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.