Background

NIWA’s TROPAC group has developed an automated system for extraction of dissolved inorganic carbon (DIC) from sea water. The goal was to improve the consistency and reduce the processing time involved in making the corresponding ¹⁴C and ¹³C measurements for our own research projects. Because of the interest others involved in marine carbon research have shown in this equipment we now make, test and install additional systems to order.

System Description

The DIC Extraction System (DICES) is based on the standard techniques of acid evolution and cryogenic trapping of the released CO₂. New features are computer controlled operation using a PC running LabVIEW for Windows and a high-efficiency automated cryogenic trap.

A repeatability of 0.02‰ for δ¹³C is achieved routinely and the effect of the extraction system on δ¹³C repeatability is smaller than that of most stable isotope mass spectrometer systems.

Automated Cryogenic Trap

Automation, together with careful design of the overall system, enables us to achieve a much better repeatability than earlier manual systems and enables a technician to process many more samples per day.

In typical use, the operator fills a reaction vessel with seawater and a side arm with phosphoric acid. A gas piston system is used to transfer sea water from field/cruise sampling bottles to the reaction bottle and duplicate aliquots can be taken simultaneously with a "Y" junction arrangement.

Reaction Vessel

The reaction vessel is coupled to the extraction system, phosphoric acid is introduced from the side arm, and evolution is assisted with a magnetic stirrer. The system extracts the headspace gas using vacuum distillation to purify evolved CO₂ through a series of cold traps. Initial traps remove water vapour and the computer controlled LN₂ trap removes CO₂. The extraction process for a typical 100 ml sea water sample for AMS ¹⁴C work takes about 20 minutes. Larger sea water samples have been accommodated with some minor redesign.

The evolved CO₂ yield is measured with a manometer in a calibrated volume. The system has not been optimised for yield, however accuracy has been checked against international DIC standard materials (CO₂ CRM from A G Dickson, Scripps Instutution of Oceanography) and is better than 1%.

The Complete Extraction Line System

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Typical Yields

Several sample runs were taken to determine reproducibility of CO₂ yields. Surface seawater collected from Wellington Harbour was analysed for total dissolved inorganic carbon (C_T) using the Automated Seawater Line. The sample was not poisoned. Replicate aliquots taken sequentially using an air displacement technique and analysed within 3 hours of each other gave the following results (see Table I):

C_T = 16.32 0.09 mg C kg_sw^-1 (0.6%) (mean precision)

Duplicate aliquots taken from a different sample using a simultaneous air displacement technique and analysed within 40 minutes of each other gave the following results (see Table II):

C_T = 17.42 0.02 mg C kg_sw^-1 (0.1%) (mean precision)

The current manometer pressure transducer has an accuracy of +/- 2 Pa. The manometer volume is calibrated to 0.1% precision and the manometer temperature is measured with a NIST calibrated thermistor, certified to +/- 0.05 °C.

Sample Runs

Further Information

See also the Operating Instructions and a summary of CO₂ yield equations.

Contact

Dr Dave Lowe [ d.lowe@niwa.co.nz ]
NIWA
P O Box 14-901
Wellington
New Zealand
Phone: +64 4 386 0300
Fax: +64 4 386 2153