Extrography
As extrography refers to a separation process comprising a combination of extraction and chromatography is.
application
The extrography process was developed for the fractionation and characterization of petroleum distillation residues ( bitumen ). It is used to separate complex mixtures whose ingredients cover a wide range of polarities . The complex mixture is separated into a manageable number of fractions with a narrower polarity range using a coarse step gradient . This results in an accumulation of substances of a certain polarity in the respective fraction. This procedure was modified for the separation of plant extracts , whereby the sample zone in particular was shortened to a few centimeters. In this way, large amounts of extract can be fractionated in a relatively short time.
In extrography, the extract is first dissolved in a suitable solvent and applied to about five times the amount of coarse silica gel . To do this, the silica gel is combined with the clear extract solution, this batch is treated with ultrasound and the solvent is then removed on a rotary evaporator while slowly turning the flask until a dry, free-flowing material remains.
The extremely polar silanol groups of the silica gel first adsorb the sample molecules of the highest polarity on their surface. This new surface of polar compounds, in turn, adsorbs slightly less polar substances from the extract. This creates several layers of sample molecules of decreasing polarity in the silica gel pores. The most apolar substances are found on the new pore surface. This also means that the dissolved extract has to be added completely to the silica gel in a single portion and not in portions, since all polarities of the substances would then be redrawn with each portion after the solvent has been removed.
The silica gel with the extracted extract is packed in the separation column in front of the chromatographic bed of fine silica gel. If the solvent gradient is then started with a lipophilic eluent , initially only the lipophilic substances are dissolved on the surface and fed to the chromatographic separating bed. In the course of the gradient, the polarity of the eluent is increased so that more and more polar substances are now also made available for chromatography . The pre-sorting of the sample molecules on the silica gel enables large amounts of substances to be fractionated. Overloading of the separation bed is prevented, since the sample molecules do not get into the separation bed all at once, but gradually in groups of different polarity.
Web links
literature
- Lean, Kilpatrick, Comparison of Precipitation and Extrography in the Fractionation of Crude Oil Residua . In: Energy & Fuels 1997, 11 (3), pp 570-585, doi : 10.1021 / ef9601125 , Link .