Page 458 - Tandem Techniques
P. 458
Page 442
The flow control and sampling unit is designed to provide three alternative methods of operation. First
the eluent from the column can be made to flow directly from the UV detector to the NMR sample tube.
Under these circumstances the spectra can be continuously monitored during the development of the
separation. The success of this alternative will depend on a large number of factors: the cell volume
(which will also determine the residence time of the solute and thus the measuring period), the sample
size, the column flow rate, the resolution of the NMR spectrometer and the rate of data acquisition by
the computer. In general, unless the new micro-cell facilities mentioned earlier are exploited, this
procedure will rarely be successful, particularly if microbore columns are used and multi-component
mixtures are being examined.
Figure 11. 4
The Flow Control Device for the LC/NMR System
Courtesy of Varian Inc.
The second alternative is to direct only those samples that are of particular interest to the NMR. Each
sample can then be trapped in the cell and data acquired on a stop-flow basis, allowing an adequate
number of pulses to be
