Download - National Science Foundation Denitrification Research Coordination Network Training Module
National Science Foundation Denitrification Research Coordination Network
Training Module
Horn Point LaboratoryUniversity of Maryland
Center for Environmental Science
The use of membrane inlet mass spectrometry (MIMS) for the measurement of high precision
N2/Ar ratios.
ADVANTAGES:• High precision• Direct dissolved gas interface
RESULT: • Detection of ≤ 0.03% dissolved N2 in < 2 minutes
Kana 2007. MIMS Denitrification NSF-RCN Training
Dr. Todd M. Kana
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Basic principals of MIMS inlet
high vacuum
Water Sample
Kana 2007. MIMS Denitrification NSF-RCN Training
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Quadrupole mass spectrometer
Ionization
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Separation Detection
Semipermeable membrane
CO2
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ee e
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Vacuum pump
N2O2
Ar
CO2
H2O
OH-
N+
CO
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500 mmSample Vacuum
Concentration
Factors affecting mass spectrometer signal
MEMBRANE MATERIAL
Silicone – high permeability to air gases, water vapor and LMW VOCs
Teflon – very low permeability to water vapor and VOCs.
TEMPERATURE
Thermal equilibration of samples
Keep analysis temperature close to saturation temperature of the water samples
WATER FLOW: Uniformity and stability of flow pattern
Kana 2007. MIMS Denitrification NSF-RCN Training
vacuum
vacuum
h membrane
h boundary layer
Tube membrane (longitudinal section)
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Mass spectrometer
Water bath
Pump
Cryotrap
Furnace (optional)
Gases that permeate the silicone membrane include:Water vapor, nitrogen, oxygen, argon, carbon dioxide, and low-molecular-weight organic compounds.
A liquid nitrogen cryotrap is used to freeze out components other than nitrogen, oxygen and argon.
A heated copper column can be used to eliminate O2
which reacts in the ion source.
Kana 2007. MIMS Denitrification NSF-RCN Training
MIMS system(modified from Kana et al. 1994)
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DGA data display and recording software
Kana 2007. MIMS Denitrification NSF-RCN Training
Ar
N2
O2
N2/Ar
O2/Ar
N2O2Ar
N2/ArO2/Ar
N2O2Ar
N2/ArO2/Ar
N2O2Ar
N2/ArO2/Ar
• The DGA operates in steady state
• Look for stable signals
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Calibration of instrument signals
Kana 2007. MIMS Denitrification NSF-RCN Training 6/12
• Water is air-equilibrated at known temperature and salinity close to that of the samples.
• Headspace air is saturated with water vapor in semi-closed flask.
• Local barometric pressure is recorded if individual gas concentrations are to be determined.
• Set up standard water at least 2-3 hours before it will be used.
• Conduct triplicate measurements for statistical assessment.
• Calibrate the signals at 10-60 minute intervals, depending on degree of drift.
Measurement precision and solubility
Kana 2007. MIMS Denitrification NSF-RCN Training
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0 2 4 6 8 10
Core ACore BCore CBlank
Incubation Time (h)
0.2%
Cornwell and Owens
• Poor precision is usually related to poor technique in acquiring the water sample.
• Denitrification studies require <0.1% resolution for N2 measurements.
• Dissolved gas concentrations change by ca. 1.0-1.5% per degree C.
• Ar is used as an internal standard.
• N2/Ar ratios change by ca. 0.1% per degree C.
• Nitrogen is half as soluble as oxygen or argon. Therefore, bubbles will affect N2 and Ar differently.
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Sample collection
Kana 2007. MIMS Denitrification NSF-RCN Training
• Sample water should be well mixed.
• Minimize contact with air and avoid making bubbles.
• Sample container should be tall and narrow for small volumes.
• Fill from the container bottom and overfill.
• Add preservative before capping if sample is to be stored.
• After capping, check the container for bubbles. Resample if bubbles are present.
• Store sample at or below sample water temperature and underwater.
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Instrument start-up
Kana 2007. MIMS Denitrification NSF-RCN Training
• Prepare standard water the day before.
• Evacuate the inlet line.
• Attach pump tubing and pump water through the line. Stop pump.
• Put liquid nitrogen around trap.
• Close roughing valve then open mass spectrometer valve slowing while monitoring MS pressure.
• Turn on peristaltic pump and leave it running.
• Unplug cold cathode gauge.
• Start up computer.
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Signal evaluation
Kana 2007. MIMS Denitrification NSF-RCN Training
• After startup, ca. 1 hour needed for signals to stabilize.
Normal N2 signal decline
Fluctuating N2/Ar
Peaks from microbubbles
N2/Ar
O2/Ar
• Erratic signals are usually caused by a dirty membrane or particles in the standard water.
• Clean membrane with 1% soap solution.
• Erratic signals are usually caused by a dirty membrane or particles in the standard water.
Measuring samples
•Move outflow tube to waste.
• Turn of peristaltic pump between samples.
• Suspend dip tube above any sediment.
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DGA shut-down
• Close the primary valve to the mass spectrometer first!
• Pump the water out of the capillary tubing.
• Remove the liquid nitrogen.
• Release the tubing from the peristaltic pump.
• Save your data and turn off computer.
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Acknowledgments
I wish to thank Rosalynn Lee and Chava Weitzman for assistance with this training module for NSF’s Denitrification Research Coordination Network.
Kana 2007. MIMS Denitrification NSF-RCN Training
More informationOriginal instrument description:Kana, T.M., C. Darkangelo, M.D. Hunt, J.B. Oldham, G.E. Bennett, and J.C. Cornwell. 1994 . A membrane inlet mass spectrometer for rapid high precision determination of N2, O2, and Ar in environmental water samples. Anal. Chem. 66:
4166-4170.
Current operational methods:Kana, T.M., J.C. Cornwell, L. Zhong. 2006. Determination of denitrification in the Chesapeake Bay from measurements of N2 accumulation in bottom water. Estuaries and Coasts 29:222-231.
These papers and others by the author may be found at:www.hpl.umces.edu/~kana
Horn Point LaboratoryPO Box 775Cambridge, MD 21613
[email protected] 221-8481
Bay Instruments, LLC6180 Waterloo DrEaston, MD 21601
[email protected] 924-3507
Contact:
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