CAN CGSB 3.0 No. 20.9-2005 cor1-2005: Determination of Sulfur Content in Petroleum Products by Wavelength Dispersive X-Ray Fluorescence Spectrometry

Scope

CAN CGSB 3.0 No. 20.9-2005 cor1-2005 establishes a test method for the determination of total sulfur content in petroleum products and related materials. This method applies to liquid hydrocarbons such as gasoline, diesel fuels, fuel oils, kerosene, residual fuels, and other distillates, as well as solid petroleum products like waxes and lubricating oils. The method is applicable to sulfur concentrations ranging from approximately 0.0030 mass% to 5.00 mass%.

The standard is aligned with internationally recognized techniques and is harmonized with the test method outlined in ASTM D2622. The corrigendum (cor1-2005) provides editorial corrections and clarifications to the original 2005 edition, ensuring consistency in operational procedures and data interpretation.

Technical Requirements

Principle of the Method

The sample is placed in a suitable X-ray cell and exposed to primary X-rays from a rhodium (Rh) target tube operated at a power sufficient to excite the sulfur Kα line. The intensity of the sulfur Kα radiation at a wavelength of 5.373 Å is measured using a wavelength dispersive spectrometer. The measured count rate is corrected for background and inter-element effects, and the sulfur concentration is derived from a calibration curve established using certified reference materials.

Instrumentation and Specifications

The method requires a wavelength dispersive X-ray fluorescence (WDXRF) spectrometer equipped with a rhodium target X-ray tube, a suitable analyzing crystal (e.g., germanium or pentaaerythritol), and a proportional or scintillation detector. The instrument must be capable of achieving a vacuum of less than 2.7 Pa (0.02 torr) in the sample chamber to minimize absorption of soft X-rays.

Calibration and Standards

At least five calibration standards covering the expected sulfur concentration range must be prepared using a sulfur-free base oil and certified sulfur compounds (e.g., dibenzothiophene, di‑n‑butyl sulfide). The standard includes a requirement for verifying calibration linearity and for performing a daily check with a control sample.

Sample Preparation

Solid samples must be heated above their melting point and handled as liquids. Liquid samples are poured directly into X-ray cells with a thin polypropylene or Mylar film window. The cell must be filled to a consistent depth (minimum 3 mm) to ensure infinite thickness for the sulfur Kα line. The sample should be free of water and sediment.

Measurement and Calculations

The net peak intensity (I_net) is obtained by subtracting the background intensity measured at a suitable offset angle. The concentration is calculated using a quadratic calibration equation:

C = a + b·Inet + c·(Inet)2

where a, b, and c are coefficients derived from the calibration standards. The standard also permits the use of internal standardization if spectral overlaps cannot be resolved otherwise.

Implementation Highlights

Corrigendum cor1-2005 introduced the following important amendments:

• Clarified cell window thickness requirements: The original text implied that only Mylar film was acceptable; the corrigendum explicitly allows polypropylene windows and specifies minimum thickness (6 µm) to avoid sample contamination.
• Corrected background measurement position: The offset angle for background was changed from +0.5° to +0.7° 2θ to avoid residual sulfur peak interference.
• Updated precision statements: The repeatability and reproducibility limits were recalculated based on a larger interlaboratory study. The corrected values are shown in the table below.
• Removed ambiguous safety language: The corrigendum replaced the outdated X‑ray safety warning with current regulatory references (e.g., Health Canada’s X‑Ray Safety Code).

Compliance Notes

Conformity assessment bodies and testing laboratories must ensure that their quality system references the specific corrigendum edition. The following aspects are critical for compliance:

Laboratory Accreditation

Accreditation to ISO/IEC 17025 is required for laboratories issuing compliance certificates based on this standard. The laboratory must demonstrate that the method is validated and that all modifications introduced by the corrigendum are incorporated into the standard operating procedure (SOP).

Quality Control

Routine QC must include:

• Daily check with a control sample (analyzed at least once per 10 samples)
• Blank determination (sulfur-free base oil) to verify spectral cleanliness
• Replicate analysis every 20 samples for repeatability assessment
• Two‑point calibration verification (low and high standards) every shift

Health and Safety

Operators must comply with local radiation safety regulations. The X‑ray tube must be shielded and interlocked. Personal dosimetry is mandatory for personnel operating the spectrometer. The standard requires inclusion of Material Safety Data Sheets (MSDS) for all sulfur compounds used in calibration.

Reporting

Results must be reported to three significant figures or as required by the applicable fuel regulation (e.g., Canadian Sulphur in Diesel Fuel Regulations). The test report must state the full standard designation including the corrigendum (CAN CGSB 3.0 No. 20.9-2005 cor1-2005) and any deviations from the method.

Frequently Asked Questions