Argon specific radioactivity

The electron capture detector is another type of ionization detector. Specifically, it utilizes the beta emissions of a radioactive source, often nickel-63, to cause the ionization of the carrier gas molecules, thus generating electrons that constitute an electrical current. As an electrophilic component, such as a pesticide, from the separated mixture enters this detector, the electrons from the carrier gas ionization are captured, creating an alteration in the current flow in an external circuit. This alteration is the source of the electrical signal that is amplified and sent on to the recorder. A diagram of this detector is shown in Figure 12.13. The carrier gas for this detector is either pure nitrogen or a mixture of argon and methane. 

In a typical experiment, 50 mCi of [18F]F2 (specific activity 1 Ci/mmol i.e., containing 50 mkmol of non-radioactive 19F2) diluted with neon (100 mmol) was bubbled into a solution of the lactam (65 mmol) in Freon 113 (15 mL) at 0 C over a period of 15 min. The solvent was evaporated at rt with a stream of dry argon, and the oily product was dissolved in Et20 (2 mL). If necessary, the Af-[uF]fluorolactams can by purified by HPLC. 

Lai (13) gives the specific activity of argon-37 in the atmosphere produced by cosmic particle reactions as 0.19 d.p.m./kg. air in the stratosphere and 2.1 X 10 2 d.p.m./kg. air in the troposphere. The radioactivity expected in a 10-liter sample of purified argon from the troposphere would be 3.98 X 10"2 d.p.m. It should be possible to measure less than twice this radioactivity level using this detector. 

The FDD chromatograms show a great similarity to the classical FID detector and offers comparable performance without the use of a flame, radioactive emitter or combustible gases. The FDD in helium photoionization mode is an excellent replacement for FIDs in petrochemical or refinery environments, where the flame and use of hydrogen can be problematic. In addition, when the helium discharge gas is doped with a suitable noble gas, such as argon, krypton or xenon (depending on the desired cut-off point), the FDD can function as a specific photoionization detector for selective determination of aliphatics, aromatics, amines, as well as other species. 

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