In this study we have set up and made separate calibrations in accordance with the methods stipulated in both ASTM D2622-10 and ISO 20884:11 norms.
The latest 2010 version of ASTM D2622 is the most stringent test method in terms of repeatability and reproducibility.
Sulfur occurs naturally in crude oil, in concentrations typically ranging from 0.5 - 5.0 wt%. However, research has shown that the presence of sulfur in road fuels, at just a few hundred mg/kg, makes automobile pollution a major contributor to greenhouse gas emission and smog formation. Exhaust emissions of sulfur as sulfur dioxide and sulfate particulates directly contribute to the pollution load. However, the main drive for reducing the sulfur level of fuel is to maximize the CO2 reduction potential of new fuel-efficient engine technology and to improve the efficiency of emission control devices, such as catalytic converters, reducing emissions of NOx and particulates.
Sulfur content in fuels has decreased significantly during the last decades. Most automobile manufacturers now develop cars aiming at the maximum sulfur content for diesel and gasoline at 10 mg/kg (ppm) for category 4 and 5, as proposed in the worldwide fuel charter 2012.
X-ray fluorescence spectrometry is used extensively for the analysis of sulfur in petroleum products. Simple sample preparation, high accuracy and precision, and good to excellent detection limits (0.2 - 1 mg/kg) are the principal reasons for this choice, and make it an excellent technique for production control.
International standard test methods used for the determination of sulfur in petroleum products include ASTM 2622, ISO 14596 and ISO 20884. Both ASTM 2622 and ISO 20884 methods are broadly analogous, employing mathematical matrix corrections, and are distinct from ISO 14596, which employs a Zr internal ratio correction method.
The latest 2010 version of ASTM D2622 is the most stringent test method in terms of repeatability and reproducibility. In this study we have set up and made separate calibrations in accordance with the methods stipulated in both ASTM D2622-10 and ISO 20884:11 norms.
Commercially available synthetic standards, prepared according to the test methods, were used for this study NIST Diesel SRM 2723a was used as purchased. Fifteen milliliters (15 ml) of sample material (standard or routine) were poured into 38 mm diameter disposable liquid cells, constructed using Mylar X-ray film.
The Petro edition of the Zetium spectrometer is especially configured to meet the needs of XRF analysis in the petrochemical industry - today and in the future. Offering the highest levels of reliability in this demanding environment, the Petro edition of the Zetium spectrometer is an XRF platform for all XRF applications, from sulfur through catalysts to wear metals. It is simple to operate, and the compact design is easily integrated into today’s laboratories.
The precision and repeatability of the Petro edition of the Zetium XRF spectrometer is excellent. For comparison, the counting statistical error (CSE) calculated in mg/kg is also shown in Table 1.
For the ISO method, 20 consecutive measurements of a sample demonstrate standard deviations better than 2 % relative at the 33 mg/kg level, e.g. 32.9 ± 0.6 mg/kg S, and this includes the error associated with preparing twenty different liquid cells.
Table 1. Analytical precision for sulfur analysis; repeatability of 20 consecutive measurements. Differences in the RMS and CSE values for the ISO and ASTM data in Table 1 can be attributed to different measurement times and different X-ray film thickness.
The repeatability requirements of both test methods require that successive test results should not exceed certain limits more than one case in twenty.
For samples with less than 60 mg/kg sulfur these limits are according to the equations below:
1.7 + (0.0248*mean(mg/kg))
0.1462* mean(mg/kg)0.8015
The repeatability of measurements for samples containing 33 mg/kg and 8.3 mg/kg sulfur, together with the repeatability limits for the ISO and ASTM methods are illustrated in Figures 1 and 2, respectively.
Figure 1. Repeatability of sulfur measurements in diesel according to ISO 20884:11. Shaded area represents the precision limits set in the method.
Figure 2. Repeatability of sulfur measurements in oil according to ASTM D2622-10. Shaded area represents the precision limits set in the method.
Repeated measurements on NIST diesel SRM 2723a for over 120 days have a maximum difference between two successive measurements of 0.5 mg/kg*. The results demonstrate excellent long-term stability for the Zetium Petro edition.
Figure 3. Repeatability measurements of sulfur in NIST diesel SRM 2723a for over 120 days. *data obtained on Zetium Petro edition 4.0 kW
The accuracy of the calibrations is illustrated in Figure 4. In this plot the calibration RMS value (0.23 mg/kg) is a statistical comparison (1 sigma) of the certified chemical concentrations of the standards with the concentrations calculated by regression in the calibration procedure.
Figure 4. Calibration plot for sulfur in oil analyzed according to ASTM 2622-10
Components typeset in bold were present in the spectrometer used to obtain the data in this note.
The detection limits for sulfur in diesel and oil are given in Table 2. Sub-mg/kg detection limits are possible for comparatively short measurement times and show that the analytical method is easily capable of satisfying the more stringent regulations governing sulfur in road fuels from 2009 and beyond.
Table 2. Detection limits for sulfur in petroleum products
The analysis of sulfur in road fuels by XRF is a fast, relatively simple analytical technique. As demonstrated, the Petro edition of the Zetium spectrometer is capable of accurate and precise analysis of sulfur in petroleum products, easily fulfilling the requirements of the internationally certified ISO and ASTM test methods. Sub-mg/kg detection limits ensure that the Petro edition of the Zetium spectrometer will be able to meet the requirements of industry, even as more stringent government regulations come into force.
