Microbore liquid chromatography with electrochemical detection for the control of phenolic antioxidants in drugs and foods.

Antioxidants are added to foods and drugs to inhibit their oxidation. As these additives are somewhat toxic, it is necessary to control the amount added to any food or drug. Liquid chromatography (LC) is a powerful tool for this purpose. Many antioxidants are electroactive molecules which enables the advantages of electrochemical detection or selectivity and sensitivity to be realized. The interest of analysts in microbore LC arises from the low mobile phase volumetric flow rates involved, the reduced on-column samples together with reduced chromatographic dilution and high efficiency. Coupling of microbore LC with electrochemical detection adds another advantage: the decrease of electrode ageing. The problem of extra-column band broadening with microbore column is discussed in the present communication. A micro LC-electrochemical detection system is constructed and tested using catecholamines. The limit of detection (LOD) for noradrenaline using a 0.7 mm bore column is found to be 0.1 pg injected in 0.2 microliter (0.6 femtomoles). Three phenolic antioxidants are studied: tert-butyl-p-hydroxyanisole (BHA), di-tert-butyl-hydroxytoluene (BHT) and n-propyl gallate (3,4,5-trimethoxybenzoic acid propyl ester). The dynamic range is four orders of magnitude with LODs down to 0.1 femtomoles (20 fg injected) with a 0.3 mm bore column. No electrode response change is observed after 60 injections of 3 ng BHA over 6 days. Antioxidants are determined in different pharmaceutical preparations and foodstuffs (chewing gums, dried potato flakes). The agreement between the manufacturer stated concentrations and observed results is found to be satisfactory.