Liquid Chromatography Detectors - Transport Detectors > Page 63

Transport Detectors

A transport detector consists of a carrier such as a metal chain, wire or disc that passes continually through the column eluent extracting a sample of the mobile phase containing the solute as a thin film adhering to its surface. The mobile phase is eliminated by evaporation leaving the solute as a coating on the carrier. The carrier is then scanned by an appropriate sensing technique to monitor the residual solute. For example, an FID could be used to sense the pyrolysis products of the solute by heating the carrier and most of the pyrolysis products containing carbon would be detected. The system is obviously restricted to those solutes that are involatile and, in addition, the solvents used for the mobile phase must be volatile (and extremely pure). The former condition is usually met in LC, otherwise the analysis would probably be carried out by GC. The latter condition is normally easy to achieve as there is a wide choice of solvents readily available for LC.

The system appears to be ideal but the early models had some disadvantages. The instruments were bulky, expensive and some incorporated a 90strontium source (for the argon detector) all of which were unpopular. In addition, as a result of the basic design, the anticipated high sensitivity was not realized and the apparatus was clumsy and difficult to operate. However, as it was a universal detector and was unaffected by the solvents used, it was readily accepted by the soap and cosmetic industry.