Earth's Field NMR Gradient/Field Coil System by Teachspin: The Instrument

Earth's Field NMR Gradient/Field Coil System

The EFNMR Gradient/Field Coil System has two types of field coils, Gradient and Helmholtz. The coil system is mounted in a non-magnetic frame that allows the z-axis to be aligned along the ambient field. This is accomplished with the aid of a permanent magnet dip needle placed inside the coils during alignment.

The magnitude and direction of the current passing through each of the three gradient coils is controlled by a 10 turn potentiometer on the front panel of the controller. These currents can be individually monitored by measuring the voltage across a built-in 0.1 ohm standard resistor in series with the coil selected on the front panel.

Fig. 1: FID in Ambient Earth's Field

Fig. 2: FID Optimized by Gradient Coils

The primary function of the three gradient coils is to cancel the three relevant first-order gradients in the local magnetic field:
It has often been difficult for instructors to locate a space within a teaching laboratory where the local Earth's magnetic field is sufficiently uniform over the sample. In such environments, the free-induction decay (FID) time is limited by non-uniformity of the magnetic field rather than by intrinsic spin-spin interactions within the sample. Severe local field gradients may cause the precession signal to decay to an imperceptible level in a time comparable to the 50 msec ring-down time after the polarization field has been turned off, making the FID impossible to observe.

Using the gradient coils at the TeachSpin development laboratory dramatically improved our signals. Consider the free-induction decay signal shown at the above right in Figure 1. This signal, from a distilled water sample, was taken in the ambient magnetic field of one of our "best" locations at the TeachSpin development laboratory. Now, examine the signal shown below, taken in the same location with the same sample. Here, the currents in the three gradient-coils were adjusted to maximize the decay time of the FID signal. The ambient magnetic field had been made more homogeneous by a factor of about twenty!

Control of the magnetic field gradients also allow the creation of deliberate one dimensional gradients of the magnetic field to be imposed along the sample. This, in turn, maps spatial locations in the sample to precession-signal location in frequency space. Using TeachSpin's "Segmented Sample Holder," and a user supplied Fourier transform analysis, students can perform experiments in one-dimensional magnetic resonance imaging.

The Helmholtz coils, which can be accurately modeled from their geometry, can produce a uniform magnetic field of up to 270 micro tesla which can be used to change the magnitude of the ambient Earth's magnetic field across the sample. This increased range of magnetiac fields allows more nuclei to be brought into the tuning range (1.6 - 2.5 KHz) of the apparatus. Now students can experiment with nuclei including ³¹P, and ²H.

Learn about Earth's Field NMR.