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A comparison of two methods used for determination of tritium concentration in urine samples by liquid scintillation counter

  • Serdar Dizman
  • Recep Keser
Original Paper

Abstract

Background

Tritium (3H) emits low-energy beta particles with a maximum energy of 18.6 keV. Liquid scintillation counting technique (LSC) is mostly used for the detection and quantification of low-energy emitters like H-3. The most widely used method to determine the level of tritium in humans is testing urine.

Method

In this study, tritium activity concentrations in urine samples taken from 20 adults were measured. Their ages range from 23 to 50. Eight of them are female, and others are male. The tritium activity concentrations in urine samples were determined with two different methods. Also, two standard samples were used to compare urine samples.

Result

The efficiency values were found with two different methods (26.07% for the first method and 26.14% for the second method). According to the comparison results, the tritium concentration differences between these methods were found in a negligible level for urine samples and in an acceptable level for standard samples.

Conclusion

The tritium activity concentrations in urine and standard samples were calculated using two different methods. According to the comparison results, these two methods can be used for determination of tritium concentrations in urine samples.

Keywords

Comparison Liquid scintillation counter Tritium Urine 

Introduction

Tritium (\(^{3}\hbox {H}\)) emits low-energy beta particles with a maximum energy of 18.6 keV, and with a half-life of 12.3 years [1]. It is found in nature, and it can also be produced by human activities. Naturally produced tritium is the result of nuclear reactions between atmospheric atoms and cosmic rays in the upper atmosphere or the result of spontaneous fission of natural uranium in the Earth’s crust [2, 3]. Artificial tritium is produced by past atmospheric nuclear tests, or it is the result of continuous releases from nuclear power plants in normal operation, incidental releases from nuclear facilities and commercial products (e.g., self-luminous light sources) that use tritium isotopes [3, 4].

Once released to the atmosphere, inorganic tritium is rapidly oxidized to tritiated water (HTO). HTO can enter the human body by inhalation, absorption through the skin, or ingestion of food and drinking water [5, 6]. The activity concentration of tritiated water in urine is the equilibrium value reached by the long-term (more than 4 h) equilibration of body fluids and soft tissue [7, 8]. Thus, analysis of tritiated water in a single sample of urine represents the activity concentration in body water at the time the sample was collected. Liquid scintillation counting technique (LSC) is mostly used for the detection and quantification of low-energy \(\beta \) emitters like H-3 and C-14 [9]. There are many studies related to the optimization of tritium analysis in LSC [10, 11, 12, 13, 14]. These studies focus on the sample/cocktail ratio, quenching, sample stability, efficiency, and other parameters.

In this study, efficiency values were found by two different methods and then the concentrations of tritium activity in urine samples were determined using these values. The main aim of this comparison study is to determine more suitable method between two methods used for determination of tritium radioisotope in urine samples by the liquid scintillation counting. At this point, this study will be a reference to researchers working on the relevant issues.
Table 1

Activity value of certified tritium solution, stages of preparation of STD1 and STD2 standards, and their activity values

(A) Certified tritium solution

(B) Main stock solution

Amount of A solution (mL)

20.66

Amount taken from A solution (mL)

5

Amount of B solution (mL)\(^{\mathrm{a}}\)

12

Reference date

1.06.2013

Reference date

30.07.2013

Activity (kBq/L)

\(4995 \pm 160\)

Activity (kBq/L)

\(2062 \pm 66\)

(C) STD1 standard solution

(D) STD2 standard solution

Amount taken from B solution (mL)

4

Amount taken from C solution (mL)

200

Amount of C solution (mL)\(^{\mathrm{a}}\)

2000

Amount of D solution (mL)\(^{\mathrm{a}}\)

2000

Reference date

30.07.2013

Reference date

30.07.2013

Activity (Bq/L)

\(4125 \pm 132\)

Activity (Bq/L)

\(412.5 \pm 13.2\)

\(^{\mathrm{a}}\)Including amounts taken from previous solution

Fig. 1

Efficiency dependence of tSIE (quench curve)

Table 2

Tritium activity concentrations found by M1 and M2 methods in the urine samples

Sample code

Calculated tritium activity (Bq/L) with M1 method

Calculated tritium activity (Bq/L) with M2 method

1

\(6.07 \pm 1.06\)

