Gas Chromatography Detectors - Early Gas Chromatography Detectors > The Gas Density Balance > Page 23

 

The increase in pressure at the base of tube (C) due to the presence of solute in (C) applied a pressure to the bottom of tube (A). This caused a flow of gas through the anemometer from tube (A) to tube(B)providing an output that was fed to a recording milliammeter. Subsequently all flows exited from the top and bottom of tube (C).

The anemometer was particularly unique. It consisted of a cylindrical chamber, 1.5 cm in diameter and 4 mm wide. A length of 0.001 in O.D. copper wire, containing 2 mm of 0.001 in Constantan wire arc welded to the copper wire in the center, passed through the conduit connecting the chamber to tubes (A) and (B). (The construction of these dual thermocouples with the equipment available in 1952 was a feat in itself). Beneath the copper Constantan junctions was situated a heater loop that raised the temperature of both junctions by convection currents circulating round the cylindrical chamber as shown in the center of the anemometer diagram. When a flow of gas passed through the anemometer as a result of solute vapor being present in tube (C), the convection currents above the heater loop were displaced so that one junction was cooled and the other heated as shown in the right–hand side of the anemometer diagram. The differential output from the two thermocouples was passed to an appropriate recording milliammeter.

The detector was quite robust, but initially difficult to set up. The sensor had a linear response (0.98<r<1.02) over about 3 orders of magnitude of concentration range and a sensitivity (minimum detectable concentration) of 5 x 10–7 g/ml (n-heptane). Unfortunately, the detector was very difficult to construct for the general chromatographer (indeed also for the instrument manufacturers) and even after many attempts, was never produced commercially as an effective GC detector. The lack of an alternative, simple detector provoked the development of alternatives. As a consequence, a number of very effective highly sensitive GC detectors were developed over a relatively short period of time of which many were manufactured commercially.