Effect of carbon on major element analysis of carbonaceous silicate rocks by WD-XRF: An evaluation of error and its correction

Geological and environmental silicate samples, such as soils and sediments, often contain high total carbon (carbonaceous) that poses difficulty in the precise analysis by certain XRF techniques. The presence of carbon in the matrix in excess of c. 2-3% results in pronounced enhancements of fluorescent X-ray intensities of analytes leading to overestimation by as much as >5% depending upon carbon content of the sample. In order to quantify this overestimation and its matrix dependence, XRF measurements were performed on reference silicate standards by doping with varying amounts (2-20%) of carbon. Precise measurement of carbon X-rays is commonly not used in routine analysis of silicate rock samples, which limits the use of internal matrix correction for carbon. The enhancement effect is found to be more serious on high-Z elements compared to lighter elements. The error of estimation exhibits a linear correlation with the carbon content of the sample. External correction factors, therefore, derived from this linear relationship were applied to the test samples that reasonably improved the accuracies of estimation to within an acceptable limit.