Tritium quenching

It is of interest that the kinetic theories of Tait and co-workers, Yermakov et al., and Bohm all involve a two-stage propagation reaction and reversible adsorption of alkylaluminium compounds on the active centres the active centre concentrations being determined by tritium quenching, CO tagging and molecular weight data, respectively. 

Active Centre Determination. A number of recent publications involving the use of molecular weight data, tritium quenching, - and CO radio tagging " have appeared recently in the scientific literature. Since the validity of these methods has been assessed recently by the present author" no further details will be given in this Report. The as yet unsettled controversy concerning the use of "CO tagging method should be noted. ... 

Measurement of the amount of tritium in the quenched samples was made by reheating them in a normal hydrogen ambient and measuring the evolved radioactivity. Since the samples contained tritium not only dissolved in the bulk crystal but also attached to the surface, mainly in a thin oxide film, measurements were made with and without removal of the latter by etching with HF, a procedure verified to remove the surface radioactivity. Their final results for the solubility s fell nicely on the Arrhenius line, ... 

For general purpose tracer work, however, and particularly in polymer chemistry, the liquid scintillation counter surpasses all other instruments in its sensitivity and adaptability. There is no question on the author s mind that at the present time such an instrument would be the first choice, particularly where tritium, carbon-14 or sulphur-35 were involved. Samples for assay are dissolved in a phosphor whose major solvent usually consists of toluene, toluene-alcohol, or dioxan. Many polymers and low molecular weight compounds are readily soluble in these solvents. Prospective users should not be deterred by alleged complications due to "variable quench effects" as these effects are readily corrected for via internal or external standards or the channels ratio method (7, 46, 91). Dilution quench corrections, though valid, are tedious and unnecessary. Where samples are insoluble in phosphor they may be suspended (e.g. as gels or as paper cut from chromatograms, etc.) or they can be burnt and the combustion products absorbed in a suitable phosphor solution. A modification of the Schoniger flask combustion technique is particularly suitable for this purpose (43—45). 

There may be occasions when it is difficult to decide whether a polymerization is anionic or cationic. The question can readily be resolved by the use of labelled quench agents (77, 78). In particular the use of a doubly labelled methanol (14CH3OT) will yield the information in one experiment. Thus, with anionic polymerization, tritium would combine with the polymer (Reaction 11) while with cationic polymerization, carbon-14 would be combined (Reaction 12). 

Table 6. Tritium isotope effects for hydroxylic quench agents...

Quenching. The emission from tritium is so weak that it is readily absorbed by a filter paper or chromatographic strip on which it is adsorbed, or b> precipitates. Fewer problems occur with 14C, but the position of the energy... 

The sample is purified by distillation to separate the tritium-containing water from both non-radioactive and radioactive impurities. Various substances can cause scintillations by means other than radionuclide emission - by chemical fluorescence or luminescence - or interfere with ( quench ) detection of scintillations due to radionuclides. Even after purification, both processes are inevitable, but to a limited extent. Luminescence due to visible light will decay when the sample is stored in a darkened region of the LS system before the sample is counted. The degree of quenching, notably due to water in the sample, is determined instrumentally by reference to comparison sources and recorded, so that any deviation from the quenching observed for the tritium standard can be taken into account. 

Although our initial concerns about possible interferences due to chemiluminescence and quenching effects have been somewhat mitigated by recent disclosures of Blank and Kidwell s tritium counting procedures, an entirely different problem exists concerning their isotopic method, one that in our opinion casts serious doubt upon the general suitability of their approach for doing hair contamination/decontamination studies. 

Q = Mw/Mn = Polymer polydispersity index ROT Quenching with tritium labeled alcohol 1 1/mol sec... 

The rate constant of this reaction k ) can be estimated according to Eq. (VII) from the data on the polymer molecular mass vs. cocatalyst concentration. However, k may also be determined directly by measuring the number of aluminium-polymer bonds (Cai) formed in reaction (19). This determination may involve quenching of polymerization by alcohol with tritium-labelled hydroxyl The parameter... 

In these experiments a tritiated substrate was mixed with 1 -labeled protonated (for H/T KIE experiment) or deuterated (for D/T KIE experiment) substrate. The mixture was allowed to react in the presence of enzyme, under defined conditions of pH and temperature, and quenched at different fractional conversions. The quenched mixtures were analyzed by HPLC and liquid scintillation counting to determine the fractional conversion (f, determined from the counting) and tritium to ratio in the products ([ H/I cjy and pH/l4C]oo for the ratio at the time point and the infinity point, respectively). The L/T KIE (also denoted by (V/K)l or kt/kj) were calculated by equation 1 (Melander Saunders, 1987) ... 

Since the radioactive sample material from most methods of sample preparation is in intimate contact or in actual solution with the phosphor, the detection of emitted particles or radiation is highly efficient and may even approach 100%. Problems of self-absorption of the emissions are thus absent, or considerably smaller than those associated with planchette counting of solid samples. This is of particular importance for the measurement of low energy beta emitters such as tritium and carbon-14. On the other hand, the measurement method has intrinsic drawbacks such as quenching and chemiluminescence. 

It should quench as little as possible. This is particularly important when measuring radioisotopes such as tritium whose beta particles are of low energy. 

Liquid Scintillation Detector, This detector measures radioactivity by recording scintillations occurring within a transparent vial that contains the unknown sample and liquid scintillator. Because the radionuclide is intimately mixed with, or actually dissolved in, the fiquid scintillator, the technique is ideal for the pure -emitters, such as H, and Typical efficiencies for liquid scintillation counting in the absence of significant quenching are 60% for tritium and 90% for... 

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