On-line technique for preparingand measuring stable carbon isotopeof total dissolved inorganic carbonin water samples ( d13CTDIC)

G. Capasso, R. Favara, F. Grassa, S. Inguaggiato, L. Longo

Abstract


A fast and completely automated procedure is proposed for the preparation and determination of d13C of total
inorganic carbon dissolved in water ( d13CTDIC). This method is based on the acidification of water samples transforming
the whole dissolved inorganic carbon species into CO2. Water samples are directly injected by syringe
into 5.9 ml vials with screw caps which have a pierciable rubber septum. An Analytical Precision «Carbonate
Prep System» was used both to flush pure helium into the vials and to automatically dispense a fixed amount of
H3PO4. Full-equilibrium conditions between produced CO2 and water are reached at a temperature of 70°C
(± 0.1°C) in less than 24 h. Carbon isotope ratios (13C/ 12C) were measured on an AP 2003 continuous flow mass
spectrometer, connected on-line with the injection system. The precision and reproducibility of the proposed
method was tested both on aqueous standard solutions prepared using Na2CO3 with d13C=-10.78 per mil versus
PDB (1 s= 0.08, n = 11), and at five different concentrations (2, 3, 4, 5 and 20 mmol/l) and on more than
thirty natural samples. Mean d13CTDIC on standard solution samples is ?10.89 < per mil versus PDB (1 s= 0.18,
n = 50), thus revealing both a good analytical precision and reproducibility. A comparison between average
d13CTDIC values on a quadruplicate set of natural samples and those obtained following the chemical and physical
stripping method highlights a good agreement between the two analytical methods.

Keywords


carbon isotope composition;Total Inorganic Carbon;Dissolved in water (TDIC);stable isotopes;continuous flow IRMS measurements;on-line extraction technique

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References


DOI: https://doi.org/10.4401/ag-3190
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X