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Techniques and Methods 10–C15

Determination of the (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


Summary of Procedure

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.

Table of Contents

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

Suggested citation:

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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