The fundamental parameter method, also called reference-free is based on theoretical calculations that relate the measured X-ray intensities to the concentrations of elements in the sample. This approach considers the physical characteristics of the X-ray interaction with the sample and employs mathematical models to estimate element concentrations . The internal standard calibration method is predominantly employed in TXRF analysis, leveraging the thin film approximation to assume a consistent matrix effect. In this approach, a known concentration of an element not naturally present in the sample, often Ga or Y, is introduced into the liquid sample. The quantity of other elements in the sample is subsequently determined in relation to the added element, which serves as an internal standard for calibration .