Volume 4, Issue 2 (February 2007)
A Method for the Preparation of NIST Traceable Fossil Fuel Standards with Concentrations Intermediate to SRM Values
A procedure is presented whereby the fossil fuel community may design and mix SRMs in different proportions and thereby produce in their laboratory standards for sulfur in distillate fuel oil, residual fuel oil, and coal of almost any desired concentration with uncertainties that are calculable and traceable to NIST SRM certified values. The expanded uncertainty, U, of a binary blend is always less than the U of the component with the largest uncertainty, and in some cases it is less than either of the SRM components. Because the sulfur content of all fossil fuel SRMs was certified at NIST with high accuracy and precision by isotope dilution thermal ionization mass spectrometry, in almost all cases the total expanded uncertainties of the standards produced from binary mixtures are an order of magnitude smaller than the reproducibility of current methods used in commercial laboratories. The use of this method gives the SRM user a continuum of concentrations available for calibrants and quality control test samples. Unlike calibrants prepared from high purity components, this method enables the SRM user to create a customized series of calibrants in the fossil fuel matrix of interest. This should reduce or eliminate biases that result from differences in matrix composition among standards and unknowns.