\(6.00 \pm 1.05\)

2

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

3

\(6.33 \pm 0.82\)

\(6.24 \pm 0.80\)

4

\(3.90 \pm 0.81\)

\(4.06 \pm 0.84\)

5

\(9.97 \pm 1.29\)

\(9.90 \pm 1.28\)

6

\(11.51 \pm 0.97\)

\(11.46 \pm 0.96\)

7

\(7.16 \pm 0.88\)

\(7.13 \pm 0.82\)

8

\(7.10 \pm 0.82\)

\(7.32 \pm 0.91\)

9

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

10

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

11

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

12

\(3.77 \pm 0.88\)

\(3.85 \pm 0.90\)

13

\(3.52 \pm 0.77\)

\(3.67 \pm 0.81\)

14

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

15

\(4.67 \pm 0.84\)

\(4.65 \pm 0.83\)

16

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

17

\(3.13 \pm 0.80\)

\(3.18 \pm 0.81\)

18

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

19

\(10.04 \pm 1.08\)

\(10.34 \pm 1.12\)

20

\(<\hbox {MDA}\)

\(<\hbox {MDA}\)

Table 3

Activity values in the measurement date of the standard samples and their calculated activity values

Standard samples

STD1

STD2

Tritium activity at the measurement date (Bq/L)

\(3958.01 \pm 131.99\)

\(395.81 \pm 13.20\)

Calculated tritium activity (Bq/L) with M1 method

\(3952.11\pm 184.68\)

\(375.98 \pm 19.25\)

Calculated tritium activity (Bq/L) with M2 method

\(3720.35\pm 161.70\)

\(363.96 \pm 17.74\)

Relative bias (%) for M1 method

0.15

5.01

Relative bias (%) for M2 method

6.00

8.05

Experimental studies

Urine samples were collected randomly from 20 adult participants living in Rize Province of Turkey. Eight participants were female, and ages of all the participants ranged from 23 to 50. About 50 mL spot urine sample was collected in a 100-mL polyethylene bottle. The samples were treated rapidly to avoid unexpected contamination. The samples were prepared for analysis according to the procedure described by Dizman et al. [15]. 10 mL of prepared samples was added into the measuring plastic vial (Zinsser Analytics, 20 mL), and then the rest of vial was filled with a scintillation cocktail (Ultima Gold LLT, Perkin Elmer Inc.) up to total volume of 20 mL. Also, the vials were shaken for about 1 min in order to make homogeneous solutions and the outer surface of vials was cleaned with ethanol to prevent contamination.

Two standard samples (STD1 and STD2) were prepared from a certified liquid tritium solution (Eckert and Ziegler, P.O. No.: P700723, Source No.: 1676-44). The preparation procedure of STD1 and STD2 standards with the activity of the certified tritium solution is given in Table 1. The vials of the standard samples were prepared at the same mixing ratio (10:10 mL) as the urine samples. Tritium concentrations in urine and standard samples were determined using liquid scintillation counting (Perkin Elmer Life and Analytical Sciences, Low Activity Liquid Scintillation Analyzer, Tri-Carb 2910 TR, USA). The measurement time was set to 60 min and repeated six times to evaluate the detection uncertainty and minimum detectable activity (MDA). The measurement uncertainties were carried out at the 95% confidence level and according to the method defined in IAEA-TECDOC-1401 [16]. The background sample was prepared by using twice-distilled groundwater with a low tritium concentration.

