Page 469 - Tandem Techniques

Page 453

Acquiring with additional scans, as shown in (C), decreases the sensitivity, because the additional scans
are acquired after the solute has left the detection cell, but dramatically increases the apparent
chromatographic efficiency.

Synopsis

NMR is the most difficult technique to operate on-line with a liquid chromatograph. The earlier NMR
instruments had very limited sensitivity relative to other spectroscopic techniques. Even with the
introduction of the 600 and 750 MHz spectrometers, sensitivity was still a problem with the in-line
combination of the spectrometer with the liquid chromatograph. However, with the recent introduction
of over-sampling, and digital filtering techniques, in data acquisition regimes spectral windows can be
constructed to include only regions of interest, without the problem of signals from outside the region
folding in. It is still necessary to accommodate the changing composition of the sample in the NMR cell
due to the flow of mobile phase, and to design flow-through cells that do not interfere with
homogeneity of the magnetic field immediately around the sample. The problem of interfering solvents
still remains, although techniques have been developed to abrogate the interfering signals, which have
certainly permitted more flexibility in the choice of the mobile phase. Modern LC/NMR systems are
largely off-line devices, where the solutes are captured in a sample loop and passed to the NMR
spectrometer for examination when convenient. The valving system can be quite complex, allowing
on-line measurement if so desired, specific peak selection, or complete peak storage. Nanoliter flow
cells have been developed to allow the use of microbore columns or packed capillaries that have
provided complete spectra from 1 nmol of sample. These small cells also allow expensive deuterated
solvents to be used, and thus eliminate solvent interference without excessive cost. For optimum
sensitivity, the cell volume and peak volume must be matched, which is extremely difficult in practice.
The use of the 600 and 750 MHz spectrometers with the micro-flow cell, has produced satisfactory
spectra from as little as 50 ng of material. The quest for higher sensitivity