The Earth field NMR device has been developed in-house based on the design presented in . It is built in a way that it can be used in a normal laboratory environment with disturbances from electromagnetic fields and deterioration of the field homogeneity by steel in reinforced concrete or furniture. Self compensation against external alternating fields is achieved by a first order gradiometer construction. The temperature of the sample is kept constant (+/- 1°C) by means of temperated air.
The aluminium shielding box with 3 Maxwell pairs and polarisation/detection coil in the center:
Although the Earth’s magnetic field is very homogeneous, it is disturbed in the laboratory environment. This disturbance can be minimized by an appropriate location in the laboratory. The probe head of the NMR device is placed in the dark blue zone in the figure below.
The rest inhomogeneity can be almost completely cancelled out by shimming using three pairs of Maxwell coils. During the shimming process, the current through each Maxwell pair is adjusted till a maximum signal (long T2*) is achieved. The picture belows shows a single FID of 25 ml water with and without shimming. Because the Larmor frequency of the protons in the Earth's magnetic field is about 2 kHz, the FID can be heard: FID of 25 ml water.
The poor sensitivity arising from the low Larmor frequency is compensated by enhancing the initial magnetisation by means of pre-polarizing (Method of Packard and Varian ), represented in the figure below:
This method also allows to measure the longitudinal relaxation time by changing the polarisation time tp. By varying the polarisation Bp, T1 can be measured at different frequencies (Field Cycling Relaxometry). With the Earth Field NMR device, T1 can be measured from 3 kHz up to 3 MHz. An example of the longitudinal relaxation time dispersion of a MnCl2 solution is given in the figure below:
Some advantages over conventionel NMR systems are:
Low manufacturing and operational costs (magnet is not required)
Due to the homogeneity of the Earth‘s field relatively large samples can be used (max. sample volume = 25 ml)
The magnetization decay can be analysed with resolution of 250 µs
Accessibility of the sample makes temperature and drying experiments possible
T1 dispersion and T2 can be measured with the same apparatus