Possibilities of energy dispersive X-ray fluorescence techniques (EDXRF) for the determination of trace amounts of sulfur in biodiesel samples

Abstract

Sulfur is a low-toxicity element in its elemental form, but when combusts with air becomes sulfur dioxide, an irritating and toxic compound. This compound is released from many combustion processes because of the fact that different fuels such as coal, petroleum, diesel or natural gas contain certain amounts of sulfur. Therefore, nowadays there is a clear need to develop analytical methodologies that allow the determination of trace amounts of sulfur (<100 ppm) in fuel samples. Even though there are some standardized methods for the determination of sulfur in these types of samples, these methods consume noticeable amounts of sample and generate high amounts of waste. Alternative methods published in the scientific literature for the determination of sulfur in fuel samples, based on the utilization of plasma based techniques, include a laborious treatment of the sample such as microwave-induced combustion. The aim of the study is to develop a fast, accurate, trustworthy and environmentally friendly method for the determination of trace amounts of sulfur in biodiesel samples. For this reason, the analytical possibilities of energy-dispersive X-ray fluorescence (EDXRF) as well as the total-reflection X-ray fluorescence (TXRF) have been tested for this purpose. In the EDXRF developed methodology, the treatment of the sample is based on a simple deposition of few microliters of the liquid sample (50 μL) onto a commercially organic holder (Ultra-Carry®) and a later dryness step. Then, the sample holder can be directly analyzed. Using this methodology, we have achieved a limit of detection of 9 ppm for sulfur as well as acceptable accuracy and precision of the results if taking into account the working concentration range. Besides, the results regarding to the determination of sulfur by means TXRF have not been as promising as the ones obtained by EDXRF. It has been demonstrated that the limit of detection of the technique do not permit the determination of trace amounts of sulfur (LOD 1000 ppm). This fact can be explained due to the low amount of sample ( 1μL) that can be poured onto the reflector holder needed for performing the analysis using TXRF and due to the handicap of not being able to work under vacuum conditions during the analysis, a particularly critical fact when determining light elements like sulfur