Ion-pair chromatography in practice

Ion-pair chromatography is suitable for all ionic compounds but not all counter ions are equally good in every case. Table 13.1 lists some counter ions suitable for reversed-phase chromatography with methanol-water or acetonitrile-water.  

There are a number of suitable reagent mixtures in buffer solutions on offer, although many workers prefer to mix their own mobile phase cocktails to suit their own particular problem. A few examples are as follows (Waters)  

Quaternary amines, e.g. tetramethyl-. Strong and weak acids, sulfonated dyes. 

Alkyl- and arylsulfonates, e.g. methane- and Strong and weak bases, benzalkonium 

Perchloric acid Very strong ion parrs with many basic samples 

Quaternary amines, e.g. tetramethyl-, tetrabutyl-, palmityltrimethylammonium Tertiary amines, e.g. trioctylamine Alkyl- and arylsulphonates, e.g. methane-and heptanesulphonate, camphorsulphonic acid Perchloric acid Alkylsulphates, e.g. laurylsulphate Strong and weak acids, sulphonated dyes, carboxylic acids Sulphonates Strong and weak bases, benzalkonium salts, catecholamines Very strong ion pairs with many basic samples Similar to sulphonic acids, different selectivity 

Amphoteric molecules such as p-aminobenzoic acid (H2NC6H4COOH, where NH2 is a basic fuuction and COOH is acidic) may be chromatographed with both of the above reagents. The acid function forms an ion pair with the former and the amino group remains undissociated as a result of buffering. The reverse applies with the latter reagent. 

The only problems are caused by mixtures of strong and weak acids with strong and weak bases. Either the strong bases or the strong acids are in ionic form. However, this type of mixture is rare. 

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