Химия и химические технологии/ 9. Радиохимия

Samoilov I.V.¹, Shatrov A.N.¹, Ibraeva L.S.²

^{1 "}Institute of Radiation Safety and Ecology" branch of the NNC RK, Kurchatov, Kazakhstan

² Shakarim State University of Semey, Kazakhstan

Optimizing the determination technique of organically bound tritium in biological objects

Tritium (H³) is widely distributed radionuclide in nature, which can be attributed to a number of the major dose-forming radionuclides. Human activity has led to an increase of H³ revenues into environment due to atmospheric testing of nuclear weapons and nuclear reactors. In components of ecosystem tritium exists mainly in two forms - as tritiated water (HTO) and as organically bound tritium (OBT). Easily volatile OBT can accumulate in plants, animals and humans. OBT can exist in human body much longer than tritiated water; biological half-life is 4 times greater than HTO, and therefore relative contribution in exposure dose will be higher.

The most common methods for OBT determination are following: 1) direct method for biological sample measurement by liquid scintillation spectrometer; 2) measurement by using a proportional counter; 3) mass spectrometric method; 4) measurement in ionization chamber. Each of this method has disadvantages. For example: direct method of biological sample measurement by liquid scintillation spectrometer has chemiluminescence quenching effects which affected to the accuracy of measurements; mass-spectrometric method - expensive equipment and complexity with samples preparation; ionization chamber and proportional counter methods – low accuracy.

As a result of possible methods analysis the most optimal method is combustion of the sample in continuous stream of oxygen, water vapor trapping with subsequent measurement of the obtained sample by liquid scintillation spectrometry. In the present work for OBT determination was used commercial PerkinElmer Sample Oxidizer model 307 ("Oxidizer") device - a system of automatic sample preparation for samples containing tritium and carbon-14 and liquid-scintillation counter - Quantulus 1220.

Sample preparation on «Oxidizer» for spectrometric analysis and spectrometric analysis has several methodological features and for their studies was raised a number of experiments. Beta spectrometer was calibrated by detection efficiency, was studied the effect of direct sunlight and heat on obtained counting samples, was identified an optimal weight for a number for some kind samples, impact of cyclicality on the measurement results and also was studied the presence of organic matter in the obtained counting sample, which can has negative affect on the results.

The samples should be stored in a cool, dark, out of direct sunlight, place to avoid getting pseudo signals. In the case of exposure to sunlight or heat the sample must be kept at a temperature 13-15⁰C within 12 hours. Under influence of sunlight and heat to the counting samples the background count rate increased more than 500 times.

For a number of some biological objects was identified an optimal sample weight based on the smallest value SQP (standard quenching parameter for LSC Quantulus). All biological objects were divided into the following categories: 1) proteins (meat, cheese) - sample weight to 0.8 g; 2) fats (animal and vegetable fats) - sample weight 0.3 g; 3) carbohydrates (plants) - sample weight to 1 g. With the sample mass which is greater than the above mentioned values, there is incomplete sample decomposition with the formation of soot, the presence of which leads to a strong quenching of the sample. With strong quenching the obtained samples can go beyond constructed quench curve, and as a consequence, for such samples cannot be obtained correct results.

Is not allowed the presence of organic material in obtained counting samples, because this leads to pseudo signals appearance, and as a result, not getting correct results. Adding to the background counting sample of organic matter (vegetable oil) leaded to his quenching and a sharp increase in the counting rate.

Also was received some specifications for the Sample Oxidizer device (output, leak, memory) required for further studies, which were as follows: output - 96.5%, memory - 0.41% leak - 0.13%.