NUCLEAR

MAGNETIC

RESONANCE

CONTENT

History

Definition

Principles of NMR

NMR Spectroscopy

Applications of NMR

Advantages of NMR

Disadvantages of NMR

HISTORYNuclear magnetic resonance was first described and

measured in molecular beams by Isidor Rabi in 1938

and in 1944. In 1946,FelixBloch and Edward Mills

Purcell expanded the technique for use on liquids

and solids. Purcell had worked on the development of

radar and detection of radio frequency power and on the

absorption of such RF power by matter laid the

background for Rabi's discovery of NMR.

The develpoment of NMR as a technique in analytical

chemistry and biochemistry parallels the development of

EM technologies.

DEFINITION

Nuclear Magnetic Resonance (NMR) is the phenomenon

whereby a magnetic nuclei absorbs and emits energy in

the presence of a magnetic field.

This energy is at a specific resonance frequency which

depends on the strength of the magnetic field and the

magnetic properties of the isotope of the atoms.

The principle of NMR usually involves two sequential

Steps:

The alignment (polarization) of the magnetic nuclear spins in

an applied, constant magnetic field H0.

DEFINITION

The perturbation of this alignment of the nuclear spins by

employing an electro-magnetic, usually radio frequency (RF)

pulse. The required perturbing frequency is dependent upon

the static magnetic field (H0) and the nuclei of observation.

The two fields are usually chosen to be perpendicular to

each other as this maximizes the NMR signal strength. The

resulting response by the total magnetization (M) of the

nuclear spins is the phenomenon that is exploited in NMR

spectroscopy and magnetic resonance imaging.

PRINCIPLES of NMR

All subatomic particles (neutrons, protons, electrons) have the

intrinsic property of spin

This spin corresponds to a small magnetic moment

In the absence of a magnetic field the moments are randomly

aligned

When a static magnetic field, Bo is applied this field acts as a

turning force that aligns the nuclear spin axis of magnetic

nuclei with the direction of the applied field

PRINCIPLES OF NMR

When a torque is applied to a spinning object, the axis of the object moves perpendicularly to the applied torque in motion called precession

So the nucleus will precesses around Bo with a frequency called the Lamour frequency

This proton can be in 2 energy states depending on the orientation of the axis

The difference between the number of protons in each state gives the bulk magnetization which provides the signal measured by NMR devices

PRINCIPLES OF NMR

The alignment of these protons is called polarization but this does not happen immediately it grows with a time constant called longitudinal relaxation time T1

After T1 an oscillating magnetic field is applied, sending pulses of radio-frequency energy into the formation

The initial pulse is perpendicular to Bo and aligns the spins in the transverse direction in phase with one another

As the pulse dies, the magnetisation caused buy the precession decreases as the spins return out of phase and the signal seen in the receiver decays

This very rapid decay is referred to as free induction decay (FID)

NMR SPECTROSCOPY

NMR spectroscopy is one of the principal techniques used to

obtain physical, chemical, electronic and structural

information about molecules due to either the chemical

shift, Zeeman effect, or the Knight shift effect, or a

combination of both, on the resonant frequencies of the

nuclei present in the sample.

It is a powerful technique that can provide detailed

information on the topology, dynamics and three-

dimensional structure of molecules in solution and the solid

state.

Types of NMR spectroscopy

Continuous wave (CW) spectroscopyThe Continuous Wave Spectroscopy laboratory is specialized

in the observation of equilibrium states in semiconductor nanostructures.

Fourier transform spectroscopyFourier transform spectroscopy is a measurement technique whereby spectra are collected based on measurements of the coherence of a radiative source, using time-domain or space-domain measurements of the electromagnetic radiation or

other type of radiation.

Types of NMR spectroscopy

Multi-dimensional NMR Spectroscopy

Multi-dimensional nuclear magnetic resonance spectroscopy

is a kind of FT NMR in which there are at least two pulses

and, as the experiment is repeated, the pulse sequence is

systematically varied

In multidimensional nuclear magnetic resonance there will

be a sequence of pulses and, at least, one variable time

period. In three dimensions, two time sequences will be

varied. In four dimensions, three will be varied.

Types of NMR spectroscopy

Solid-state NMR spectroscopy

This technique complements X-ray crystallography in that it

is frequently applicable to molecules in a liquid or liquid

crystal phase, whereas crystallography, as the name implies,

is performed on molecules in a solid phase.

APPLICATIONS OF NMR

MEDICINE

The application of nuclear magnetic resonance best known to the

general public is magnetic resonance imaging for medical

diagnosis magnetic resonance microscopy in research settings.

NMR is used to generate metabolic fingerprints from biological

fluids to obtain information about disease states or toxic insults.

Biochemical information can also be obtained from living tissue (e.g.

human brain tumors) with the technique known as in vivo magnetic

resonance spectroscopy or chemical shift NMR Microscopy.

APPLICATIONS OF NMR

CHEMISTRY

By studying the peaks of nuclear magnetic resonance

spectra, chemists can determine the structure of many

compounds.

NMR spectroscopy is used to unambiguously identify known

and novel compounds, and as such, is usually required by

scientific journals for identity confirmation of synthesized

new compounds.

A chemist can determine the identity of a compound by

comparing the observed nuclear precession frequencies to

known frequencies.

APPLICATIONS OF NMR

PETROLEUM INDUSTRY

Another use for nuclear magnetic resonance is data

acquisition in the petroleum industry for petroleum

and natural gas exploration and recovery.

A borehole is drilled into rock and sedimentary strata into

which nuclear magnetic resonance logging equipment is

lowered.

Nuclear magnetic resonance analysis of these boreholes is

used to measure rock porosity, estimate permeability from

pore size distribution and identify pore fluids (water, oil and

gas).

ADVANTAGES OF NMR

Only fluids are visible to NMR technology so porosity measurement is independent of the lithology.

Producible zones with high percentage of clay-bound water can be identified.

A better measurement of permeability is possible than traditional plots.

In-situ measurement of oil viscosity

Differentiation of oil/gas zones

DISADVANTAGES OF NMR

Any diamagnetic or paramagnetic ions present in the formation can affect the tool response.

Expensive

Slower logging speeds

Slimhole tools are not available

Shallow depth of penetration

Permeability measurement is actually an empirical measurement and should only be used to compare to permeabilities