An Effective Method of UV-Oxidation of Dissolved Organic Carbon in Natural Waters for Radiocarbon Analysis by Accelerator Mass Spectrometry

XUE Yuejun, GE Tiantian, and WANG Xuchen, 2), *



An Effective Method of UV-Oxidation of Dissolved Organic Carbon in Natural Waters for Radiocarbon Analysis by Accelerator Mass Spectrometry

XUE Yuejun1), GE Tiantian1), and WANG Xuchen1), 2), *

1)Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China,Qingdao 266100, P.R. China 2)Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao266100,P.R. China

Radiocarbon (14C) measurement of dissolved organic carbon (DOC) is a very powerful tool to study the sources, transformation and cycling of carbon in the ocean. The technique, however, remains great challenges for complete and successful oxidation of sufficient DOC with low blanks for high precision carbon isotopic ratio analysis, largely due to the overwhelming proportion of salts and low DOC concentrations in the ocean. In this paper, we report an effective UV-Oxidation method for oxidizing DOC in natural waters for radiocarbon analysis by accelerator mass spectrometry (AMS). The UV-oxidation system and method show 95%± 4% oxidation efficiency and high reproducibility for DOC in both river and seawater samples. The blanks associated with the method was also low (about 3μgC) that is critical for14C analysis. As a great advantage of the method, multiple water samples can be oxidized at the same time so it reduces the sample processing time substantially compared with other UV-oxidation method currently being used in other laboratories. We have used the system and method for14C studies of DOC in rivers, estuaries, and oceanic environments and have received promise results.

radiocarbon; dissolved organic carbon; UV-oxidation; natural waters; AMS

1 Introduction

Dissolved organic carbon (DOC) is the second largest organic carbon reservoir on earth and the largest exchangeable organic carbon pool (about 660PgC) in the ocean (Hedges, 1992). DOC plays important roles not only for the global carbon cycle but affecting the nutrients and many elements cycles and microbial activities in the ocean as well (Hansell and Carlson, 2001, 2002; Middelbore and Lundsgaard, 2003; Jiao., 2010; Nelson and Carlson, 2012). In the last two decades, although we have gained a great knowledge about the distribution and cycling of DOC in the world oceans (Druffel., 1992; Hansell., 2009; Bauer., 2013). Its sources and bioavailability in the ocean especially in the ocean’s deep regions (>1500m), however, are still not fully understood (Jiao and Azam, 2011; Becker., 2014; Follett., 2014). It thus remains as an on-going active research area to study the sources, transformation and bioavailability of DOC in the ocean.

Measurement of natural occurring radiocarbon (14C) compositions in DOC is a very powerful tool to identify the sources and cycling processes of DOC in natural waters (Williams and Druffel, 1987; Williams., 1992; Druffel., 1992; Raymond and Bauer, 2001; Wang., 2012). Based on the abundances of14C, the radiocarbon age (year before present) of DOC can be then calculated so it provides insight information about the sources and cycling time scales of DOC in the ocean. Measurements of14C in DOC have revealed that the14C age of DOC in the deep ocean ranged 4000-6000 years old, suggesting that a large fraction of DOC in the ocean is cycling on a very long time scales (Williams and Druffel, 1987; Druffel and Bauer, 2000). However, a recent DOC-14C study by Follett. (2014) reported that up to 30% of DOC in the deep ocean could be modern which represents a significant fraction of recent-fixed modern carbon flux in the deep ocean. In the river and coastal waters,14C age of DOC also varied widely. Our previous study have measured the14C compositions of DOC in the Huanghe and Changjiang Rivers and the14C age ranged from 305 to 1570 years with great seasonality as affected by different organic matter inputs (Wang., 2012). These14C studies certainly provide promise information to our understanding of the sources and cycling of DOC in the river and ocean that other chemical methods could not possibly provide.

To measure14C in seawater DOC is a complicated process that requires the first step for complete oxidation of sufficient DOC to CO2. This is rather difficult for seawater analysis because the very high proportion of salts and extremely low DOC concentrations in the deep ocean (about 40μmolL−1). Contamination during sample processing is often a big consideration. With the application of accelerator mass spectrometry (AMS) in the last 20 years, the amount of carbon required for high precision14C analysis has been sustainably reduced to about 100μgC or less. Even though, 300-500mL seawater usually is needed for oxidation in order to collect enough CO2for high precision14C analysis. The most commonly used method for sufficient DOC oxidation is by ultraviolet light oxidation (UV-Oxidation) (Williams and Druffel, 1987; Bauer., 1992). More recently, Beaupre. (2007) developed a UV-oxidation method for DOC based on the modification of the conventional UV-oxidation method. This method could oxidize 30mL to 500mL seawater and gives low blank and high precision for14C measurements. However, the disadvantage of the method is that only one sample can be oxidized at same time using the system and it takes at least 6h to finish one sample. This time consuming process certainly has limitations for studies when large number of DOC samples need to be processed throughput.

