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The determination of trace metals in lubricating oils by atomic spectrometry

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Abstract

The determination of trace metals in lubricating oils using atomic spectrometric methods is reviewed. The importance of such analyses for technical diagnostics as well as the specific sample characteristics related to the analyte form (metallo-organic and metal particles) is discussed. Problems related to sample pre-treatment for appropriate sample introduction and calibration are addressed as well as the strategies to overcome them. Recent trends aimed at simplifying sample manipulation are presented. The applications and scope of AAS, ICP OES and ICP MS techniques for the determination of trace metals in lubricating oil is individually discussed, as well as some present instrumental trends.

Introduction

Engine and turbine components undergo continual wear under normal operating conditions; this is minimized by the use of lubricating oils. Lubricating oils from petroleum are mainly composed of paraffinic, naphthenic and, to a lesser extent, aromatic hydrocarbons. Several additives, including metallo-organic ones, are also part of the final composition of commercial lubricating oil. Wear has both physical (friction between metallic parts, high temperature and pressure) and chemical (corrosion) sources. Chemical wear may produce not only metallic particles but also soluble metallo-organic species, whereas physical wear generates metallic particles of varying sizes (up to a few micrometers) [1], [2]. Part of the metallic debris is not retained by filters and collectors, remaining in the lubricating oil and being transported through the whole system [3]. Increasing amounts of some key elements in the lubricating oil may indicate the extent of the wear of wetted components. For instance, an abrupt increase of Ni, Sn or Cr indicates corrosion in bearings, valves and pistons, Fe indicates corrosion in various parts, Na indicates oil contamination with anti-freeze fluids and so on [4], [5]. The diagnosis based on elemental analysis of used lubricating oils may indicate the need for preventive maintenance of engines and turbines before irreversible damage occurs. Besides bringing economic benefits, such diagnoses save lives. The Spectroscopy Oil Analysis Program (SOAP) of the United States Air Force was the pioneer program based on this type of monitoring [4].

Elements such as Ag, B, Ba, Bi, Ca, Cd, Co, Cr, Fe, Hg, Mg, Mo, Ni, P, Sb, Se, Sn, Ti and Zn, are also deliberately introduced in small portions to lubricating oils to address requisites for special applications [4]. In these cases, metallo-organic compounds containing these elements act as additives, improving lubricating capability and properties such as antioxidant, anticorrosive, dispersing, antiwear, and others. The improvement of performance of the oil is dependent upon the amount of the additive introduced; therefore, these amounts must be strictly controlled.

Sensitive techniques are required for analysis of used lubricating oils since the capability of monitoring small concentration changes in the key elements is needed. The complexity of the oil matrix, its viscosity and the high organic load impose serious difficulties for elemental analysis. Such determinations are also an analytical challenge because all wear metals (solid particles of different sizes and metallo-organic species) must be accurately determined for information purposes [4], [6].

Apart from the existence of a few electroanalytical [7], [8], [9], [10], [11] and XRF [12] methods for the determination of Zn, Cu, Pb, Fe, Cr, Ni, As and Cd in lubricating oils, the majority of analytical methods reported in the literature are based on atomic spectrometric techniques such as FAAS, ET AAS, DC or ICP OES, ICP MS and AFS.

Section snippets

Sample pre-treatment, introduction and analyte calibration

Despite advances in sample introduction utilizing a minimum of previous treatment (solid sampling, electrothermal vaporization, laser ablation, etc.), most methods based on atomic spectrometric techniques still require some sample pre-treatment. In the case of viscous complex oil-based organic materials, major problems arise due to the difficulty in introducing them directly into the atomizers and selecting proper calibration standards [13]. As a consequence, proper preparation of oil samples,

Atomic fluorescence and atomic absorption methods

Pioneering work within the SOAP program used AFS for the determination of Ag, Cu, Fe, Mo and Mg in jet engine oils [36], [37], [38], [39]. Sample atomization from a graphite rod or in a low-quenching H2–Ar or Air–C2H2 flame was employed. Because of low-radiance light sources, resonance fluorescence schemes were chosen; therefore, a high stray and scattering background environment was expected. Limits of detection at the μg g 1 level were reported. Determinations of Pb, Cr and Ni were plagued by

Final remarks

Table 1 summarizes the referenced analytical procedures for determination of metals in lubricating oils. In spite of the difficulties of this type of analysis caused by sample heterogeneity and matrix complexity, modern instrumentation in combination with a proper sample pre-treatment, has greatly improved the reliability of trace metal results. FAAS remains as an important technique for metal determination in lubricating oil due to its lower costs and robustness in relation to interferences

Acknowledgements

The authors thank CNPq (Brazil) for scholarships and Petrobras for research funding. FINEP (Brazil) is also acknowledged.

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    This paper was presented at the 9th Rio Symposium on Atomic Spectrometry, held in Barquisimeto, Venezuela, 5–10 November 2006, and is published in the special issue of Spectrochimica Acta Part B, dedicated to that conference.

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