An Equilibrator System to Measure Dissolved Oxygen and Its Isotopes

Lauren Elmegreen Rafelski Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

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Bill Paplawsky Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

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Ralph F. Keeling Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

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Abstract

An equilibrator is presented that is designed to have a sufficient equilibration time even for insoluble gases, and to minimize artifacts associated with not equilibrating to the total gas tension. A gas tension device was used to balance the pressure inside the equilibrator with the total gas tension. The equilibrator has an e-folding time of 7.36 ± 0.74 min for oxygen and oxygen isotopes, allowing changes on hourly time scales to be easily resolved. The equilibrator delivers “equilibrated” air at a flow rate of 3 mL min−1 to an isotope ratio mass spectrometer. The high gas sampling flow rate would allow the equilibrator to be interfaced with many potential devices, but further development may be required for use at sea. This system was tested at the Scripps Institution of Oceanography pier, in La Jolla, California. A mathematical model validated with performance tests was used to assess the sensitivity of the equilibrated air composition to headspace pressure and makeup gas composition. Parameters in this model can be quantified to establish corrections under different operating conditions. For typical observed values, under the operating conditions presented here, the uncertainty in the measurement due to the equilibrator system is 2.2 per mil for δ(O2/N2), 1.5 per mil for δ(O2/Ar), 0.059 per mil for δ18O, and 0.0030 per mil for Δ17O.

Corresponding author address: Lauren Rafelski, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0244. E-mail: lauren.rafelski@gmail.com

Abstract

An equilibrator is presented that is designed to have a sufficient equilibration time even for insoluble gases, and to minimize artifacts associated with not equilibrating to the total gas tension. A gas tension device was used to balance the pressure inside the equilibrator with the total gas tension. The equilibrator has an e-folding time of 7.36 ± 0.74 min for oxygen and oxygen isotopes, allowing changes on hourly time scales to be easily resolved. The equilibrator delivers “equilibrated” air at a flow rate of 3 mL min−1 to an isotope ratio mass spectrometer. The high gas sampling flow rate would allow the equilibrator to be interfaced with many potential devices, but further development may be required for use at sea. This system was tested at the Scripps Institution of Oceanography pier, in La Jolla, California. A mathematical model validated with performance tests was used to assess the sensitivity of the equilibrated air composition to headspace pressure and makeup gas composition. Parameters in this model can be quantified to establish corrections under different operating conditions. For typical observed values, under the operating conditions presented here, the uncertainty in the measurement due to the equilibrator system is 2.2 per mil for δ(O2/N2), 1.5 per mil for δ(O2/Ar), 0.059 per mil for δ18O, and 0.0030 per mil for Δ17O.

Corresponding author address: Lauren Rafelski, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0244. E-mail: lauren.rafelski@gmail.com
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