Page 451 - Tandem Techniques

Page 435

Chapter 11
Liquid Chromatography/Nuclear Magnetic Resonance Spectroscopy (LC/NMR)
Tandem Systems

In principle, the association of the liquid chromatograph with the NMR spectrometer should be a very
powerful analytical tool for the separation and identification of unknown substances. There are,
however, some serious difficulties in the association of the two techniques, more so, perhaps, than with
other spectroscopic techniques. The main problems that must be solved were outlined many years ago
by Bayer et al. [1] who were one of the first research groups to combine the liquid chromatograph on-
line with the NMR spectrometer. More recently the basic problems have been again reiterated by
Nicholson et al. [2]. The general challenges facing the designer of an LC/NMR tandem instrument are
as follows.

1/ As a rule, the sensitivity realized in NMR measurements has been significantly lower than that
achieved in UV spectroscopy. However, with the increase in magnetic field strengths, and the
introduction of analog/digital converters having higher dynamic ranges, there has been a marked
increase in the sensitivity available for LC/NMR tandem systems. The recent introduction of over-
sampling and digital filtering techniques in data acquisition regimes has allowed spectral windows to be
contracted to include only regions of interest, without the problem of signals from outside the region
folding in. Furthermore, it will be seen later that the use of NMR micro-cells, if constructed correctly
and fabricated from the materials having appropriate magnetic properties, can also provide a significant
increase in sensitivity.