Normal Phase Ion-pair Partition Liquid Chromatography

Two types of system are used for ion-pair liquid chromatography. When polar stationary phase materials, such as silica gel, are used an ion-pair partition mechanism is applied. When non-polar stationary phase materials, such as octadecyl-bonded silica gel and polystyrene gel, are employed a paired-ion adsorption mechanism is involved. The former is called normal-phase ion-pair partition liquid chromatography, and the latter is called reversed-phase ion-pair liquid chromatography. 

The following conditions should be satisfied to perform satisfactory ion-pair liquid chromatography. 

Normal-phase Ion-pair Partition Liquid Chromatography 

Analytes in the organic phase (eluent) form ion-pairs with counter-ions existing in the hydrophilic stationary phase. The ion-pairs are then partitioned into the organic phase. Usually, porous silica gel is used as the support, and water or formamide is the stationary phase. The requirements are that a specific amount of counter-ions must be contained in the stationary phase, and the pH must be controlled to ionize the analytes. The eluent is not miscible with water, and counter-ions must stay in the stationary phase. 

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The molecular absorption intensity of polar compounds is usually small, but highly sensitive detection can be obtained after pre- or post-column derivatiza-tions. The use of ultraviolet absorption or fluorescence-active counter-ions makes it possible to achieve highly sensitive detection of polar compounds and enhance the capability of ion-pair liquid chromatography. For example, N,N-dimethylprotriptyline has been used as a counter-ion for carboxylic acids12 and picric acid for quaternary amines13 in normal-phase ion-pair partition liquid chromatography. Phenethylammonium, cetylpyridinium, l-phenethyl-2-pyco-linium, and naphthalene-2-sulfonic acid have been used for sulfonic acid and alkyl amines detection.14,15 Ion-pair post-column extraction was applied on-line for fluorescence detection.16... 

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