Portable IC system enabled with dual LED-based absorbance detectors and 3D-printed post-column heated micro-reactor...

The on-site and simultaneous determination of anionic nitrite (NO₂^-) and nitrate (NO₃^-), and cationic ammonium (NH₄⁺), in industrial and natural waters, presents a significant analytical challenge. Toward this end, herein a 3D-printed micro-reactor with an integrated heater chip was designed and optimised for the post-column colorimetric detection of NH₄⁺ using a modified Berthelot reaction. The system was integrated within a portable and field deployable ion chromatograph (Aquamonitrix) designed to separate and detect NO₂^- and NO₃^-, but here enabled with dual LED-based absorbance detectors, with the aim to provide the first system capable of simultaneous determination of both anions and NH₄⁺ in industrial and natural waters.

Incorporating a 0.750 mm I.D. 3D-printed serpentine-based microchannel for sample-reagent mixing and heating, the resultant micro-reactor had a total reactor channel length of 1.26 m, which provided for a reaction time of 1.42 min based upon a total flow rate of 0.27 mL min^-1, within a 40 mm² printed area. The colorimetric reaction was performed within the micro-reactor, which was then coupled to a dedicated 660 nm LED-based absorbance detector. By rapidly delivering a reactor temperature of 70 °C in just 40 s, the optimal conditions to improve reaction kinetics were achieved to provide for limits of detection of 0.1 mg L^-1 for NH₄⁺, based upon an injection volume of just 10 μL. Linearity for NH₄⁺ was observed over the range 0-50 mg L^-1, n = 3, R² = 0.9987. The reactor was found to deliver excellent reproducibility when included as a post-column reactor within the Aquamonitrix analyser, with an overall relative standard deviation below 1.2 % for peak height and 0.3 % for peak residence time, based upon 6 repeat injections.

The printed post-column reactor assembly was integrated into a commercial portable ion chromatograph developed for the separation and detection of NO₂^- and NO₃^-, thus providing a fully automated system for the remote and simultaneous analysis of NO₂^-, NO₃^-, and NH₄⁺ in natural and industrial waters. The fully automated system was deployed externally within a greenhouse facility to demonstrate this capability.