Utilization of analytical technique for evaluation of δ13cdic in groundwater using ea-Irms under the condition of Vietnamese lab - Thinh Thi Hong Nguyen

Journal of Science and Technology 54 (4B) (2016) 48-55 UTILIZATION OF ANALYTICAL TECHNIQUE FOR EVALUATION OF δ13CDIC IN GROUNDWATER USING EA-IRMS UNDER THE CONDITION OF VIETNAMESE LAB Thinh Thi Hong Nguyen1, Minh Hoang Tran2, Tuan Dinh Phan3, Hung Cam Ly3, * 1 Institute for Nuclear Science & Technology, 179 Hoang Quoc Viet street, Cau Giay district, Hanoi, Vietnam 2Binh Duong Department of Natural Resources and Environment, 7th Floor, Tower B, Intergrated Political Administration Center of Binh Duong, Hoa Phu ward, Thu Dau Mot City, Binh Duong Province, Vietnam 3Hochiminh City University of Natural Resources and Environment, 236B Le Van Sy, Ho Chi Minh City, Vietnam *Email: lchung@hcmunre.edu.vn Received: 15th August 2016; Accepted for publication: 10th November 2016 ABSTRACT As the demand of extraction and consumption of groundwater increases, its quality has been facing the issue of water contamination caused by industrial as well as domestic activities. Isotopic ratio analysis is a modern and effective technique widely used to trace the source of groundwater contamination. In this study, an analytical technique for carbon stable isotopic ratio of dissolved inorganic carbon in groundwater using elemental analyzer - isotopic ratio mass spectrometer was utilized under the condition in Vietnamese lab. As a result, correlation and calibration equations were established in which good reproducibility of the international standards was achieved with the value of first standard deviation less than 0.3 ‰ as well as the accuracy and precision of the measurements were obtained with errors less than 0.12 ‰. Based on these equations, 15 samples of groundwater collected from Cu Chi (Viet Nam) district were analyzed and calculated. Keywords: isotopic ratio, carbon, dissolved inorganic carbon, EA-IRMS, groundwater. 1. INTRODUCTION Carbon has three isotopes that are useful tracers in the terrestrial ecosystem: stable 12C and l3C; and radioactive 14C. Many investigations, particularly in the water management, biological sciences and climate change, have focused on quantifying variation in carbon stable isotope Utilization of analytical technique for δ13CDIC in groundwater using EA-IRMS under the 49 ratios 13C/12C [1-3]. Although natural variations in the 13C/12C ratio are very small, the differences in this ratios are sufficient to trace various chemical, physical and biological processes; specially the source of groundwater contamination. The relative variation of stable isotope ratio in natural compounds is expressed in term of a well-known notation δ using the unit of parts per thousand (‰) due to its quite small value. The equation is presented as: δ13C (‰) = (RS / RR – 1) × 1000 (1) where RS and RR are the isotopic ratio 13C/12C in the sample and in the Pee Dee Belemnite (V- PDB) standard, respectively [4]. According to the published data about the carbon isotopic ratio, carbonate rocks typically have δ13C values of ± 5 ‰. Plants convert atmospheric carbon with δ13CCO2 value of -7 ‰ to organic compounds via photosynthesis. Then, C3 plants (such as pine and apple trees) have δ13C values in the range from -22 to -33 ‰, whereas C4 plants (e.g. corn) have δ13C values of around -10 to -20 ‰. For dissolved inorganic carbon (DIC), the δ13CDIC value in catchment waters is generally in the range of -5 to - 25 ‰ [5]. The EA-IRMS (elemental analyzer - isotopic ratio mass spectrometer) technique has been widely accepted for the routine analysis of isotopic ratios, such as 15N/14N and 13C/12C [6-8] as well as 34S/32S [9-13]. However, the implementation of EA_IRMS technique to determine the 13C/12C ratio has not been utilized in Vietnamese laboratory. So far, the Institute for Nuclear Science & Technology in Vietnam is the only lab having EA_IRMS system which is able to support for isotopic ratio analysis. Hence, the analytical technique for determination of δ13CDIC in groundwater using EA_IRMS need to be utilized in order to establish the correlation and calibration equations to enhance the reproducibility of this method as well as improve the accuracy for the calculation of the δ13CDIC in groundwater. In this study, the international standards IAEA CO-8, IAEA CO-9 and NBS-19 were used to optimize the operation condition of EA-IRMS system by which the correlation and calibration equations were obtained and 15 samples of groundwater were then analyzed and calculated. 