3B Scientific Teltron NMR Supplementary Set User Manual

Page 4

7.4 Experiment procedure
• Insert the glycerine sample (yellow top) into

the sample chamber (see Fig. 9).

Fig. 9 Basic unit with glycerine sample inserted

• Set a frequency of about 13 MHz on the

control console (since the frequency knob is

a 10-turn potentiometer, it may need to be

turned by multiple revolutions).

• Set the sensitivity to medium and adjust if

necessary.

At the optimum setting, the LED can be seen to

flicker slightly. If the LED lights up fully, the sig-

nal is overloaded.
• Carefully adjust the fine setting using the

frequency selector knob seeking out a peak

in the signal between about 1 ms to 1.5 ms

in width.

Note:

When looking for the peak, it can be helpful to

loosen the knurled screws a bit to change the

strength of the magnetic field and thus the sig-

nal. In order to optimise the signal refer to secti-

on 7.5.
• Vary the frequency to bring it to the middle

of the peak and write down what that fre-

quency is.

• Carry out the experiment again with different

material samples.

For the polystyrene sample (green top) the fre-

quency will be in the same range as for the

glycerine sample. For the Teflon sample (blue

top) the frequency will be lower (see Figs. 13 to

15).
Another experiment can be carried out in which

the stalk of a plant can be inserted into the sam-

ple chamber for its resonant frequency to be

determined.

7.5 Optimisation of signal
If the signal is fuzzy (width of signal > 2ms),

there are several ways it can be improved. The

basic requirement for this is that some kind of

signal, no matter how poor its quality, needs to

be obtained for the glycerine sample. The objec-

tive is to obtain a signal with a median width of 1

ms.

7.5.1

Use the two knurled screws to vary the

pressure on the assembly discs and observe the

signal as you do so. It may be necessary to

tighten the two screws to differing degrees.
7.5.2

Pull the probe out some of the way (up to

5 mm) and observe the signal.
7.5.3

Slightly loosen the two knurled screws and

move the magnets about 1 to 2 mm away from

their end positions. To do this, use your thumbs

to push back the two coils while resting your

fingers on the base unit (Fig. 10). Tighten the

knurled screws while observing the peak.

Fig. 10 Moving the magnets

Fig. 11 Magnet moved out of its end position

7.5.4

Slightly loosen the two knurled screws and

move the magnets about 1 to 2 mm away from

their end positions, then move them back to the

ends. In doing so, move the two discs a little

further forward. Tighten the knurled screws

while observing the peak.

7.6 Evaluation
Resonant frequencies of material samples
Glycerine (

H) 42.58

MHz/T

Polystyrene (

H) 42.58

MHz/T

Teflon (

F) 40.06

MHz/T

Plant stalk (

H) 42.58

MHz/T

The following therefore applies:

Polystyren

Glycerine

Teflon