Determination of iron in highly-saline matrices by FIA-ICP-MS

Analysis of iron by inductively-coupled-plasma mass-spectrometry (ICP-MS) may be significantly improved by applying a protocol of flow-injection analysis. The iron species of the sample was preconcentrated by an ammonia buffer at pH = 9.2 on a filterless nylon-knotted reactor, and the adsorbed species were subsequently eluted by hydrochloric acid and analysed by ICP-MS. During the FIA step of preconcentration, a high degree of salinity did not influence the adsorption mechanism of iron, which may be related to formation of iron-hydroxide complexes at the sites of amide moieties of the nylon-tube material. Such complexes may form because of the hydrophilic character of the walls of the nylon reactor. Accordingly, the method allowed determination of iron at low levels in complex matrices and the detection limits obtained mainly depended on the purity of chemicals used for digestion of samples and for dilutions. Enrichment up to approximately a factor of thirty was obtained following a preconcentration period of 40-120 seconds. Detection limits of 1-4 ppb were obtained at a sampling frequency of 30 per hour. The main interferences were constituted by cations that form hydroxide species that precipitate in alkaline medium and by anions that complex to iron species in alkaline solution. Owing to the absence of columns in the manifold, an excellent repeatability was obtained, which allowed constant flow and long-time stability of the knotted reactor without loss of preconcentration efficacy.