50 chromatography • spectroscopy
Benchtop NMR spectroscopy N
Dr Kevin Nott and Dr Andreas Bergner outline the potential of a benchtop nuclear magnetic resonance instrument for structural characterisation in the chemistry laboratory.
uclear magnetic resonance (NMR) Spectroscopy, along
with mass spectrometry, is an important technique for structural characterisation thus has become a standard tool for organic chemistry.
Unfortunately, time on an NMR instrument is expensive, even for routine analysis, because of the associated running costs.
Te super-conducting magnet requires regular cryogen fills of liquid helium and nitrogen, the former becoming an increasingly scarce and therefore expensive commodity. In addition, it normally requires an expert to run and maintain the instrument..
Fig. 1. Schematic representation of Benchtop NMR Relaxometry (also known as Time Domain NMR) and Spectroscopy.
Low running costs In contrast, benchtop NMR instruments based on permanent magnet technology have
low running costs as they are cryogen-free; these instruments are commonly used for measurement of oil, moisture, solid fat and fluorine content for process and quality control.
Permanent magnets constructed from rare earth metals have been developed recently which are smaller, lighter and have much better performance than traditional AlNiCo magnets.
For example, the MQC benchtop NMR analyser has the smallest magnet footprint as well as the largest sample capacity, 14ml, for a magnet of it class (magnetic field >0.5 Tesla, 20MHz in frequency). Tis results in increased signal-to- noise and better sampling leading to improved accuracy.
In addition, the combination of larger sample size and innovative accessories leads to
easier sample preparation for many applications, including measurement of oil content in snack food, spin finish on fibre and soluble fluoride in toothpaste. .
Time domain signal Te MQC uses the time domain signal to distinguish between solid and liquid, or oil/fat and bound water by their different decay time constants.
Tus the bulk NMR signal from an oil, for example in snack food, may be determined immediately after signals from the both the solids and bound water have decayed; this signal, normalised by mass, may be used to calibrate for oil content.
More recently, a bench-top NMR instrument has been developed that is capable of acquiring high resolution spectra in the frequency domain.
Pulsar differs from the MQC in that (1) the magnet has better field homogeneity which leads to sharper, well resolved peaks, and (2) the field strength is higher, 1.45 Tesla (vs. 0.55 Tesla) required for better spectral resolution, or separation between the peaks.
Te Pulsar gives spectral information by applying a Fourier transform to the Free Induction Decay (FID). A schematic representation of NMR Relaxometry (or Time Domain NMR) and spectroscopy is given in Fig. 1.
Fig. 2. shows the type of information that can be obtained from bulk vegetable oils using the Pulsar spectrometer. Te 1H NMR Spectrum of glyceryl trilinoleate shows all the resonance peaks associated
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