Page 452 - Tandem Techniques

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2/ The intensity of the NMR signal with on-line monitoring depends on the flow rate, and as the flow
rate increases, the signal decreases. However, according to Bayer et al. [1], the reduction in signal can
be restricted to a reasonable level at flow rates between 0.5 and 2 ml/min.

3/ In order to realize high NMR resolution, the magnetic field throughout the sample must be very
homogeneous and to achieve this, the sample tube is usually spun at fairly high speeds. So far, this has
proved impossible in flow-through cells and consequently, in the past, considerable resolution has been
lost in these types of cell.

4/ The dynamic range of the NMR measurements is impaired due to H NMR signals from eluting
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solvents. Consequently, efficient solvent suppression techniques are necessary that will also
accommodate the solvent composition changing during the period of chromatographic development.
This problem, will also apply to C nuclei and the solvent can again interfere with the spectrum of the
13
solute. In the case of proton spectroscopy, solvents may be chosen that do not contain protons, such as
carbon tetrachloride or deuterated solvents, but the former restricts chromatographic performance and
the latter can become very expensive if standard LC columns are used.

One of the more common solvents used in reversed phase LC is acetonitrile, which gives rise to a single
resonance in the H NMR spectrum at about , which can be easily suppressed. However, this
1
suppression leaves the 13C satellite peaks, from the 1.1% of molecules with the naturally abundant 13C
isotope at the methyl carbon. These satellite peaks are often larger than those from the sample and thus
they must also be suppressed. Suppression has been achieved in two ways by setting the suppression
radiation frequency over the central peak and the two satellite peaks in a cyclic procedure or,
alternatively, if an inverse geometry probe is used which includes a 13C coil, the 13C decoupling is
possible and this collapses the satellite peaks under the central peak, enabling conventional single peak
suppression. The use of small-bore columns would significantly reduce the solvent consumption, and
render the use of special and expensive solvents more economically viable, but