Gas Chromatography Detectors - The Simple Gas Density Balance > Page 34

Figure 14. The Separation of the Compounds of Hydrogen, Deuterium and Tritium

Despite its sensitivity shortcomings the katharometer can be used in most GC analyses that utilize packed columns and where there is no limitation in sample availability. The device is simple, reliable, rugged and relatively inexpensive. An example of the use of a katharometer to monitor the separation of various compounds of hydrogen, deuterium and tritium, employinggas solid chromatography is shown in figure 14. The stationary phase was activated alumina [treated with Fe(OH)2], and the column was 3 m long and 4 mm I.D. The carrier gas was neon, the flow rate 200 ml/min (at atmospheric pressure) and the column temperature was -196oC.


The Simple Gas Density Balance

The original gas density balance has already been described. It was complicated, difficult to fabricate and its manufacture was notacommercialsuccess. Intheearlydays of chromatography GOW-MAC developed some elegantly designed filaments for use in the construction of katharometers, which, in due course, were used in many other manufacturer's katharometer products. These sensing filaments were rugged and highly reliable and were used by GOW–MAC to emulate Martin's density balance in a simple form. A diagram of the GOW-MAC gas density balance is shown in figure 15. The sensor consists of a pneumatic bridge of tubes containing three vertical tubes all connected by horizontal tubes at the top and the bottom. Pure carrier gas enters the center of the right hand vertical tube and splits into two streams one passing along the lower horizontal tube and the other along the upper horizontal tube. The eluent from the column enters the center of the middle tube and the flow also splits into two streams and each meets the respective flow from the right-hand tube. The flows in the two horizontal tubes finally pass up and down the left-hand vertical tube to meet at the center and then exit to waste. Flow sensors are situated in the horizontal tubes between the right-hand vertical tube and the center vertical tube. When only carrier gas is present in the system, the horizontal flows are equal and the temperature and thus the potential across the filaments of the two sensors are the same. When a solute is eluted from the column, vapor will be present in the center vertical tube and the pressure at the top and bottom of the tube will differ.