Isotopic shift

The isotopic shift (also called isotope shift) is the shift in various forms of spectroscopy that occurs when one nuclear isotope is replaced by another.
Atomic spectra.
Isotope shifts in atomic spectra are minute differences between the electronic energy levels of different isotopes of the same element. Today they are the focus of a multitude of theoretical and experimental efforts due to their importance for atomic and nuclear physics. If atomic spectra also have hyperfine structure the shift refers to the center of gravity of the spectra.
From a nuclear physics perspective, isotope shifts combine different precise atomic physics probes for studying nuclear structure, and their main use is nuclear-model-independent determination of charge-radii differences. 
There are two effects which contribute to this shift:
NMR spectroscopy.
In NMR spectroscopy, Isotopic effects on chemical shifts are typically small, far less than1 ppm the typical unit for measuring shifts. The NMR signals for and ("HD") are readily distinguished in terms of their chemical shifts. The asymmetry of the signal for the "protio" impurity in arises from the differing chemical shifts of and .
Vibrational spectra.
Isotopic shifts are best known and most widely used in vibration spectroscopy where the shifts are large, being proportional to the ratio of the square root of the isotopic masses. In the case of hydrogen, the "H-D shift" is (1/2)1/2 or 1/1.41. Thus, the (totally symmetric) C-H vibration for and occur at 2917 cm−1 and 2109 cm−1, respectively. This shift reflects the differing Reduced mass for the affected bonds.