『Abstract
　Mass spectrometric analysis of O-isotopic composition of nitrate has many potential application in studies of environmental processes. Through this work, rapid, reliable, precise, broadly applicable, catalyst-free, low-priced and less labor intensive procedure for measuring δ¹⁸O of nitrate using Isotope Ratio Mass Spectrometer has been developed and implemented. The conditions necessary to effect complete nitrate recovery and complete removal of other oxygen containing anions and dissolved organic carbon (DOC) without scarifying the isotopic signature of nitrate were investigated. The developed procedure consists of two main parts: (1) wet chemistry train for extraction and purification of nitrate from the liquid matrix; (2) off-line pyrolysis of extracted nitrate salt with activated graphite at 550℃ for 30 min. The conditions necessary to effect complete nitrate recovery and complete removal of other oxygen containing compounds were investigated. Dramatic reduction in processing times needed for analysis of δ¹⁸O of nitrate at natural abundance level was achieved. Preservation experiments revealed that chloroform (99.8％) is an effective preservative. Isotopic contents of some selected nitrate salts were measured using the modified procedure and some other well established methods at two laboratories in Egypt and Germany. Performance assessment of the whole developed analytical train was made using internationally distributed nitrate isotopes reference materials and real world sample of initial zero-nitrate content. The uncertainty budget was evaluated using the graphical nested hierarchal approach. The obtained results proved the suitability for handling samples of complicated matrices. Reduction of consumables cost by about 80％ was achieved.

Keywords: ¹⁸O isotope; Nitrate extraction; IRMS; Cl^- interference; Performance assessment; Uncertainty evaluation』

1. Introduction
2. Materials and methods
　2.1. Preparation and loading anion exchange columns
　2.2. Stripping of bound nitrate
　2.3. Preparation of NO3^--bearing eluent
　2.4. Effect of competing ions
　2.5. Isotopes measurements
　2.6. Loading of the reagents in the pyrolysis tube
　2.7. Activation of graphite
3. Results and discussion
　3.1. Wet chemistry
　　3.1.1. Nitrate capturing and elution efficiencies
　　3.1.2. Effect of competing ions
　　3.1.3. Precipitation of sulfate
　　3.1.4. Effect of dissolved organic carbon (DOC)
　　3.1.5. Removal of dissolved organic carbon
　　　3.1.5.1. Protonation using cation exchange resins
　　　3.1.5.2. Adsorption using activated charcoal and activated carbon
　　3.1.6. Neutralization
　　3.1.7. Samples preservation
　　　3.1.7.1. Waste water zero nitrate samples
　　　3.1.7.2. Preservation onto anion exchange resin column
　3.2. Pyrolysis experiments
　　3.2.1. Effect of activation of graphite
　　3.2.2. Pyrolysis temperature
　　3.2.3. Pyrolysis time
　　3.2.4. Effect of cooling rate
　　3.2.5. Combustion blanks
　3.3. Performance assessment and uncertainty evaluation
　　3.3.1. Performance assessment of the pyrolysis stage
　　3.3.2. Performance assessment and scale factor
　　3.3.3. Uncertainty evaluation
　3.4. Method characteristics
　3.5. Cost analysis
4. Conclusions
　4.1. Merits of the developed procedure
Acknowledgements
References