Freezing prevents ⁶⁰Co‑γ irradiation artefacts in DOM concentration, composition and δ¹³C

Area of Science:

• Environmental Chemistry
• Biogeochemistry
• Analytical Chemistry

Background:

• Gamma (γ)-irradiation is standard for sterilizing environmental water samples for transport and biosecurity.
• The effects of γ-irradiation on the molecular and isotopic integrity of dissolved organic matter (DOM) are not well understood.
• Understanding these effects is crucial for accurate biogeochemical studies.

Purpose of the Study:

• To assess the impact of γ-irradiation (50 kGy) on dissolved organic carbon (DOC) concentration, DOM composition, and stable isotope signatures (δ¹³C, δ¹⁵N, δ¹⁸O, δ²H).
• To compare the effects of irradiation on unfrozen versus frozen natural waters and organic matter standards.
• To determine optimal methods for preserving sample integrity during sterilization.

Main Methods:

• Analysis of DOC concentration using established methods.
• Characterization of DOM composition using fluorescence spectroscopy (humic-like fluorophores) and assessment of aromaticity.
• Measurement of stable isotope ratios (δ¹³C, δ¹⁵N, δ¹⁸O, δ²H) using isotope ratio mass spectrometry.
• Comparison of irradiated unfrozen samples with irradiated frozen samples and controls.

Main Results:

• Unfrozen water irradiation caused significant DOC loss, oxidative fragmentation of humic-like substances, and δ¹³C enrichment due to preferential ¹²C oxidation.
• Irradiation of unfrozen samples led to increased low-molecular-weight acids and microbial protein-like components, with decreased aromaticity.
• Frozen samples showed no significant changes in DOC, DOM composition, or δ¹³C, indicating ice immobilization preserves DOM.
• Gamma irradiation acidified samples and altered SO₄ and NO₃ concentrations, with increased SO₄ likely from dissolved organic sulfur oxidation.

Conclusions:

• Freezing environmental water samples before γ-irradiation effectively preserves DOC concentration, DOM molecular integrity, and isotopic signatures.
• Gamma irradiation of unfrozen water samples significantly alters DOM characteristics and isotopic values.
• Freezing is a recommended pretreatment step to maintain sample integrity for biogeochemical research involving sterilized water samples.