In the paper, we report an effective UV-oxidation method developed in our laboratory for oxidation of DOC in natural waters. Depending on the DOC concentration, 4 to 12 samples can be oxidized at the same time and the reproducibility and precisions are good for both13C and14C measurements. We have used this method for DOC-14C studies in riverine, estuarine, coastal and oceanic waters and have obtained promise results.

2 Methods

2.1 UV-Oxidation System

The UV-oxidation system we used is shown in Fig.1. It includes an Ace Glass model 7900 UV-Oxidation apparatus with power supply (Ace Glass Inc.). A 50cm long 1200 watts medium-pressure mercury arc UV lamp (Hanovia) is vertically placed in the center of the UV- Oxidation housing. Twelve custom-made quartz sample reaction tubes (140mL, 2.6cm OD×56cm, Fig.1-1) can be vertically placed in the housing around the UV lamp. The specially designed reaction tube has a top ground-glass joint stripping probe which is used for gas purging and can be connected directly to the vacuum line for CO2extraction as shown in Fig.1. The vacuum extraction line is made from 12-15mm OD Pyrex tubing and contains mainly a (2) KI solution trap; (3) a cold water trap (dry ice/isopropanol slush); (4) a liquid nitrogen trap for CO2collection; (5) cold-finger trap with calibrated volume for CO2quantification; (6) CO2transfer tubing connection; (7) a protection liquid nitrogen trap at the end of the vacuum line leading to (10) an oil-free molecular rotary pump (TPS Compact Dry TV301, Agilent Technologies); (8) a pressure gauge for high vacuum measurement (to 1×10−4Torr, Agilent) and (9) a pressure transducer to quantify CO2volume (0-200 Torr, MKS Inc.). The major improvement of our UV-oxidation system is using the multiple Ace Glass UV-Oxidation apparatus with specially designed 12 quartz UV-oxidation tubes which can be connected directly to vacuum line. In this case, multiple water samples can be oxidized at the same time.

Fig.1 Schematic diagram of the UV-oxidation system for DOC and the vacuum extraction line. Detail information for each component is provided in the text.

2.2 UV-Oxidation of DOC

We first tested the UV-oxidation efficiency and the blank level using the system. A 1.0molL−1DOC stock solution was prepared using a reagent purity oxalic acid (Aladdin, 99.99%). Three DOC concentrations (100μmolL−1, 200μmolL−1and 400μmolL−1) were then diluted using the stock solution and Milli-Q high purity water. Measured 120mL of each DOC solution was placed in each of the pre-combusted (850℃ for 2h) quartz reaction tubes and acidified to pH 2 using 85% H3PO4. The acidified DOC solutions were purged with ultra-high purity (UHP) helium gas through the top of the reaction tube (Fig.1-1) for 20min to remove dissolved inorganic carbon (DIC). Four duplicates were prepared for each DOC standard solution. After purging, the DOC standard solutions were UV-oxidized for 5h. At each half hour, water samples were collected from each reaction tube to measure DOC concentration. We also used river water, estuarine water and seawater of different salinities to test the UV-oxidation efficiencies. The water samples were collected in Changjiang River and Huanghe River Estuaries during cruises in March and April, 2014. Water samples were filtered using GF/F filters (pre-combusted at 550℃ for 5h) and kept frozen in glass bottles until processing. The procedure of UV-oxidation of DOC for the field water samples was conducted in the same way as the DOC standard solutions as described above.

To test the system blanks on carbon isotope measurement, we used a 200μmolL−1DOC standard solution to conduct UV-oxidation and extracted CO2generated from DOC oxidation for carbon isotope measurement. As described above, 120mL solution was placed into a quartz reaction tube in four duplicates and acidified to pH 2 using 85% H3PO4. The acidified standard solutions were purged with UHP helium gas for 20min and UV-oxidized for 5h. Following the UV-oxidation, gaseous CO2generated from DOC oxidation was purged again with UHP helium gas through the vacuum extraction line and CO2was purified and collected cryogenically and flame- sealed inside 6mm OD Pyrex break-seal tubes for14C and13C analysis.

We also conducted the UV-oxidation blank test using field water samples. Three samples (from Huanghe River, Changjiang River and coastal seawater outside Changjiang Estuary) were tested for reproducibility on isotope measurements. As described above, 120mL water sample was placed into a quartz reaction tube in triplicate and acidified to pH 2 using 85% H3PO4. The acidified water samples were purged with UHP helium gas for 20min and UV-oxidized for 5h. After UV-oxidation, generated CO2from DOC oxidation was purged again with UHP helium gas through the vacuum extraction line and CO2was purified and collected cryogenically. After measuring the volume, CO2was then flame-sealed inside 6mm OD Pyrex break-seal tubes for14C and13C analysis.

