Liquid Chromatography Detectors - Transport Detectors > The Moving Wire Detector > Page 64
The Moving Wire Detector
The original wire transport detector was developed by James et al. (33) and subsequently manufactured and marketed by Pye Unicam. A diagram of the Pye Unicam detector is shown in figure 41.
Figure 41 The Pye Unicam Moving Wire detector
The carrier wire from a spool passes through an oven at 750˚C and any residual lubricants remaining on the wire after drawing are burnt off. After passing round a pulley the wire then goes through a coating block where it is coated with a thin film of mobile phase. The wire then passes through an evaporator at about 105˚C (or a temperature appropriate for the solvent(s) employed). In the evaporation chamber a nitrogen stream is allowed to flow counter-current to the wire movement to improve the rate of solvent evaporation. The wire carrying the solvent-free solute then enters (via a restriction) a pyrolysis tube that is maintained at about 500˚C. The pyrolysis tube has a nitrogen supply entering at either end which sweeps the tube contents, including the pyrolysis products that are formed, out through a center tube into the FID.
The FID was a standard detector and the device functioned well and, as expected, was completely independent of the nature of the solvents used for the mobile phase. The detector sensitivity was disappointing, and found to be little better than the average refractive index detector viz. 5 x 10-6 g/ml. The poor sensitivity resulted from excessive noise (not a weak signal) which may have been caused by high boiling impurities in the solvents, fluctuations in the nitrogen flow and/or irregularities in the pyrolysis process. The linear dynamic range was also found to be less than two orders of magnitude. However, the device did establish the viability of the transport method for LC detection.