The inaccuracies and validity of the approximate linear relation between ln(fluorescent intensity/concentration) and ln(fluorescent energy) are presented and discussed. We then present the derivation of a new linear relation between ln(fluorescent intensity ratio of analyte to pure analyte/ concentration of analyte) and ln(fluorescent energy) which is valid for all elements that can be excited by radiation sources, whether by tube x-ray or by radioisotope sources.
C. T. Yap and Younan Hua, "Theoretical Studies in EDXRF on a New Linear Relation: ln(Fluorescent Intensity Ratio of Analyte to Pure Analyte/Concentration of Analyte) Versus ln(Fluorescent Energy)," Appl. Spectrosc. 47, 2052-2057 (1993)