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The invention of gas chromatography by James and Martin was evoked by their work on the synthesis of fatty acids in plants. To aid in their research, a method was needed to separate the fatty acids extracted from plant tissue and to quantitatively estimate the different fatty acids present. As a consequence, the technique suggested by Martin and Synge in 1941 (GC) was developed into a practical separation procedure. Subsequently, the synthetic pathways for the different fatty acids were examined using 13C and 3H markers. Thus, having established a technique to separate the fatty acids, those that were radioactive needed to be identified and the relative activity of each peak compared and to do this successfully, a proportional radioactive detector was required. James and Piper described a radioactivity detector 1961-3 suitable for this purpose is still in use today, although the detector has been fabricated in various different forms by a number of different manufacturers. A diagram of the radioactivity detector based on the device of James and Piper is shown in figure 46.


There are two basic forms of the radioactivity detector, one that measures 13C only and the other that measures both 13C and 3H. In both systems the carrier gas used must be helium or argon and the column eluent is fed through a furnace packed with copper oxide to oxidize all the solutes to carbon dioxide and water.



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.

The Katharometer Detector
The katharometer was developed in the late 1940s for measuring carbon dioxide in the flue gasses produced from various types of industrial furnaces. A knowledge of the carbon dioxide content allowed the combustion conditions to be changed to improve burning efficiency. With the introduction of gas chromatography, its use as a possible GC detector was explored by Ray . T he sensor is a simple device and is depicted in figure 12.


figure 12


A filament carrying a current is situated in a tubular cavity through which flows the column eluent. Under equilibrium conditions, the heat generated in the filament is equal to the heat lost and consequently the filament assumes a constant temperature. The heat lost from the filament will depend on both the thermal conductivity of the gas and its specific heat. Both these parameters will change in the presence of a different gas or solute vapor and as a result the temperature of the filament changes, causing a change in potential across the filament. This potential change is amplified and either fed to a suitablerecorder or passed to an appropriate data acquisition system.As the detector filament is in thermal equilibrium with its surroundings and the device actually responds to the heat lost from the filament, the detector is extremely flow and pressure sensitive. Consequently, all katharometer detectors must be carefully thermostatted and must be fitted with reference cells to help compensate for changes in pressure or flow rate.

The Katharometer Detector
Figure 13. The Off-Line Katharometer Sensor

There are two types of sensor design, the "in-line" sensor where the column eluent actually passes directly over the filament (as shown in figure 12) and the "off-line" cell where the filaments are situated away from the main carrier gas stream and the gases or vapors only reach the sensing element by diffusion.(as shown in figure 13). Due to the high diffusivity of vapors in gases, the diffusion process can be considered as almost instantaneous. The filament wire is usually made from tungsten or platinum as both metals have high temperature coefficients of resistance and at the same time are relatively inert. The column and reference filaments are situated in the arms of a Wheatstone Bridge and a suitable current is passed through the filaments to heat them significantly above ambient temperature. To ensure temperature stability, the sensors and their conduits are installed in a high thermal conductivity metal block which is thermostatted by means of a separate oven. The performance of the in-line sensor is almost identical to that of the off-line sensor.

For maximum sensitivity hydrogen or helium is used as the carrier gas. The katharometer sensitivity is only about 10-6 g/ml (probably the least sensitive of all GC detectors) and has a linear dynamic range of about 500 (the response index being between 0.98 and 1.02)

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 -196℃.

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