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Analytical Chemistry > NMR > Flashcards

Flashcards in NMR Deck (12):
1

Basics

- Radiofrequency (RF) radiation used to excite atoms with magnetic moments
- Atoms with odd masses/odd atomic numbers possess magnetic moments (also known as spin)

2

what does RF radiation do

switches 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

3

NMR active isotopes

1H (99.985%) 7Li (92.58%) 13C (1.11%) 15N (0.37%) 23Na (100%) 27Al (100%) 31P (100%)

4

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
- the environment of the nucleus

5

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

6

NMR advantages

- The analytical technique for summative and specific molecular information
- Provides much more information about molecular structure than any other technique, i.e. its specificity is very high

7

NMR disadvantages

- Quite intensive, but now with high sensitivity benefits
- Very expensive instruments which often require specialist staff

8

biomedical advantages of high resolution NMR analysis

1) Virtually non-invasive technique for the study of metabolic diseases, toxicological processes and drug metabolism
2) Provides simultaneous multicomponent information regarding the metabolic status of biofluids and appropriate tissue sample extracts,
i.e > 80 metabolites detectable in human plasma
> 100 metabolites detectable in human urine
~ 80 metabolites detectable in human saliva
3) Minimal sample preparation, fast analysis and therefor rapid sample throughput time- acquisition of an 1H NMR spectrum of biofluid in 10-15 min
4) For components containing > 1 class of 1H nucleus, we observe > 1 signal per compound (couple if adjacent to each other ), facilitating rapid identification
5) A high degree of spectral dispersion and sensitivity (<= umol/l) at, for example, an operating frequency of 600-750 MHz
6) We can identify molecules which we would not necessarily anticipate to be present in biological samples e.g. methanol present in human saliva derived from direct or passive cigarette smoking

9

NMR theory

- Certain atomic nuclei act as atomic ‘spinning tops’ giving them properties associated with magnetic vector
- Common nuclei have odd atomic numbers and include 1H, 13C, 15N, and 19F, as well as many more
- 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

deltaE= hyBo (look on powerpoint for proper equation)
- Where h is plank’s constant and y is a constant specific to each nucleus, called the magnetogyric ratio

(look on powerpoint for picture)
- Arrows represent alignment of spin
- Pulses 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 therefor magnetic environment
- When nuclei excited to the high spin state relax back to the ground state, radiation is emitted, which is measured

10

NMR instrumentation

- Sample placed in magnetic field and RF pulses excite the nuclei
- As the magnetic nuclei realign, the detector picks up the radio 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 int NMR spectra which we can interpret

11

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 (boiling point -2969 degrees Celsius, 4K)
- The liquid helium chamber is surrounded by liquid nitrogen (boiling point -196 degrees Celsius, 77k)
- The sample and spinner are lowered using ait from the top, down through the bore until it rests within the probe top

12

NMR spectrometers usually have operating frequencies ranging from...

100-900 MHz