Mode of Operation
Free Induction Decay (FID) is the magnetic resonance due to Larmor precession that results from the stimulation of nuclei by means of either a pulsed dc magnetic field or a pulsed resonant frequency (rf) magnetic field, somewhat analogous respectively to the effects of plucking or bowing a stringed instrument. Whereas a pulsed rf field is usual in conventional (high field) NMR spectrometers, the pulsed dc polarising field method of stimulating FID is usual in EFNMR spectrometers and PPMs.
EFNMR equipment typically incorporates several coils, for stimulating the samples and for sensing the resulting NMR signals. Signal levels are very low, and specialised electronic amplifiers are required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better the signal-to-noise ratios. The main trade-offs are performance versus portability and cost.
Since the FID resonant frequencies of NMR active nuclei are directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectroscopy data to analyse suitable substances in the Earth's magnetic field.
An important feature of EFNMR compared with high-field NMR is that some aspects of molecular structure can be observed more clearly at low fields and low frequencies, whereas other features observable at high fields may not be observable at low fields. This is because:
- Electron-mediated heteronuclear J-couplings (spin-spin couplings) are field independent, producing clusters of two or more frequencies separated by several Hz, which are more easily observed in a fundamental resonance of about 2 kHz. "Indeed it appears that enhanced resolution is possible due to the long spin relaxation times and high field homogeneity which prevail in EFNMR."
- Chemical shifts of several ppm are clearly separated in high field NMR spectra, but have separations of only a few milliherz at proton EFNMR frequencies, and so are undetectable in an experiment that takes place on a timescale of tenths of a second.
For more context and explanation of NMR principles, please refer to the main articles on NMR and NMR spectroscopy. For more detail see proton NMR and carbon-13 NMR.
Read more about this topic: Earth's Field NMR
Famous quotes containing the words mode of, mode and/or operation:
“Literature exists at the same time in the modes of error and truth; it both betrays and obeys its own mode of being.”
—Paul Deman (1919–1983)
“I have no scheme about it,—no designs on men at all; and, if I had, my mode would be to tempt them with the fruit, and not with the manure. To what end do I lead a simple life at all, pray? That I may teach others to simplify their lives?—and so all our lives be simplified merely, like an algebraic formula? Or not, rather, that I may make use of the ground I have cleared, to live more worthily and profitably?”
—Henry David Thoreau (1817–1862)
“You may read any quantity of books, and you may almost as ignorant as you were at starting, if you don’t have, at the back of your minds, the change for words in definite images which can only be acquired through the operation of your observing faculties on the phenomena of nature.”
—Thomas Henry Huxley (1825–95)