Liquid Chromatography Detectors - Transport Detectors > The Modified Moving Wire Detector > Page 67

A diagram of the original moving wire detector modified in this way is shown in figure 43. The hydrogen lines entering the detector body were enlarged to reduce the flow impedance and permit the satisfactory operation of the aspirator. The detector was connected to a 2 in. length of 1/2 in I.D. thin walled stainless steel tube filled with about 2 g of nickel catalyst and then to the aspirator. The wide tube was closed with a loose plug of quartz wool and .

Figure 43 The Modified Moving Wire Detector

The nickel catalyst was prepared by absorbing a saturated solution of nickel nitrate onto 20/40 BS mesh brick dust, decomposing the nitrate at 500˚C for 3 hours followed by reduction of the nickel oxide so produced to metal in a stream of hydrogen at 250˚C. The jet/venturi aspirator (supplied commercially as a molecular entrainer) was placed in line with the hydrogen flow to the detector.

To improve the aspirating efficiency, the gas used was a mixture of hydrogen and argon, and with this mixture, the jet/venturi pressure drop continuously sucked the combustion gasses into the hydrogen stream. In figure 43 the reduced pressure side of the aspirator is shown connected to the side limb of the oxidation tube and the two tube system used in the original moving wire detector, was replaced by a single tube. The oxygen or air is fed into the center of the tube and, thus, provided both the evaporator flow and the oxidation flow. All tubes were constructed of quartz. The linear dynamic range of the system was determined to be about four orders of magnitude a considerable improvement on the pyrolysis instrument. The response index for a series of compounds of different chemical types ranged from 0.96 to 1.04. and the actual response was found to be proportional to the carbon content of the solutes.