formula_27 is the chemical shift of the "i"th chemical species which contains the nucleus and formula_28 is the concentration /mole fraction (formula_29 "c" is a concentration / mol dm−3) 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 1H,13C and 31P. It is usual to use a deuterated solvent when measuring 1H chemical shift values.