NMR spectroscopy

Nuclear Zeeman effect

Several isotopes (e.g., 1H, 13C, 15N, 31P, 27Al, 29Si, etc.) carry a spin. This fundamental quantum-mechanical property can be described by an inherent spin angular moment of the atomic nucleus. In combination with its electric charge, the spin generates a magnetic moment which may interact with an external magnetic field or with other spins.

If a sample containing magnetically active nuclei of, for example, the above mentioned isotopes is inserted into the strong magnetic field of a superconducting magnet (e.g., at 9.4 T, approx. 200,000-fold stronger than earth's magnetic field) the spins will align and the energetic degeneracy of different spin states is lifted. This effect has been discovered in 1896 by Pieter Zeeman for the electron spin and has been awarded with the Nobel prize in Physics in 1902. Zeeman observed the splitting of optical absoprtion or emission lines in atomic vapors due to a magnetic field. In comparison to the Zeeman effect which relies on the relatively large spin moment of unpaired electrons, the nuclear Zeeman effect is about three order of magnitude smaller. Therefore, modern NMR spectroscopy requires very large magnetic fields at the edge of technical possibilities.

Nuclear magnetic resonance

A nuclear spin inside a strong magnetic field which is also subject to an electromagnetic field oscillating at the nuclear Larmor frequency will show resonance. Thus, a nuclear spin transition is induced a an appropriate photon is absorbed. The resulting induction can be measured by the same radiofrequency pickup coil which has been used for pulse excitation, and the signal is detected and electronically stored. Because the nuclear spin of each atom inside a molecule senses a different magnetic environment (for example, due to local fields of other spins or shielding by electronic orbitals), a unique and individual resonance line will be measured in the NMR sepctrum.

At typical magnetic fields of 7–24 T, Larmor frequencies between 300 and 1000 MHz are obtained for protons (1H nuclei of hydrogen). The chemical shift (i.e., the relative frequency shift due to electronic shielding) is only on the order of 10–20 ppm. FOr other nuclei the respective Larmor frequency is considerably smaller, however, the chemical shift range can take up much larger ranges (with spans of up to thounsands of ppm). This results in numerous application scenarios for NMR.

The NMR spectrometer

Based on the above-described basics, an NMR spectrometer contains the following components: (1) magnet, (2) NMR probe, (3) console, (4) work station.

  1. The magnet generates the required magnetic field through the induction of a superconducting coil. Once cooled below its critical temperature and charged up to field, the current flows persistently and essentially without electrical resistance. This not only reduces the electrical power loss which makes operation with conventional magnets extremely cost expensive due to the electricity and cooling required; it also ensures an extremely stable magnetic field due to the purely direct current. This allows for spectral separation of NMR lines with an extremely high resolution in the ppb range and below. However, the magnet requires constant cooling with liquid Helium at a temperature of 4.2 K (–269 °C). In order to reduce cryogen losses, the helium cryostat is shielded by an insulating vacuum and an additional vessel cooled by liquid nitrogen. This increases the laboratory footprint, often resulting in the need for specialized spaces and multi-story clearance.
  2. The NMR probe is holding the sample tube inside the sweet spot of the vertical magnet bore. More importantly, it contains the means to generate and pickup the radiofrequency fields crucial for NMR. For this, special inductive coils are utilized (e.g., saddle or solenoid). These are part of electromagnetic resonance circuits for increasing the conversion factor between magnetic field and electric signal. Furthermore, NMR probes often contain equipment for temperature variation or spinning of the sample and, particularly for analytical high-throughput applications, with automatic sample changers.
  3. The console contains the electronic components for generation of the radiofrequency pulses and detection of the nuclear spin signals which are emitted or picked up, respectively, by the NMR coil. For this, frequency synthesizers, pulse or abitrary waveform generators as well as duplexers, preamplifiers and audio amplifiers are utilized in the homodyne method. Additionally, the console contains dedicated control components for the magnet (e.g., cryogen level sensors, shim coil systems for field homogeneity optimization, etc.) and for communication with the work station.
  4. The work station acts as the interface between the spectroscopist and the spectrometer. This is typically a desktop PC operating a dedicated control and processing software in direct communication with the console via ethernet. Through the software, the operator may program special pulse sequences, vary experimental parameters or post-process obtained spectroscopic data for scientific analysis.