As for obtaining the kinetic isotope effects, mixtures of substrates containing stable isotopes may be analyzed using a mass spectrometer, which yields the ratios of the isotopic molecules in the initial substrate ( defined here as [ A 2 ] 0 / [ A 1 ] 0 = R 0 ), in the substrate after some conversion ( [ A 2 ] / [ A 1 ] = R ), or in the product ( [ P 2 ] / [ P 1 ] = R P ).