Quantitative characterization of dynamic exchange between various conformational states provides essential insights into the molecular basis of many regulatory RNA functions. Here, we present an application of nucleic-acid-optimized carbon chemical exchange saturation transfer (CEST) and low spin-lock field R-1 rho relaxation dispersion (RD) NMR experiments in characterizing slow chemical exchange in nucleic acids that is otherwise difficult if not impossible to be quantified by the ZZ-exchange NMR experiment. We demonstrated the application on a 47-nucleotide fluoride riboswitch in the ligand-free state, for which CEST and R-1 rho RD profiles of base and sugar carbons revealed slow exchange dynamics involving a sparsely populated (p similar to 10%) and shortly lived (tau similar to 10 ms) NMR "invisible" state. The utility of CEST and low spin-lock field R-1 rho RD experiments in studying slow exchange was further validated in characterizing an exchange as slow as similar to 60 s(-1).