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

kana@hpl.umces.edu

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Basic principals of MIMS inlet

high vacuum

Water Sample

Kana 2007. MIMS Denitrification NSF-RCN Training

+

Quadrupole mass spectrometer

Ionization

++

+

-

Separation Detection

Semipermeable membrane

CO2

+

-e

ee e

ee

12 1616

1612

16

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

466

467

468

469

470

471

472

473

474

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

kana@hpl.umces.edu410 221-8481

Bay Instruments, LLC6180 Waterloo DrEaston, MD 21601

kana@bayinstruments.com410 924-3507

Contact:

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