2. MATERIALS AND METHODOLOGY 2.1. Chemicals and instruments All chemicals and instruments used for this study such as Cromium oxide (Cr2O3 – 99 %), silvered cobaltous cobaltic oxide (Co3O4/Ag), copper (99.5 %), magnesium perchlorate (MgClO4- 99.5 %), quartz fused (SiO2 – 99.5 %), tin capsule, quartz tube reactor were commercially purchased from Microanalysis (UK), and used as received without any further purification unless otherwise noted. The international standards IAEA CO-8, IAEA CO-9 and NBS-19 supplied by International Atomic Energy Agency and National Bureau of Standards have been used in this study for carbon calibrations. All groundwater were collected from Cu Chi district, Hochiminh City, Vietnam and stored in bottles which have been sealed with paraffin to minimize atmospheric CO2 contamination. The alkalinity of groundwater will be determined by Vietnamese standard (TCVN6636-1:2000) before going to analyze the DIC. Hung Cam Ly, et al 50 The instrumentations used in the study were the Elemental Analysers (EA from EuroVector, Italy) and Isotope ratio mass spectrometers (IRMS from IsoPrime, UK) together with a Reference Gas Injector system (RGI). 2.2. Methodology 2.2.1. Determination of DIC Depending on the value of alkalinity, the volume of samples will be calculated in order to obtain at least 0.5 mg of BaCO3. The calculated volume of sample is poured from the full bottle, and DIC of the water sample is precipitated by the addition of Ba(OH)2. The bottle is immediately capped. The sample is allowed to precipitate at ambient temperature for at least 24 hours before further sample processing takes place. After samples have been treated with Ba(OH)2, they are filtered through 0.45 μm glass filters to separate the BaCO3 from the water and any excess Ba(OH)2 in the sample bottle. The sample is collected on the filter paper and then flushed many time with deionized water to bring the pH of the carbonate to a neutral level (pH between 7 and 8). Once the pH is neutral, the filter paper with sample is transferred to a glass Petri dish and dried overnight at 90 °C. A representative sample is placed in a 2 ml dry, clean, air-tight vial and ready for analysis by EA-IRMS. 2.2.2. Determination of δ13CDIC Prepared samples are loaded in Sn tightly crimped capsules to avoid any trapping of moisture before the combustion. After O2 pulsed injection, the capsule is dropped into a combustion furnace containing chromium oxide and silvered cobaltous oxide and operating at a temperature of 1030 °C. When O2 is introduced, the Sn oxidation creates an exothermic reaction that ensures a complete combustion and oxidation of the sample. Then, the mixture flows into the reduction chamber containing copper and operating at 650 °C. The reduced Cu absorbs the excess O2 and the He gas carries the products of the combustion (N2, CO2, and H2O) via a water trap for subsequent phase separation on the gas chromatographic column (GC) where separation of CO2 and N2 is performed. It should be noticed that the background signal obtained in the elemental analyzer (showed on the thermal conductivity detector - TCD) should be set to its minimum level and a good chromatographic separation of the gases needs to be obtained with complete H2O absorption by the water trap before going into the IsoPrime IRMS where the isotope ratios δ13C are reported relative to a reference gas standard. IAEA CO-8, IAEA CO-9 and NBS 19 are three international standards which the values of δ13CV-PDB are known as -5,749 ‰; -47,119 ‰ and +1,95 ‰, respectively [4]. These values will be used for carbon calibration. This calibration is obtained by plotting raw δ13C values of standards measured by EA-IRMS as a function of their known isotopic compositions. Furthermore, it is necessary to determine the correlation between raw δ13C values of standards and δ13C value of CO2 reference from which δ13CDIC (‰) of groundwater can be calculated. 