NMR analysis Flashcards
basics
radiofrequency radiation to excite atoms with magnetic moments
atoms with odd masses/odd atomic numbers (1H, 13C, 31P) possess magnetic moments (also known as spin)
NMR theory
the RF radiation switched the atomic spins from being aligned, to being aligned against an applied magnetic field
the range of frequencies used for excitation, as well as the complex splitting patterns observed are inherently characteristic of the chemical structures being studied
NMR active isotopes
1H (99.985%) 7Li (92.58%) 13C (1.11%) 15N (0.37%) 23Na (100%) 27Al (100%) 31P (100%)
Spin-flips
The exact frequency of the r.f. required to cause the spin-flips depends upon:
- The strength of the magnetic field B0
- The isotope observed e.g. 1H, 13C, 15N etc
- The chemical environment of the nucleus
NMR applications
Important technique to characterise the exact structure of a compound
Can determine impurities, including enantiomers, all without separation
Useful for fingerprinting
Applied to quantitative analysis of drugs and metabolites
Can be applied to the direct multicomponent analysis of complex biological samples e.g. human urine and blood plasma
give 2 NMR advantages
The analytical technique for summative and specific molecular information
Provides much more information about molecular structure than any other technique
give 2 NMR disadvantages
Quite intensive, but now with high sensitivity benefits
Very expensive instruments which often require specialist staff
6 biomedical advantages of high-resolution NMR analysis
non-invasive technique for the study of metabolic diseases, toxicological processes and drug metabolism
Provides simultaneous multicomponent information regarding the metabolic status of biofluids and appropriate tissue sample extracts
Minimal sample preparation, fast analysis and therefore rapid sample throughout time
For components containing > 1 class of 1H nucleus, we observe > 1 signal per compound, facilitating rapid identification
A high degree of spectral dispersion and sensitivity
can identify molecules which we would not necessarily anticipate to be present in biological samples
NMR theory
Certain atomic nuclei act as atomic “spinning tops”, giving them properties associated with a magnetic vector
Common nuclei have odd atomic numbers and include 1H, 13C, 15N, and 19F
These nuclei, when placed in a magnetic field will tend to align with the field itself
The energy difference between nuclei aligned parallel to the field and antiparallel depends on the strength of the magnetic field, B0
delta E = hyB0
Where h is plank’s constant and y is a constant specific to each nucleus, called the magnetogyric ratio
Arrows represent alignment of spin
Pluses of RF radiation reverse the alignment of nuclear spin from the low energy state to the high energy state
Absorption of RF often called resonances as RF of the radiation matches the frequency at which the nuclei spin
The exact resonance frequency of a given magnetic nucleus depends on its chemical and therefore magnetic environment
When nuclei excited to the high spin state relax back to the ground state, radiation is emitted, which is measured
NMR instrumentation
Sample placed in magnetic field and RF pluses excite the nuclei
As the magnetic nuclei realign, the detector picks up the ratio signals due to relaxation
The RF pulse is repeated multiple times to distinguish the output signal from background noise
Like in IR, Fourier Transform protocols convert the radio output into NMR spectra which we can interpret
what is an NMR magnet
Niobium-tin-copper clad coil wound in the manner of a spool of thread. The current runs through this coil, and this gives rise to the magnetic field.
This coil is submerged in liquid helium
The liquid helium chamber is surrounded by liquid nitrogen
The sample and spinner are lowered using air from the top, down through the bore, until it rests within the probe top.
