With nuclear magnetic resonance (NMR) spectra the observed chemical shift value, , arising from a given atom contained in a reagent molecule and one or more complexes of that reagent, will be the concentration-weighted average of all shifts of those chemical species. Chemical exchange is assumed to be rapid on the NMR time-scale. 
terms of mole fractions,
 is the chemical shift of the ith chemical species which contains the nucleus and  is the concentration /mole fraction ( c is a concentration / mol dm) of that species. This expression has the same mathematical form as Beer's law. Chemical shift values may be obtained for more than one nucleus in an analogous way that absorbance may be measured at more than one wavelength. Typical isotopes that may be used in this context are H,C and P. It is usual to use a deuterated solvent when measuring H chemical shift values.