Geiger-Muller counting

Since most of the radioisotopes used in biochemical research are (3 emitters, only methods that detect and measure /3 particles will be emphasized. Two counting techniques are in current use, scintillation counting and Geiger-Muller counting... 

Gas Ionization Counters A common gas ionization counter is the Geiger-Muller counter where the electronic pulses derived from the ionization process are registered as counts. The instrument can be adjusted to detect only radiation with a desired penetrating power. 

Lost counts correction is necessary because Geiger-Muller tubes have a dead-time in their operation. 

Use a Geiger-Muller counter (and periodically use paper discs to swipe surfaces for scintillation counting) to determine the radioisotope as well as the sites and amounts of contaminating radioactivity. 

A schematic representation of a Geiger-Muller counter. The high-energy radioactive particle enters the window and ionizes argon atoms along its path. The resulting ions and electrons produce a momentary current pulse, which is amplified and counted. 

Finally, the influence of the dead time D in eq. (7.3)) has to be taken into account, particularly if the dead time of the detector is high (as in the case of Geiger Muller counters) and if the counting rates of the sample and the calibration source are markedly different. 

Adsorption onto activated charcoal (Norit A) has been used for small oligonucleotides, up to tetranucleotides (Crane and Lipman 1953 Mandeles and Kammen 1966). The method is quicker and more convenient than DEAE-cellulose chromatography. Mandeles and Kammen (1966) used Norit A suspended in 1 mM phosphate 1 mM pyrophosphate buffer at pH 6.0 at a concentration of 100 mg/ml. The nucleotide solution is adjusted to pH 3 and 5 mg Norit for every optical density unit ( 40 pg) of nucleotides is added. The charcoal is collected on a filter paper and washed with water. (A good quantitative measure of or radio-activity may be obtained by counting the dry filter paper in a Geiger-Muller thin window counter.) The nucleotide material is eluted with a small volume of a mixture of ethanol, water and ammonia (600 400 6.5 v/v/v) which is then removed by drying under reduced pressure at 40°C. 

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