Analysis of ammonia and methylamines in natural waters by flow injection gas diffusion coupled to ion chromatography

Flow injection gas diffusion-ion chromatography (FIGD-IC), is a new hyphenated technique for the simultaneous analysis of nanomolar levels of ammonia (NH3) and methylamines (MAs) in < 50 ml of marine, estuarine and freshwaters. Alkaline EDTA is added on-line to flowing sample to achieve a sufficiently high pH (> 12.0) needed to deprotonate > 95% of the amines to their uncharged volatile forms. In addition the reagent chelates Mg2+ and Ca2+ to prevent their precipitation as Ca(OH)2 and Mg(OH)2. The amines diffuse selectively across a gas-permeable microporous PTFE Goretex® membrane into a recirculating flow of acidic ‘acceptor’ in which they are reprotonated and preconcentrated. The acceptor solution is then injected onto an ion Chromatograph (IC) where NH+4 and MA cations are separated within 15 min and detected by chemically suppressed conductimetry using cyclopropylamine as an internal standard for quantification. The response of the coupled FIGD-IC system was sensitive (ca. 3–5 nM for MAs, 20–40 nM for NH3), linear (R2 = 0.99, 0–2000 nM in seawater) and precise (R.S.D. = 1–6% at 1 μM) for all analytes. The applicability of FIGD-IC is demonstrated through laboratory analysis of NH3 and MAs in a range of natural water samples, on-board a research vessel, and through inter-comparisons with fluorimetric assay of NH3.