Inline monitoring of high ammonia concentrations in methanol with a customized 3D printed flow cell

A novel system for inline monitoring of ammonia (NH₃) suitable for methanol is presented. An optical ammonia sensor with a response time t₉₀ of 33 s was combined with a tailor-made, 3D printed flow cell and allowed efficient measurements under continuous flow. The optical sensor includes a fluorescent indicator dye that is physically immobilized into a polyurethane hydrogel. A protective layer made of hydrophobic polyether sulfone (PES) shields the ammonia sensitive material against interfering substances and guarantees long-term stability in methanol. The sensor can be read out via a compact phase fluorimeter. Measurements in continuous flow are enabled by a flow cell manufactured via selective laser melting (SLM) of stainless steel. Stainless steel was chosen for the flow cell due to its good heat transfer properties and relatively good chemical resistance of NH₃ in methanol. The measurements were successfully carried out with ammonia concentrations between 0.3 and 5.6 mol L^{− 1} NH₃ in methanol at 25 °C up to 80 °C. Additionally, different flow-rates (0.5–2.0 mL min^{− 1}), varying internal pressure (0.5–2.0 bar) as well as reversibility of the measurements at 25 and 60 °C were studied in detail. The sensor did not degrade indicated by sufficient signal and low drift over a period of two weeks, thus indicating the high potential of the novel set-up for real-time measurements in continuous flow applications. Graphical abstract: [Figure not available: see fulltext.].