Determination of Wear Metals in Marine Lubricating Oils by Microwave Digestion and Atomic Absorption Spectrometry

Abstract

The dif®culties associated with the development of a microwave-assisted acid digestion of lubricating oils in determination of wear metals are presented. The interest of this sample treatment lies in its basis for determining the total metal contents with enough sensitivity by¯ame atomic absorption spectrometry (FAAS). It allows earlier diagnosis of the engine state than is obtained by the widely applied simple dilution procedure. Another advantage is the avoidance of contamination and loss of the metals to be determined. The procedure employs nitric acid and hydrogen peroxide in a four-stage programme. Fe, Cu, Cr and Pb are determined by FAAS as representative of engine wear. The limits of detection are 0.1 mg/g for Fe and Cr and 0.05 mg/g for Cu and Pb. A comparison with other procedures is presented for spiked samples. Different types of used lubricating oils supplied by an oil company were analysed to prove the suitability of the procedure proposed.The determination of wear metals in used lubricating oils is of great interest in preventive maintenance of engines. The characteristics and behaviour of used marine engine oils are in many cases different from those of other lubricating oils [1,2]. The characteristics of the sample preparation procedures commonly used for these oils are as follows. Dilution with organic solvents and acidi®cation followed by dilution with organic solvents [3±8] leads to problems concerning repeatability, owing to the appearance of solid species (resolved in some cases with surfactants), and often the total metal content is not analysed. The traditional dry ashing and acid dissolution ASTM±IPC standard method is time-consuming and carries risk of loss and contamination. Microwave digestion with acid mixtures in closed te¯on vessels avoids these problems [9] and ensures that the total metal content can be analysed, but it may give lower sensitivity in comparison to other procedures [10].The wear metal determination requires a complicated microwave treatment, owing to the matrix and the metal concentrations in these oils. In the present work, the treatment of a larger amount of oil than that proposed by the commercial suppliers for detection of wear metals [10] was achieved in spite of the problems presented by the rapid development of a great deal of gases. The oil was digested with nitric acid and hydrogen peroxide. The metals determined were Fe, Cu, Cr and Pb as representative of marine engine wear [1]. Our aim was to destroy the organic matter and achieve total dissolution of the sample, to allow work in an aqueous medium and use of inorganic compounds as standards. This sample preparation procedure is also adequate for voltammetric metal determinations and for determining semimetals [11,12] such as Sn and Sb, which can also be found as wear elements and require use of hydride generation methods [13].The sample treatment procedure proposed is an interesting alternative to the classical ashing procedure and can also be used as a comparison method when dilution with organic solvents...

Determination of Wear Metals in Lubrication Oils: A Comparison Study of ICP-OES and FAAS

“…The method's drawbacks are high sample consumption, narrow linear determination range and difficulties in determining refractory elements. [1][2][3][4][5][6][7][8][9] However, these are seldom of interest to maintenance personnel (except phosphorus, which is an important additive element in lubrication oils).…”

Optimization of an open-focused microwave oven digestion procedure for determination of metals in diesel oil by inductively coupled plasma optical emission spectrometry

“…A restricted number of papers report the use of closedvessel microwave ovens for this task [9][10][11][12][13][14], while very few papers describe the utilization of open-focused microwave ovens for the same purpose [15][16][17][18]. It is important to remark that only three papers were found in the current literature reporting the microwave-assisted digestion of diesel oil.…”

“…Determination of metals in organic liquids like oils by plasma-based techniques can be performed utilizing three approaches: (1) dilution of sample in a convenient organic solvent and introduction of the solution obtained into the instrument [2][3][4]; (2) preparation of an emulsion with the sample and its introduction into the spectrometer [5][6][7] and (3) digestion of the sample before injection using microwave radiation, hot plate or dry ashing [8][9][10][11][12][13][14][15][16][17][18]. The use of the first two approaches is suitable when a low manipulation of the samples is required, but it is limited by the small number of metallic standard solutions prepared in oil available in the market [19,20].…”

Evaluation of Different Methods for Copper Determination in Insulating Oils by Graphite Furnace Atomic Absorption Spectrometry

“…5 Estes problemas são particularmente críticos quando se trata da determinação de qualquer espécie em nível traço. No caso de amostras derivadas do petróleo, diversos procedimentos vem sendo empregados para o pré-tratamento visando a determinação de elementos traços tais como a extração induzida por quebra da emulsão, [6][7][8] a digestão ácida, 9-11 a diluição com solventes adequados, [12][13] a conversão das amostras em emulsões ou microemulsões [14][15][16][17][18] que possam, posteriormente, ser introduzidas em diferentes instrumentos.…”