Background
The sensitivity of NMR signal detection depends on the gyromagnetic ratio (γ) of the nucleus. In general, the signal intensity produced from a nucleus with a gyromagnetic ratio of γ is proportional to γ3 because the magnetic moment, the Boltzmann populations, and the nuclear precession all increase in proportion to the gyromagnetic ratio γ. For example, the gyromagnetic ratio of 13C is 4 times lower than the proton, so the signal intensity it produced will be 64 times lower than that of a proton. However, noise also increases as the square root of the frequency, the sensitivity therefore becomes roughly proportional to γ5/2. A 13C nucleus would be 32 times less sensitive than a proton, and 15N around 300 times less sensitive. Sensitivity enhancement techniques are therefore desirable when recording an NMR signal from an insensitive nucleus.
The sensitivity can be enhanced by artificially by increasing the Boltzmann factors. One method may be through nuclear Overhauser effect (NOE), for example, for 13C signal, the signal-to-noise ratio can be improved three-fold when the attached protons are saturated. However, for NOE, a negative value of K may result in a reduction in signal intensity; for 15N which has a negative gyromagnetic ratio, if the dipolar relaxation has to compete with other mechanisms, the observed 15N signal can be near zero. Alternative methods are therefore necessary for nuclei with a negative gyromagnetic ratio, and one such method using the INEPT pulse sequence was proposed by Ray Freeman in 1979.
Read more about this topic: Insensitive Nuclei Enhanced By Polarization Transfer
Famous quotes containing the word background:
“They were more than hostile. In the first place, I was a south Georgian and I was looked upon as a fiscal conservative, and the Atlanta newspapers quite erroneously, because they didn’t know anything about me or my background here in Plains, decided that I was also a racial conservative.”
—Jimmy Carter (James Earl Carter, Jr.)
“I had many problems in my conduct of the office being contrasted with President Kennedy’s conduct in the office, with my manner of dealing with things and his manner, with my accent and his accent, with my background and his background. He was a great public hero, and anything I did that someone didn’t approve of, they would always feel that President Kennedy wouldn’t have done that.”
—Lyndon Baines Johnson (1908–1973)
“In the true sense one’s native land, with its background of tradition, early impressions, reminiscences and other things dear to one, is not enough to make sensitive human beings feel at home.”
—Emma Goldman (1869–1940)