Desenvolvimento de métodos analíticos para quantificação de elementos em óleos lubrificantes

Abstract

Lubricating oils, according to the Brazilian regulatory agency, ANP - Agenda Nacional de Petroleo, Gas Natural e Biocombustiveis, is a mixture composed by mineral based oil and additives (ANP, 2015). The standard methods recommended by the ANP (2105) for various element analysis in these oils employ different analytical techniques, using direct dilution of the matrix with organic solvents. In this work, three methodologies were developed and validated for the determination of different elements in this matrix. In the first one, Ca, Zn, Mg, Cu and Fe quantification by Flame Atomic Absorption Spectrometry (FAAS) were performed after the acid-digestion of the sample in a closed-vessel microwave digestion system. A 241 fractional factorial design was performed to optimization of the procedure, the parameters studied were sample mass, composition of acid mixture (nitric acid, hydrogen peroxide and water), time of pre-digestion and microwave heating program. This method was validated with a reference material of lubricating oil sample of ASTM interlaboratory test (LU-1209) for the above mentioned elements. Recovery ranged from 88 to 106% and coefficients of variation between 0.3 and 5.4%, for all metals. The second methodology proposed was the Zn and Ca quantification by FAAS after aqueous two phase system (ATPS). Studies were carried out in order to find a higher percentage of zinc extraction in ATPS, among them, the influence of the pH, extractor concentration, nature of the electrolyte, nature of the copolymer, tie-line length (ILL), and mass ratio between the top and the bottom phases. The optimized process led to a maximum extraction of Zn(H) of (107 ± 4)%, using a system composed of copolymer L64 + sodium sulfate + water. In addition, the optimized method was used for quantification of Ca (II) and validated for both metals. Recoveries ranged from 89 to 100% and coefficients of variation from 0.9 to 6.8%. Finally, the third methodology describes the use of small volumes of lubricating oil, organic solvents and yttrium as internal standard for determination of Ca, P, Zn and S by Total Reflection X-Ray Fluorescence CFXRF). The method was optimized using an ASTM sample (LU-1209) and xylene as a solvent. The experimental parameters studied were the sample volume, the measurement time and the volume deposited on the quartz disk. Optimized conditions using xylene were analyzed, followed by kerosene and hexane. The analytical merit figures were used to evaluate the performance of the analytical method, for all solvents. Recoveries ranged from 98 to 111% and the relative standard deviation remained between 1.8 and 10.1%. For all elements, the results obtained applying TXRF method were in agreement with certified values. For all methodologies, after validation, the quantification of the elements was realized in eight new and four used lubricating oil samples. As much as possible, the results obtained in the proposed methodologies were compared with those from standardized methodology. In these situations, no significant differences were observed at the 95% confidence level, according to the t-Student test.