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Nuclear magnetic resonance (NMR) spectrometer

Introduction

NMR spectroscopy works by detecting the nucleus of hydrogen (or carbon or certain other types of atomic nuclei) using radiowaves as the samples are placed in a strong magnetic field. The exact chemical environment of hydrogen determines the energy of radiowaves that can be absorbed (resonance frequency). For instance, in a given “static” magnetic field, hydrogen atoms in water absorb a different radio frequency (energy) than those in other types of molecules such as methane or alcohol. Even hydrogen atom nuclei located in different parts of the same molecule absorb slightly different radiowave energies.

NMR relies on a fundamental property of atomic nuclei called spin which causes the atoms that are NMR-active to behave like small bar magnets. Within the instrument, those atoms are subjected to two magnetic fields. The first field is produced by a magnet and has a constant intensity (static field).

The second field is produced by an electromagnetic radiation as it passed through a coil (oscillating field) that surrounds the sample under investigation, in a cavity called the probe. This latter field is approximately 10,000 times weaker in intensity than the static field. Atoms within the sample can absorb this electromagnetic radiation at a given frequency that depends on their chemical environment.

Complex organic molecules, for example, will contain many atoms in different chemical environments, leading to absorption at numerous frequencies. By plotting the intensity and frequency of these absorptions, the NMR spectrum is produced. The NMR spectroscopist can identify the different types of hydrogen environments present in a molecule and determine its composition (eg CH2, CH3, OH etc).

Applications

NMR spectroscopy is used for the characterisation and identification of solids and liquids either in solution state (made up in a NMR-inactive solvent) or solid state (for powders, tissue samples and thick gels). Solids and liquids are routinely characterised and identified by using a range of 1D and 2D spectra that involve measuring the following isotopes: 1H, 11B, 13C, 15N, 19F, 31P or 19Si.

The technique can be utilised for elucidating the chemical structure of pure samples but is increasingly used to study the behaviour of mixtures such as protein-drug interactions, drug stability, quantitative determinations and other diffusion studies. NMR is therefore of great interest to the organic and inorganic chemists but also to biochemists and drug formulation scientists who see in it a new, non-destructive, way to look at complex mixtures with little or no sample preparation.

Instrumentation

  • Brüker Avance III 400 two-channel Fourier Transform NMR spectrometer (FT-NMR) and Brüker Avance III 600 three-channel FT-NMR spectrometer. Both spectrometers are controlled by HP z400 workstation running TopSpin 3.2 pl6 for control and processing and Icon NMR 4.7.5 for automation, under Windows 7 Professional 64-bit operating system. Both instruments offer solution state NMR capability as summarised below.
  • AVIII400 FT-NMR spectrometer (400MHz for 1H) is equipped with a 5 mm BBFO+ (direct observe) probehead which allows observation of a range of nuclei: 1H, 11B, 13C, 15N 19F, 31P, 19Si and many others as well as 19F with 1H decoupling. The probe is fitted with automatic tuning and matching control and Z-gradient coils enabling gradient spectroscopy. A 60-position sample changer allows fully automated experiments to be run round-the clock. Finally, a variable temperature control system that comprises a 26 L liquid nitrogen Dewar allows work in the temperature range of the probehead -150ºC to 150ºC.
  • AVIII600 FT-NMR spectrometer (600MHz for 1H) is equipped with a 5 mm 1H{13C, 15N} TXI probehead. It is an inverse geometry triple resonance probe (TXI) for 1H observation with simultaneous pulsing/decoupling of 13C and 15N and 2H lock. The probehead is equipped with shielded z-gradient coils for gradient spectroscopy and a thermocouple oven for optimal temperature control and stability for long experiments (protein NMR capability). The instrument is typically used from 0°C to 40°C.

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