Evaluation of Reagentless pH Modification for in Situ Ocean Analysis: Determination of Dissolved Inorganic Carbon Using Mass Spectrometry

Abstract

Rationale

In situ analytical techniques that require the storage and delivery of reagents (e.g., acidic or basic solutions) have inherent durability limitations. The reagentless electrolytic technique for pH modification presented here was developed primarily to ease and to extend the longevity of dissolved inorganic carbon (DIC) determinations in seawater, but can also be used for other analytical methods. DIC, a primary carbon dioxide (CO₂) system variable along with alkalinity, controls seawater pH, carbonate saturation state, and CO₂ fugacity. Determinations of these parameters are central to an understanding of ocean acidification and global climate change.

Methods

Electrodes fabricated with electroactive materials, including manganese(III) oxide (Mn₂O₃) and palladium (Pd), were examined for potential use in electrolytic acidification. In-line acidification techniques were evaluated using a bench-top membrane introduction mass spectrometry (MIMS) setup to determine the DIC content of artificial seawater. Linear least-squares (LLSQ) calibrations for DIC concentration determinations over a range between 1650 and 2400 µmol kg^–1 were obtained, using both the novel electrolytic and conventional acid addition techniques.

Results

At sample rates of 4.5 mL min^–1, electrodes clad with Mn₂O₃ and Pd were able to change seawater pH from 7.6 to 2.8 with a power consumption of less than 3 W. Although calibration curves were influenced by sampling rates at a flow of 4.5 mL min^–1, the 1σ measurement precision for DIC was of the order of ±20 µmol kg^–1.

Conclusions

Calibrations obtained with the novel reagentless technique and the in-line addition of strong acid showed similar capabilities for DIC quantification. However, calculations of power savings for the reagentless technique relative to the mechanical delivery of stored acid demonstrated substantial advantages of the electrolytic technique for long-term deployments (>1 year). Copyright © 2013 John Wiley & Sons, Ltd.