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Earth's field NMR

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This article is about Earth's field NMR (EFNMR). For Electric field NMR (same acronym EFNMR), seeElectric field NMR.

Nuclear magnetic resonance(NMR) in thegeomagnetic fieldis conventionally referred to asEarth's field NMR (EFNMR).EFNMR is a special case oflow field NMR.

When a sample is placed in a constant magnetic field and stimulated (perturbed) by a time-varying (e.g., pulsed or alternating) magnetic field, NMR active nuclei resonate at characteristic frequencies. Examples of such NMR active nuclei are theisotopescarbon-13andhydrogen-1(which in NMR is conventionally known asproton NMR). The resonant frequency of each isotope is directly proportional to the strength of the applied magnetic field, and themagnetogyric or gyromagnetic ratioof that isotope. The signal strength is proportional to the stimulating magnetic field and the number of nuclei of that isotope in the sample. Thus, in the 21teslamagnetic field that may be found in high-resolution laboratoryNMR spectrometers,protons resonate at 900 MHz. However, in the Earth's magnetic field the same nuclei resonate at audio frequencies of around 2 kHz and generate feeble signals.

The location of a nucleus within a complex molecule affects the 'chemical environment' (i.e. the rotating magnetic fields generated by the other nuclei) experienced by the nucleus. Thus, differenthydrocarbonmolecules containing NMR active nuclei in different positions within the molecules produce slightly different patterns of resonant frequencies.

EFNMR signals can be affected by magnetically noisy laboratory environments and natural variations in the Earth's field, which originally compromised its usefulness. However, this disadvantage has been overcome by the introduction of electronic equipment which compensates changes in ambient magnetic fields.

Whereaschemical shiftsare important in NMR, they are insignificant in the Earth's field. The absence of chemical shifts causes features such as spin-spin multiplets (separated by high fields) to be superimposed in EFNMR. Instead, EFNMR spectra are dominated by spin-spin coupling (J-coupling) effects. Software optimised for analysing these spectra can provide useful information about the structure of the molecules in the sample.

Applications[edit]

Applications of EFNMR include:

  • Proton precession magnetometers (PPM) orproton magnetometers,which produce magnetic resonance in a known sample in the magnetic field to be measured, measure the sample's resonant frequency, then calculate and display the field strength.
  • EFNMR spectrometers, which use the principle ofNMR spectroscopyto analyse molecular structures in a variety of applications, from investigating the structure of ice crystals in polar ice-fields, to rocks and hydrocarbons on-site.
  • Earth's fieldMRIscanners, which use the principle ofmagnetic resonance imaging.

The advantages of the Earth's field instruments over conventional (high field strength) instruments include the portability of the equipment giving the ability to analyse substances on-site, 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.

Although those commercial EFNMR spectrometers and MRI instruments aimed at universities etc. are necessarily sophisticated and are too costly for most hobbyists, internet search engines find data and designs for basic proton precession magnetometers which claim to be within the capability of reasonably competent electronic hobbyists or undergraduate students to build from readily available components costing no more than a few tens of US dollars.

Mode of operation[edit]

Free induction decay(FID) is the magnetic resonance due toLarmor precessionthat results from the stimulation of nuclei by means of either apulsed dc magnetic fieldor apulsed 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 electronicamplifiersare required to amplify the EFNMR signals to usable levels. The stronger the polarising magnetic field, the stronger the EFNMR signals and the better thesignal-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 theEarth'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 heteronuclearJ-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."[1]
  • Chemical shiftsof several parts per million (ppm) are clearly separated in high field NMR spectra, but have separations of only a few millihertz 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 onNMRandNMR spectroscopy.For more detail seeproton NMRandcarbon-13 NMR.

Proton EFNMR frequencies[edit]

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

Larmor precession frequency =magnetogyric ratiox magnetic field
Proton magnetogyric ratio= 42.576 Hz/μT (also written 42.576 MHz/T or 0.042576 Hz/nT)
Earth's magnetic field: 30 μT near Equator to 60 μT near Poles, around 50 μT at mid-latitudes.

Thusproton(hydrogen nucleus) EFNMR frequencies areaudio frequenciesof about 1.3 kHz near the Equator to 2.5 kHz near the Poles, around 2 kHz being typical of mid-latitudes. In terms of theelectromagnetic spectrumEFNMR frequencies are in theVLFandULFradio frequencybands, and theaudio-magnetotelluric(AMT) frequencies ofgeophysics.

Examples of molecules containing hydrogen nuclei useful in proton EFNMR arewater,hydrocarbonssuch asnatural gasandpetroleum,andcarbohydratessuch as occur inplantsandanimals.

See also[edit]

References[edit]

  1. ^Robinson J. N.; et al. (2006)."Two-dimensional NMR spectroscopy in Earth's magnetic field"(PDF).Journal of Magnetic Resonance.182(2): 343–347.Bibcode:2006JMagR.182..343R.doi:10.1016/j.jmr.2006.06.027.PMID16860581.

External links[edit]

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