National Science Foundation Denitrification Research Coordination Network Training Module

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. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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 kana@bayinstruments.com 410 924-3507 Horn Point Laboratory PO Box 775 Cambridge, MD 21613 kana@hpl.umces.edu 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