Spectrophotometric calibration procedures to enable calibration-free measurements of seawater calcium carbonate saturation states.

A simple protocol was developed to measure seawater calcium carbonate saturation states (Ω) spectrophotometrically. Saturation states are typically derived from the separate measurement of two other carbon system parameters, with each requiring unique instrumentation and often complex measurement protocols. Using the new protocol, the only required equipment is a thermostatted laboratory spectrophotometer.

Spectrophotometric Determination of Carbonate Ion Concentrations: Elimination of Instrument-Dependent Offsets and Calculation of In Situ Saturation States.

This work describes an improved algorithm for spectrophotometric determinations of seawater carbonate ion concentrations ([CO]) derived from observations of ultraviolet absorbance spectra in lead-enriched seawater. Quality-control assessments of [CO] data obtained on two NOAA research cruises (2012 and 2016) revealed a substantial intercruise difference in average Δ[CO] (the difference between a sample's [CO] value and the corresponding [CO] value calculated from paired measurements of pH and dissolved inorganic carbon). Follow-up investigation determined that this discordance was due to the use of two different spectrophotometers, even though both had been properly calibrated.