Testing for Sulfur in Petroleum Products Using Wavelength Dispersive X-Ray Fluorescence

Sulfur can almost always be found in feedstocks and petroleum products.¹ As it is thought to be an undesirable contaminant that results in the formation of harmful pollutants like sulfur oxides, it is typically removed during crude oil processing.

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While the removed sulfur can be utilized to synthesize other chemicals, like sulfuric acid, crude oil that has a high sulfur content usually has a lower monetary value and is often known as ‘sour crude.’

The lower value denotes the higher costs which come with refining crude oil with this chemical composition. The sulfur content can cause further expenses through damage to refinery equipment as it can also prove corrosive.

The potential health and environmental effects of the sulfur content in petroleum products and the influence on the value of materials mean there is an urgent requirement for analytical techniques able to detect and quantify this sulfur content, which is compatible with various petroleum products and crude oils.

Regulations on Sulfur Content

A limitation of 10 ppm of sulfur for all diesel and gasoline is stated in The European Union’s 2009/30/EC directive states². Ultra-low-sulfur diesel fuel is now the standard for on-road diesel in the US. This means the sulfur content permitted is below 15 ppm.³

The International Maritime Organization stated that fuel oil used in ships operating outside emission control areas must decrease sulfur content to 0.5% m/m from 1 January 2020.⁴ Other regions have lower limits of 0.1% m/m.⁵

The ASTM method D2622 is one way to measure sulfur content and ensure product compliance. It is a standard test technique for quantifying sulfur concentration by utilizing wavelength dispersive X-ray fluorescence (WDXRF).⁶

ASTM Method D2622

The ASTM method D2622 guides sample preparation for measurement with WDXRF. This technique has a practical limit of quantification of 3 mg/kg of sulfur, equipment-depending, and is compatible with an extensive range of fuel types, including kerosene, crude oil, jet fuel, diesel fuel and biodiesel.

Relatively straightforward sample preparation, excellent sensitivity and fast data acquisition speeds are all benefits of using WDXRF for measuring sulfur concentrations. Often, samples can be measured in minutes.

As a number of new requirements, especially for fuels, stipulate that only very low sulfur concentrations are acceptable, all of these advantages are vital for regulatory compliance.

Matrix Compatibility

One of the drawbacks of making accurate measurements with WDXRF is the sample matrix's influence on the measured fluorescence. The sample matrix is a combination of all the elements present in the sample, including C, H and O.

Matrix effects in the form of attenuation, where the matrix absorbs some of the fluorescence emitted by S, could be a result of differences in the concentrations of these components.

As the fluorescence yield is already low for the low energy lines and the presence of other elements will result in the absorption of the emitted fluorescence signal, this is especially problematic for measuring XRF for sulfur.⁷

To avoid discrepancies that could arise from matrix effects, one key assumption for the ASTM technique is that the sample and standard matrices are well-matched or that any differences are accounted for correctly.

Some of the differences between the sample and standard matrices, which can lead to errors from such matrix effects, can happen from C/H ratio differences and interfering heteroatoms or other species.

LGC’s Reliable Certified Reference Materials

Instrument calibration for WDXRF instruments is typically carried out by utilizing a series of sulfur-containing certified reference materials (CRMs). These standards must cover a variety of concentrations and match the real samples' matrix conditions for optimal accuracy.

LGC provides a wide variety of CRMs for this purpose.^8,9 It supplies matrix blanks with different isooctane to toluene ratios to enable optimal C/H ratio matching between sample and matrix to avoid matrix effect problems.

Sulfur concentrations begin at 5 µg/g for catalog products, so calibrations can be carried out over a complete range of concentrations. This ensures that all sample measurements are inside the calibration range.

The standards are available as individual bottles or in a comprehensive kit that includes a full range of common calibration concentrations, in addition to a drift monitoring sample, perfect for numerous uses as a calibration update.

Reference materials can be created in-house, but the advantage of using CRMs from LGC is that these comply with the D2622 technique and are ready for use immediately, making calibration a less labor-intensive process.

All of LGC’s standards have to pass rigorous quality control processes and are traceable to an ISO/IEC 17025-certified laboratory. In order to maintain confidence in data quality and instrument performance, LGC also provides blanks that can be introduced as part of the analysis processes.

Compliance with ISO 14596:2007⁹ and ISO 20884:2019¹⁰ standards for establishing sulfur content in petroleum products with XRF and numerous other regulations requires traceability to CRMs. Customers can rely on LGC’s expertise to create optimal quality CRMs, which can be used to minimize analysis time and boost measurement precision and accuracy.

References

1. Bajia, S. C., Singh, R. J., Bajia, B., & Kumar, S. (2017). Determination of sulfur content in petroleum products–an overview. Journal of Sulfur Chemistry, 38(4), 450–464. https://doi.org/10.1080/17415993.2017.1289530
2. 2009/30/EC, European Union (2021), https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:32009L0030, accessed 4 January 2021
3. Diesel Fuel Standards, EPA (2021), https://www.epa.gov/diesel-fuel-standards/diesel-fuel-standards-and-rulemakings, accessed 4 January 2021
4. Sulfur Content, IMO (2020), https://www.imo.org/en/MediaCentre/HotTopics/Pages/Sulphur-2020.aspx, accessed 4 January 2021
5. Global Sulfur Cap, DNV GL (2020), https://www.dnvgl.com/maritime/global-sulphur-cap/index.html, accessed 4 January 2021
6. D2622-16, ASTM (2020), https://www.astm.org/Standards/D2622.htm, accessed 4 January 2021
7. XRF for Analysis of Sulfur, Shimadzu (2020), https://www.ssi.shimadzu.com/sites/ssi.shimadzu.com/files/About/Literature/pittcon2020/622-2_EDXRF_Sulfur_Petroleum_Products.pdf
8. Standards, LGC (2020), https://us.lgcstandards.com/US/en/search/?text=VHG-SISO7, accessed 4 January 2021
9. ISO 17034:2016, ISO (2020), https://www.iso.org/standard/29357.html, accessed 19 January 2021
10. ISO 14596:2007, ISO (2020), https://www.iso.org/standard/42636.html, accessed 4 January 2021
11. ISO 20884:2019, ISO (2020), https://www.iso.org/standard/74314.html, accessed 4 January 2021

This information has been sourced, reviewed and adapted from materials provided by LGC Limited.

For more information on this source, please visit LGC Limited.