Elutrope range
The elutropic series (eluotropic series) sorts the most common organic solvents according to their elution effect (elution selectivity) in chromatography . The elution effect is the ability of the solvent to let a substance run along. The arrangement is empirical and depends on the stationary phase used. The elutropic series, however, mostly correlates with the permittivities of the solvents.
For the most common adsorbent in chromatography, silica gel , there is therefore the following series (increasing elution effect):
| substance | Elution force E 0 |
|---|---|
| n -pentane | 0.00 |
| n -hexane | 0.00 |
| Cyclohexane | 0.03 |
| Cyclopentane | 0.04 |
| Carbon tetrachloride | 0.14 |
| Diisopropyl ether | 0.22 |
| toluene | 0.22 |
| benzene | 0.25 |
| Diethyl ether | 0.29 |
| Dichloromethane (methylene chloride) | 0.30 |
| chloroform | 0.31 |
| acetone | 0.43 |
| Dioxane | 0.43 |
| Ethyl acetate (ethyl acetate) | 0.45 |
| Tetrahydrofuran | 0.48 |
| MTBE ( tert -butyl methyl ether) | 0.48 |
| Dimethyl sulfoxide | 0.48 |
| Acetonitrile | 0.5 |
| Pyridine | 0.55 |
| 2-propanol (isopropanol) | 0.60 |
| Ethanol | 0.68 |
| Methanol | 0.73 |
| acetic acid | |
| water |
From the list you can see that a polar substance such as B. Acetylsalicylic acid runs very slowly on a silica gel column with n -hexane, but very quickly with methanol. In general, solvents or solvent mixtures (also called running agents or eluents) are used to separate substances that lead to a moderately fast running speed for the most interesting sample components. High running speeds, correspondingly high R f values in thin-layer chromatography , generally lead to poor separation results (R f values in TLC greater than 0.5 or 0.6).
When it comes to determining a suitable mobile phase for preparative column chromatography from preliminary tests with the help of thin layer chromatography , it is advisable to observe the following rule: groups of substances whose separation is important and difficult should R f values in the region of 0, 3 have. This is a good compromise with regard to the separation efficiency and the duration of the chromatography (if you work isocratically , i.e. the composition of the solvent is kept constant during the separation).
The concept of the elutropic series, in which the solvents are ordered according to polarity or permittivity, tries to reduce their properties in chromatography to a single parameter. This simplification is very useful for a first consideration. When it comes to optimizing the selectivity of chromatographic separations, however, it is advantageous to take a closer look at the specific interactions between the individual molecules (the sample, the mobile phase and the adsorbents). For example, there are parameters that characterize the ability of solvent molecules as acceptors in hydrogen bonds or as donors. This is also the reason why in tricky cases it can make sense to use three instead of two solvents for a mixture. If there was only one dimension of polarity, two would be sufficient.
It is a competition between the molecules of the solvent or the solvent mixture with the molecules of the individual sample components for the adhesion points on the adsorbent. In classical liquid chromatography , adsorbents are used that have polar points of adhesion (silica gel, aluminum oxide). The problem is that a small part of these points of adhesion has a higher polarity (at certain points of failure of the adsorbent). This can be remedied in the following way: Instead of dry mobile solvents, mobile solvents are used which have a certain very low water content. This deactivates the disruptive, highly polar adhesion points (because they are occupied by the highly polar water molecules). To produce such mobile solvents, for example, dry and water-saturated mobile solvents can be mixed (in a ratio of 1: 1 or 3: 1 depending on the adsorbent). This procedure is only permitted if the sample molecules are more or less insensitive to water. If the solvent mixtures contain highly polar components such as ethanol or methanol, water additives are usually unnecessary.
literature
- G. Duve, O. Fuchs, H Overbeck, "Solvent Hoechst" 6th ed. 1976, Hoechst Aktiengesellschaft
Web links
- Eluotropic series - Elutrope series
Individual evidence
- ↑ Michael Wächter: Chemielabor - Introduction to Laboratory Practice , Wiley-VCH Verlag, Weinheim, 1st edition, 2011, p. 87, ISBN 978-3-527-32996-0 .