X-ray fluorescence (XRF) is a powerful analytical instrumental method used in a wide variety of industries to determine the elemental composition of various materials. In oil analysis especially, XRF techniques have gained wide acceptance. Among other applications, XRF is used to determine sulfur in petroleum products and residual catalysts, monitor additives in lubricating oils, analyze regular wear metal in lubricants and analyze wear debris.
During the 50 years since its commercialization, XRF instrumentation has evolved into a wide family of different analytical instruments (Figure 1).
Figure 1. An Early Commercial Wavelength
Dispersive XRF Spectrometer System.
The spectrometer can be seen on top of the generator (left). The counting electronics and the paper-chart recorder are in the cabinet on the right-hand side (Norelco, USA and Philips, the Netherlands, mid 1950s).
Generally, XRF instruments can be categorized into two classes. The first class uses a crystalline structure mounted on a goniometer (an instrument used to measure angles) to diffract X-ray photons of a selected wavelength/energy (much like X-ray diffraction). These instruments are referred to as wavelength dispersive spectrometers (WDS) (Figure 2a).
Figure 2a. Wavelength Dispersive Spectrometer
The second class of XRF instruments is based on the detector system’s capability to determine the energy of the photons (Figure 2b).
Figure 2b. Energy Dispersive Spectrometer
These are called energy dispersive instruments. Energy dispersive XRF instruments do not require a goniometer, and are thus of a much simpler mechanical design and are consequently less expensive.
In several cases, WDS systems offer the analyst an alternative to ICP or atomic absorption spectrometers, without the sample preparation complexity and particle size limitations commonly encountered with emission-type spectrometers.
Energy dispersive systems are identified primarily by their relatively simple optical path and unlike optical spectrometers, do not rely on diffraction. The most popular systems are bench top systems. In this case, their ultimate performance is largely determined by the quality of the X-ray source and the detector.
When only a verification of the presence (or absence) of one or more analytes in the sample is required, qualitative analysis is sufficient. Modern computer programs, based on a physical description of the X-ray fluorescence processes, allow concentrations to be obtained from samples and do not need standards similar to the unknown for calibration. These methods are in essence providing semiquantitative analysis and are often called standardless methods. The actual concentration of each analyte in the sample is required for true quantitative analysis. In this case, to adequately set-up the analysis, the level of required accuracy and precision must also be known.
Quantitative analysis by means of XRF requires proper calibration standards. First, standards are analyzed and intensities obtained. Subsequently, a calibration curve (X-ray intensity versus concentrations) for each analyte is determined. XRF instruments then compare the spectral intensities of unknown samples to those of known standards.
These provide traceability to primary standards and reference materials such as those provided by the National Institute of Standards and Technology (NIST) and other regulatory bodies. Establishing a calibration is simplified by the fact that XRF is essentially a multielement technique and thus does not require single element standards. A single set of calibration standards can be used to calibrate several elements, covering concentrations from trace level to high percentages.
Sulfur is a hazardous element, both mechanically and environmentally. Sulfur causes corrosion and rust on metallic parts in engines and the emission of sulfur dioxides into the air is a big environmental concern. Consequently, environmental protection agencies have issued strictly controlled sulfur emission limits for industry, automobiles and domestic applications.
The permitted levels of sulfur in both diesel and gasoline fuels is decreasing rapidly each year. XRF has been used to analyze sulfur in automotive fuels for many years. It is, for instance, the prescribed method in ASTM D2622, ASTM D6334 and ASTM D4294 methods. WDXRF spectrometers are used for regular sulfur analysis in petroleum products even at the low concentration levels in the ppm range, without compromise on the precision of the analysis (Figure 3).
Figure 3. Repeatability Results for an
Oil Sample Containing Less Than 4 ppm Sulfur.
Today, almost all commercial lubricants contain chemical additives. Depending on lubricant type, additive concentrations range from 0.1 percent to 30 percent of formulated oil by volume.
The XRF technique is a valuable instrumental technique and has been applied for elemental analysis of new and used lubricating oils. XRF can be used for quality control, product development and product performance classification.
For example, zinc, phosphorous and less commonly copper are used in antiwear additives. Sulfur, phosphorous and molybdenum are common components of extreme-pressure additives. Calcium, barium and magnesium are components of detergent additive packages used in engine oils. For quality assurance (QA) purposes, the multielement capability, excellent reproducibility and the limited or nonexistent sample preparation requirements (Figure 4) are key to making XRF an ideal QA tool.
Figure 4. Simplicity of Sample Preparation
Reduces the Total Time Required for an Analysis.
XRF is effective for analysis of wear metals in lubricating oils. The primary reasons are simple sample preparation, high accuracy (Table 1) and good to excellent detection limits (Figure 5). Some of the main advantages of XRF over other wear debris techniques such as ICP include a greater sensitivity to larger particles and simpler sample preparation.
Table 1. Calibration Accuracy of
Some Common Elements in Wear Metal Analysis.
Figure 5. Typical Lower Limit Detection
(100 seconds counting time) Values for Wear Metals
Obtained on WDXRF Spectrometer.
One of the big advantages of the XRF is its ability to analyze samples in different shapes or forms. The sample may not be in the form of only a liquid and/or suspension, but it may also be a solid or a powder, offering obvious advantages. For example, when dealing with the analysis of wear particles that have been filtered from the oil, removing the oil from the sample by filtration reduces the analytical complications such as those caused by the intense scattering of the primary X-rays by the oil matrix.
Furthermore, the foil on the specimen holder (Figure 4), which is required when dealing with liquids, can be eliminated. This simplifies specimen presentation and reduces scatter as well. Matrix effects (differences in absorption of X-rays) due to different base oils are also eliminated by filtration. These filters can be analyzed directly, without further preparation.
Especially with debris analysis, XRF’s ability to analyze even coarse particles (diameters greater than 10 microns) is important to diagnosing abnormal wear which, if left unattended, could lead to catastrophic failures. Instrumental methods that use nebulizer techniques (such as ICP) are at a disadvantage, as they typically detect only particles with diameters of less than 10 microns.
In contrast, larger particles are always detected by XRF, as long as they end up on the filter material. XRF, however, might miss the very small particles that emit radiation below the detection limit of the instrument. In this case, XRF and ICP can be complementary techniques. It is important to note for traceability and future reference that the samples are not destroyed nor consumed during XRF analysis, unlike ICP and other emission-based instruments.
One of the main advantages of energy dispersive X-ray analysis (EDXRF) is that it’s possible to view the X-ray spectrum (Figure 6) observing all elements at once.
Spectrum analysis allows for quick identification of elements that are present in a particular sample no matter what their sources. Also, concentration differences between two or more samples can be assessed in a qualitative manner. This allows spotting and identifying problems and trends, without specifically having to set-up the system. In addition, the spectra can be compared to spectra of the alloys from which the machinery under investigation is made, allowing component specific wear problems to be determined.
This “fingerprinting” is an essential part of identifying the particular component of the equipment that is wearing. The combination of the elemental XRF analysis and filtergrams used in ferrrography will further enhance the diagnostic capabilities of debris analysis in used lubricants.
XRF is a powerful analytical tool that is complementary to more commonly used methods such as ICP and RDE spectrometry. Deployed either as a root cause analysis tool, or in conjunction with other techniques to characterize abnormal wear patterns, XRF and in particular EDXRF offers a new tool in the arsenal of weapons available to oil analysis labs and end-users alike.
L. Kempenaers, PhD, PANalytical, the Netherlands.