Hydrogen and Oxygen isotope analysis (2
18O) of water by
Thermal Conversion “pyrolysis” – Continuous Flow – Isotope
Ratio Mass Spectrometry
Isotope Science Laboratory
Department of Physics and Astronomy
University of Calgary
Overview
The 2H/1H and 18O/16O ratios of low TDS water samples are determined using a
high temperature “pyrolysis” reactor coupled to an isotope ratio mass
spectrometer in continuous flow mode. The system is comprised of a Finnigan
Mat TC/EA reactor interfaced to a Finnigan Mat Delta+XL mass spectrometer via
a Conflow-III open split/interface. Approximately 100nL of water is injected into
the septum port of the TCEA using an A200S liquid autosampler (CTC). Upon
injection the water rapidly vapourizes and is carried down into the hot zone of the
TCEA with the helium carrier. H2O is quantitatively converted to H2(gas) & CO(gas)
at 1400 ˚C as the water vapour interacts with the glassy carbon packing of inside
the TCEA. After conversion, all gases are swept through a small, acid gas trap,
the TCEAs GC (90˚C) and finally into the ion source via the Conflow-III open
split. Measurement of 2H and 18O are made in two separate sequences,
optimizing the ion source first for HD and then for 18O respectively. Raw data is
corrected for drift and normalized to the international VSMOW-VSLAP scale
using LIMs (USGS). Final results are expressed in the usual per mil notation.
Further comments on technique
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A “bottom feed” helium adapter is used on the TCEA
A stainless steel needle guide/adaptor is used at the top of the inner
glassy carbon tube (length ~1.5cm, orifice=2mm)
The syringe used is an SGE 0.5BNR-5/00.63 (p/n 000300)
To overcome “memory”, two sequence lines are used for each
sample/standard. The first line “pre-conditions” the reactor with 3 rapid
~300uL injections. The data from this line is not collected. Then a single
injection of the sample/standard is made and the raw delta values
measured.
Helium carrier flow thorough the system is ~110 ml/min
Normalization standards are run at the beginning and end of each
sequence.
Internal lab standards are analyzed repeatedly within each sequence at an
interval of 1 standard per 5 samples to guarantee quality control.
Mass Spectrometric Measurements
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The “raw” 2HH2Oand18OH2O values are normalized using internal,
laboratory water standards which are have been calibrated against
International reference materials: VSMOW, VSLAP & VGISP. All internal
lab standards are periodically checked against fresh VSMOW, VSLAP &
VGISP
USGS LIMs is used for drift correction, normalization and data
management (http://water.usgs.gov/software/LIMS/).
Corrected 2HH2Oand18OH2O values are reported in the usual per mil (‰)
notation
Accuracy and Precision:
Accuracy and precision of 2H determinations are generally better than 2.0‰
(one standard deviation based on n=50 lab standard).
Accuracy and precision of 18O determinations are generally better than 0.2‰
(one standard deviation based on n=50 lab standard).
References:
M. Gehre, H. Geilmann, J. Richter, R. A. Werner, W. A. Brand. Continuous flow
2H/1Hand18O/16Oanalysis of water samples with dual inlet precision. Rapid
Communications in Mass Spectrometry 2004, 18, 2650.
R.A. Werner and W.A. Brand, Referencing strategies and techniques in stable
isotope ratio analysis, Rapid Communications in Mass Spectrometry, 2001: 15:
501-519
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