Abstract: Quantitative analysis of the ultra-light element fluorine (F) has always been one of the difficulties in electron probe microanalysis (EPMA). High-resolution qualitative analysis and proper subtraction of spectral interferences from major elements are prerequisites for accurate F quantification. This study focuses on F in monazite samples with very low F content or even close to 0. Through comparison and analysis of high-resolution qualitative analysis spectra with apatite, InP, CeAl₂, and LaF₃ samples, it is found that the third-order line of high-content P K_α does not interfere with the F K_α peak, but the spectral line M_ζ of the M energy level series of Ce interferes with the F K_α peak. Monazite with high Ce content needs accurate quantitative analysis of F through interference peak subtraction. The interference of the M-level spectral lines (including M_α, M_β, M_ζ) of rare earth elements on ultra-light element peaks needs to be taken seriously, and the necessary interference correction should be made through qualitative analysis of the spectra.