Liquid Analysis in the Petroleum Industry: Precision, Accuracy and Instrument Safety

One of the largest concerns for laboratories using X-ray fluorescence (XRF) is the safety of their instrument during liquid analysis. An accident can damage an XRF system, costing days of productivity and thousands of dollars in parts and service. It is for these reasons that liquid analysis safety features are at the forefront of analytical instrument development.

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Sample preparation for liquid analysis is rather simple for XRF. The liquid sample is contained in a disposable cylindrical polymer cell (Fig. 1) with a thin film for X-ray and fluorescence transmission. The film used depends upon several key factors: liquid matrix type (oil, solvent, acid, etc), desired limits of detection, and contaminates contained in the films. The liquid is then added to the cells by either volume or weight. Keeping a consistent analysis volume is important in the measurement of non-infinitely thick samples by XRF.

An alternative method of sample preparation involves placing the liquid samples onto specially designed filter pads and allowing the sample to dry. The benefit of this method can be seen in improved detection limits, especially regarding light elements, i.e., with atomic numbers less than chlorine.

There are many elements that are considered to be important for control of petroleum processes. One of the most requested is sulfur. Sulfur (S) can range from sub-ppm to 5% in petroleum samples. Fortunately, XRF is able to analyze S at these varying concentrations using a sample calibration curve corrected for self absorption (Fig. 2/Table 1).

Other common elements in petroleum analysis are lead (Pb), nickel (Ni), vanadium (V) and chlorine (Cl). As shown in Table 2, the repeatability of analysis is excellent even at ppm levels.

Common limits of detection in oil are normally less than 1.0ppm, which can be seen in the table 3 results. These results were obtained using 100 seconds counting time per element in an oil standard sample. Sample preparation was perform by using the standard liquid cell with thin film. Elements which will be above the 1.0ppm detection limits are light elements such as sodium (Na) and magnesium (Mg). Elements with lower atomic numbers than Na will not be possible to measure using this standard preparation method due to absorption of their characteristic XRF photons by the supporting film.

For trace elemental analysis of Na and Mg or analysis of boron (B), carbon (C), nitrogen (N) or fluorine (F), a complete different sample preparation method must be employed. This method utilizes a specially designed absorbing pad as the analysis media. The liquids, whether it be aqueous, oils or solvents, are pipetted onto the pad and allowed to either dry in air or an oven. Once dried, the pads can then be analyzed under vacuum in the X-ray spectrometer. The process of using these pads allows the analysis to occur without any film between the material and the goniometer and for the material to be concentrated for much better detection limits. Examples can be seen in Fig. 4 and 5. The regression analysis illustrates extraordinary limits of detection of 0.2 ppm of Na and Mg.

Petrochemical regulations are becoming increasingly strict and are demanding lower levels of quantification for key elements such as sulfur, nickel, vanadium and lead in products. Compliance with various norms, including ASTM D2622, ASTM D4927 and ISO 20884, requires high sensitivity and stability to ensure content conforms to regulatory limits. The ARL PERFORM’X wavelength dispersive XRF analyzer provides excellent repeatability and resolution. The innovative way of using an absorbing pad for sample preparation markedly enhances limits of detection, particularly for light elements such as sodium and magnesium. In addition, it allows oils to be measured directly without dilution, which significantly reduces sample preparation time and increases speed and throughput of analysis.