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Instrument: Multiparameter Inorganic Carbon Analyzer (MICA)

Introduction

Measurements of four seawater inorganic carbon system parameters - pH, fugacity or partial pressure of carbon dioxide (fCO₂ or pCO₂, respectively), total dissolved inorganic carbon (DIC), 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. In conventional methodologies, the four core parameters of the seawater inorganic carbon system are measured using diverse instrumentation (for example, potentiometry, spectrophotometry, gas chromatography, nondispersive infrared analysis, and coulometry). It is highly desirable, however, for all four parameters to be measured simultaneously and continuously with high temporal resolution and with high precision and accuracy. Among all available methodologies for measurements of inorganic carbon species in seawater, spectrophotometric methods are especially promising because they can simultaneously measure multiple parameters at relatively low cost. Moreover, spectrophotometric methods have many advantages for measuring inorganic carbon species in seawater: high sensitivity, good stability and selectivity, simplicity, and low rates of sample and reagent consumption. The available spectrophotometric methodologies developed over years for measurements of inorganic carbon parameters were engineered into a single system that is intended for high-resolution underway monitoring onboard research vessels. The MICA was designed as an autonomous multiple parameter flow-through system for simultaneous measurement of surface seawater TC, pH, and fCO₂.

Figure 1. Schematic of the MICA.

Table 1. MICA characteristics.

[Abbreviations: mL, milliliter; μL, microliter; h, hour; min, minute; μatm, microatmospheres; μmol/kg, micromoles per kilogram]

Measurements

All of the measurements are based on spectrophotometric determinations of solution pH at multiple wavelengths using sulfonephthalein indicators (fig. 1 and table 1). The pH and TA optical cells are machined from PEEK polymer rod, and have a 15-cm optical pathlength. The fCO₂ and DIC optical cells consist of Teflon AF 2400 (DuPont) capillary tubing sealed within a borehole also machined from PEEK. The Teflon AF tubing, filled with a standard indicator solution that has a fixed total alkalinity, forms a liquid core waveguide (LCW). The LCW functions as both a long pathlength (15 cm) optical cell and a membrane that equilibrates the internal standard solution with either external air (providing air pCO₂ measurements), seawater (providing seawater fCO₂ measurements), or acidified seawater (providing DIC measurements). Both pCO₂ and DIC are then determined by measuring the pH of the internal solution. The MICA system makes repetitive observations with measurement frequencies of seven samples per hour. Measurement precisions in the field are ±0.0008 units for pH, ±0.9 μatm for pCO₂, and ±2.4 μmol kg^-1 for DIC. These precisions are close to those obtained with conventional methods in the laboratory (fig. 2).

Figure 2. Comparison of MICA data with analysis of discrete samples.

[Abbreviations: UM, University of Miami; USF, University of South Florida; AOML, Atlantic Oceanographic and Meteorological Laboratory; μatm, microatmospheres; μmol kg^-1, micromoles per kilogram]