Analysis of Sulphur in Petroleum Products According to ASTM D4294 & ISO 8754 using Shimadzu’s EDX-7000

The sulphur content of fuels is widely regulated to reduce atmospheric emissions of sulphur dioxide, or SO₂, and other sulphur oxides, or SOx, during combustion of the fuel. Sulphur dioxide has wide-ranging negative health effects, particularly impacting the respiratory system as well as contributing to the formation of acid rain.

The primary sources of SO₂ in the atmosphere are from sulphur contained in fossil fuels that are burned for power generation and transportation as well as naturally-occurring volcanic emissions. To limit SO₂ discharge into the atmosphere, various organisations and governments have imposed increasingly strict limits on the sulphur content of fuels. A recent example of this is the International Maritime Organisation’s (IMO) regulation set to take place in 2020 limiting the content of sulphur in marine fuel at 0.5%, reduced from the current 3.5% limit.

Method
The method applied in this application note adheres to the methodology described in ASTM D4294, Standard Test Method for Sulphur in Petroleum and Petroleum Products by Energy-Dispersive X-ray Fluorescence Spectrometry, and ISO 5784, Petroleum Products – Determination of Sulphur Content – Energy Dispersive X-ray Fluorescence.
The measurement conditions for each standard method as well as the actual parameters used in this study are outlined in Table 1.
Several factors were assessed for method compliance, including repeatability (r), reproducibility (R), blank measurement, and a lower limit of detection (LLD).
For ASTM D4294, repeatability was assessed by analysing one sample 20 times on the same instrument and performed by the same operator. Reproducibility was assessed by analysing one sample 20 times on one instrument in one laboratory, and then the same sample was analysed a further 20 times on a different instrument (of the same model) and in a different laboratory by a different operator. The result is the magnitude of the range of results for the 20 replicated about the true value, and the results were then compared to the calculated, statistical limit as per the method specifications, calculated based on equations 1 and 2. The blank was assessed by analysing a blank sample 20 times. The blank measurement must be below 2 ppm. When analyte concentrations were ≥500 ppm, analysis was conducted with the analytical chamber purged with He and under standard atmospheric conditions. At concentrations <500 ppm, analysis was conducted only with the chamber purged with He.
r =0.4347 × S0.6446
r =1.6658 × S0.3300
Equation 1: Repeatability (r), in ppm, according to ASTM D4294 for base oil (a) and diesel fuel (b). The target concentration is represented as S.
R =1.9182 × S0.6446
R =1.6658 × S0.3300
Equation 2: Reproducibility (R), in ppm, according to ASTM D4294 for base oil (a) and diesel fuel (b). The target concentration is represented as S.
For ISO 8754, repeatability was assessed by performing two sets of 20 analyses (40 total analyses) of the same sample on the same instrument in the same laboratory by the same operator. Reproducibility was assessed in the same manner as that for ASTM D4294. The results were then compared to the calculated, statistical limit as per the method specifications, calculated based on equations 3 and 4. The analytical chamber was operated under ambient atmospheric conditions for all analyses.
r =0.045 ×(S + 0.05)
r =0.0215 ×(S + 0.15)
Equation 3: Repeatability (r) in weight percent (wt. %) according to ISO 8754 for sulphur in concentration range of 0.03 to 0.05 wt. % (a) and greater than 0.05 wt. % (b). The target concentration is represented as S.

R = 0.1781 ×(S + 0.05)
R = 0.0812 ×(S + 0.15)
Equation 4: Reproducibility (R) in weight percent (wt. %) according to ISO 8754 for sulphur in concentration range of 0.03 to 0.05 wt. % (a) and greater than 0.05 wt. % (b). The target concentration is represented as S.

Results
A summary of results of this study are presented in Tables 2 and 3 for ASTM D4294 and Tables 4 and 5 for ISO 8754.
In addition to the above results, the results of measurement of 20 analyses of the blank sample yielded a maximum result of 1.9 ppm. Lower limits of detection (LLD) calculated using a 200 second integration time yield results of 5.61 ppm with a non-purged sample chamber, and 2.49 ppm with a helium-purged chamber.

Discussion
Calibration of the EDX-7000 with known standards yielded a linear and steep calibration curve, critical to effective quantitation (Fig. 1). Such a predictable and easily-modelled response on the instrument results in accurate and precise quantitation of unknown samples.
Repeatability results per ASTM D4294 for base oil show that the EDX-7000 can comply with the method at sulphur concentrations of ≥500 ppm using a non-purged analytical chamber and ≥50 ppm when purging the chamber with helium (e.g., Fig. 2). Furthermore, for sulphur concentrations of ≥500 ppm, the analytical time can be reduced by 50%, more stringent than the scope of the method, and still comply with repeatability requirements. For diesel fuel, repeatability complied with ASTM D4294 at sulphur concentrations of ≥100 ppm.
Reproducibility results per ASTM D4294 for base oil show that the EDX-7000 can comply with the method at sulphur concentrations of ≥500 ppm using both a non-purged and a helium purged chamber. For diesel fuel, the concentration range is lower at ≥100 ppm sulphur using a helium purge.
For both repeatability and reproducibility as per ISO 8754, the EDX-7000 complied across the entire concentration range of 0.03 to 4 weight percent sulphur using ambient (non-purged) conditions.
Calculations of the LLD demonstrate sensitivity of the EDX-7000 for sulphur as low as 2.49 ppm. This concentration is far lower than the upcoming requirements of > 0.5 wt. percent (5000 ppm). Even when operating the instrument without the use of the helium-purged chamber, sensitivity was only diminished slightly, to 5.61 ppm. Again, this value is well within the scope of upcoming IMO regulations.

Conclusion
The method and data outlined above demonstrate the applicability of Shimadzu’s EDX-7000 towards ASTM D4294 and ISO 8754 for measuring total sulphur in petroleum products. As regulations on sulphur in fuels and petroleum products become stricter, fast and easy methods for testing become increasingly important. Shimadzu’s EDX-7000 provides a simple way for analysis of sulphur, and other elements, in petroleum products.