GB/T 17040-2019Determination of sulfur in petroleum and petroleum products—Energy dispersive X-ray fluorescence spectrometry (English PDF)

1 Scope

This Standard specifies a test method for the determination of sulfur content in petroleum and petroleum products using energy dispersive X-ray fluorescence spectrometry.

This Standard is applicable to the determination of petroleum and petroleum products that are single-phase and liquid at ambient or moderately elevated temperatures, or are soluble in hydrocarbon solvents; these include automotive gasoline, ethanol-blended gasoline, diesel fuel, biodiesel and its blends, jet fuel, kerosene, other distillate fuels, naphtha, residual fuel, crude oil, lubricating base oils, hydraulic fluids, and similar petroleum products.

This Standard is applicable to the determination of sulfur mass fractions in the range of 0.0017% (17 mg/kg) to 4.6% (46 000 mg/kg).

2 Normative References

The following documents are indispensable for the application of this document. In terms of references with a specified date, only the version with that specific date is applicable to this document. In terms of references without a specified date, the latest version (including all the modifications) is applicable to this document.

GB/T 4756 Method for manual sampling of petroleum liquids

GB/T 8170 Rules of rounding off for numerical values & expression and judgement of limiting values

GB/T 27867 Petroleum liquid - Automatic pipeline sampling

NB/SH/T 0843 Standard practice for evaluating analytical measurement system performance of petrochemical industry (Statistical techniques)

ASTM D6259 Practice for determination of a pooled limit of quantitation for a test method

ASTM D7343 Practice for optimization, sample handling, calibration, and validating of X-ray fluorescence spectrometry methods for elemental analysis of petroleum products and lubricants

3 Method Summary

The specimen is placed in the X-ray beam emitted from an X-ray source, and the intensity of the excited sulfur Kalpha characteristic X-rays (energy: 2.3 keV) is measured. The accumulated counts are compared with those of pre-prepared standard samples to determine the sulfur content expressed as a mass fraction (see ASTM D7343). At least three sets of standard samples are required to cover the following mass fraction ranges: 0.0% to 0.1%, 0.1% to 1.0%, and 1.0% to 5.0%.

4 Conditions for Use of the Method

This Standard assumes that the matrix of the standard samples matches that of the samples to be tested, or that differences in the matrix have been taken into account (see A.2). Differences in the carbon-to-hydrogen mass ratio (mC/mH) between the samples to be tested and the standard samples (see Appendix A), or the presence of other heteroatoms, may result in a matrix mismatch. Table 1 specifies the permissible contents of heteroatoms, as well as ethanol and methanol, in the samples to be tested.

This Standard may also be used for the analysis of oxygenated fuels containing ethanol and methanol at levels exceeding the ranges specified in Table 1; however, the precision and bias data do not apply (see Appendix B).

For samples with relatively high oxygen content (mass fraction greater than 3%), the samples shall be diluted or matrix-matched standard samples shall be used to ensure the accuracy of the determination results.

5 Instruments and Equipment

6 Reagents and Materials

6.1 Reagent purity: All chemical reagents used in this Standard shall be of analytical reagent grade or higher purity.

6.2 Di-n-butyl sulfide (DBS): Not lower than guaranteed reagent grade; molecular formula: (C4H9)2S. In this Standard, it is used as a reference material with a certified sulfur content [typically 21.91% by mass]; this value is used to calculate the precise sulfur content of the prepared standard samples (see 8.1). Warning - n-Butyl sulfide is a flammable and toxic compound.

NOTE: It is necessary to know the sulfur content of the di-n-butyl sulfide, rather than just its purity, because the impurities may also be sulfur-containing compounds.

