Hydrogen and Oxygen isotope analysis (2H & 18O) of water by
Headspace equilibration – 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 high TDS waters (> 10,000 mg/L) such as saline,
brackish or brine waters are determined using headspace equilibration with H2
and CO2 gas mixtures respectively, and analyzed using Continous Flow – IRMS.
Sample preparation:
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Water samples should be collected in Nalgene, HDPE or glass sample
bottles. Cap liners should be plastic or rubber (cardboard or foil coated
cardboard are to be avoided)
20 ml is plenty
Bottles should be filled to capacity to minimize headspace
Samples should be filtered to at least 0.45uM,
No biocide is needed
Online (CF-IRMS) technique:
250uL of water is placed with a Hokko bead stick into a Exetainer vial (Labco p/n:
038W), then capped and flushed with either a 0.5% CO2 + balance helium or 2%
H2 + balance helium gas mix respectively. If samples contain dissolved organics,
activated charcoal may also be added to the auto-sampler vial. If samples
contain H2S, copper rods may also be added to the auto-sampler vial. Vials are
flushed for ~10minutes at ~70ml/min then placed in the heated block of the
GasBenchII at 25ºC and left to react for ~72hrs and ~12hrs for CO2 and H2
respectively. The CO2 or H2 headspace is then sampled automatically by the Gas
Bench using a sample loop (typically 50uL) and inlet to the ion source of a
DeltaVPlus stable isotope ratio mass spectrometer for analysis of 18O/16O and
2H/1H ratios. The headspace of each vial is sampled 6 times by loop injection.
The first peak is discarded and the subsequent 5 injections are acquired. If the
first peak is >30 [V], the subsequent 5 injections are automatically diluted by ~
1/3. 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.
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Further comments on technique:
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Normalization standards are used throughout the sequence to correct for
drift
Internal lab standards are analyzed repeatedly within each sequence at
approximately 1 standard per every 7 unknowns
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 4.0‰
(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:
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
Hilkert A.W., et al., Thermo application note: 30048, 18O-Equilibration on Water,
Fruit Juice and Wine Using Thermo Scientific GasBench II
Duhr A, et al., Thermo application note: 30049, Automated H2/H2O Equilibration
for δD Determination on Aqueous Samples Using Thermo Scientific GasBench II
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