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Nuclear magnetic resonance (NMR) in thegeomagnetic field is 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-13 andhydrogen-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 ratio of 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 the21 tesla magnetic field that may be found in high-resolution laboratoryNMR spectrometers, protons resonate at900 MHz. However, in the Earth's magnetic field the same nuclei resonate at audio frequencies of around2 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, differenthydrocarbon molecules 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 shifts are 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 of EFNMR include:
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 are too costly for most hobbyists (one simple instrument is $9400 as of March 2025),[1] 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. Two open source designs are available with PCB layouts and firmware for megaAVR microcontrollers, as well as a circuit layout and design in a peer reviewed journal.[2][3]
An optical earth field spectrometer has also been described.[4]
Free induction decay (FID) is the magnetic resonance due toLarmor 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 electronicamplifiers are 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:
For more context and explanation of NMR principles, please refer to the main articles onNMR andNMR spectroscopy. For more detail seeproton NMR andcarbon-13 NMR.
The geomagnetic field strength and hence precession frequency varies with location and time.
Thusproton (hydrogen nucleus) EFNMR frequencies areaudio frequencies of about1.3 kHz near the Equator to2.5 kHz near the Poles, around2 kHz being typical of mid-latitudes. In terms of theelectromagnetic spectrum EFNMR frequencies are in theVLF andULFradio frequency bands, and theaudio-magnetotelluric (AMT) frequencies ofgeophysics.
Examples of molecules containing hydrogen nuclei useful in proton EFNMR arewater,hydrocarbons such asnatural gas andpetroleum, andcarbohydrates such as occur inplants andanimals.
Earth's Field NMR with Gradient Field Coils - $9,400.00