Sulfur is nearly always found in petroleum products and feedstocks.1 It is usually removed during crude oil processing as it is considered an undesirable contaminant that leads to the formation of harmful pollutants such as sulfur oxides.
While the removed sulfur can be used to synthesize other chemicals, such as sulfuric acid, crude oil with a high sulfur content often has a lower monetary value and is sometimes known as ‘sour crude’. The lower value reflects the higher costs associated with refining crude oil with this chemical composition. The sulfur content can also prove corrosive, causing additional expenses through damage to refinery equipment.
The impact on the value of materials and the potential health and environmental effects of the sulfur content in petroleum products means there is a compelling need for analytical methods to detect and quantify this sulfur content, which are compatible with crude oils and various petroleum products.
Regulations on Sulfur Content
The European Union’s 2009/30/EC directive stipulates a limitation of 10 ppm of sulfur for all diesel and gasoline.2 In the US, ultra-low-sulfur diesel fuel is now the standard for on-road diesel. This means the allowable sulfur content is less than 15 ppm.3
From 1 January 2020, the International Maritime Organization declared that fuel oil used in ships operating outside emission control areas must reduce sulfur content to 0.5% m/m.4 Other regions have lower limits of 0.1% m/m.5
One approach to measuring sulfur content and ensuring product compliance is the ASTM method D2622, a standard test method for quantifying sulfur concentration using wavelength dispersive X-ray fluorescence (WDXRF).6
ASTM method D2622
The ASTM method D2622 guides sample preparation for measurement with WDXRF. The method has a practical limit of quantification of 3 mg/kg of sulfur, equipment-depending. The technique is compatible with an extensive range of fuel types, including diesel fuel, jet fuel, kerosene, crude oil, and biodiesel.
The advantage of WDXRF for measuring sulfur concentrations is the excellent sensitivity, relatively straightforward sample preparation, and fast data acquisition speeds. Samples can often be measured in just minutes. All of these benefits are crucial for regulatory compliance as many new requirements, particularly for fuels, stipulate that only very low sulfur concentrations are acceptable.
One of the challenges with making accurate measurements with WDXRF is the sample matrix's effect on the measured fluorescence. The sample matrix is a combination of all the elements present in the sample, including C, H and O. Variations in the concentrations of these components may lead to matrix effects in the form of attenuation, where the matrix absorbs some of the fluorescence emitted by S. This is particularly problematic for measuring XRF for sulfur, as the fluorescence yield is already low for the low energy lines and the presence of other elements will lead to absorption of the emitted fluorescence signal.7
A fundamental assumption for the ASTM method is that the standard and sample matrices are well-matched or that any differences are correctly accounted for to avoid discrepancies that could arise from matrix effects. Some of the differences between standard and sample matrices that can contribute to errors from such matrix effects can occur from C/H ratio differences and interfering heteroatoms or other species.
LGC’s Reliable Certified Reference Materials
Instrument calibration for WDXRF instruments is usually performed using a series of sulfur-containing certified reference materials (CRMs). Such standards need to cover a range of concentrations and, for optimal accuracy, match the real samples' matrix conditions.
For this purpose, LGC offers an extensive range of CRMs.8,9 It provides matrix blanks with different isooctane to toluene ratios to allow for optimal C/H ratio matching between matrix and sample to avoid matrix effects issues.
For catalog products, sulfur concentrations start at 5 µg/g, so calibrations can be performed over a full range of concentrations, ensuring all sample measurements are within the calibration range. The standards are available as individual bottles or in a comprehensive kit that includes a full range of common calibration concentrations as well as a drift monitoring sample, ideal for multiple uses as a calibration update.
Reference materials can be created in-house, but the advantage of using CRMs from LGC is that these are compliant with the D2622 method and are immediately ready for use, making calibration a less labor-intensive process. All of LGC’s standards are traceable to an ISO/IEC 17025-certified laboratory, where they must pass rigorous quality control processes. LGC also supplies blanks that can be introduced as part of the analysis processes to maintain confidence in instrument performance and data quality.
Compliance with ISO 14596:20079 and ISO 20884:201910 standards for determining sulfur content in petroleum products with XRF and many other regulations requires traceability to CRMs. To minimize analysis time and boost measurement accuracy and precision, customers can rely on LGC’s expertise to produce optimal quality CRMs.
References & Further Reading
- 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
- 2009/30/EC, European Union (2021), https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:32009L0030, accessed 4th January 2021
- Diesel Fuel Standards, EPA (2021), https://www.epa.gov/diesel-fuel-standards/diesel-fuel-standards-and-rulemakings, accessed 4th January 2021
- Sulfur Content, IMO (2020), https://www.imo.org/en/MediaCentre/HotTopics/Pages/Sulphur-2020.aspx, accessed 4th January 2021
- Global Sulfur Cap, DNV GL (2020), https://www.dnvgl.com/maritime/global-sulphur-cap/index.html, accessed 4th January 2021
- D2622-16, ASTM (2020), https://www.astm.org/Standards/D2622.htm, accessed 4th January 2021
- 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
- Standards, LGC (2020), https://us.lgcstandards.com/US/en/search/?text=VHG-SISO7, accessed 4th January 2021
- ISO 17034:2016, ISO (2020), https://www.iso.org/standard/29357.html, accessed 19th January 2021
- ISO 14596:2007, ISO (2020), https://www.iso.org/standard/42636.html, accessed 4th January 2021
- ISO 20884:2019, ISO (2020), https://www.iso.org/standard/74314.html, accessed 4th January 2021