Separation of Amine Cations

As in anion chromatography, the IC separation of organic cations has long been known, or at least suspected, that its mechanism involved more than simple ion exchange. Hoffman and co-workers [9,10] have shown that two mechanisms occur in such cases ion exchange and hydrophobic interaction between the sample cations and the resin matrix. For example, these authors showed that the slopes of the linear plot of log k vs. carbon number for protonated amine cations decrease going from 30 % acetonitrile (70 % water) to 70 % acetonitrile in the eluent. This is due to lower hydro-phobic interaction in the 70 % acetonitrile. 

Dumont, Fritz and Schmidt studied cation chromatography in organic solvents containing little if any water [11]. Under these conditions solvation of the lipophilic part of the cation should be sufficient to virtually eliminate the hydrophobic interaction between the sample cations and the ion-exchange resin. In this way the true ion-exchange selectivity could be measured. 

After trying several different inorganic acids, methanesulfonic acid was selected as the eluting acid for the separation of protonated amine cations. In IC of cations with as the eluting cation, k should vary according to the following equation  

The effect of solvent was studied by measuring the retention factor, k, for a series of protonated alkylamine cations with 25 mM methanesulfonic acid in the appropriate solvent as the eluent [11]. Ordinarily a plot of log k vs. the number of carbon atoms in such a homologous series would be linear. The slope of such a plot is at least in part an indication of the effect of the carbon chain on the retention factor. The retention factors were measured under identical conditions in each of four organic solvents. The k values of the alkylamines increased according to the solvent used in the following order methanol, ethanol, 2-propanol, acetonitrile. However, in any given solvent the k values of the individual amines were almost constant from Cl to CIO. Separation of the individual amines was not possible. 

Two other types of organic cations did show enough difference in their retention factors for practical separations. Separation of aniline, /V-methylaniline and N,N-dimethylaniline in 100 % methanol is shown in Fig. 7.10. Significant differences in k values of octylamine, dioctylamine and trioctylamine were observed in methanol, ethanol and 2-propanol. 

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Two ion exchange resins from Dionex contain carboxyl groups. The CS14 column is recommended for gradient separation of amine cations. 

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