Electrodialysis as a sample processing tool for bulk organic matter and target pollutant analysis of seawater

Abstract

Electrodialysis (ED) is an advancing seawater sample processing tool that enables the separation of analytes from the often interfering salt matrix. In this study, we present the evaluation of a laboratory scale ED system for both dissolved organic matter (DOM) and target pollutant analysis of seawater.The developed sample processing protocol yields reproducible data and was found to be robust towards moderate changes in sample composition. At the final salinity of 0.1, the average recovery of DOM in the form of dissolved organic carbon, nitrogen and phosphorus (DOC, DON and DOP) was 44, 53 and 89%, respectively. DOM loss occurred mainly in the late stage of the ED process.When investigating specific ED processing parameters, it was discovered that the initial sample salinity does not influence DOM recovery. The final salinity, by contrast, is a dominant influence factor on DOM recovery. Furthermore, DOC and DOP recoveries could be improved by 8% by refining the electrical current in the ED cell. Surprisingly, adjustments of the sample pH did not lead to any improvements in DOM recovery.The experiments with target analytes showed that the recovery of individual molecules is determined by their n-octanol water partition coefficients logKow. High recoveries > 80% were achieved for compounds with medium logKow of −1 to 3. Hydrophobic compounds with logKow > 3 were lost through surface adsorption to the system walls and tubing. Small, polar and charged compounds with logKow < −1 are prone to loss via ED membrane passage, which occurred predominantly in the late stage of the ED process. Consequently, sample processing with ED was deemed beneficial for the LC-MS or GC–MS analysis of polar target compounds, because they are often difficult to enrich from seawater. Furthermore, during LC-MS or GC–MS analyses, matrix-dependent ion suppression was reduced in ED isolates, giving rise to increased signal responses of 25 to 620%, which resulted in improved instrumental sensitivity.