Anal. Chem. 1986, 58,3052-3054
3052
Carbon Dioxide-Water Oxygen Isotope Fractionation Factor Using Chlorine Trifluoride and Guanidine Hydrochloride Techniques Joseph P. Dugan, Jr.,* and James Borthwick Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275
A new value for the C0,-H,O oxygen isotope fractionation factor of 1.041 45 f 0.000 15 ( l a )has been determined. The data have been normalized to the VSMOWNSLAP scale and were obtained by measuring isotopic compositions with the guanidine hydrochloride and chlorine trifluoride technlques.
Multiple determinations of three carbonate reference standards over 2 years agree extremely well with the recommended values (12) and clearly demonstrate that our results are not subject to systematic errors of measurement (Table I). The guanidine method (which is described in detail elsewhere ( 3 ) )offers the advantages of reproducibility, batch preparation, and the ability to store the reaction tubes indefinitely after either step of reaction (2)
Oxygen isotope work with small ( < 5 pL) water samples in sedimentary diagenetic, ore deposit, and paleoclimate studies has necessitated a more precise determination of t h e fractionation factor between carbon dioxide and water because the most common technique for such determinations involves equilibrating a small aliquot of COz of known isotopic composition with a small sample of water. The fractionation factor cy C02-H20 is defined as
(NHZ)&:NH*HCl+ 2H2O
aCO2-HzO =
('80/160)CO* ('s0/'60)H20
- 1000 + 6C02
1000 + 6H2O
(1)
where
6 =
(180/160sarnple
-
180/160standard)
x 1000
180/1609tandard
Until now there have been ten different determinations of the oxygen isotope fractionation factor ranging from 1.0406 ( I ) t o 1.0424 ( 2 ) . Of these only one (3)has been made by use of isotope d a t a that are explicitly stated to b e normalized to a per mil scale in which 6lSO V-SLAP = -55.5 relative t o VSMOW(0) as discussed by Coplen e t al. ( 4 ) . Measurements made earlier a t t h e Southern Methodist University stable isotope laboratory gave a value for aCO,-H,O of 1.041 43 f 0.000 32 (217) (3). As this differs significantly from t h e values of 1.0412 (5-8) and the value of 1.041 11 f 0.00007 (9) determined by careful work in other laboratories, this paper seeks to improve upon the accuracy with which our earlier determinations have been made.
EXPERIMENTAL SECTION In this study we have determined the starting isotopic compositions of reference waters, before and after equilibration with COz, by the guanidine hydrochloride and chlorine trifluoride techniques, the methodologies of which are described below. Oxygen isotope ratios were measured on a Finnigan MAT 251 mass spectrometer having internal precision for C 0 2 isotopes of