3. RESULTS AND DISCUSSION 3.1. The recovery of DIC Utilization of analytical technique for δ13CDIC in groundwater using EA-IRMS under the 51 As mentioned in the determination of DIC, the volume of samples needs to be calculated in order to obtain at least 0.5 mg of BaCO3. To do that, after the alkalinity of the samples are measured in the range of 400-600 mg HCO3-/L (not shown in this paper), the recovery of DIC needs to be determined. The procedure for precipitation of DIC is applied for the samples with known concentrations. The results presented in Table 1 show that the recovery of DIC can reach an average value of 93.14 %. Hence, the minimum volume of samples is calculated as 40 ml to ensure required amount of samples ready for analysis by EA-IRMS. Table 1. The recovery of DIC. No Samples Volume mBaCO3 known mBaCO3 measured The recovery (ml) (mg) (mg) (%) 1 NaHCO3 15 49.335 46.129 93.50 2 NaHCO3 15 49.335 46.585 94.43 3 NaHCO3 15 49.335 46.326 93.90 4 Na2CO3 15 45.0478 41.903 93.02 5 Na2CO3 15 45.0478 41.205 91.47 6 Na2CO3 15 45.0478 41.678 92.52 3.2. Isotopic ratio analysis of international standards Before starting isotopic analyses of the samples, the operation of EA-IRMS system needs to be optimized such as the reference gas carried by He should generate a stable signal in the mass spectrometer source, the background signal needs to be obtained in the elemental analyzer, a good chromatographic separation of the gases and completed H2O absorption by the water trap are required. Measured 13C/12C ratios are obtained by comparing integrated peak areas for atomic number 44 and 45 of the sample CO2 pulses compared to those for the sample and reference gases. The final isotopic ratio measurements of IAEA CO-8, IAEA CO-9, NBS 19 and CO2 reference gases are expressed in term of δ13CV-PDB and δ13CCO2-ref. The results together with the known values of IAEA CO-8, IAEA CO-9 and NBS 19 are presented in Table 2. Table 2. δ13C values of the C isotopic standards used for the EA-IRMS technique. The average values of 13CV-PDB measured for IAEA CO-8, IAEA CO-9 and NBS 19 samples are obtained as 5.68 ‰, -47.17 ‰ and 1.83 ‰, respectively. The reproducibility of the standards is acceptable as shown by value of the first standard deviation in the range of 0.07 to 0.27. Isotopic standards Nature δ13CV-PDB known [4] (‰) δ13CV-PDB measured (‰) δ13CV-PDB average (‰) Standard deviation (σ1) δ13CCO2_ref 1st 2nd 3rd IAEA-CO 8 calcite -5.749 -5.55 -5.7 -5.8 -5.68 0.13 18.11 IAEA-CO 9 BaCO3 -47.119 -47.35 -47.31 -46.86 -47.17 0.27 -22.63 NBS-19 calcite 1.95 1.79 1.91 1.8 1.83 0.07 26.50 Hung Cam Ly, et al 52 Furthermore, the accuracy and precision of the EA-IRMS isotopic ratio measurements can be observed by comparing the measured values with the known values of δ13CV-PDB (error < 0.12 ‰). 3.3. Correlation and calibration 3.3.1. The correlation of δ13C values Based on the analysis results, a plot of δ13CV-PDB-avg and δ13CCO2-ref can be performed and shown in Figure 1. A correlation equation for this relationship can be obtained as in equation (2): δ13CV-PDB-avg = 1.0041 δ13CCO2-ref – 24.365 (2) Figure 1. Correlation of δ13C values of the isotopic standards. According to this figure, the values of δ13CV-PDB-avg vary from 1.95 ‰ to -47 ‰ while the values of δ13CCO2-ref change from 26.50 ‰ to -22.63 ‰ and the correlation equation has a very good R-squared value (R2 = 0.9997) which indicates the regression line fit well the data. Later on, this correlation equation will be used to calculate the values of δ13C measured from the samples of groundwater. 3.3.2. The carbon calibration The calibration can be obtained by plotting δ13CV-PDB-avg values of international standards (IAEA CO-8, IAEA CO-9 and NBS 19) measured by EA-IRMS as a function of their known isotopic compositions, as shown in Figure 2. Figure 2. Calibration determined using δ13C values of the isotopic standards. Utilization of analytical technique for δ13CDIC in groundwater using EA-IRMS under the 53 A good calibration equation can be generated based on these data. Later on, all measured data will be corrected by the following equation: δ13CV-PDB (corrected) = 0.9999 δ13CV-PDB (measured) + 0.0329 (3) It is important to have standards with a wide spread of isotopic compositions in order to generate an accurate and robust calibration equation. All measured results show very good agreement with the known values of δ13CV-PDB, the error is around 0.05 ‰ to 0.12 ‰. Here, the good calibration obtained offers a precise correction along all of this range (from -50 ‰ to +5 ‰) for a wide spread of sample C-isotopic values. 