Liquid Chromatography Detectors - Transport Detectors > The Modified Moving Wire Detector > Page 66

Combustion takes place, ions are produced in the expected manner, and the ion current processed in the usual way. Unfortunately, due to the occlusion of local, high concentrations of solute between the links of the chain, the detector output was extremely noisy and thus the system exhibited a relatively poor sensitivity. A chromatogram obtained with the chain detector is shown in figure 42. The noise spikes on each peak are obvious, which, besides affecting the overall sensitivity of the detector, makes quantitative analysis extremely difficult.

The Modified Moving Wire Detector

In the early 1960s, a number of attempts were made to improve the performance of the transport detector. In 1966 Karmen (35) introduced an aspirating system to draw the pyrolysis products into the hydrogen flame detector. Later in 1970, Scott and Lawrence (36) developed the Karman's system further and introduced a modified form of the original moving wire detector.

The sensitivity of the original detector, in addition to being degraded by the high noise level, was also limited by the proportion of the pyrolysis products that entered the FID. Excepting synthetic polymers, (which often quantitatively produced monomers) many compounds yielded only a few percent of volatile compounds on pyrolysis. Thus the FID could only sense a very small fraction of the products from the solute deposited on the wire.  If the solutes, were completely combusted in oxygen or air, however, then all the carbon in the solute would be converted to carbon dioxide. Furthermore, if the carbon dioxide was then reduced to methane by mixing it with hydrogen and passing it over a nickel catalyst, the carbon dioxide would be quantitatively converted to methane which could be detected by the FID. This procedure would increase the sensitivity of the detector to those substances that gave poor yields on pyrolysis and, in addition, increase the linear dynamic range and possibly provide a predictable response.