Water ICE: Ion Exclusion Chromatography of Very Weak Acids with a Pure Water Eluent

Separation of ions or ionizable compounds with pure water as eluent and detecting them in a simple fashion has been an elusive goal. It has been known for some time that carbonic acid can be separated from strong acids by ion chromatography in the exclusion mode (ICE) using only water as the eluent. The practice of water ICE was shown feasible for very weak acids like silicate and borate with a dedicated element specific detector like an inductively coupled plasma mass spectrometer (ICPMS), but this is rarely practical in most laboratories. Direct conductometric detection is possible for H2CO3 but because of its weak nature, not especially sensitive; complex multistep ion exchange methods do not markedly improve this LOD. It will clearly be impractical in acids that are weaker still. By using a permeative amine introduction device (PAID, Anal. Chem. 2016 , 88 , 2198 - 2204 ) as a conductometric developing agent, we demonstrate that a variety of weak acids (silicate, borate, arsenite, cyanide, carbonate, and sulfide) cannot only be separated on an ion exclusion column, they can be sensitively detected (LODs 0.2-0.4 μM). We observe that the elution order is essentially the same as that on a nonfunctionalized poly(styrene-divinylbenzene) column using 1-10% acetonitrile as eluent and follows the reverse order of the polar surface area (PSA) of the analyte molecules. PSA values have been widely used to predict biological transport of pharmaceuticals across a membrane but never to predict chromatographic behavior. We demonstrate the application of the technique by measuring the silicate and borate depth profiles in the Pacific Ocean; the silicate results show an excellent match with results from a reference laboratory.