Stable Isotope Facility

Oxygen (¹⁸O) and Hydrogen (D/H) Analysis of Solids by TC/EA-IRMS

The SIF provides ¹⁸O or D/H isotope analyses of solid samples using a TC/EA system interfaced to IRMS.

Analysis

¹⁸O and D/H are analyzed separately. If both analyses are needed, duplicate sets of samples must be sent. Solids are analyzed for ¹⁸O or D/H isotopes using a Hekatech HT Oxygen Analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). Samples are thermally decomposed to CO and H₂O in a molybdenum-lined cermanic reactor filled with glassy carbon at 1345°C . Oxygen is analyzed as CO, and D/H is analyzed as H₂.

During analysis, samples are interspersed with several replicates of at least two different laboratory standards. These laboratory standards, which are selected to be compositionally similar to the samples being analyzed, have been previously calibrated against NIST Standard Reference Materials (including IAEA-CH7, IAEA-601 and IAEA-602). A sample’s preliminary isotope ratio is measured relative to reference gases analyzed with each sample. These preliminary values are finalized by correcting the values for the entire batch based on the known values of the included
laboratory standards.

The final delta values, delivered to the customer, are expressed relative to international standard V-SMOW (Vienna Standard Mean Ocean Water) both hydrogen and oxygen. For information on delta notation and the international references, please refer to a stable isotope reference such as Sharp, Z. (2005) Principles of Stable Isotope Geochemistry (Prentice Hall).

Memory Effect and Oxygen

When analyzing solid samples for ¹⁸O, it is not uncommon for a sample to affect the isotope value of the next sample - particularly if the samples have significantly different isotope values. Because of this memory effect, it is recommended that oxygen samples be run in triplicates.

Exchangeable Hydrogen

Hydrogen in solid samples is susceptible to exchange with ambient water vapor. This fraction can be substantial - up to 30%. The desire is to determine the D/H for the non-exchangeable fraction of a given sample. For keratin samples (such as feather and hair), this exchange is well understood. Keratin samples are allowed to equilibrate with laboratory keratin standards for at least 96 hours before analysis, as described by Wassenaar and Hobson, 2003¹. Following analysis the laboratory keratin standards are used to establish the D/H for the non-exchangeable hydrogen fraction of the customer's samples.

For hydrogen samples other than keratin, the only reliable method to determine D/H for the non-exchangeable fraction is to equilibrate the samples and standards with two waters with different D/H values. From the two datasets, the fraction of exchangeable hydrogen and the D/H for the non-exchangeable fraction can be calculated (see Chesson 2009²). SIF is finalizing its implementation of the dual equilibration method for non-keratin solid hydrogen samples and will be accepting these samples in mid-2012.

¹Wassenaar, L.I., and Hobson, K.A. (2003). Comparative equilibration and online technique for determination of non-exchangeable hydrogen of keratins for use in animal migration studies. Isotopes in Environmental and Health Studies, 39(3): 211-217.

²Chesson, L.A., Podlesak, D.W., Cerling, T.E., and Ehleringer, J.R. (2009). Evaluating uncertainty in the calculation of nonexchangeable hydrogen fractions within organic materials. Rapid Commun. Mass Spectrom, 23: 1275–1280.