Tritium, detection

The nonquantitative detection of radioactive emission often is required for special experimental conditions. Autoradiography, which is the exposure of photographic film to radioactive emissions, is a commonly used technique for locating radiotracers on thin-layer chromatographs, electrophoresis gels, tissue mounted on sHdes, whole-body animal sHces, and specialized membranes (13). After exposure to the radiolabeled emitters, dark or black spots or bands appear as the film develops. This technique is especially useful for tritium detection but is also widely used for P, P, and 1. 

Ioni2ation chamber -for radiopharmaceutical measurements [RADIOPHARMACEUTICALS] (Vol 20) -for tritium detection [DEUTERIUM AND TRITIUM - TRITIUM] (Vol 8)... 

In the water and oil industries, transport processes can be investigated. An example is the study of underground currents by application of tritiated water. In this application, advantage is taken of the high sensitivity of tritium detection (Fig. 20.1 T can be determined in an atomic ratio down to T H ... 

Liquid scintillation counting is by far the most common method of detection and quantitation of -emission (12). This technique involves the conversion of the emitted P-radiation into light by a solution of a mixture of fluorescent materials or fluors, called the Hquid scintillation cocktail. The sensitive detection of this light is affected by a pair of matched photomultiplier tubes (see Photodetectors) in the dark chamber. This signal is amplified, measured, and recorded by the Hquid scintillation counter. Efficiencies of detection are typically 25—60% for tritium >90% for and P and... 

Gas-flow counting is a method for detecting and quantitating radioisotopes on paper chromatography strips and thin-layer plates. Emissions are measured by interaction with an electrified wire in an inert gas atmosphere. AH isotopes are detectable however, tritium is detected at very low (- 1%) efficiency. 

Other methods of sensitive detection of radiotracers have been developed more recently. Eourier transform nmr can be used to detect (nuclear spin 1/2), which has an efficiency of detection - 20% greater than that of H. This technique is useful for ascertaining the position and distribution of tritium in the labeled compound (14). Eield-desorption mass spectrometry (fdms) and other mass spectral techniques can be appHed to detection of nanogram quantities of radiolabeled tracers, and are weU suited for determining the specific activity of these compounds (15). 

Enhanced Reaction Kinetics. For reactions involving tritium, the reaction rates are frequendy larger than expected because of the ionising effects of the tritium P-decay. For example, the uncataly2ed reaction 2T2+O2 — 2X20 can be observed under conditions (25°C) for which the analogous reaction of H2 or D2 would be too slow for detection (30). 

Tritium is readily detectable because of its radioactivity. Under certain conditions concentrations as low as 370 )-lBq/mL (10 //Ci/mL) can be detected. Most detection devices and many analytical techniques exploit the ioni2ing effect of the tritium P-decay as a principle of operation (62,63). 

Mass Spectrometer. The mass spectrometer is the principal analytical tool of direct process control for the estimation of tritium. Gas samples are taken from several process points and analy2ed rapidly and continually to ensure proper operation of the system. Mass spectrometry is particularly useful in the detection of diatomic hydrogen species such as HD, HT, and DT. Mass spectrometric detection of helium-3 formed by radioactive decay of tritium is still another way to detect low levels of tritium (65). Accelerator mass spectroscopy (ams) has also been used for the detection of tritium and carbon-14 at extremely low levels. The principal appHcation of ams as of this writing has been in archeology and the geosciences, but this technique is expected to faciUtate the use of tritium in biomedical research, various clinical appHcations, and in environmental investigations (66). 

Thermal-Conductivity Analyzer. The thermal-conductivity analy2er operates on the principle that the loss of heat from a hot wire by gaseous conduction to a surface at a lower temperature varies with the thermal conductivity of the gas, and is virtually independent of pressure between 1.3 kPa (10 mm Hg) and 101 kPa (1 atm). This technique is frequently used in continuous monitors for tritium in binary gas mixtures for immediate detection of process change. 

The emitted P particles excite the organic molecules which, in returning to normal energy levels, emit light pulses that are detected by a photomultiplier tube, amplified, and electronically counted. Liquid scintillation counting is by far the most widely used technique in tritium tracer studies and has superseded most other analytical techniques for general use (70). 

A widely used instmment for air monitoring is a type of ionization chamber called a Kaimn chamber. Surface contamination is normally detected by means of smears, which are simply disks of filter paper wiped over the suspected surface and counted in a windowless proportional-flow counter. Uptake of tritium by personnel is most effectively monitored by urinalyses normally made by Hquid scintillation counting on a routine or special basis. Environmental monitoring includes surveillance for tritium content of samples of air, rainwater, river water, and milk. 

Tritium first detected in atmospheric hydrogen (V. Fairings and P. Harteck) and later shown to be present in rain water (W. F. Libby et al, 1951). 

The triatomic hydrogen molecule ion H3+ was first detected by J. J. Thomson in gas discharges and later fully characterized by mass spectrometry its relative atomic mass, 3.0235, clearly distinguishes it from HD (3.0219) and from tritium... 

When judging by communications available in the open literature, none of effects 1 through 5 could so far be observed repeatedly and reproducibly under rigorously controlled conditions. Provisionally, all instances of published experimental confirmation can be placed into two groups (1) the observation of sporadic sufficiently pronounced manifestations, and (2) the observation of more invariant but very weak effects (as a rule, at the level of background noise, particularly in the detection of neutrons and tritium). However, there were far fewer confirmations than infirmations (i.e., work in which the successful experiments could be carefully reproduced or the method used to determine the products was analyzed and shown to be in error). Such work has been of exceptional value in the area of advancing the methods and techniques used in experimental studies. 

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