Eluotropic series

The elutrope series ( eluotropic series ) the most common organic solvents sorted by their Elutionswirkung ( Elutionsselektivität ) in the chromatography. The Elutionswirkung is the ability of the solvent to be run with a substance. The arrangement is made empirically and is dependent on the stationary phase used. However, the number elutrope correlated mostly with the permittivities of the solvents.

For the most common adsorbent in chromatography, silica gel, therefore, results in the following series (rising Elutionswirkung ):

From the list you can see that a polar substance such as Acetylsalicylic acid on a silica gel column using n- hexane is running very slowly, with methanol, however, very quickly. In general, solvents or solvent mixtures (also running or eluent ) is hereby established for the separation of substances that lead to an excessively fast speed in the most interesting sample components. High speeds, corresponding to high Rf values ​​in TLC ( Rf values ​​in TLC greater than 0.5 or 0.6 ) lead to generally poor separation results.

When it comes to determine from preliminary experiments by thin layer chromatography a suitable eluent for preparative column chromatography, it is convenient to observe the following rule. Groups of substances whose separation is important and difficult, should have Rf values ​​in the neighborhood of 0.3. This is a good compromise with regard to the separation efficiency and the duration of the chromatography ( if operating isocratically, is thus held, the composition of the eluent during the separation constant).

The concept of the eluotropic series, in which the solvent are sorted in accordance with the polarity and the permittivity, attempts to reduce their properties in chromatography on a single parameter. This simplification is very useful for a first consideration. When it comes to optimize the selectivity of chromatographic separations, it is advantageous to the specific interactions between the individual molecules ( the sample of the eluent and the adsorbents ) to take a closer look. There are, for example parameters which characterize the ability of the eluent molecules as acceptor in hydrogen bonding or donor. This is also the reason that it may be useful in difficult cases to use three instead of two solvents for a mixture. If there were only the dimension of polarity, only two would be sufficient.

There is a competition between the molecules of the eluent or mobile phase mixture with the molecules of the individual components of the sample on the adsorbent traps. In the classic liquid chromatography adsorbents are used, the traps having polar (silica, alumina). The problem is that a small part of these traps has a higher polarity (on certain impurities of the adsorbent). In the following manner, to be remedied: Instead of dry eluent eluent are used which have a certain very low water content. This disturbing, highly polar traps are disabled (because they are occupied by the highly polar water molecules). To produce such eluent can be mixed as dry and water-saturated mobile phase ( ratio of 1: 1 or 3: 1, depending on the adsorbent). This procedure is only permitted when the sample molecules are more or less insensitive to water. When the eluent mixtures containing highly polar components such as ethanol or methanol, water additives are generally unnecessary.