Simultaneous determination of passivator and antioxidants additives in insulating mineral oils by high performance liquid chromatography analysis

Insulating mineral oils frequently contain additives to improve their inherent characteristics, such as oxidation stability (antioxidants), electrostatic charging tendency and compatibility with other materials (passivators). Despite standard test methods are available for the detection of individual additives, none of them covers the simultaneous detection of additives of different kind. For the first time the simultaneous determination of antioxidants and passivators most frequently added to mineral insulating oils is provided. The compounds investigated included three inhibitors (N-phenyl-1-naphthylamine, 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-p-cresol) and two passivators (benzotriazole and Irgamet 39). A solid phase extraction step, aimed at the reduction of matrix oil components was optimized. Due to the hydrophobic characteristics of the additives, a reversed-phase chromatographic separation was used and optimized. The method here developed was applied to the analysis of inhibited and passivated transformer oils, compared with Standard IEC60666 method, which requires the use of FT-IR spectrophotometry (measurement of the adsorption corresponding to the stretching (OH) of the phenol group at 3650 cm-1), and finally applied inside an ASTM Proficiency Testing Program. In a unique analysis, the method provides a tool to assess chemical composition and concentration of oil additivation, and it allows discrimination among phenolic inhibitors.