Chemical shift - Definition and Overview

In Magnetic_Resonance_Imaging, MRI, chemical shift is caused by slight variations in the precession frequency of nuclei due to bonding. This is especially useful in chemical analysis because the precession frequency of each different bonding environment can easily be separated.

Chemical shift is usually expressed in parts per million (ppm), because it is calculated from:

<math>\delta = \frac{DifferenceInPrecessionFrequencyBetweenTwoNuclei}{OperatingFrequencyOfTheMagnet}<math>.

Since the numerator is usually in Hz, and the denominator in MHz, delta is expressed in ppm. Chemical shift was a concept invented to make the shift in precession frequency between two nucleii independent of the strength of the magnet. TMS (tetra-methyl silane) is commonly used today as the definition of chemical shift zero.

The operating frequency of a magnet is calculate from the Larmor_equation: <math>F_{larmor} = \gamma * B_0<math>, where <math>B_0<math> is the actual strength of the magnet in units like Tesla_(unit), or Gauss, and <math>\gamma<math> is the gyromagnetic ratio of the nucleus being tested.