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| Techniques and Methods 10–C15 |
(15N/14N)of Ammonium in Water: RSIL Lab Code 2898 U.S. Geological Survey
By Janet E. Hannon and John Karl Böhlke
Chapter 15
Section C, Stable Isotope-Ratio Methods
Book 10, Methods of the Reston Stable Isotope Laboratory
The purpose of the technique described by Reston Stable Isotope Laboratory (RSIL) lab code 2898 is to determine the N isotopic composition, δ(15N/14N), abbreviated as δ15N, of ammonium (NH4+) in water (freshwater and saline water). The procedure involves converting dissolved NH4+ into NH3 gas by raising the pH of the sample to above 9 with MgO and subsequently trapping the gas quantitatively as (NH4)2SO4 on a glass fiber (GF) filter. The GF filter is saturated with NaHSO4 and pressure sealed between two gas-permeable polypropylene filters. The GF filter “sandwich” floats on the surface of the water sample in a closed bottle. NH3 diffuses from the water through the polypropylene filter and reacts with NaHSO4, forming (NH4)2SO4 on the GF filter. The GF filter containing (NH4)2SO4 is dried and then combusted with a Carlo Erba NC 2500 elemental analyzer (EA), which is used to convert total nitrogen in a solid sample into N2 gas. The EA is connected to a continuous-flow isotope-ratio mass spectrometer (CF-IRMS), which determines the relative difference in ratios of the amounts of the stable isotopes of nitrogen (15N and 14N) of the product N2 gas and a reference N2 gas. The filters containing the samples are compressed in tin capsules and loaded into a Costech Zero-Blank Autosampler on the EA. Under computer control, samples then are dropped into a heated reaction tube that contains an oxidant, where combustion takes place in a He atmosphere containing an excess of O2 gas. To remove S-O gases produced from the NaHSO4, a plug of Ag-coated Cu wool is inserted at the bottom of the reaction tube. Combustion products are transported by a He carrier through a reduction furnace to remove excess O2, toconvert all nitrogen oxides to N2, and to remove any remaining S-O gases. The gases then pass through a drying tube to remove water. The gas-phase products, mainly N2 and a small amount of background CO2, are separated by a gas chromatograph (GC). The gas is then introduced into the IRMS through a Finnigan ConFlo II interface. The ConFlo II interface is used to introduce not only sample into the IRMS but also N2 reference gas and He for sample dilution. The flash combustion is quantitative, so no isotopic fractionation is involved. The IRMS is a Finnigan Delta V CF-IRMS with 10 cups and is capable of detecting ion beams with mass/charge (m/z) 28, 29, 30. The ion beams from N2 are as follows: m/z 28 = 14N14N, m/z 29 = 14N15N, and m/z 30 = 15N15N. The ion beam with m/z 30 also represents 14N16O, which may indicate contamination or incomplete reduction.
Forward
Conversion Factors
Acronyms, and Abbreviations
Symbols
Summary of Procedure
Reporting Units and Operational Range
Reference Materials and Documentatione
Reference Materials Used, Storage Requirements, and Shelf Life
Documentation
Labware, Instrumentation, and Reagents
Sample Collection, Preparation, Analysis, Retention Times, and Disposal
Sample Containers, Preservation, and Handling Requirements
Sample Preparation
Performing the Analysis
Problematic Samples
Interferences
Troubleshooting and Bench Notes
Maintenance and Maintenance Records
Sample Retention Time and Disposal
Data Acquisition, Processing, Evaluation, Quality Control, and Quality Assurance
Laboratory Information Management System for Light Stable Isotope (LIMS-LSI)
Quality-Control (QC) Samples
Acceptance Criteria for all QC Samples
Corrective Action Requirements
Responsible Parties for All QA/QC Functions for Procedures Covered in RSIL SOPs
Data Management and Records
Health, Safety, and Waste-Disposal Information
Applicable Health and Safety Issues
Personal Protection
Electrical Hazards
Chemical Hazards
Gas Cylinder Handling
Specific Waste-Disposal Requirements
Revision History
References Cited
Apendixes
| A. | Step-by-Step Procedure to Log-in Samples to LIMS-LSI. |
| B. | Step-by-Step Procedure to Convert NH4+ in Water to NH3 Gas, Then to (NH4)2SO4. |
| C. | Excel Sample Workbook. |
| D. | Step-by-Step Procedure for Placing Sample Filters in Capsules and Weighing Reference Samples. |
| E. | Step-by-Step Procedure for Zero Blank Autosampler Operation. |
| F. | Step-by-Step Procedure to Add Sample Information to Sequence Table. |
| G. | Step-by-Step Procedure to Retrieve Data from ISODAT for LIMS-LSI and for Data Backup. |
| H. | Step-by-Step Procedure to Transfer Data to LIMS-LSI, Transfer Data to Data Backup Computer, and to Reevaluate Old Data. |
| I. | Step-by-Step Procedure to Determine and Apply Correction Factors and Evaluate Data. |
| J. | Step-by-Step Procedure to Check Elemental Analyzer for Leaks. |
| K. | Daily Checklist. |
| L. | Changing the Insertion Tube. |
| M. | Changing the Water Trap. |
| N. | Changing the Reaction Tube. |
| O. | Step-by-Step Procedure to Report Data. |
Hannon, Janet E., and Böhlke, John Karl, 2008, Determination of the δ(15N/14N) of ammonium (NH4+) in water: RSIL lab code 2898, chap. C15 of Révész, Kinga, and Coplen, Tyler B., eds., Methods of the Reston Stable Isotope Laboratory: U.S. Geological Survey, Techniques and Methods, 10–C15, 30 p.
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