Teachspin - Earth's Field NMR: Experiments with a Low-Field NMR

Earth's Field NMR

Earth's Field NMR FINDING THE PRECESSION SIGNAL

The student's first job is to find the precession signal by adjusting the coil's tuning capacitor and tuning the amplifier. A typical signal from a water sample doped with CuSO₄ is shown in Figure 2. The student has two apparent experimental parameters to play with: (1) the polarizing field, (2) the polarization time.

POLARIZATION DEPENDS ON MAGNETIC FIELD STRENGTH

For a fixed polarization time, students can quickly discover the linear relation between polarization field and maximum signal amplitude. That's Curie's Law. A graph of student data is shown in Figure 3.

"DISCOVERING" SPIN LATTICE RELAXATION TIME

Examination of the maximum signal amplitude as a function of polarization time, for a fixed polarization field, yields surprising and important data for the student to ponder. For times longer than about ten seconds, the signal does not change with increasing polarization time; saturated magnetization. For shorter times, the signal decreases, but obviously not linearly.

A graph for Buffalo tap water is shown above. Analysis of these data shows it is mathematically described by the equation:

M(t) = M_sat(1- e^{-
a
t}) or ln(M_sat - M(t)) = -a t + lnM_sat

A plot of the natural log of the difference between the saturation magnetization and the magnetization at time t, versus the polarization time, yields a straight line of slope a. Alpha is the reciprocal of the spin-lattice relaxation time, T₁. This time can be dramatically changed by the addition of paramagnetic ions, such as copper sulfate.

EXPERIMENTING WITH NON-TOXIC FLUORINE SAMPLES

TeachSpin's apparatus is also capable of detecting fluorine nuclear precession. Various fluorinated liquids are available from TeachSpin with different relaxation times. A particularly interesting sample of fluorobenzene exhibits a pronounced beat signal on the proton's free precession. The beats are due to the proton-fluorine spin-spin coupling. A measure of the beat frequency accurately determines the J-coupling between the proton and fluorine spins.

Learn about the Earth's Field NMR Gradient/Field Coil System.