MRS provides a non-invasive window into an in vivo biochemistry – basically a virtual biopsy. Here, we present two novel MRS sequences (SISTEM-1 and 2) and method to selectively prepare and detect the signal of endogenous metabolites. The long-lived signal of NAA-CH2 protons was isolated by the strong suppression of H2O and lipid signals using broad-band saturation or double-quantum filter. The frequency of the modulated NAA-CH2 signal was measured and associated with a pH value. Non-invasive measurement of pH value, imaging of drug distribution are only some of the possible applications.