3.4. Isotopic ratio analysis of groundwater The optimized condition used in the procedure of isotopic ratio analysis of international standards was applied similarly to analyze the samples of groundwater collected from Cu Chi district, Ho chi minh City, Vietnam. Using the correlation and calibration equations obtained, the value of δ13CDIC were determined and corrected for 15 samples of DIC in groundwater, as reported in Table 3. Table 3. δ13C value of groundwater collected from Cu Chi district. No Samples δ13CCO2-ref (‰) δ13CDIC(measured) (‰) δ13CDIC(corrected) (‰) 1 N5 5.90 -18.44 -18.41 2 N6 9.44 -14.88 -14.85 3 N13 18.53 -5.76 -5.73 4 N16 6.38 -17.95 -17.92 5 N17 5.04 -19.30 -19.27 6 N18 7.48 -16.85 -16.82 7 N20 7.98 -16.35 -16.32 8 N21 8.35 -15.98 -15.95 9 N22 6.14 -18.20 -18.17 10 N23 5.79 -18.55 -18.52 11 N24 5.60 -18.74 -18.71 12 N25 6.77 -17.56 -17.53 13 N26 6.96 -17.37 -17.34 14 N27 7.17 -17.16 -17.13 15 N28 5.49 -18.85 -18.82 The results show that the values of δ13CCO2-ref are in the range of 5.04 ‰ to 18.53 ‰ and δ13CDIC in groundwater are corrected as varying from -19.27 ‰ to -5.73 ‰, respectively. The results are consistent with previous studies of the isotopic ratio of DIC in groundwater [5]. These values will provide information for tracing the source of carbon dissolved in groundwater which may come from carbonate rocks, degradation of organic matter, or human pollution sources. Hung Cam Ly, et al 54 4. CONCLUSIONS Analytical technique for δ13CDIC in groundwater using EA-IRMS was utilized under the condition in Vietnamese lab. As a result, the recovery of DIC in groundwater can reach an average value of 93.14 %. Good reproducibility of the international standards were achieved with the value of first standard deviation less than 0.3 ‰ as well as the accuracy and precision of the EA-IRMS isotopic ratio measurements can be obtained with the error less than 0.12 ‰ as comparing the measured values to the known values of δ13CV-PDB. Furthermore, the correlation and calibration equations were established, contributing to standardize and improve the accuracy for the calculation of the δ13CDIC in groundwater. Consequently, 15 samples of groundwater were analyzed and the values of δ13CDIC were determined in the range of -5.73 ‰ to -19.27 ‰, being consistent with the previous studies in the literature. Acknowledgements. The research was funded by the Ministry of Natural Resources and Environment in the framework of Research Science and Technology project at ministerial level in 2015, with the grant number of 12.08.2015. REFERENCES 1. Peterson B. J., Fry B. - Stable isotopes in ecosystem studies. Annu. Rev. Ecol. Syst. 18 (1987) 293. 2. Sharp Z. - Principles of Stable Isotope Geochemistry. Prentice Hall, New Jersey, 2005. 3. Dawson T. E., Siegwolf R. - Stable Isotopes as Indicators of Ecological Change. Academic Press, London, 2007. 4. Gonfiantini R., Stichler W., Rozanski K. - Standards and intercomparison materials distributed by the International Atomic Energy Agency for stable isotope measurements. Proceedings of a consultants meeting held in Vienna, 1-3 December 1993, p.13-29. 5. Kendall C., McDonnell J.J. - Isotope Tracers in Catchment Hydrology, Elsevier Science, Oxford, UK, 1998. 6. Preston T., Owens N.J.P. - Interfacing an automatic elemental analyzer with an isotope ratio mass-spectrometer – the potential for fully automated total nitrogen and N-15 analysis. Analyst 108 (1983) 971–977. 7. Barrie A., Bricout J., Koziet J. - Gas-chromatographystable isotope ratio analysis at natural abundance levels. Biomed. Mass Spectrom. 11 (1984) 583–588. 8. Lichtfouse E., Budzinski H. - 13C analysis of molecular organic substances, a novel breakthrough in analytical sciences. Analysis 23 (1995) 364–369. 9. Giesemann A., H.-J. Jager, Norman A.L., Krouse H.R., Brand W.A. - On-line sulfur isotope determination using an elemental analyzer coupled to a mass spectrometer. Anal. Chem., 66 (1994) 2816–2819. laboratories by CF-IRMS. Technical Note 309/LA version 2. Micromass UK, 1996. 10. Morrison J., Fourel F., Churchman D. - Isotopic sulphur analysis by CF-IRMS. Technical Note 509/LA. Micromass UK, 2000. 11. Grassineau N.V., Mattey D.P., Lowry D. - Rapid sulphur isotope analysis of sulphide minerals by C-GC-IRMS. Eighth Annual V.M. Goldschmidt Conference, Toulouse. Utilization of analytical technique for δ13CDIC in groundwater using EA-IRMS under the 55 12. Morrison J., Fallick A., Donelly T. - d34S measurements of standards from several 13. Mineral. Mag., 62A, Part 1 (1998) pp. 537–538. 14. Grassineau N.V., Mattey D.P., Lowry L. - Rapid sulphur isotopic analyzes of sulphide and sulphate minerals by continuous flow-isotope ratio mass spectrometry (CFIRMS). Anal. Chem. 73 (2001) 220–225.Grassineau N.V. - High-precision EA-IRMS analysis of S and C isotopes in geological materials. Applied Geochemistry 21 (2006) 756–765.