Rapid Determination of Trace Metals in Oil Using Handheld X-Ray Fluorescence Spectroscopy

Jul 01, 2014

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).


Table I: Standard methods used to measure trace metals in petroleum products (adapted from reference 5)
As shown in Table I, a number of standard methods have been developed specifically for measuring trace metals in fuel, oil, and petroleum products. The most common methods are based on atomic absorption spectrophotometry (AAS) and inductively coupled plasma–optical emission spectroscopy (ICP-OES). These methods require significant sample preparation, which involves ashing the sample in a platinum crucible for 2–12 h, mixing with flux and heating to form a fusion, acid digestion to dissolve the residues, and dilution to a known volume (12,13). This process is labor intensive, time consuming, and expensive. In addition, the process can lead to erroneous results because of volatilization of metals or inadvertent introduction of metal contaminants from reagents and sample containers (1). After the samples are prepared, they are analyzed via AAS or ICP-OES. This process involves introduction of the sample into a nebulizer as an aqueous solution, an oil–water emulsion, a microemulsion (1), or a three-component emulsion (14,15). The use of nonaqueous solvents affects nebulization efficiency, atomization efficiency, precision, and accuracy (16), and the industry consensus is that the use of organic solvents gives erroneously low results (1). A related technique called rotrode-OES, which is based on the use of a rotating graphite disk-shaped electrode to concentrate large wear and contaminant particles and detection of specific metals via OES (17), is documented in ASTM method 6595 (18) and is widely used by the US military and postal service as a qualitative screen for metals in unfiltered oils.

Energy dispersive X-ray fluorescence (EDXRF) and wavelength dispersive X-ray fluorescence (WDXRF) spectroscopy represent an alternative to the standard AAS and ICP-OES methods, and have been used to determine a variety of elements in petroleum cokes (11), petroleum oils (5,11,19,20), shale oil (21), lubricating oil (5), and residual oil (13). EDXRF systems can be purchased as benchtop instruments, portable units, or handheld analyzers. Numerous vendors have developed marketing literature based on the use of a benchtop EDXRF instrument or custom EDXRF instrument to measure trace metals in oil. It should be noted that some of this work is dated, uses nonportable laboratory-based instrumentation or obsolete products, and derives quantitative results through fundamental parameter based models (versus calibrating instrument response using authentic standards).

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.