The Analysis of Low Level Sulfur in Biodiesel According to ASTM D2622

February 1, 2009

Global warming is seen as a growing problem across the world. One of the major contributing factors to global warming is greenhouse gas emissions and particulates emitted from automobiles. In an attempt to control the particulate emissions from motor vehicles, limits have been placed on the amount of elements such as Sulfur allowed in automotive fuels. Currently in the US, the EPA tier 2 regulations have set the limit for S in road fuels (Gasoline, Diesel and Biodiesel) at 150 ppm.

Global warming is seen as a growing problem across the world. One of the major contributing factors to global warming is greenhouse gas emissions and particulates emitted from automobiles. In an attempt to control the particulate emissions from motor vehicles, limits have been placed on the amount of elements such as Sulfur allowed in automotive fuels. Currently in the US, the EPA tier 2 regulations have set the limit for S in road fuels (Gasoline, Diesel and Biodiesel) at 150 ppm. In Europe, the EU Directive 98/70/EC has set a maximum limit for S in road fuels of 50 ppm. As of January 1st, 2009, this maximum will drop to only 10 ppm. (EU2003/17/EC). There are several testing methods available to detect the level of S in fuels, one of which is the ASTM method D-2622. This method covers the analysis of S in petroleum and petroleum products. The method specifies an overall repeatability®, and reproducibility (R) across all matrix types. With the allowed levels dropping lower and lower, it will be more and more difficult for current testing methods to precisely and accurately quantify S at the levels needed. This application data note demonstrates the precise and accurate analysis of ultra low levels of S in Biodiesel fuel, in a calibrated range of zero to one hundred parts per million using the Axios Petro Wavelength Dispersive XRF.

Instrumentation and Sample Preparation

The instrument used in this study was an Axios Petro 4kW wavelength dispersive XRF(WDXRF) equipped with an SST mAX zero drift X-ray tube. The samples were prepared by pouring 10 grams of sample into 45 mm PANalytical brand sample cups, using 89mm diameter, 3.5 μm Low Sulfur Mylar sample support film (Figure 1).

Figure 1

Application Set Up and Measurement Parameters

Measurement parameters used for this application are shown in Table I. Background correction with two points was used, as instructed in the D2622 methodology. Counting times are listed in the table, and the total analysis time was 80 seconds, with 40 seconds on peak and 20 seconds at each background. All analyses were performed in a helium environment.

Table I: Measurement conditions

Table II: Repeatability(r) of 5 ppm QC sample, in accordance with D2622

Results

Calibration

The calibration for the study was performed using a set of PANalytical/VHG brand biodiesel fuel standards. The set consists of 9 calibration standards ranging from 0 to 100 ppm S in Fatty Acid Methyl Ester (B100) fuel. Verification of the calibration was performed using independent 5 ppm QC sample of the same matrix. The resulting calibration curve is highly linear with a standard deviation of 0.4 ppm. The 3 sigma lower limit of detection calculated for 100 seconds of measuring time is 0.2 ppm (Figure 2).

Figure 2

Analysis of Unknown Samples and Precision Test

ATSM D-2622 lists strict criteria for the precision of the analysis. The repeatability is defined as the difference between successive test results obtained by the same operator, under constant operating conditions on identical test material, and is calculated by the equation: Repeatability (r)= 0.1462*X0.08015 mg/kg. X is equal to the sulfur concentration in mg/kg and all values are reported in ppm (1 ppm =1 mg/kg). The repeatability requirement is that no more than one in twenty results will fall outside of the stated range. The allowed range for 5 ppm is 0.53ppm. The data show compliance with the method, as the greatest difference between consecutive results at 5 ppm is 0.501 ppm and no data was excluded as is allowed by the method. The long term repeatability over 4 months using the same calibration and measurement conditions is also excellent with a standard deviation of only 0.20 ppm for the 5 ppm QC sample (Figure 3).

Figure 3

Conclusion

The Axios Petro 4 kW wavelength dispersive XRF spectrometer is a powerful analytical tool, fully capable of meeting and surpassing the requirements of ASTM D2622 for determining low level of Sulfur in biodiesel fuels. Moreover, the data show the excellent long term reproducibility that is possible using the Axios Petro and the PANalytical Petroleum methodology for the analysis of low S in petroleum products.

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