A critical examination of sorbent extraction pre-concentration with spectrophotometric sensing in flowing systems

In this paper, the solid-phase spectrophotometric concept is critically examined in flow systems exploiting a dedicated microcolumn-based optical sensor packed with octadecyl chemically-modified silica gel. The flow configuration integrates matrix separation with pre-concentration and on-column sensing of non-polar complexes resulting from analyte derivatization. The design criteria for optimum performance of both the sorbent-packed microcolumn and optosensing instrumentation are for the first time discussed in detail. Practical considerations required to warrant the maximum sorption efficiency of the reversed-phase material by avoiding chain collapse, and avenues to overcome the effects derived from its transparency changes upon solvation with solutions of different polarity are also addressed. The noteworthy features of the flow-through enrichment system involve the sensitivity enhancement with respect to common spectrophotometric procedures in the liquid-phase, as well as the capture of a large amount of scattered light, which is the most severe pitfall of conventional solid-phase absorptiometric approaches using commercially available flow-through cells. Several common spectrophotometric assays for monitoring key inorganic and organic parameters (viz. oxidized nitrogen, ammonium, sulfite, phosphate, iron(II)/total iron, chromium(VI), nickel(II) and phenol index) in environmental samples are taken as practical examples. Plausible sorption mechanisms together with discussions for proper performance of the different investigated reversed-phase extraction optosensing methods are presented in the bulk of the text. Special emphasis is paid to problems arising in real sample analysis due to unspecific sorption of matrix constituents and potential means to circumvent them are thoroughly explored.