The use 100 of selective RF excitation is discussed. A spectrally selective, large tip angle RF pulse is used 104 to detect a weak signal from a first chemical species. A non-selective, small tip angle pulse is used in measuring a signal from a second chemical species in 110 exchange with the first chemical species. The acquired resonance data 108, 116 provides may be used to measure the ratio of the spectrally distinct species in exchange.