Transcript Slide 1

National Science Foundation
Denitrification Research Coordination Network
Training Module
The use of membrane inlet mass spectrometry
(MIMS) for the measurement of high precision
N2/Ar ratios.
Dr. Todd M. Kana
[email protected]
ADVANTAGES:
• High precision
• Direct dissolved gas interface
RESULT:
Horn Point Laboratory
University of Maryland
Center for Environmental Science
• Detection of ≤ 0.03%
dissolved N2 in < 2 minutes
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Kana 2007. MIMS Denitrification NSF-RCN Training
Basic principals of MIMS inlet
Semipermeable
membrane
Ionization
Water
Sample
Detection
Separation
+
+
high vacuum
++
-
Quadrupole mass
spectrometer
CO
N2
N+ H O
2
Vacuum pump
O2
CO2
Ar
OH-
+
CO2
e
e
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e
e
-
e
e
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Factors affecting mass spectrometer signal
3E-10
TEMPERATURE
MEMBRANE MATERIAL
2.5E-10
QMS Signal (amps)
Thermal
equilibration
of samples to air gases, water
Silicone
– high permeability
Dh boundary layer
2E-10
vapor
and LMW VOCs
Keep analysis temperature close to
Dh membrane
1.5E-10
saturation
temperature of the
water
vacuum
samples
1E-10
Teflon
– very low permeability to water vapor
WATER FLOW:
and
5E-11 VOCs.
vacuum
Uniformity and
0
stability of flow pattern Tube
0
20
40(longitudinal
60
80
100
membrane
section)
Gas concentration (%)
Concentration
Sample
500
mm
Kana 2007. MIMS Denitrification NSF-RCN Training
Vacuum
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MIMS system
(modified from Kana et al. 1994)
Furnace
(optional)
Pump
Water
bath
Cryotrap
Mass
spectrometer
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.
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DGA data display and recording software
N2
O2
Ar
N2/Ar
O2/Ar
N2
O2
Ar
N2/Ar
O2/Ar
• The DGA operates in steady state
• Look for stable signals
Ar
O2/Ar
N2
O2
Ar
N2/Ar
O2/Ar
Kana 2007. MIMS Denitrification NSF-RCN Training
N2/Ar
O2
N2
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Calibration of instrument signals
• 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.
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Measurement precision and solubility
474
N22 (mM)
800
800 473
700
700 472
600
600 471
500
500
44
44
Core A
Core B
Core C
Blank
42
42
40
40
38
38
• Denitrification studies require <0.1%
resolution for N2 measurements.
470
36
36
0.2%
469
400
400
32
32
467
200
200 466
00 0
• Dissolved gas concentrations
change by ca. 1.0-1.5% per degree C.
34
34
468
300
300
N22:Ar
900
900
• Poor precision is usually related to
poor technique in acquiring the water
sample.
• Ar is used as an internal standard.
Cornwell and Owens
30
30
10
102
20
420
30
6 30
Temperature
Temperature ((oooC)
C)
Incubation Time (h)
8
40
40
10
• 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
• 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
• 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
• After startup, ca. 1 hour needed for
signals to stabilize.
• 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.
N2/Ar
O /Ar
Fluctuating N22/Ar
Measuring samples
Normal N2 signal decline
• Move outflow tube to waste.
Peaks from microbubbles
• 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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More information
Original 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
Contact:
Horn Point Laboratory
PO Box 775
Cambridge, MD 21613
Bay Instruments, LLC
6180 Waterloo Dr
Easton, MD 21601
[email protected]
410 221-8481
[email protected]
410 924-3507
Acknowledgments
I wish to thank Rosalynn Lee and Chava Weitzman for assistance with this
training module for NSF’s Denitrification Research Coordination Network.
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