Trace analysis of volatile fatty acids in marine waters using modern high-pressure ion chromatography

Volatile fatty acids (VFAs) are key intermediates in carbon transformation in marine environments and feature widely in models for a hydrothermal origin of life. Quantifying VFAs in hydrothermal fluids is challenging due to their trace concentrations and the high inorganic ion loads of these matrices. Previous methods often rely on manual sample pre-treatment or complex instrumentation (e.g., mass spectrometry), increasing contamination risks, consumable use, and costs. To circumvent such challenges, we developed a simplified quantification method for trace VFAs in seawater-like matrices using a modern high-pressure ion chromatography (HPIC) system. This approach utilizes single-dimension ion exchange chromatography with conductivity detection alone, and a choice of two analytical column options to separate formate, acetate, propionate, butyrate, valerate, pyruvate, and lactate (measured as ∑anion) from inorganic anions. Modern HPIC systems, in addition to being versatile for other analytes (e.g., cations, nutrients), enable higher peak resolution and increased ion exchange capacity, and our tests show this allows for significantly greater trace VFA sensitivity than previous techniques. With careful sample handling and contamination control, our method achieves better absolute limits of detection for smaller sample requirements (≤ 0.3 mL), statistically determined to be below 10 ppb (~ 0.05 μmol/kg for ∑formate, ~ 0.03 μmol/kg for ∑acetate). Additionally, our study provides detailed insights into limiting VFA contamination sources, as well as their stability in storage. Initial analysis of hydrothermal fluids from the Arctic Mid-Ocean Ridges reveals formate (0.6–7.3 μmol/kg) controlled by metastable CO₂–H₂–HCOOH equilibrium, and unexpectedly low acetate (2.6–5.8 μmol/kg), likely reflecting competition between thermogenic formation and stability.