2. 1 Q. If your NMR spectrum has a distorted base line, what all
could be the problem ?
• Ans: If NMR spectrum base line distorted, some unwanted
difficulties are:
• 1.Small signals in the presences of large signals.
• 2. Spectral integration
• 3.Difference spectra
• 4. Water saturation
• 5.2D experiments, especially NOESY.
In some cases “ base line “ distortion can arise from
real NMR signals, which can be wider than the observation
sweep width in FID(free induction decay).These corrupted
data points add low frequency modulations in the Fourier
transformed spectrum, and thus formed the distorted base
line.
3. Some sources of these instruments baseline distortions are
1. Digitizer overload(clipping)
2. Preamp and or receiver overload
3. Audio filters
4. Slow recovery of RF stage from over load
5. Probe acoustical ringing.
2 Q. Why is the DSS used instead of TMS as the 1H Chemical shift
reference for a biological samples?
Ans: DSS has a higher solubility in aqueous solution.DSS has a pH
dependent shift and can associate with other solute molecules. In
proton NMR of aqueous solutions, DSS and TSP are both
susceptible to changes in their shift at low sample pH, when they
become protonated.They are also capable of intermolecular
interaction with other solute molecules. Sodium trimethyl silyl
propane sulfonate (DSS) is the organ silicon compound .
4. A white water soluble solid ,it is used as a chemical shift standard
for proton NMR spectroscopy of aqueous solutions. The proton
spectrum of DSS also exhibits peaks at 2.91 ppm(m),1.75 ppm(m)
and 0.63 ppm (m) at an intensity of 22% of the reference peak at o
ppm.
5. 3 Q. Why the c13 chemical shift has range of approximately 300
ppm,where as 1H has only 10 ppm ?
Ans: Chemical shift depends on the net magnetic field felt by the
nuclei (H,C).The net magnetic field, in turn depends upon the
electron density possessed by the particular atom(shielding
constant).In the case of hydrogen there is just a single electron and
by co-ordination with others(H-F,CH3-F) leads to the slight
change in electron density around the hydrogen nuclei there fore
results in the small range of chemical shift.
6. However carbon having six electron being tetravalent as well as
attached to diverse functionalities leads to the considerable
change in electron density around the carbon nuclei there by
,possess broader range of chemical shift values.
Basically H-NMR can give you lot of in formations
because of the multiplicity of the signals. In C-NMR instead
generally you can not observe multiplicity of the signals because
of the low natural isotopic abundance of c13 which is 1.1%.
4 Q. When is a high power rectangular RF pulse used? What
parameters with respect to the RF pulse are needed to be
specified?
Ans: When we apply high power amplifier that generates the
large current necessary to drive the RF-coils. The amplifiers used
in modern MR systems typically produce peak power in the range
of 10-30 KW resulting in maximum transmitted B1 fields on the
order of 10-50uT. Some parameters needs to change like transmit
attenuation, transmit gain, RF gain,reference amplitude,RFlevels.
