stepwise gradient elution or continuous solvent programming. 

Jandera, P. Churacek, J. Gradient elution in liquid chromatography II. Retention characteristics (retention volume, bandwidth, resolution, plate number) in solvent-programmed chromatography—Theoretical considerations. J. Chromatogr. 1974, 91, 223-235. 

Fig. 4. Gas chromatographic separation of AF-FC 2 (active fraction of estrous urine extract after flash chromatography and TLC), upper chromatogram AF-HPLC (active fraction of estrous urine extract after FC, TLC, and HPLC), lower chromatogram, ly sample. DB-1-30N column (J W)(0.25 ym 0.32 mm) 30M helium carrier gas temperature-programmed 5%C/minute, 35°C to 330°C retention time depicted in minutes. Solvent peak eluted prior to 4 minutes.

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