Page 454 - Tandem Techniques

Page 438

For historical interest, a diagram of the original flow-through cell of Bayer [1], which incorporates the
basic principles of the earlier LC/NMR systems that operated with electromagnets, is shown in Figure
11.1.

The cell-volume had to be kept as small as possible to minimize band dispersion, but, at the same time,
the geometry of the flow cell had to provide optimum synchronization with the NMR sensing coil.
Consequently, in the older electromagnetic instruments the wall of the cell had to be straight and
parallel with the axis of the coil. Such flow-through cells had volumes in excess of 400 µl (too large for
modern LC columns), and thus at a flow-rate of 1 ml/min the average residence time of the sample was
about 25 sec.

Modern high-sensitivity, high-resolution NMR instruments have superconducting magnets, and thus
have entirely different sensing coils and cells. The column eluent cannot pass vertically through the
field entering at the top of the instrument and exiting at the base and, furthermore, because of the
geometry of the superconducting coils and cryostatic environment, the coils are no longer wound
concentrically on the sample tube, but are often in the Helmholtz configuration (this configuration,
however, is not always optimum as will be seen later). Consequently, the eluent from the column
normally passed through a U-shaped conduit, one limb of which will be the actual sample cell.
Modern flow-through cells have volumes ranging from 20 to 50 µl but recently, Sweedler [3], has
claimed the successful use of sample tubes with capacities that are measured in nanoliters, with an
accompanying improved signal to noise ratio of about two orders of magnitude. The sensing coil is only
1 mm long, 0.5 mm O.D. and the capillary running through the coil has a sample capacity of 5 nl. The
secret appears to be in surrounding the coil/capillary assembly in a commercial perfluorinated organic
liquid that has the same magnetic susceptibility as the copper micro-coil. This device, which will be
discussed in more detail later, is claimed to provide a more uniform magnetic field in the sample region
leading to improved resolution and peak shape. This discovery could quickly lead to greatly improved
LC/NMR systems.