Page 463 - Tandem Techniques

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between the column and a 15 cm of 50 mm I.D. fused silica capillary tube which had sufficient flow
impedance to provided the required split ratio. The column flow then passed to an injection valve, and
then to a length of fused silica tubing, 70 cm long, and 315 µm I.D., 12 cm of which was packed with
reversed phase particles, 5 µm in diameter. A small portion of the polyimide coating was removed to
act as the NMR detection window.

Optimization of LC and NMR Operating Conditions for Tandem LC/NMR Systems

The relationship between chromatographic peak width and NMR cell volume was investigated
theoretically by Grifffiths, [5], who arrived at some conclusions that suggested that adjusting the
chromatographic peak width to fit the NMR sample tube volume would be advantageous. This is a little
difficult as the chromatographic peak width increases continuously with the retention time (or volume)
of the solute and the resolution obtained from the chromatographic column increases as the peak width
decreases. The more efficient the column, the more narrow the peak at any given retention time. For a
column of given efficiency (determined, among a number of other parameters, by the particle size and
method or preparation) the smaller the radius, the smaller the peak volume.

It was found that the interrelationship between peak width in time, peak width in volume, column
dimensions and retention time can be very complex. The maximum sensitivity was realized when the
chromatographic peak width was about one third of the cell width. It is clear that if only one solute, or a
limited number of solutes that were eluted around the same retention volume or retention time, were the
only peak or peaks of interest, then the adjustment of the chromatographic conditions to provide a peak
one third of the cell volume in order to provide optimum sensitivity, might be practical and worthwhile.
However, this would need to be arranged with a clear understanding that the chromatographic
resolution must not be compromised, or the point of using liquid chromatography in the first place
would be lost. If NMR spectra were required for a number of peaks that extended over a significant
retention period, then adjusting the operating condition would be far more difficult