Multi-parameter Inorganic Carbon Analyzer (MICA)

Project Team
Robert H. Byrne, project lead
Sherwood Liu, method development and testing
Eric A. Kaltenbacher, instrument design
Jim Patten, software

MICA instrument on the R/V Ronald H. Brown

The Multi-parameter Inorganic Carbon Analyzer (MICA) developed in the College of Marine Science is an autonomous multi-parameter flow-through CO2 system capable of simultaneously measuring pH, carbon dioxide fugacity (fCO2), atmospheric carbon dioxide partial pressure (pCO2), and the total dissolved inorganic carbon (DIC) of natural water.

Measurements of four seawater inorganic carbon system parameters - pH, carbon dioxide fugacity or partial pressure, total dissolved inorganic carbon, and total alkalinity (TA) - are essential for carbon cycle investigations on both global and local scales. Both observational and modeling efforts rely on high quality inorganic carbon data from field measurements. Extensive efforts have been devoted to improving methodologies and instruments for determination of carbon parameters in seawater. All measurements of MICA system are based on spectrophotometric determinations of pH at multiple wavelengths using sulfonephthalein indicators.

The pH optical cell is machined from a poly(etheretherketone) (PEEK) polymer rod bearing a bore-hole with an optical pathlength of about 15 cm. The fCO2 optical cell consists of Teflon AF 2400 (DuPont) capillary tubing sealed within the bore-hole of a PEEK rod. This Teflon AF tubing is filled with a standard indicator solution with a fixed total alkalinity, and forms a liquid core waveguide (LCW). The LCW functions as both an optical cell and a membrane that equilibrates the internal standard solution with external seawater. Carbon dioxide fugacity (fCO2) is then determined by measuring the pH of the internal solution. DIC is measured by determining the pH of standard internal solutions in equilibrium with seawater that has been acidified to convert all forms of DIC to CO2. The system runs repetitive measurement cycles with a 7 sample per hour frequency for each type of measurement.

The system has been extensively tested on research vessels. Field precisions were evaluated as 0.0008 units for pH, 0.9 atm for fCO2, and 2.4 mol kg-1 for DIC. These field precisions are close to those obtained in the laboratory. Direct comparison of our measurements and measurements obtained using established standard methods revealed that the system achieved field agreements of 0.00120.0042 units for pH, 1.02.5 atm for fCO2, and 2.26.0 mol/kg for DIC. This system integrates spectrophotometric measurements of multiple CO2 parameters into a single package suitable for observations of both seawater and freshwater.

This compact MICA instrument allows large-scale spatial CO2 system mapping via research vessels and high resolution temporal CO2 system monitoring on moorings. MICA is capable of both seawater and freshwater measurements.