Page 455 - Tandem Techniques

Page 439

The Modern LC/NMR Tandem Instrument

There are four methods of operating LC/NMR combination equipment and so the modern tandem
instrument should be capable of functioning in all four modes. The four regimes are, 1. Continuous
flow, 2. Stop-Flow monitoring, 3. Time-sliced stop-flow monitoring and 4. Peak collection and off-line
monitoring.

1. Continuous Flow Monitoring

Continuous-flow measurement is the simplest method of monitoring an LC eluent. Unless enriched
compounds are employed, this mode of operation is usually only suitable for H or F NMR. If gradient
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elution development is needed to separate the materials in a reasonable time, the resonance positions of
the solvent peaks will continuously change. This usually means a preliminary run must be carried out to
determine the optimum suppression program that will be necessary to obtain satisfactory spectra.

2. Stop-flow Monitoring

If either the retention times of the solutes are known or a UV detector is also used to monitor the eluent
as the separation proceeds, the solute bands can be stopped when they are situated in the NMR
measuring cell. Employing stop-flow monitoring, all the normal techniques of high-resolution NMR
can be used, and as the diffusion of solutes in liquids is very slow, little chromatographic resolution is
lost.

3. Time-sliced Stop-flow Monitoring

Time-sliced monitoring is similar to stop-flow monitoring, except that various portions of the peak are
examined as they enter the NMR sensor cell. In this way the spectra from different parts of the peak can
be examined which can be very helpful if the chromatographic resolution is not complete.

4. Peak Collection and Off-line Monitoring

This procedure is very similar to stop-flow monitoring, except that each peak is stored in its own
respective sample loop as it is eluted, and later