A novel tool for stable nitrogen isotope analysis in aqueous samples.

Area of Science:

• Environmental Science
• Analytical Chemistry
• Geochemistry

Background:

• Bulk stable isotope analysis (BSIA) of dissolved organic carbon and total nitrogen bound (TN_b) is vital for understanding nutrient cycling.
• Carbon (C) and nitrogen (N) transformations are interconnected in biological systems.
• Existing methods lack simultaneous, direct analysis of stable carbon and nitrogen isotopes in aqueous samples without offline preparation.

Purpose of the Study:

• To extend a high-temperature combustion (HTC) system for measuring TN_b stable nitrogen isotope composition.
• To develop a system for simultaneous and direct BSIA of δ¹³C and δ¹⁵N in aqueous samples.

Main Methods:

• Modified a TOC analyzer with a high-performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS) interface.
• Integrated a reduction reactor to convert NO_x to N₂.
• Developed a focusing unit for nitrogen and a degassing device for online separation of TN_b from N₂.

Main Results:

• Demonstrated proof of principle with various compound solutions.
• Achieved precision and trueness of ≤0.5‰ for δ¹⁵N_AIR-N2 values.
• Obtained good results down to 40 mgN/L and acceptable results down to 5 mgN/L for TN_b.
• Developed the first system for simultaneous and direct δ¹³C and δ¹⁵N BSIA of aqueous samples.

Conclusions:

• The developed system is the first capable of direct BSIA for both δ¹³C and δ¹⁵N in aqueous samples.
• This advancement offers new research opportunities in stable isotope analysis.