Isolation of Organic Ions

Organic compounds that are ionic or that become ionic at certain pH values can be isolated selectively from neutral organic compounds by SPE with ion-exchange materials [2]. Amines become protonated cations in acidic solution and are retained by a short cation-exchange column in the H form. Actually, the cation exchanger or exchangers can convert a neutral amine to the protonated cation. 

Neutral organic compounds that cannot exist as cations may be retained by physical adsorption but can be washed off the cation-exchange column by a brief rinse with an organic solvent The amine cation can then be eluted from the column with a 1 M solution of trimethylamine in methanol. The trimethylamine converts the amine cation to the free amine, which is no longer retained by the cation exchanger. Because of its volatility, trimethylamine is easily removed from the eluate. After acidification, the sample amines can be separated by cation chromatography. 

The cation-exchange resin used in this operation should be a macroporous rather than a microporous polymer to minimize physical adsorption. An intermediate exchange capacity (-0.6-1.2 mequiv g ) is required and the column dimensions should be small to avoid use of an excessive amount of trimethyl-amine-methanol solution to neutraUze the resin s H capacity in the elution step. 

An Empore ion-exchange disk inserted into a tube is ideal for this use. 

In a similar manner, carboxylic acid anions, phenolates, etc. can be separated from other organic matter by retention on a small anion exchanger in the OH form. 

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