Matrix removal before inductively coupled plasma spectrometric detection: Another capability of flow field-flow fractionation

A modified flow field-flow fractionation (FlFFF) configuration was investigated for its matrix removal capability before inductively coupled plasma optical emission spectrometric (ICP-OES) detection. Poly(ethylene imine) (PEI) having a molecular weight of 25000 Da was added to test solutions (buffered at pH 9) to form complexes with analyte elements (Ag, Cd, Co, Cu, Hg, Ni, Pb and Zn) but not with the matrix elements (Ca, K, Mg and Na). With the use of a poly(ether sulfone) membrane having a 1000 Da molecular weight cut-off inside the FlFFF channel, matrix elements were removed by filtering off through the membrane, whereas the PEI-analyte complexes remained in the channel and were subsequently eluted and detected. Two approaches, i.e., cross flow matrix removal (CFM) and opposing flow matrix removal-analyte preconcentration (OFM), were evaluated for matrix removal and matrix removal with simultaneous preconcentration, respectively. With the FlFFF/CFM approach, the signal intensities of matrix elements were reduced by approximately 80-90% while those of analyte elements were only slightly reduced (∼20%). With the FlFFF/OFM approach, an unlimited sample volume can be introduced into the FlFFF channel with an elution volume of approximately 1 ml, leading to analyte preconcentration.