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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. Kanakana@hpl.umces.edu • ADVANTAGES: • High precision • Direct dissolved gas interface • RESULT: • Detection of ≤ 0.03% dissolved N2 in < 2 minutes Horn Point Laboratory University of MarylandCenter for Environmental Science 1/12 Kana 2007. MIMS Denitrification NSF-RCN Training
Basic principals of MIMS inlet Semipermeable membrane Detection Separation Ionization Water Sample + high vacuum + + + - Quadrupole mass spectrometer Vacuum pump CO N2 O2 + CO2 N+ H2O Ar 16 16 16 12 12 16 OH- CO2 e - e e e e e 2/12 Kana 2007. MIMS Denitrification NSF-RCN Training
Factors affecting mass spectrometer signal TEMPERATURE Thermal equilibration of samples Keep analysis temperature close to saturation temperature of the water samples • MEMBRANE MATERIAL • Silicone – high permeability to air gases, water vapor and LMW VOCs • Teflon – very low permeability to water vapor and VOCs. Dh boundary layer Dh membrane vacuum WATER FLOW: Uniformity and stability of flow pattern vacuum Tube membrane (longitudinal section) Vacuum Sample Concentration 500 mm 3/12 Kana 2007. MIMS Denitrification NSF-RCN Training
MIMS system(modified from Kana et al. 1994) Furnace (optional) Pump Mass spectrometer Water bath Cryotrap 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. 4/12 Kana 2007. MIMS Denitrification NSF-RCN Training
DGA data display and recording software N2 N2 N2 O2 O2 O2 Ar Ar Ar N2/Ar N2/Ar N2/Ar O2/Ar O2/Ar O2/Ar • The DGA operates in steady state • Look for stable signals Ar O2/Ar N2/Ar O2 N2 5/12 Kana 2007. MIMS Denitrification NSF-RCN Training
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. 6/12 Kana 2007. MIMS Denitrification NSF-RCN Training
Measurement precision and solubility • Poor precision is usually related to poor technique in acquiring the water sample. • Denitrification studies require <0.1% resolution for N2 measurements. 0.2% • Dissolved gas concentrations change by ca. 1.0-1.5% per degree C. Cornwell and Owens • 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. 7/12 Kana 2007. MIMS Denitrification NSF-RCN Training
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. 8/12 Kana 2007. MIMS Denitrification NSF-RCN Training
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. Kana 2007. MIMS Denitrification NSF-RCN Training 9/12
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. Normal N2 signal decline Fluctuating N2/Ar N2/Ar • Erratic signals are usually caused by a dirty membrane or particles in the standard water. O2/Ar • Measuring samples • Move outflow tube to waste. Peaks from microbubbles • Turn of peristaltic pump between samples. • Suspend dip tube above any sediment. 10/12 Kana 2007. MIMS Denitrification NSF-RCN Training
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. 11/12
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: Bay Instruments, LLC 6180 Waterloo Dr Easton, MD 21601 email@example.com 410 924-3507 Horn Point Laboratory PO Box 775 Cambridge, MD 21613 firstname.lastname@example.org 410 221-8481 Acknowledgments I wish to thank Rosalynn Lee and Chava Weitzman for assistance with this training module for NSF’s Denitrification Research Coordination Network. 12/12 Kana 2007. MIMS Denitrification NSF-RCN Training