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Earth's field NMR



Nuclear magnetic resonance (NMR) in the geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR). Note that the same acronym is used for electric field NMR.

EFNMR is a special case of low field NMR.

When placed in a constant magnetic field and stimulated (perturbed) by a pulsed or alternating magnetic field, NMR active nuclei (such as 1H or 13C) resonate at frequencies characteristic of the isotope. The resonant frequencies and signal strengths are proportional to the strength of the applied magnetic field. Thus in the 21 tesla magnetic field that may be found in high resolution laborotory NMR spectrometers, protons resonate at 900 MHz. However in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz, generating very weak signals.

The location of a nucleus within a complex molecule affects the chemical environment experienced by the nucleus. Thus different hydrocarbon molecules containing NMR active nuclei in different positions within the molecule produce slightly different patterns of resonant frequencies. Analysis of the frequency spectrum allows the structure of the molecule to be determined.

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Applications

A common application of EFNMR is the proton precession magnetometer (PPM), which produces magnetic resonance in a known sample in the magnetic field to be measured, measures the sample's resonant frequency, then calculates and displays the field strength. Another application is the EFNMR spectrometer, used to analyse molecular structures in a variety of applications, from investigating the structure of ice crystals in polar ice-fields, to rocks and hydrocarbons in the field.

Advantages of EFNMR spectrometers over conventional (high field strength) NMR spectrometers include the portability of the equipment giving the ability to analyse substances in situ, and their lower cost. The much lower geomagnetic field strength, that would otherwise result in poor signal-to-noise ratios, is compensated by homogeneity of the Earth's field giving the ability to use much larger samples. Their relatively low cost and simplicity make them good educational tools. Examples (illustrated) are the TeachSpin and Terranova MRI instruments.

Mode of operation

Free Induction Decay (FID) (or Larmor precession) is the magnetic resonance 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 field method of stimulating FID is usual in EFNMR spectrometers and PPMs.

Since the FID resonant frequency of NMR active nuclei is directly proportional to the magnetic field affecting those nuclei, we can use widely available NMR spectropscopy data to analyse suitable substances in the Earth's magnetic field.

For more context and an explanation of NMR principles, please refer to the main articles on NMR and NMR spectroscopy.

Proton EFNMR frequencies

The geomagnetic field strength and hence precession frequency varies with location and time.

Larmor precession frequency = magnetogyric ratio x magnetic field
Proton magnetogyric ratio = 42.576 Hz/μT
Earth's magnetic field: 30 μT near Equator to 60 μT near Poles, around 50 μT at mid-latitudes.

Thus proton (hydrogen nucleus) EFNMR frequencies are audio frequencies of about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes.

Examples of molecules containing hydrogen nuclei useful in proton EFNMR are water, hydrocarbons such as natural gas and petroleum, and carbohydrates.

History

Early EFNMR instruments were developed in the 1950s using thermionic valve (vacuum tube) circuits. Sir Peter Mansfield's first acquaintance with NMR was an undergraduate project to develop a transistorized EFNMR spectrometer in the late 1950's [1]. Following that introduction to NMR, he went on to invent an MRI scanner, for which he shared a Nobel prize.

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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Earth's_field_NMR". A list of authors is available in Wikipedia.
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