On-site total inorganic nitrogen determination in soil (pore-water) samples using portable ion chromatography

A portable ion chromatograph (pIC) designed for nitrite and nitrate analysis has been modified with a post-column 3D-printed heated microreactor enabling simultaneous ammonium determination. This modification allows for the simultaneous detection of all three inorganic nitrogen nutrients in a single run within 15 minutes, using a sample volume as low as 10 μL. Coupled with a soil pore-water sampler and a portable vacuum pump, this method was tested in various field studies, demonstrating successful detection of all three nutrients. The modified system achieved a LOD of 0.1 mg L^-1 and a LOQ of 0.25 mg L^-1 for ammonium, showing a strong correlation with benchtop ion chromatography (IC): Pearson correlation coefficients reached 0.999 for NO₃^-, 0.991 for NO₂^-, and 0.993 for NH₄⁺. To evaluate the robustness of this method, three experiments were conducted: Validation and detection of nitrogen nutrients in the field; intraday comparative analysis between benchtop IC and the pIC, and; on-site farm analysis over a 5 days period, comparing pIC results with the conventional grab-and-lab approach. The application of synthetic fertiliser at the sampling site before pore-water collection resulted in a 223% increase in NO₃^- concentrations and a 197% increase in NH₄⁺ concentrations. The detected nutrient levels in fertilised soil pore-waters reached a maximum of 3.7 mg L^-1 for NO₂^-, 1000.9 mg L^-1 for NO₃^-, and 49.5 mg L^-1 for NH₄⁺.