!"# ! 2 between the reference protonated molecule and its major isotope (𝑖 = 1 or 𝑖 = 2) and the observed intensity values in the matching peaks in the mixture spectrum, where matching implies a peak in the mixture spectrum within a mass tolerance window (±𝜖) and with signal intensity above the noise threshold. If there is no peak matching the protonated molecule in the mixture spectrum, the LFPM Isotope Ratio Difference is reported as NA.