A METROLOGICAL AND STATISTICAL USE OF ISOTOPE DILUTION INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY FOR SULFUR DETERMINATION IN BIODIESEL

Sulfur content in fuels has been worldwide controlled for specification requirements, as a means to show the evolution of “cleaner petroleum fuels” and biofuels, such as biodiesel. Under this assumption, there has been a growing need for analytical methods with the ability to determine low sulfur concentrations (up to 10 mg kg-1). This work describes the evaluation of isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS)1 as a metrological tool to provide reference values for commercial biodiesel samples. Closed vessel microwave-assisted digestion was employed using nitric acid and hydrogen peroxide as the decomposition medium, followed by isotope spike with 34S dissolved in n-propanol. In order to overcome the spectral interference due to the presence of diatomic oxygen molecules in the ion beam, a reaction cell was used. Under optimized conditions, the determination of sulfur could be carried out at m/z +16, i.e., monitoring SO+ signal for both sulfur isotopes2. In order to achieve the reference values, uncertainty budgets based on ISO GUM3 and Eurachem/CITAC4 rules were employed, using the isotope dilution equation variables. The sample set has combined commercial methyl esters, derived from soybean oil and beef tallow. The results were useful to correlate the current worldwide standard methods – ultraviolet fluorescence (UVF)5 and inductively coupled plasma optical emission spectrometry (ICP OES)6 - in a large concentration range (about 1 to 300 mg/kg, with uncertainties up to 14%). In addition, different sulfur species (organic and inorganic) could be studied in this correlation. Therefore, the technical standards associations, standardization committees and national regulation agencies may be able to accept UVF and ICP OES test methods for total sulfur determination without any doubt about accuracy.