Micellar liquid chromatography of terephthalic acid impurities

•Eight impurities in industrial terephthalic acid separated by micellar HPLC.•Sodium dodecylsulfate mobile phase was run with a flow rate gradient.•Solute-micelle partition coefficients calculated for isomeric aromatic acids.•Three C18 columns from different companies tested.

The production of terephthalic acid (TPA) by oxidation of p-xylene is an important industrial process because high purity TPA is required for the synthesis of polyethylene terephthalate, the primary polymer used to make plastic beverage bottles. Few separation methods have been published that aim to separate TPA from eight major aromatic acid impurities. This work describes a “green” micellar liquid chromatography (MLC) method using a C18 column (100 × 2.1 mm, 3.5 μm), an acidic 1% sodium dodecyl sulfate (SDS) mobile phase, and a simple step flow rate gradient to separate TPA and eight impurities in less than 20 min. The resulting chromatogram shows excellent peak shape and baseline resolution of all nine acids, in which there are two sets of isomers. Partition coefficients and equilibrium constants have been calculated for the two sets of isomers by plotting the reciprocal of the retention factor versus micelle concentration. Quantitation of the nine analytes in an actual industrial TPA sample is possible. Limits of detection for all nine acids range from 0.180 to 1.53 ppm (2.16-19.3 pmoles) and limits of quantitation range from 0.549 to 3.45 ppm (6.48-43.0 pmoles). In addition, the method was tested on two other reversed phase C18 columns of similar dimensions and particle diameter from different companies. Neither column showed quite the same peak resolution as the original column, however slight modifications to the mobile phase could improve the separation.