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Methodology of tritium determination in aqueous samples by Liquid Scintillation Counting techniques

Liquid scintillation counting (LSC) is the most commonly used technique for measurement of tritium activity concentration in aqueous samples, therefore, this chapter offers an overview of advances in measurement methods and applications of different LSC methods for tritium determination, presents development and optimization of those methods as well as comparison of their advantages and limitations. With respect to low levels of tritium in the environment, although LSC technique in general is adequate for low-level measurements of environmental samples, certain conditions have to be met in order to obtain accurate and reliable results of tritium activity measurements, as discussed in this chapter. LSC methodology is convenient for aqueous samples since it assumes their mixing with liquid scintillation cocktails. Ultra-low background liquid scintillation spectrometer Quantulus 1220TM has been used in all presented experiments. Presented methods differ in sample preparation techniques: electrolytic enrichment, distillation, direct mixing of sample with scintillation cocktail (without any sample pretreatment), and sample combustion, which altogether result in different detection limits and range of application. Each of described methods is discussed in terms of its application, advantages, cost and limits of detection. Relevant optimal parameters were investigated and established for each method. Presented results enable evaluation of influence that scintillation cocktail/vial/blank sample selection have on figure of merit value and obtained tritium activity concentrations. Optimization of sample-to-cocktail ratio, choice of appropriate scintillation cocktail based on the comparison of their efficiency, background and minimal detectable activity determination were performed. Additionally, the effect of chemiluminescence and photoluminescence and different combinations of scintillator/vial were analyzed. Quench phenomena and their origins were described in brief as well, since they can seriously decrease detection efficiency and, consequently, obtained tritium activity concentrations during counting. Analysis of quench influence on detection efficiency resulted in quench correction curve that is presented also, with discussion on how various quenchers with different strengths affect LS measurements. From the presented results it could be concluded that different scintillation cocktails do not behave similarly in the presence of various quenchers and have different resistance, which indicates that quench correction curve is applicable only for specific experimental conditions, i.e. for a certain sample composition and LS counter. It should be developed/determined in each laboratory for each specific application. Liquid scintillation counting is irreplaceable in environmental tritium monitoring and research. Furthermore, one can choose the most adequate method for certain application that compromises the laboratory requirements and limitations among the presented sample preparation techniques, based on comparative study of their effectiveness, cost and minimum detectable activity concentration typical for each of presented methods.

Tritium ; measurement methods ; enrichment ; Liquid Scintillation Counting (LSC) ; optimization ; Quantulus 1220TM

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