Chemical shift

In nuclear magnetic resonance and its associated applications - NMR spectroscopy and magnetic resonance imaging, chemical shift consists of slight variations in the precession frequency of nuclei, and is considered better than the SI unit of frequency (Hertz) in that it is independent of the magnetic field strength used.

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

${\displaystyle \delta ={\frac {\mbox{difference in precession frequency between two nuclei}}{\mbox{operating frequency of the magnet}}}}$

Since the numerator is usually in hertz, and the denominator in megahertz, delta is expressed in ppm.

The detected frequencies (in Hz) are usually referenced against TMS (tetramethylsilane), which is given the chemical shift of zero.

The operating frequency of a magnet is calculate from the Larmor equation: ${\displaystyle F_{\mathrm {larmor} }=\gamma *B_{0}}$, where ${\displaystyle B_{0}}$ is the actual strength of the magnet in units like teslas or gauss, and ${\displaystyle \gamma }$ is the gyromagnetic ratio of the nucleus being tested. Chemical shifts are different for dfferenty types of atoms; fro example, in 1H-NMR methanol will give two peaks beacause there are two different types of hydrogen atoms in methanol molecule.

See also

• Nuclear magnetic resonance
• MRI
• Larmor equation
• Gyromagnetic ratio
• physics
• electromagnetism
• chemistry
• medicine
• materials science