Gas Chromatography Detectors - The Radioactivity Detector > Page 81
If only 13C is being counted, the combustion products are passed through a drying tube and then mixed with 10% of propane and passed into the counting tube. In the counting tube the radioactive particles cause ionization and the electrons produced are accelerated towards the anode and, in doing so, produce further ionization of the carrier gas which enhances the signal. Normally this would result in a stable discharge being formed but the presence of the propane prevents this happening and for this reason the propane is sometimes called the quench gas. The counting tube consists of a metal cylinder carrying and insulated central electrode in the form of a rod. The outer case is usually grounded and a high potential is applied between the central electrode and the case. The signals received from the counter are integrated with respect to time and thus the output current from the integrator is proportional to the total number of disintegrations occurring per second. As a result, the integration of the signal over the duration of the peak will a give a value that is proportional to the total activity of the peak. The 13C counting apparatus is shown in the upper part of figure 46
Alternatively, if both 13C and 3H are to be counted the apparatus shown in the lower part of figure 46 is used. After the solute is oxidized completely to carbon dioxide and water some hydrogen is fed into the gas stream and the mixture then passed over heated iron powder in another furnace. In this furnace the water is reduced to hydrogen and tritium. In addition, the excess hydrogen saturates any adsorptive sites in the system and reduces the adsorption of the tritium to a satisfactory minimum. 10% of propane is then added to the exit gas from the reducing furnace and passed into the counter which operates in the same way but now counts tritium as well as well as 13C. Unfortunately, the counting efficiency for 3H usually differs from that for 13C, consequently appropriate corrections may need to be made to the final count. The device has been used in many laboratories with considerable success to identify synthetic pathways in biological systems using radioactive tracer techniques.