Quenching with tritium

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... 

The alkaline solution of [ Hj-NaBI was added to irradiated samples to a final concentration of 0.6 mM (specific activity= 156 mCi/mMole). The samples were allowed to react for one hour at 25 C. The reaction was quenched with an equal volume of 12 N HCl and the samples were desiccated in vacuo (<0.1mm Hg) at room temperature to near dryness to remove volatile tritium. To each sample was Aen added 0.5 ml of 0.1 N HCl and the desiccation repeated. The samples were resuspended in 4% SDS and subjected to centrifugation through a 5-ml column of Sephadex G-50 as described by Penefsky (4). The samples were then counted for tritium in an aqueous counting scintillant (Amersham Corp.) using a Beckman LS9000 liquid scintillation counter. 

Proof that the growing polybutadienyl ion might be cationic was evidently obtained by Childers (1963) using radioactive tracers. The catalyst was cobalt octoate and (C2H5)3Al2Cl3 (Al/Co = 10). One run was quenched with tritiated butanol, another with carbon-14 methanol. The tritium activity of the polymer was negligible compared to the carbon-14 activity which can be accounted for by reaction of the methanol with macromolecular carbonium ions. 

The preparation of samples labelled with tritium always requires more careful attention than samples labelled with carbon-14. Furthermore, when the samples are low in activity, then a procedure which results in a heavily quenched sample may necessitate a prohibitive amount of counting. Quenching is probably the greatest obstacle in sample preparation and is the term applied to any effect which reduces the light output by the system. The determination of quenching is synonymous with the determination of counting efficiency. The most common methods used to determine counting efficiency are (1) internal standard (2) channels ratio, and (3) the external standard method. As these methods have been described in detail recently, they are not further discussed [74, 78, 205]. 

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, ... 

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). 

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. 

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. 

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... 

The full details of the synthesis of squalene by Biellmann and Ducep have appeared (see Vol. 1, p. 161). This synthesis can be readily adapted for the introduction of tritium by quenching the ylide (8) with tritiated water before the alkylation step. 

The distilled water is counted in an LS system at a selected water-to-cocktail ratio (usually 1 1). Samples may be counted without purification if other radionuclides are known to be absent or can be differentiated clearly from tritium by pulse-height discrimination in the detection system, and if chemicals that cause excessive quenching or fluorescence are known to be absent. Section 15.4.3 illustrates an extension of this measurement technique, where is measured in flowing water with a scintillation counter. 

Tritium labelled compounds can also be used as true tracers for following the rates of homogeneous reactions. Thus, in the reaction of amines with epoxides, one can label the amine and by taking samples at fixed time intervals, quenching and subjecting them to radio-HPLC one can follow the disappearance of amine, the increase in the formation of a 1 1 complex and in some cases, the appearance of further complexes until reaction is complete (Fig. With so much information becoming available it is... 

This results in an error not correctable by external standardization or by internal standardization unless the internal standard has a distribution coefficient identical with the labeled molecule in the sample. Further, since phase contact in some solgel scintillators varies with the aqueous sample concentration, the accuracy of the quench correction will vary with sample size. In short, phase contact in solgel scintillators is at present an undefined variable which Ccin result in large and unsuspected errors in experimental results in the radioassay of tritium-labeled compounds. 

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