Two procedures are described for calculating the composition and mass thickness of thin samples from measured x-ray intensities. One procedure is suitable for use with a programmable hand calculator but gives correct results only for very thin samples. The other procedure utilizes the NRLXRF program and produces correct results for thin and thick films.
van Willigen, J. H. H. G., Weber, H. T., Bos, M., & van der Linden, W. E. (1982). Two calculation procedures for the determination of composition and mass thickness of thin samples by x-ray spectrometry. Analytica chimica acta, 136, 379-384. https://doi.org/10.1016/S0003-2670(01)95400-5