Long-term Time-series
Measurements of Dissolved Noble Gases

D.E. Lott, W.J. Jenkins, and B. Baschek

Woods Hole Oceanographic Institution, MA, USA

Introduction

Dissolved gas concentrations can be interpreted as fingerprints of different air-sea gas exchange mechanisms like diffusion, gas bubble injection, or biogeochemical processes. The contribution of these different sinks/sources can be estimated by measuring a variety of dissolved gases with different physical and chemical properties.

For this purpose, a Noble Gas Sampler has been developed. It can autonomously collect 50 dissolved gas samples in the upper ocean during long-term deployments of weeks to months. These water samples are stored in stainless steel chambers until retrieval. The gas concentrations in the water samples are determined later in the laboratory with a quadrupole mass spectrometer.

The Noble Gas Sampler is used in combination with high resolution time series measurements of dissolved O2 and N2 taken with a Seabird CTD, Aanderaa Oxygen Optode, and Pro-Oceanus GTD/pN2-system.

The Noble Gas Sampler

The Noble Gas Sampler is a closed-loop system filled with distilled water (Figure 1). Due to the gas-permeable membrane of Teflon tubing it is in diffusive equilibrium with the surrounding sea water (Figure 2). Dissolved gas samples are then taken by pumping this water into 1mL-stainless steel sample chambers where it is stored until retrieval. A diffusion barrier prevents the contamination of the sample (Figure 3). A rotary valve allows it to take up to 50 discrete samples.

Figure 1: The Noble Gas Sampler.
Figure 2: Block Diagram of the Noble Gas Sampler. For description of components see text.

Components of Noble Gas Sampler

Computer Control: A Tattletale-8 controller is programmed to take dissolved gas samples by:

  • selecting a sample chamber with the 50-port valve
  • powering the pump to flush the sample chamber
  • recording the flushing pressure as verification.

Battery: An alkaline D-cell battery pack supplies 9V to the pump and 28V to the computer and rotary valve.

Pump: A modified Micropump produces at 9V a pressure differential of 40 PSI yielding a sample chamber flushing rate of 0.6 mL/min.

Pressure Sensor: An Omega pressure transducer is used to record the flushing pressure of the system to verify the sample chamber flushing rate.

Filter: A 316 stainless steel 60 mm frit filter is used to trap particulates.

Distilled Water Reservoir: A 105 mL distilled water reservoir is common to all 50 gas-permeable tubes.

Figure 3: Teflon tubing and sample chambers.

Gas Permeable Tubing: Each section of FEP Teflon tubing (1/16” ID, 1/8’’ OD) is 760 cm long with a volume of 14.7 mL (Figure 4).

Sample Chambers: Each of the 50 1mL-sample chambers (Figures 4&5) and their corresponding Teflon tubes form an independent circuit between the reservoir and the 50-port sample selection valve. A diffusion barrier prevents the contamination of the sample.

Sample Selection Valve: The 50-port single-head rotary valve, designed by D. Lott, WHOI, is used to complete the circuit at the high pressure side of the pump for one selected sample vessel and its associated Teflon tube.

Ambient Pressure Equilibration Volume: This variable volume allows the total volume of the closed-loop system to achieve pressure equilibrium with the environment.

Figure 4: Sketch of Sample Chambers.
Figure 5: Gas permeation characteristics for PTFE tubing at 25°C and a 10% step in gas concentration. The marks indicate the equilibration time for the different gases: e.g. 9min for He and 12h for Kr.
Figure 6: Diffusive contamination of a sample in the chamber as a function of time. The calculations were done for warm water (25°C) to show the maximal effect. Note: Because a time series is being measured, this effect can be corrected for to a very small fraction of a percent.

References

HOOD, M. 1998. Characterization of air-sea gas exchange processes and dissolved gas/ice interactions using noble gases. Ph.D. thesis, WHOI/MIT, USA.

LOTT, D.E. 2001. Improvements in noble gas separation methodology: A nude cryogenic trap. Geochem. Geophy. Geosy 2. Art No. 2001GC000202.