Nuclear Magnetic Radiation (NMR) Spectroscopy

What is the process of an NMR instrument?

Processing Nucleus → RF Pulse → Time domain/Free induction decay → Fourier transform → frequency domain


1. Superconducing magnet => colled by liquid helium and liquid nitrogen
2. Sample tube spins within the probe coils in the hollow bore at the center of the magnet
3. The radio frequency excitation pulse and resulting NMR signals are sent through cables between the probe coils in the magnet and the computer
4. Radio frequency (RF) generator and computer operating console
5. Fourier transformation of the signal from the time domain to the frequency occurs at teh computer console

How are the NMR sample and spinner used?

A few milligrams of material is dissolved in a deuterated solvent and placed in a 5 mm tube. The NMR sample tube is inserted into the spinner and positioned using a depth gauge

What is happening within the Magnet Can?

The superconducting magnet is immersed in liquid helium at 4K. The outer insulated chamber is filled with liquid nitrogen at 77K

What is happening under the magnet?

the probe and the shim stack are instered into the magnet from below. Spectrometer connections to the Shim stack and the probe are made here

What is happening within the magnet

The sample sits inside the shim stack, which adjusts the magnetic field for maximum homogeneity

What is happening within the shim stack?

the samples sits in the probe which sits inside the shim coils

What is happening within the probe?

The NMR sample is located within two RF coils. The inner and outer coils transmit RF to the sample and measure the RF received from the sample

What is a Probe?

The probe is the part of an NMR spectrometer that does much of the work, in terms of exciting the nuclear spins, and detecting the NMR signals. The Probe goes into the center of the magnetic field, and the sample is inserted into the probe to perform the NMR experiment. The probe contains the RF coils, tuned at specific frequencies for specific nucleu in a given magnetic field. The probe also contains the necessary hardware to control the sample temperature (when combiend with an external temperature controller)

What is the structure of a Probe?

often, probes are constructed with two observe coils, one closest to the sample (the inner coil), and another further from the sample (the outer coil). This allows the probe to respond to multiple frequencies, adn to allow the excitation/irradation of multiple nuclei. The nuclei that use the inner coil are detected with the highest sensitivity.

What is a Broadband Observe probe?

constructed with the inner-coil tuned to a “broadband” nucleus, for maximum senesitivy for the nuclei

What is an Inverse or Indirect Detection probe?

probe is construcnted such that the inner coild is tuned for 1H. These probes give the highest proton senesitiviy, with much lower senesitivity to directlyu observe lower frequency nuclei

What does modern NMR probes also include that older NMR probes do not?

an actively-sheliding Pulsed Field Gradient (PFG) coil, which allows the application of field-gradient pulses. Many modern NMR experiments rely on the use of PFG pulses

After removing the PFG coil assembly, you can see the Radioofrequency (Rf) coils that create the “B1” fields that we call _____, and these same coils pickup (or detect) the NMR signal.

pulses

What do we know about the outer coil?

The coil furthest from the sample. This is the proton (1H) and Fluorine (19F) observe/decouple coil. This coil is the type often called a “saddle coil” because the current runs up one side, and down the other side. These coils are less efficient than Helmholtz-style coils; however, they are better suited fro a secondary coil that surrounds the main detection coil. Both the saddle coil, and the Helmholtz coil create magnetic fields that are perpendicular to the vertical axis (or main Bo field), which is, of course, a requirement. The outer coil is perpendicular to the B1 field generated by the inner coil. This is important to prevent the Rf current from one coil from inducing a current in the other coil (or, to isolate them from one another)

What do we know about the inner coil?

Second Rf could that gives the highest sensitivity; coil closest to the sample. This is the broadband observe coil, normally tuned to observe carbon (13C) and phosphorous (31P) This is a Helmholtz-style could, which gives better Fr pulse performance, and more sensitivty than the saddle coil. there are two turns on each side (wound in complimentary directions), which create a B1 field that would be side by side. The extra sensitivity of the inner coil arise both from the fact that the coils is closer to the sample (higher filling factor), and due to the fact that the multi-turn coil is more efficient.

To pulse and observe any NMR nucleus, the probe circuit (consisting of the RF coils and a network of capacitors) must be tuned for what?

the proper frequency response, and the proper impedance

Nearly all NMR systems operate at 50 OHMs impedance, so all RF circuits must be tuned such that the impedance in and out of all components in the signal path is exactly 50-OHMs at the specific signal frequency. How do we adjust the capacitance?

Since the inductance of the coil is fairly constant, changing the capacitance of the capacitors changes the frequency at which the circuit is tuned. The sticks and knobs that you turn (on the bottom of the probe) change the overlap of an inner and outer sleeve in a variable capacitor (the vertical silver and white cylinder) the more the inner-cylinder overlaps with the outer cylinder, the higher the capacitance.

What is shimming?

process that corrects for imperfections in the main field and for introduced perturbation. It is enacted by virtue of resistive room-temperature coils (the RT shim tube) which carry electric current and produce correction fields concentrated at the point of the probe and sample. The localized magnetic fields are designed to cancel out spatial inhomogeneities in the main field

What is Relaxation?

re-establishment oof the equilibrium stat of the near spin magnetization

We always measure it by half height (h/2)

R=1/T2\* where T2 (time constant) = transverse spin relaxation (most important)

Sensitivity of NMR Experiments rely on a signal-to-noise ratio (S/N). Many factors affect the S/N ratio. What are the easiest ways to increase S/N?

1. Increase the sample concentration


1. Acquire more scans

The sample must be soluble in order to perform NMR measurements in solution. What are the solvent requirements?

1. should not contain hydrogen atoms => use deuterated solvents
2. Should be inert
3. should have a low boiling point so it can be easily removed
4. inexpensive

What are some common solvents used in NMR?

CDCl3 (d-chloroform) = cheapest and most commonly used

D2O (d2-water)

C6H6 (d6-benzene)

CD3OD (d4-methanol)

CD3COCD3 (d6-acetone)

Field frequency lock system is employed in commercial NMR instruments. How is this done?

A reference nucleus is continuously irradiated and monitored. Corrections are made for a drift in the field. and the reference signal is the deuterium in the solvent.

What are some things that will give impurity peaks?

1. Traces of common laboratory solvents
2. traces of grease and plasticizers are also common
3. trace amounts of ferromagnetic impurities cause sever broadening of NMR peaks because of reduction of relaxation times


1. common sources include: rust particles from tap water, etc; particles form metal spatulas, and raney nickel

The magnetic field is along the z-axis is the the magnetization vector. The vector is moved by transmitting a signal (Rf signal) from a coil in the detector in the x,-plane. What is the problem we deal with and how do we deal with it?

The problem that we have to deal with is that the induced magnetic field that we deal with is much stronger than any electrical signal that can be transmitted through the probe coil of the spectrometer. Instead of using a fixed frame of reference, from here on we use a coordinate framework where the x,y-plane rotates about the z-axis at the observation frequency (usually hundreds of millions of times per second) close to the alarm or precession rate of the material under study

You can pulse at different times and get different flips

What is the difference between a Hard Pulse and a Soft Pulse?

Hard pulse = get the entire spectrum (what we normally do)

Soft pulse = get a specific range of the spectrum (get better sensitivity and resolution)

What is acquisition?

When the magnetization is not at equilibrium, it returns slowly (typically over a period of seconds) to the equilibrium magnetization along the z-axis. In this process, radio frequency radiation is emitted. This is acquired at the end of the pulse sequence and is called the Free Induction Decay (FID)