Isotope Dilution Mass Spectrometry for Highly Precise Determination of Dissolved Inorganic Carbon in Seawater Aiming at Climate Change Studies

Dissolved
inorganic carbon (DIC) is one of the most important parameters
to be measured in seawaters for climate change studies. Its quantitative
assessment requires analytical methodologies with overall uncertainties
around 0.05% RSD for clear evaluation of temporal trends. Herein,
two alternative isotope dilution mass spectrometry (IDMS) methodologies
(online and species-specific) using an isotope ratio mass spectrometer
(IRMS) and two calculation procedures for each methodology have been
compared. As a result, a new method for the determination of DIC in
seawaters, based on species-specific IDMS with isotope pattern deconvolution
calculation, was developed and validated. A 13C-enriched
bicarbonate tracer was added to the sample and, after equilibration
and acidification, the isotope abundances at CO2 masses
44, 45, and 46 were measured on an IRMS instrument. Notably, early
spiking allows correcting for evaporations and/or adsorptions during
sample preparation and storage and could be carried out immediately
after sampling. Full uncertainty budgets were calculated taking into
account all the factors involved in the determination (initial weights,
concentration and isotope abundances of standards, and final IRMS
measurements). The average DIC value obtained for CRM seawater agreed
very well with the certified value. Propagated precision obtained
ranged from 0.035 to 0.050% RSD for individual sample triplicates.
Reproducibility, assessed by three independent experiments carried
out in different working days, was excellent as well (−0.01%
and 0.057%, error and full combined uncertainty, respectively). Additionally,
the approach proposed improves on established methods by simplicity,
higher throughput (15 min per sample), and lower volume requirements
(10 mL).