Determination of some inorganic species in edible vegetable oils and fats by ion chromatography

Abstract A procedure has been developed for the ion chromatographic determination of total chlorine, phosphorus and sulphur and of iron, copper, nickel, zinc, cobalt, lead and cadmium in edible vegetable oils and fats. The organic matrix which strongly interferes in analytical procedures, is completely removed by saponification followed by oxidative UV photolysis. This method is simpler and requires less reagents compared with other sample pretreatment procedures. To oil and fats, ethanol and potassium hydroxide were added, and they were saponified for half an hour. Hydrogen peroxide was added and the sample was subjected to UV photolysis at 85±5°C. In less than 1 h organic constituents were completely degraded, while inorganic constituents, except nitrates, iodides and manganese, remained unaffected by UV radiation. Chlorine, phosphorus and sulphur, present in different organic compounds, can be quantified as total amounts only, without speciation. The clear sample solutions were directly injected on to an ion chromatograph equipped both with a conductivity detector, for the determination of total chloride, phosphate and sulphate ions using a carbonate–hydrogencarbonate eluent, and a post-column reactor and a variable-wavelength UV–Vis detection system, for the determination of lead and cadmium using an oxalate eluent and iron, copper, nickel, zinc and cobalt using a pyridine-2,6-dicarboxylic acid eluent. The method has been tested on spiked and unspiked samples of vegetable oils and fats and was found to be satisfactory for the determination of the previously cited elements. Ion chromatographic responses were compared to voltammetric ones and were found to be in good agreement (±3%).