The matrix calibration uses regression of experimental data to calculate the calibration curve relating the XRF emission intensity (ordinate) of an element with its concentration or mass (abscissa). Experimental data are obtained measuring samples with known concentrations of elements of interest. The calibration curve equation is then used to convert into the element concentration or mass the XRF emission intensity of each element identified in the spectrum of a sample with unknown composition . This methodology is often referred as external calibration or empirical approach and it is widely used in many analytical techniques other than XRF.