Determination of trace metals in oil and petroleum products is typically based on inductively coupled plasma–optical emission spectroscopy (ICP-OES), which requires significant sample preparation and expensive instrumentation. This method, based on the use of a handheld energy-dispersive X-ray fluorescence analyzer, involves minimal sample preparation, uses authentic standards for calibration, gives low parts-per-million detection limits, and provides significant time and cost savings.
Metal contaminants in combustion engines can come from a number of sources, including oil, oil additives, and engine wear. In some cases, metals are intentionally added to oil and fuels to improve antioxidant, anticorrosive, dispersing, and anti-wear properties (1). During the refining process, metal contaminants decrease catalyst activity and selectivity, alter the product distribution (2,3), and cause auto-oxidation and decomposition of hyperoxides (4,5). In engines, metal contaminants can cause corrosion of parts such as bearings, valves, and pistons (1,6), eventually leading to engine failure (2). Early warning and appropriate corrective action are essential to improve engine performance, longevity, and cost savings (6,7). Moreover, the presence of toxic metals in automotive fuels and oils represents a potential source of environmental contamination (7–12).
More recently, inductively coupled plasma–mass spectrometry (ICP-MS) (22–27) and total reflection X-ray fluorescence (TXRF) (4–6) have been used for this application. TXRF is a specialized configuration of EDXRF in which a small amount of sample is dissolved or suspended in a liquid matrix, applied to a quartz disk, dried, and analyzed as a thin film at a very low grazing angle. The use of a drying step and a low grazing angle provides detection limits that are two to three orders of magnitude lower than EDXRF methods. However, the use of small sample volumes and the various preparation steps can lead to nonrepresentative and erroneous results, respectively.
Recent developments in handheld EDXRF analyzers merit their reconsideration for elemental analysis applications (28,29). Compared to AAS, ICP-OES, and TXRF, handheld EDXRF requires much simpler sample preparation procedures and provides the convenience of handheld analysis for off-site or in-field work. The goal of this study was to develop and characterize a new handheld EDXRF method for this application based on the use of authentic standards to provide accurate quantification of V, Cr, Fe, Ni, and Zn. This article provides details for the method, analytical figures of merit (linearity, detection limits, precision, accuracy, robustness), and a comparison of results from the analysis of a set of oil samples using both handheld EDXRF and ICP-OES methods.