Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/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 (unit)|tesla magnetic field that may be found in high resolution laboratory 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.

Applications

Applications of EFNMR include:

• Proton precession magnetometers (PPM) or proton 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 of NMR spectroscopy 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.

• Earth's field MRI scanners, which use the principle of magnetic 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.

Examples (illustrated) are the TeachSpin and Terranova MRI instruments.

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

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.

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 (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.

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. These frequencies are in the ULF radio frequency band.

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

History

Early NMR instruments were developed in the 1950s using thermionic valve (vacuum tube) circuits (see for example Scientific American's, Amateur Scientist, by C. L. Stong April, 1959). 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.