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frequency coil was wound. The tube served both as the sample chamber and the coil former. The actual
detector coil was 1 mm in length, and the cells were fabricated from tubing that had internal diameters
ranging from 75 to 530 µm, which provided detecting volumes that ranged from 5 to 200 nl. Employing
such cells in the static mode, detection limits of less than 50 ng were achieved for amino acids for one-
minute data acquisition times. Although the concentration sensitivity of the system was poor for the 75
mm I.D. capillary (ca 30 mM) such coils allow less than 100 ng (0.1-1.0 nmol) amounts of amino acids
to be detected with the 60-second acquisition time. The NMR line widths were in the several 100 Hz
range for thin-walled tubes (30 µm walls) but could be reduced to about 10 Hz using thicker walled
tubes (140 µm walls). Since thermal noise in the windings of the coil, not sample noise, is the primary
noise contributor, the coil resistance must be minimized. Coil resistance depends on winding geometry,
inter-turn spacing and the specific resistance of the material from which the wire is constructed. The
authors used 42 gauge copper wire in conjunction with spaced winding. Two wires were wound onto
the tube in parallel, and after completion, one winding was removed, leaving the coil with evenly
spaced windings equivalent to the diameter of the wire. It should be noted that the coil resistance could
be reduced very significantly by using superconducting materials.

The coil was wound from varnished copper magnet wire with the aid of a pin vice, micro-manipulator
and stereo-microscope. Typically the coil contained 14 to 17 turns of wire extending over 0.9 to 1.1
mm. This produces a cell having a volume of about 50 nl. Generally, as the sensor coil is reduced in
size, the mass sensitivity improves, because of the increase in strength of the rf field per unit of current
flow. It has been reported that as the coil diameter is reduced from 1000 µm to 50 µm, there is a twenty-
fold reduction in the limit of detection. This would correspond to a 400-fold increase in measurement
time for the larger coil to obtain the same signal to noise ratio as the smaller coil. The layout of the
tandem apparatus is diagramatically depicted in Figure 11.16. The micro coil was contained within the
NMR probe so that the microcoil/capillary assembly could be positioned reproducibly in the bore of the