6.3 Drift monitoring samples (optional): Any sulfur-containing substance that remains stable under repeated X-ray irradiation - such as polysulfide oil, glass, or metal samples - may be used as a drift monitoring sample. Liquids, pressed powders, and solid materials that decompose under repeated X-ray irradiation shall not be used. Available sulfur-containing substances include liquid petroleum products suitable for long-term use, metal alloys, or fused glass discs. With an appropriate counting time, the count rate exhibited by the monitoring sample shall show a relative standard deviation of less than 1%. The count rate of the monitoring sample shall be determined during calibration (see 8.2.1) and re-determined during analysis (see 11.2). The count rate will be used to calculate the drift correction factor (see 14.6).

Although the calculation can be performed manually, drift correction is usually carried out automatically by software. For X-ray spectrometers with relatively good stability, the magnitude of the drift correction factor does not change significantly overall.

6.4 Polysulfide oil: Nonyl polysulfide with a known sulfur content, typically diluted with a hydrocarbon-based solvent. Warning - It may cause skin sensitization.

NOTE: Polysulfide oil is a high-molecular-weight oil with a sulfur content of up to 50% by mass. It retains excellent physical properties - such as low viscosity, low volatility, and a relatively long shelf life - even after thorough mixing with white oil. The sulfur content of the polysulfide oil is determined by diluting it with white oil and then directly comparing it against a corresponding certified reference material.

6.5 White oil: Sulfur content less than 2 mg/kg. In this Standard, it serves as the diluent for preparing standard samples (the sulfur content of any other diluents used in this Standard is also less than 2 mg/kg). When performing low-sulfur testing (mass fraction less than 200 mg/kg), the calculation of the sulfur content in the standard sample shall include the sulfur content of the matrix material (see 8.1). If the sulfur content of a solvent or reagent is unknown, it must be determined, or a reagent of higher purity must be used to prepare the standard sample.

6.6 X-ray transparent films: Any film that is resistant to the sample's chemical action, sulfur-free, and allows sufficient X-ray transmission may be used. Suitable options include polyester, polypropylene, polycarbonate, and polyimide films. However, samples with high aromatic hydrocarbon content may dissolve polypropylene, polycarbonate, and polyester films.

6.7 Helium (optional): Purge gas; select the appropriate specifications according to the instrument manufacturer's recommendations when required.

6.8 Counting gas: For instruments equipped with a flow-proportional counter, the purity of the counting gas shall meet the instrument manufacturer's specification requirements.

6.9 Calibration check sample: One or more liquid petroleum products or standard samples with known sulfur content (e.g., polysulfide oil, di-n-butyl sulfide, thiophene) that were not used to establish the calibration curve. Calibration check samples can be used to determine the accuracy and precision of the initial calibration curve (see Clause 8).

6.10 Quality control (QC) samples: Stable petroleum products or other solid samples analyzed periodically to ensure that the determination system is in a satisfactory state (see Clause 14). It is recommended to verify the system status using QC samples and quality control charts. The quality control process shall be carried out independently by each laboratory. Samples that are typical and remain stable over a long period may be used as QC samples. Solid materials are recommended for use as drift monitoring samples.

7 Preparation of Sample Cups

7.1 If reusable sample cups are used, ensure that they are clean and dry before use. Disposable sample cups shall not be reused. Polyester or polycarbonate films (see 6.6) with a thickness of less than 10 µm are commonly selected as window films due to their high transmittance for characteristic sulfur X-rays. A new window film shall be used for the determination of each sample. Avoid touching the interior of the sample cup, the window film exposed to X-rays, and the instrument's window. When measuring samples with relatively low sulfur content, oils from fingers can affect the determination results. Wrinkles in the window film can affect the transmittance of sulfur X-rays. Therefore, to ensure reliable determination results, the window film shall be kept flat and clean.

7.2 Recalibrate the instrument whenever the type or thickness of the window film changes, or before using a new batch of polyester film.

7.3 For samples with high aromatic hydrocarbon content, it is advisable to use a film free of impurity elements as the window film; a 6 µm thick polyimide film is the preferred choice.

8 Calibration