Patent GB-1396971-A (GB)
Title: Method and apparatus for measuring radioactivity of organic substance
IPC: G01T7/02
Filed: 1972-11-09
Abstract: 1396971 Radiochemical analysis; analysis of radio-active material SAGAMI CHEMICAL RESEARCH CENTER 9 Nov 1972 [10 Nov 1971 10 June 1972 (2)] 51731/72 Headings G1A and G1B [Also in Division H1] Gas mixtures containing at least one radioactive component are analyzed using a gas flow type proportional counter having a flow passage whose length is at least twenty times its diameter. This reduces errors in count rate produced by variations in gas flow-rate and effective passage volume and produces an output curve having a plateau (constant counting rate) which can be used directly as a measure of the radio-activity. A system for analyzing compounds labelled with radio-active elements is shown in Fig. 3. A sample of Triphenylmethane labelled with C-14 is introduced at port A into a dry combustion furnace 12 where it is split up into <SP>14</SP>CO 2 , <SP>3</SP>H 2 O and N 2 . He may be added as a carrier gas. The gas mixture passes through a cooling trap 16 surrounded with dry ice which condenses the <SP>3</SP>H 2 O and allows the other gases to pass via a needle valve 20 to a gas chromatography column 24 containing silica gel. The output of the column is connected to a thermal conductivity detector 26 and propane as a counter gas is added to the flow which passes through a proportional counter 30. The outputs of the thermal conductivity detector 26 and proportional counter 30 are recorded by a graph plotter the N 2  being measured first followed by the <SP>14</SP>CO 2 . After a suitable time a cock 18 is operated to pass the gas flow to a reducing furnace and simultaneously the cooling trap is heated to release the <SP>3</SP>H 2 O which enters the furnace where it reacts with C to release CO which passes to the measuring system while the <SP>3</SP>H 2  is occluded in the furnace. A quantity of light water is then inserted into the sample input A to flush any trapped <SP>3</SP>H 2 O to the reducing furnace and to dilute and release the occluded <SP>3</SP>H 2  which is then passed to the measuring system. If desired a second thermal conductivity detector (31) may be inserted between the furnace 12 and the cooling trap 16 to measure the overall quantity of <SP>14</SP>CO 2 , <SP>3</SP>H 2 O and N 2  produced, Fig. 4 (not shown). The proportional counter 30 comprises three gas flow counter tubes connected in series, each tube having a length of 95 cm and a cross-sectional area of 0À75 cm<SP>2</SP>. The carbon reducing furnace Fig. 5 (not shown), comprises a vertical porcelain tube (34) surrounded by a heating furnace (50) and containing a platinum mesh (44) resting on a bed of porcelain sand. A layer of carbon rests on the platinum mesh and the quantity present is maintained constant by means of a porcelain filler tube connected to a reservoir. The gas is passed up through the sand to the platinum mesh where it reacts with the carbon. In a modification Fig. 6 (not shown), a layer (52) of quartz cotton or fibres is placed between the platinum mesh and the porcelain sand to prevent the carbon falling through into the latter. If the proportional counter of the invention is used to measure C-14 and H-3 gases which are insufficiently separated by a gas chromatography column to produce separate counting peaks, the individual counting rates may still be determined by the plateaux forming shoulders at either side of the large central peak representing the combined counting rate.