Detecting Ultra-Low Sulfur in Automotive Fuels

Regulatory vehicle emissions limits for sulfur content of petroleum-based fuels are decreasing in allowable concentration levels around the globe. This development stems from the environmental concern related to pollutants like fine particulate matter (PM), which is produced by the combustion of diesel fuels, and sulfur oxides (SO_x).

In order to reduce these pollutants, the automotive sector has installed catalytic converters on vehicle exhausts, but when the fuel contains high concentrations of sulfur, the material that is used in the converters gets depleted over time. Therefore, to overcome this issue, the allowable limits of sulfur in fuel have been decreased to just a few ppm.

The word ultra-low sulfur diesel, or ULSD, refers to present standards for on-road vehicle diesel fuels from the U.S. EU and EPA authorities limit sulfur content to 10 ppm. This is a significant reduction when compared to the formerly allowed sulfur levels of 50 ppm or even 350 ppm within recent history.

ASTM D2622 — the test standard that has been updated in 2016 — is the preferred method in the industry for analyzing ultra-low sulfur in diesel and other types of fuels by wavelength dispersive X-ray fluorescence (WDXRF) spectrometry. One advantage of this method is that samples can be prepared easily and quickly with excellent precision.

Instrument

In this analysis, the Thermo Scientific™ ARL™ PERFORM’X X-ray fluorescence series spectrometer (Figure 1) used was a 4200 W system.

Figure 1. ARL PERFORM’X X-ray fluorescence spectrometer.

A dual water cooling system with tap water is used by the 4200 W version that cools down the deionized water circuit employed for the X-ray tube cooling. This spectrometer has been configured with four collimators, six primary beam filters, two detectors, up to nine crystals, helium purge, and Thermo Scientific’s 5GN+ Rh X-ray tube for optimum performance from ultra-light to heaviest elements due to its 50 micron Be window.

Equipped with a low current filament, the latest X-ray tube ensures an unparalleled analytical stability month after month. The ARL PERFORM’X spectrometer provides the ultimate in performance and sample analysis safety. Its special LoadSafe design comprises of a series of features that prevent any inconvenience at the time of sample flushing and loading. Accidental exposure of liquid samples to vacuum is prevented by liquid cassette recognition. Incase X-ray exposure time is too long, overexposure safety automatically ejects a liquid sample. In addition, the Secutainer system safeguards the primary chamber by vacuum collecting any loose powders in a uniquely designed container, which can be effortlessly removed and cleaned by any operator.

To ensure the protection of the spectral chamber, the ARL PERFORM’X employs a helium shutter designed for the complete protection of a goniometer at the time of liquid analysis under helium operation. An exclusive X-ray tube shield in the “LoadSafe Ultra” optional configuration offers complete protection against liquid cell rupture or sample breakage.

Calibration Ranges and Results

Very low X-ray intensities have to be detected to analyze ultra-low sulfur levels. For optimal analysis, the parameters and conditions applied in this measurement were set to activate the sulfur peak while maintaining the background as low as possible. Peak-to-background signal ratio was increased by collimator choice, and power settings were adjusted to increase the peak count rates even more. The final calibration used settings of 30 kV–80 mA power, a flow proportional counter (FPC), a PET analyzing crystal, and a medium collimator (0.4°).

Besides the analysis of the sulfur peak, one background measurement point was also implemented. Since the most difficult function in trace sulfur analysis is at the ultra-low level, a calibration was produced for 0 to 20 ppm (see Figure 2). Using blank oil as the base solvent, a 0.300% sulfur certified reference material was diluted to a concentration of 2.5 ppm, 4 ppm, 14 ppm, and 20 ppm. A blank oil was examined as a blank standard. Table 1 shows the conditions of the experiment.

Figure 2. Typical trace sulfur regression calibration curve.

Table 1. Analytical conditions

From the calibration, the measured detection limit is 0.1 ppm at 100 seconds counting time and sensitivity of 351 kcps/% (kilo counts per second per percent).

These limits are well within the regulations for ultra-low sulfur, indicating that the ARL PERFORM’X spectrometer is the perfect device for this kind of analysis. The repeatability results shown below demonstrate the stability of the ARL PERFORM’X spectrometer. To obtain these results, up to 20 different liquid cells were prepared and equal amounts of the same sample were poured into each. The samples examined were diesel fuel and gasoline.

Tables 2 and 3 demonstrate the repeatability results attained for S with ARL PERFORM’X 4200 W at 7.3 ppm and 8.7 ppm, respectively. In accordance with the ASTM 2622-16 standard, the highest permissible difference from one run to the next is set at 0.72 ppm and 0.83 ppm, respectively.

Table 2. Repeatability obtained for S on a gasoline 95 octane

Table 3. Repeatability obtained on a diesel

Conclusion

These results demonstrate that the ARL PERFORM’X sequential XRF spectrometer can be used to perform ultra-low sulfur analysis in an easy way. The precision data are shown to be consistent with the ASTM 2622-16 limits, which work well in these types of matrix for R&D or routine analysis.

In addition, operation is made easy through the advanced Thermo Scientific OXSAS WDXRF software, which operates with the new Microsoft Windows^® 10 package.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.