Traditionally, this type of analysis was performed using ICP-OES, following test methods EN 14538:2006 and EN 16294:2012.
However, this technique is labor-intensive and does not cover all relevant elements, such as S and Cl.
XRF analysis offers several advantages, including minimal sample preparation, the ability to analyse all key elements and to screen incoming goods for unwanted elements.

When analyzing FAME (Fatty Acid Methyl Esters), key elements of interest include:

• Sulphur

• Soap-forming metals: Na, Mg, K, Ca

• Phosphorus

• Chlorine

For Used Cooking Oils and other recycling products, these same elements are relevant, but additional attention must be given to contaminants that may point to unintentional or fraudulent mixing with other materials. Monitoring these elements helps ensure the integrity and quality of the recycled biofuel.

Several test methods exist for determining sulphur content in fuel using XRF (see the section on sulphur in petrochemical products). However, these methods are typically limited to fuels with a maximum of 10% FAME (3.5% oxygen content) due to the matrix effects of oxygen on the signal of other elements. To use XRF effectively for higher FAME concentrations, we need to take the oxygen content into account.

There are two approaches:

Use a system that analyses oxygen and automatically corrects for its content:

At the launch of the SPECTRO XEPOS, equipped with the unique Pd/Co tube, a new method was developed in cooperation with XRS to indirectly measure the oxygen content using the cobalt excitation line. The SPECTRO XEPOS can calculate the oxygen concentration by measuring the intensity of the cobalt Kα-Compton peak and making a relation of this intensity of with oils with a known oxygen concentration. By determining the oxygen concentration (indirectly) and by applying a Fundamental Parameter Correction, the influence of oxygen on the determination of all trace elements can be fully  compensated. Calibration standards are prepared with known oxygen concentrations, enabling a dedicated calibration curve for oxygen from 0 up to 20%. Due to this wide range, the method is designed to cover all steps of the process – from incoming raw FAME, Used Cooking Oils and even animal fat up to the final refined product. It allows a consistent monitoring and quality control at each stage of the purifying process. A total of 25 elements are included
in the method. The key additive elements – Na, Mg, P, S, Cl, K, and Ca – are calibrated in a relatively wide range using high-purity single-element standards, ensuring accurate quantification. All other trace elements are calibrated using the well-established Conostan S-21 standard in various concentrations. This setup of this method offers both flexibility and precision for comprehensive XRF analysis in the field of biofuel analysis.

Use matrix matched standards that match with your unknown samples:

When using an XRF analyser that doesn’t automatically correct for varying oxygen levels, we can offer all standard sets with a fixed oxygen content to support matrix matching with the customers unknown samples. The fixed oxygen concentration can be defined between 0 and 20%, which offers a lot of flexibility for customer.