What is Earth’s Field NMR?
Conventional high field NMR instruments require an extremely uniform magnetic field for polarisation of nuclear spins and the detection of the resultant magnetisation. It is difficult and expensive to make a magnetic field of the required homogeneity (much less than 1 ppm in some cases) and so sample sizes are usually quite small. However, because the polarisation and detection fields are large, the resulting signal to noise ratio (S/N) can also be large. Earth’s field NMR (EFNMR) uses the globally available, homogeneous Earth’s magnetic field for detection. The homogeneity of this field means that large samples can be used – partially compensating for the very low detection field. To improve the initial magnetization, a relatively crude electromagnet with a field about 350 times larger than the Earth’s field is used to polarise the sample. This pre-polarization method greatly enhances the S/N of the EFNMR signal.
Why use Earth’s Field NMR?
Two of the most significant advantages of EFNMR over conventional NMR systems for teaching and some research applications are low purchase and operational costs and portability. This is because a large homogeneous magnet is not required for an EFNMR system. Despite the low detection field in EFNMR, the pre-polarisation methodology sufficiently increases sensitivity such that it is possible to perform a large range of modern and sophisticated NMR techniques. For example, the Terranova-MRI Earth’s field system can be used to acquire spin echoes or CPMG (Carr-Purcell-Meiboom-Gill) echo trains. It can measure T1 and T2 relaxation times and can employ the methods of phase cycling and signal averaging. A pulsed gradient coil can be used in conjunction with the Terranova EFNMR system to measure diffusion coefficients using the PGSE (pulsed gradient spin echo) method.
Teaching and training using EFNMR
Despite the inherently low sensitivity of EFNMR when compared to a high-field system, there are advantages to teaching and training NMR at very low magnetic fields. One advantage is the relative simplicity of a low-field system. The Earth’s field has a magnitude on the order of 0.05 mT and therefore the Larmor frequency for 1H is a couple of kHz. At higher fields, and hence higher Larmor frequencies – on the order of many MHz, the NMR signal is mixed down with intermediate frequencies before sampling because direct sampling of signals over 20 MHz is very difficult. In the case of EFNMR, no mixing is necessary and so the observed signal is simply an amplification of the actual signal induced in the coil. This is advantageous because it allows students to directly observe the NMR signal and to explore properties such as signal phase in a more intuitive manner. In addition, the low-frequency excitation pulse can be observed directly on most laboratory oscilloscopes, thus providing the student with an interactive means of observing directly the physical meaning of such NMR parameters as the “phase”, “power”, “frequency” and “duration” of the excitation pulse.