Sub topics of NMR spectroscopy.Volume and Concentration Consideration in NMR Spectroscopy,Spectrometer setup,Deuterium Lock,Probe Tunning for optimizing NMR signal,Sample Tune Placement,

2. Sub topics of NMR spectroscopy
By
Hajira Mahmood
Ph.D. Scholar

3. Volume andConcentration Consideration
in NMR Spectroscopy
• Biomolecule samples.
• Prepare at least 0.05mm. higher than 1 mm , preferred as long as solution viscosity
does not increase significantly.
• Small molecule samples.
• About 3mm is sufficient for 13C direct NMR measurements, and10mM for 13C
measurements. If MW is 200, 10mg/500uL gives 100mM concentration.
• Note that for quantification, avoid preparing too high concentration which may
cause baseline and line shape issues.

4. Conti………
Sample volume should be minimum of 250 µL for a Shigemi tube
 160µL for a 3mm/5mm tube
 500 µL for a regular NMR tube.
Insufficient volume may cause bad shimming result, consequently spectra with
poor resolution & line shape.
In many cases the sample volume should weight more than the concentration
for a good NMR measurement.
The NMR sample should be homogeneous, free of air bubbles and insoluble
substances. High salt concentration and paramagnetic ions should be
avoided

5. Spectrometer setup
A typical NMR spectrometer setup consists of a superconducting
magnet, a probe, a console, and a computer.
The superconducting magnet is often a solenoid made from niobium
titanium alloy.
A large current flows around the loop, creating the strong continuous
magnetic field required to align the nuclei
The sample is placed in an NMR tube and dropped into the magnet on a
cushion of compressed air.
The strong magnetic field causes the nuclei to become aligned or
opposed to it.

6. Conti….

7. Deuterium Lock
The lock system keeps the spectrometer operating at a constant net
magnetic field.
Usually the liquid NMR uses deuterium solvent for stabilizing magnet
field (lock) & shimming (gradient shimming).
If you have to use samples without deuterium solvent, you are not able
to lock the field.
The magnet field may drift during the accumulation, resulted in broad
lines.

8. Probe Tunning for optimizing NMR signal
The probe is at the heart of an NMR spectrometer, in the centre of the
applied magnetic field. The probe holds the:
Sample and sample spinning apparatus
Radio frequency (RF) coils
Temperature controller
Detector coils

9. Conti…
The term tuning involves two connected activities:
Tuning the coil to the required frequency
Matching the coil to the correct impedance ensuring its efficient
operation
The circuit requiring tuning is composed of inductive (L) and
capacitive (C) elements and the L and C values determine the
frequency of the RF circuit.
A circuit is tuned by adjusting the capacitor values until the required
frequency is reached.

10. Sample Tune Placement
An NMR tube is a thin glass walled tube used to contain samples in nuclear magnetic
resonance spectroscopy.
Typically NMR tubes come in 5 mm diameters but can also range from 10 mm to
3 mm samples.
 It is important that the tubes are uniformly thick & well-balanced to ensure that NMR
tube spins at a regular rate (i.e. that they do not wobble usually about 20 Hz in an NMR
spectrometer.
For 1H, 0.5 - 5.0 mg of the sample is sufficient.
For high molecular weight samples, more concentrated solutions are recommended.
However, too concentrated solution leads to lower resolution due to saturation and/or
increased viscosity.
For 13C and other nuclei five times the concentration of 1H is recommended (20-100
mg.