Tritium activities of the samples and standards were obtained using Eq. (1).
$$\begin{aligned} A\left( {Bq/L} \right) =\frac{\hbox {cps}_{\mathrm{s}} -\hbox {cps}_{\mathrm{b}}}{\varepsilon \cdot \hbox {V}} \end{aligned}$$
(1)
where \(\hbox {cps}_{\mathrm{s}}\) is the counts per second of the analyzed sample or standard (cps), \(\hbox {cps}_{\mathrm{b}}\) is the counts per second of the background sample (cps), \(\hbox {V}\) is the volume of the sample (liter), and \(\varepsilon \) is the efficiency.
The efficiency value in Eq. (1) was calculated with two different methods. Efficiency value of the first method was calculated using Eq. (2).
$$\begin{aligned} \varepsilon =\frac{{\text {cps}}_{\mathrm{st}} -{\text {cps}}_{\mathrm{b}}}{A_{\mathrm{st}}}, \end{aligned}$$
(2)
where \(\hbox {cps}_{\mathrm{st}}\) is the counts per second of the tritium standard (STD1), \(\hbox {cps}_{\mathrm{b}}\) is the counts per second of the background sample, and \(\hbox {A}_{\mathrm{st}}\) is the activity of the standard (STD1) in dps (disintegrations per second) unit.
The efficiency value of the second method is obtained according to the procedure described by Dianu et al. [17]: One of the spectral analysis methods in Packard’s TriCarb Liquid Scintillation Analyzers is to measure quench by the transformed Spectral Index of the External standard (tSIE). Ten standard tritium samples with different quenching levels were prepared. The same tritium concentration was used in all samples. To obtain different quenching effects, various volumes of the quenching agent (carbon tetrachloride) were added into vials. Then, vials were counted in the tritium (0–18.6 keV) counting windows, and the quench curve (Fig. 1) of tritium was obtained by using the tSIE parameter. Quench curve was fitted with third-degree polynomial function, and the following efficiency Eq. (3) was obtained.
$$\begin{aligned} {\text {Efficiency}}= & {} 3.04\times 10^{-8} (\hbox {tSIE})^{3}- 13.75\times 10^{-5} (\hbox {tSIE})^{2}\nonumber \\&+\,0.1695 (\hbox {tSIE}) - 7.3651 \end{aligned}$$
(3)
tSIE value for each sample is given in the result report of LSC. According to tSIE values, efficiency and activity values for samples are calculated, respectively. These two methods defined by Eqs. (2) and (3) were called as M1 and M2 in the rest of the paper, respectively.

Results and discussion

The tritium concentrations in the urine samples are shown in Table 2 for both M1 and M2 methods. The efficiency value for M1 method was found as 26.07% by Eq. (2). In the M2 method, the average efficiency value is calculated (26.14%) since the tSIE value for each sample changes. The MDA for M1 method was 2.64 Bq/L. MDA for M2 method is calculated separately for each sample, and average MDA is found as 2.63 Bq/L. Thus, the tritium activity values in the samples are determined to be above or below the MDA. The activity concentrations of eight samples (40%) for both M1 and M2 methods were below the MDA. The average tritium activity concentrations in the urine samples for M1 and M2 methods were found as \(6.43 \pm 1.63\) and \(6.48 \pm 1.62\) Bq/L, respectively. The results show that the average tritium activity value found by M2 method is only 0.05 more than the value found by M1 method.

The tritium activity concentrations of STD1 and STD2 by M1 method were found as \(3952.11 \pm 184.68\) and \(375.98 \pm 19.25\) Bq/L, respectively. The tritium activity concentrations of STD1 and STD2 by M2 method were found as \(3720.35 \pm 161.70\) and \(363.96\pm 17.74\) Bq/L, respectively. The relative bias and tritium activity values found by M1 and M2 methods for STD1 and STD2 are given in Table 3. These results show that the values found by M1 method for STD1 and STD2 are 6.23 and 3.20%, respectively, which are higher than the values found by M2 method. Also, the relative bias values found for STD1 and STD2 by M1 and M2 methods are in an acceptable level. Taking account of these results, these two methods (M1 and M2) can be used for determination of tritium activity concentrations in urine samples.

Conclusion

The tritium activity concentrations in urine and standard samples were calculated using two different methods. The results of these methods were compared with each other. According to the comparison results, a difference was found in a negligible degree between the tritium activity values found by these two methods for urine samples. Besides, a difference between the tritium activity values found by these two methods for standard samples was found in an acceptable degree. Therefore, these two methods can be used for determination of tritium concentrations in urine samples.

Notes

Acknowledgements

This study was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK) under the Project No. 214S221.

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Copyright information

© Institute of High Energy Physics, Chinese Academy of Sciences; Nuclear Electronics and Nuclear Detection Society and Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.Department of Physics, Faculty of Sciences and ArtsRecep Tayyip Erdogan UniversityRizeTurkey

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