A brief description of NMR sub topics regarding to Volume and Concentration, Consideration in NMR Spectroscopy, Biomolecule samples, Small molecule samples, Sample volume, Spectrometer setup, NMR instrumentation, Deuterium Lock, Probe Tunning for optimizing NMR signal, Sample Tube Placement

2. NMR sub topics
• Hajira Mahmood
• Ph.D. Scholar (Chemistry)
• Analyst
• Chemist

3. Volume andConcentration Consideration
in NMR Spectroscopy
• Biomolecule samples. Prepare at least 0.05mm. higher than 1 mm is
preferred as long as solution viscosity does not increase significantly.
• Small molecule samples. About 3mm is sufficient for 13C direct NMR
measurements on CP800, and10mM for 13C measurements on CP600. 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. Sample volume
 should be a minimum of 250 µL for a Shigemi tube, 160µL for a
3mm/5mm tube, or 500 µL for a regular NMR tube. Insufficient volume
may cause bad shimming result, and consequently spectra with poor
resolution and 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. NMR instrumentation

7. DeuteriumLock
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) and 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 center of the applied
magnetic field. The probe holds the:
 Sample and sample spinning apparatus
 Radio frequency (RF) coils
 Temperature controller
 Detector coils

9. Probe Tunning
 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 and 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 sometimes recommended.
 However, too concentrated solution leads to lower resolution due to saturation and/or increased
viscosity.
 For 13C and other nuclei ca. five times the concentration of 1H is recommended (20-100 mg.