Characterizing the 13C/12C Ratio Obtained from Oxidation Techniques Used in Isotopic Total Organic Carbon (TOC) Measurements0 pages

Application Note 37820312

Keywords
iTOC-CRDS - Isotopic Carbon
Analyzer
Total Organic Carbon
1030S TOC Solids Module

Characterizing the 13C/12C Ratio Obtained from
Oxidation Techniques Used in Isotopic Total
Organic Carbon (TOC) Measurements

Introduction
Stable isotope measurements of organic carbon found in fresh and
terrestrial waters, soil samples, plant tissues, foodstuffs and synthetics are
of interest for provenance, metabolism, and climatology, among other
applications. These measurements are typically made by oxidizing the
carbon present in the sample and determining the 13C/12C ratio of the
resultant CO2 gas. Oxidation techniques employed in total organic carbon
(TOC) analysis can have effects on the overall CO2 recovery, particularly
for large, poorly soluble, or structurally robust molecules. What is less
well understood is the oxidation techniques’ intrinsic effects on the
observed isotope ratio for a particular species.
In this work, the aqueous TOC measurements were carried out using an
OI Analytical Aurora 1030W TOC Analyzer, which operates on the
heated sodium persulfate oxidation principle. Resultant gases are directed
through a solid-state non-dispersive infrared (SSNDIR) detector set to
monitor CO2. The response of this detector is filtered to ignore the
presence of water vapor and other gaseous interferants.
High-temperature catalytic oxidation (HTCO) was conducted using the
1030S Solids Module, which combusts samples in quartz crucibles by
flowing O2 over them at 900 °C. Resulting gases are directed into the
SSNDIR detector in the Aurora 1030 for determination of TOC content.
Measurements of the stable carbon isotope ratios of samples are
performed by cavity ringdown spectrometry, which monitors the decay of
a pulse of light trapped in a gas-filled highly-reflective cavity. Based on
the amount of time it takes the light pulse to decay, the concentration of
gas species that absorb the light can be determined. As the energies of
vibrational bands are highly isotope-dependent, it is easy to distinguish
between lines absorbed by 12CO2 and 13CO2, so the relative abundance of
those species can be measured in pseudo-real time.
The iTOC-CRDS is an analytical system (Figure 1) that combines all
three of the above devices and an automatic concentration controller into
a system that can analyze a sample (aqueous, neat, slurry, or solid) for its

Presented at the 2012 Pittsburgh
Conference on Analytical
Chemistry and Applied
Spectroscopy Orlando, Florida,
March 11–15, 2012