DETERMINATION OF COPPER IN EDIBLE SOYBEAN OILS

Soybean oils have been analyzed for their copper content before and after hydrogenation with copper-containing catalysts. A low-temperature dry asher, an apparatus in which oxygen plasma is generated in a radio frequency field under high vacuum, was adopted for ashing glyceride oils. The residues were analyzed by a colorimetric procedure using zinc dibenzyldi-thiocarbamate as the reagent. Identical samples were analyzed without ashing by neutron activation and atomic absorption techniques. The accuracy of the methods was determined by adding known amounts of copper at four different levels to two different soybean salad oils. Plots of copper found versus added copper showed that results were consistent over the range 0.04-5.0 ppm for all three methods, but that the atomic absorption results were low. The relative error of a single determination was ±13% and that of the mean of duplicate determinations ±9%. Analysis of natural soybean oils showed a copper content of about 0.03-0.10 ppm, whereas the same oils hydrogenated with cooper-containing catalysts and without metal removal treatments had levels of 3-5 ppm. © 1968 AOCS Press.