Investigation of retention of undesirable organic matter from aqueous samples in stopped-flow dialysis procedures for inorganic anions.

Sample preparation is a key issue in the analysis of many real samples using ion chromatography (IC). One of the many means of sample preparation that has received considerable attention in recent years is the stopped-flow in-line dialysis approach hyphenated to IC. Using appropriate dialysis membranes undesirable high-molecular weight compounds, colloids and dispersed particulate matter are retained whereas the analyte ions pass through the membrane and are separated and quantified by IC. However, in the many papers about analytical use of in-line dialysis (not only in connection with IC) the retention of undesirable compounds has never been quantified. This study is a first attempt to fil this gap. The breakthrough (BT) of humic acids (HA) and lignin as model compounds often contained in natural water samples was investigated in the stopped-flow dialysis procedure using different dialysis membranes. BT was measured and quantified by UV-absorption in the receiver solution leaving the dialysis cell. The most important factor for retention of undesired compounds is the pore size or the molecular weight cut-off (MWCO) of the dialysis membrane. In addition, the recovery of analytes in acceptable time needs to be considered. The optimized system addresses these two aspects and permits using the proper membrane almost quantitative recovery of several inorganic ions and at the same time almost complete retention of the model interferents. Artificial natural water samples were prepared and analyzed containing a mixture of HA, lignin and a surfactant and a number of anions. In long-term operation no alterations of the dialysis process occurred and no adverse effects in IC separations were observed.