Concentration of DOC was measured by high temperature catalytic oxidation (HTCO) method (Sharp., 1993) using a Shimadzu TOC-L analyzer equipped with an ASI-L auto-sampler. The instrument was calibrated using 5-point calibration curves derived from prepared DOC standard (KHP) solution. Instrument blank DOC value was checked against reference low carbon water and seawater (CRM, University of Miami, Rosenstiel School of Marine and Atmospheric Sciences). Blank subtraction was carried out using Milli-Q water which was analyzed before each sample ran. Total blanks associated with DOC measurement was about 5μmolL−1and the analytic precision on triplicate injections were < 3%.

2.3 Isotopic Measurement

Carbon isotope (14C and13C) compositions were measured for the purified CO2gas generated from UV-oxidation of DOC at the National Ocean Science Accelerator Mass Spectrometry (NOSAMS) facilities at Woods Hole Oceanographic Institution (WHOI) in USA. A small split fraction of CO2was measured for13C using a VG IR-MS and the rest CO2was graphitized for14C analysis using AMS. Values of13C are reported in ‰ relative to the PDB standard and values of14C measured as the fraction modern based on modern reference material used and14C values are calculated and reported in ‰ relative to the standard as well.

3 Results and Discussion

3.1 UV-Oxidation Efficiency

The results of UV-oxidation efficiency tested using DOC standard solution and field samples were summarized in Table 1 and plotted in Fig.2. The UV-oxidation efficiencies were 97% to 98%±2% for DOC standard solutions and 94% to 96%±4% for the field samples determined at the end of the UV-oxidation (Table 1). During the time series UV-oxidation test, about 40%-60% of DOC were oxidized after one hour and 94%-98% of DOC oxidation efficiencies were reached at 4h. These oxidation efficiencies obtained in our experiments are quite consistent with the results reported using a different setting of UV-oxidation method (Beaupre., 2007), indicating that our UV-oxidation system is effective for DOC oxidation in natural waters with good reproducibility. We therefore set 5 consecutive hours as standard oxidation time for all samples to ensure a completed DOC oxidation. The recovery of CO2collected cryogenically on the vacuum extraction line was slightly lower than that of UV-oxidized, probably due to the small calibration errors of CO2pressure volume or lost during pumping of the uncondensed gases. In any cases, this small differences will not affect the measured values of both13C and14C (Table 1).

The values of bothd13C andD14C measured for the DOC standard solution after UV-oxidation showed good agreement with the values of solid oxalic acid standard (within 2% differences), indicating the system blanks arevery low and the reproducibility of UV-oxidation is good. For the three field DOC samples, the reproducibility and precisions ofd13C andD14C measurements are slightly lower than that of the DOC standard solution but still considered as reasonably well as compared with the results of Beaupre. (2007) using a different UV-oxidation method.

Table 1 UV-oxidation efficiency and isotope results reproducibility

Note:aUV-oxidized (%) is calculated based on DOC concentrations measured before and after UV-oxidation;bRecovery (%) is calculated based on the CO2collected at the end of the UV-oxidation.

Fig.2 (a) DOC concentration changes and (b) DOC oxidation efficiencies during the time series UV-oxidation of DOC standard solutions; (c) DOC concentration changes and (d) DOC oxidation efficiencies during the time series UV-oxidation of riverine, estuarine and coastal waters with different salinity.

As discussed by Beaupre. (2007), the biggest pro- blems and challenges for measuring marine DOC isotopic ratios are the high proportion of salts in seawater, low DOC concentration and blanks associated with sample processing. Since the application of AMS, the amount of carbon needed for high precision14C analysis has been reduced substantially to 100μg or less. Thus the sample volume needed for DOC oxidation can be reduced to 100-200mL for estuarine and coastal waters and 300mL for deep ocean waters accordingly. Our results suggest that with carefully designed UV-oxidation systems and sample handling, this can be successfully achieved. We have used our UV-Oxidation system to oxidize DOC in natural waters including rain and snow samples for14C studies and have received promise results (Wang., 2015).

As a summary, our UV-oxidation system showed high efficiency for DOC oxidation in natural waters. The blanks associated with the system are low and the reproducibility is high for DOC isotopic measurement. As a great advantage of our UV-Oxidation system, multiple samples (4 to 12 depending on DOC concentration) can be oxidized at same time so it reduces the sample processing time substantially compared with other UV-oxidation methods. This system and method can be used for radiocarbon studies of DOC in natural waters especially when large amount of samples need to be processed in timely.

Acknowledgements

We thank the advices and technical assistants from Dr. Xu Li in the NOSAMS facility at Woods Hole Oceanographic Institution in USA. We also thank the staff at NOSAMS for high precision measurements of14C of the samples. Financial support for this work was provided by Ocean University of China (841312004) and National Natural Science Foundation of China (Grant Nos. 41476057 and 41221004).

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(Edited by Ji Dechun)

DOI 10.1007/s11802-015-2935-z

ISSN 1672-5182, 2015 14 (6): 989-993

© Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2015

(January 20, 2015; revised August 4, 2015; accepted September 15, 2015)

* Corresponding author. Tel: 0086-532-66782831 E-mail:xuchenwang@ouc.edu.cn