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Can you analyse biofuels using a standard oil calibration, typically based on a CH₂ matrix?

Renewable fuels and biofuels are becoming increasingly important in the automotive fuel market. Fatty Acid Methyl Ester (FAME), usually produced from vegetable oil, is commonly blended into diesel at concentrations ranging from 5% up to 20%. In addition to fresh vegetable oil, recycled streams such as used cooking oil are widely used as a more economical and environmental friendly alternative.

With this growing variety of raw materials and blends, keeping an eye on elemental composition is critical. From pure FAME and waste oils to processed intermediates and final fuels, consistent and reliable analysis is needed to ensure compliance within the industry specifications.

Traditionally, this analysis has been performed using ICP-OES, following methods such as EN 14538 and EN 16294. While accurate, these techniques are labour-intensive and typically limited in scope, as they do not cover all relevant elements like sulphur and chlorine. That’s where XRF spectrometry comes in as a practical alternative. With minimal sample preparation and the ability to analyse multiple elements simultaneously, XRF spectrometry is ideal for fast screening and routine quality control.

But this raises an interesting question: can you analyse biofuels using a standard oil calibration, typically based on a CH₂ matrix?

What makes biofuels different from traditional fuels:

Biofuels differ fundamentally from traditional petroleum-based fuels. They originate from biological sources such as vegetable oils, waste cooking oils or animal fats, each introducing variability in composition. More importantly, unlike fossil fuels, biofuels contain a significant amount of oxygen within their molecular structure.

In XRF spectrometry, matrix matching is essential. Unknown samples must behave similarly as the calibration standards in order to obtain accurate results. A calibration built on a CH₂ matrix assumes that the sample matrix consists only of carbon and hydrogen. This works well for conventional fuels, but not for biofuels, where oxygen is present in considerable amounts.

This difference has a direct impact on the measurement. Oxygen absorbs more X-ray radiation than carbon and hydrogen, which leads to a suppression of the detected signal. The effect is especially pronounced for light elements, where the measured intensity decreases as the oxygen content increases.

The impact of Oxygen on your results:

The influence of oxygen becomes very clear when looking at its effect on the phosphorus signal. As the oxygen content increases, the measured intensity decreases in a predictable way. This relationship is illustrated in the table below, where the suppression of the phosphorus signal is shown as a function of oxygen content:

Suppression of P XRF signal in presence of oxygen in oil

A clear trend emerges: the higher the oxygen content, the stronger the signal suppression. In other words, oxygen introduces a systematic negative bias in the measurement.

In practice, a typical vegetable oil contains around 12.5% oxygen. This results in roughly 20% signal suppression, which directly translates into an underestimation of about 20% in the reported concentration if no correction is applied. This clearly shows why applying a standard CH₂ calibration to biofuels inevitably leads to an underestimation of the results. To obtain accurate and consistent data, the matrix effect of oxygen must be taken into account.

So how to compensate for this Oxygen effect?

There are two main strategies to overcome this challenge, depending on the capabilities of the instrument and the application.

The most advanced approach involves an automatic oxygen correction. In this case, the XRF system indirectly determines the oxygen content and applies fundamental parameter corrections. This makes it possible to compensate for matrix effects across a wide range of samples, typically covering oxygen contents from 0 up to 20%. As a result, one single calibration can be used for different sample types, from raw vegetable oils and FAME to finished biofuel blends. This approach also enables direct screening of used cooking oils, allowing operators to verify whether the oxygen content is within the expected range or if further investigation is needed. More information on this approach and its implementation in biofuel analysis can be found on the XRS biofuel application page.

A second approach is based on matrix-matched standards. When direct oxygen correction is not available, calibration standards can be prepared with a fixed and known oxygen content. By matching the matrix of the standards to that of the samples, a higher accuracy can be achieved for specific product types. While this method is less flexible, it remains a reliable solution for routine analysis when sample composition is relatively consistent.

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