Gases

WARNING: We DO NOT ACCEPT samples with enriched 14C (often used as a tracer).  Any contamination of any amount of enriched 14C ("hot") material may cause thousands of dollars in clean-up and other costs. If you submit "hot" samples to this lab, you may be liable.

Quantitative - N2, O2+Ar, CO, CH4, CO2, C1-C5 [H2 / He] (alone or dissolved)

Requirements:

  • About 10 ml are used to flush the the 1 ml sample loop of the instrument.
  • 500, 750 or 1000mL Wheaton Bottles or other pre-approved containers must have a thick butyl rubber septum for extracting the gas.
  • Exact bottle volume must be known. If Isoflasks are used, weight of Isoflask and weight of water should be provided.
  • Bottles should be filled full without any air bubble.
  • Samples should be stored at 4oC.
  • Consult with Paul before sampling to make sure you have the appropriate container, etc.

Methodology:

At room temperature, a head space is created by gently displacing the water with high purity Helium, typically 10% of the total bottle volume. The bottle is shaken for 5 minutes and equilibrated for 30 minutes. The head space is analysed for gaseous components on an SRI GC 8610C of N2, O2+Ar, CO2, CO, methane, ethane, propane, butane and pentane percentages, a calibration curve is created with known gas concentrations prior to sample analysis. We cannot analyse for Hydrogen and Helium using this approach.

Corrections are applied for volume, temperature and gas solubility using Henry’s law as per the three references listed below.

Corrections:

TC = CAH + Cwhere:

  • TC = total concentration of gas in original aqueous sample
  • CAH = aqueous concentration in head space after equilibrium in mg/L = (55.5mol/L)*(pg/H)*MW(g/mol)*103mg/g
    where pg= partial pressure of gas in atm from GC analysis, H = Henry’s law constant, MW = molecular weight in g/mol
  • CA = aqueous concentration in water after equilibrium in mg/L
    = [(Vh/(Vb-Vh)]*Cg*(MW(g/mol)/(22.4L/mol))*[273K/(T+273K)]*103mg/g
    where Vh = head space volume, Vb = bottle volume, Cg = gas concentration (also equal to pg ), T = temperature in Kelvin

Expected error has not been determined yet, we believe it should be around 10% of the absolute value.

References:

  • Analysis of dissolved methane, ethane and ethylene in ground water by standard gas chromatographic technique, Journal of Chromatographic Science, Vol 36, May 1998.
  • Sample preparation and calculations for dissolved gas analysis in water samples using a GC head space equilibration technique, RSKSOP-175, revision No 2, May 2004.
  • Compilation of Henry’s Law Constants for Inorganic and Organic Species of Potential Importance in Environmental Chemistry. http://www.henrys-law.org/henry-3.0.pdf
  • SRI Chromatogragh Operating notes, Peak simple version 4.44
Isotopes of Gas - N2, CO, CH4, CO2, C1-C5 / H2 (alone or dissolved)

Requirements

  • 500, 750 or 1000mL Wheaton Bottles or other pre-approved containers must have a thick butyl rubber septum for extracting the gas.
  • Bottles for gases should be slightly pressurized to avoid the creation of a vacuum when the sample is drawn out.
  • Samples should be stored at 4oC.
  • Analysis should be done within 2 weeks.
  • Consult with Paul before sampling to make sure you have the appropriate container, etc.

Methodology (Gas only)

 

Methodology (Dissolved Gas)

Additional preparation is needed for dissolved gases.  Samples are warmed up to room temperature then 10% of water volume is removed and simultaneously replace by UHP helium. Samples are shaken and equilibrated for 30 minutes before the head space is sampled for GC concentration (SRI GC 8610C) or isotope analysis (GC Isolink).

For isotopes, the gas is injected into the GC Isolink and the gas of interest (C1-C3 or other) is isolated and converted to CO2,H2 or N2.

References

  • Hudson F., RSKSOP-175, Sample Preparation and Calculation for Dissolved Gas Analysis in Water Samples Using GC Headspace Equilibration Technique, EPA document, May 2004, 17 p.
  • Kampbell D., Vandegrift S.,Analysis of dissolved methane, ethane and ethylene in ground water by a standard gas chromatographic technique, Journal of chromatographic science, vol 36, May 1998, pp 253-256.
  • Thermo Fisher Scientific, GC Isolink Operating Manual, Revision D-1222980, 2011.
  • SRI GC Chromatogram, Operating manual for Peak Simple version 4.44.
Isotopes of CO2 (alone, in air or breath)

Requirements

  • Container should have a septa, butyl is best.
  • Best container is exetainer, with about 0.2 to 2% of CO2. Do not use vacutainers.
  • Extra fee may be applied to transfer proper concentration or into exetainer.

 

Methodology

Exetainers are analysed on the Gas Bench, where CO2 is separated from air and send to IRMS, as per normal CO2 analysis. Carbonate standards are used for 13C and 18O calibration.

 

Reference

  • Instruction Manuals for the GasBench and the Thermo Finnigan DeltaPlusXP IRMS.

 

 

Isotopes of N2+O2+Ar
Isotopes of Pure Gas in 6mm Pyrex Break Seals

Requirements:

  • Pyrex tubes should be 6mm or 1/4" in outer diameter, and about 14 cm in length.
  • Sealed tips should be smooth; no thin, sharp bits!
  • Sealed tips should be relatively small, and stay within the width of the Pyrex tube.

Methodology:

Break seals are loaded into the cracker tubes of the 10-port manifold on the Thermo Finnigan DeltaV+, and pumped for 15 minutes.  The break seals are snapped to release the contained gas.  The gas is run in dual-inlet against a calibrated reference gas.  If there is less than about 20 µM of gas, then the microvolume (cold finger) must also be used to pull the gas into a smaller volume before analysis.

Dual Inlet precision:

The analytical precision (2 sigma) for carbon isotopes is +/- 0.05 permil.
The analytical precision (2 sigma) for oxygen isotopes is +/- 0.10 permil.
The analytical precision (2 sigma) for hydrogen isotopes is +/- 2.0 permil.
The analytical precision (2 sigma) for sulphur isotopes is +/- 0.2 permil.

NOTE: Dual Inlet analysis offers the best precision. But it is also possible to transfer CO2 or H2 gas from a break seal into an Exetainer flushed with helium.  It is then run as a regular headspace sample on the GasBench + Thermo Finnigan DeltaPlusXP IRMS. Aliquots of the calibrated reference gas used in dual-inlet are run with the samples to enable normalization of the data.  This analysis is less precise but easier.

References:

  • Instruction Manual for GasBench and DeltaPlus XP IRMS
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