Sample Preparation Based on Dynamic Ion-Exchange Solid-Phase Extraction for GC/MS Analysis of Acidic Herbicides in Environmental Waters Nanqin Li and Hian Kee Lee* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Republic of Singapore 117543 The newly established enrichment technique, dynamic ion-exchange solid-phase extraction (DIE-SPE), was stud- ied for sample preparation for GC/MS analysis of 16 acidic herbicides in environmental waters. C 18 bonded silica was the solid-phase material used. The optimal sample pH was weakly acidic to neutral. However, for common tap water and surface water, which run pH 6-9, all the acidic herbicides except for Chloramben could be effectively extracted from a sample of 1000-mL volume without pH adjustment. The humic acid could be concur- rently extracted from water, but most of it was separated from the sample by using 3 mL of 10% methanol in acetone as the eluent, which would completely elute the analytes and leave a large part of the humic acid on the cartridge. The selective elution reduced the interference of humic acid and made the DIE-SPE an effective ap- proach for the analysis of the acidic herbicides in surface water. Comparing DIE-SPE with conventional reversed- phase SPE (RP-SPE), the former gave higher recoveries for the acidic herbicides and was less affected by sample matrixes. A tandem-cartridge system combining RP- and DIE-SPE in sequence was set up for the simultaneous isolation of the acidic herbicides and removal of the interfering substances. Despite some minimal retention on the upper RP-SPE cartridge, most of the acidic herbicides could be extracted on the lower DIE-SPE cartridge with recovery over 80% except for Chloramben (50%), fenoprop (73%), MCPB (67%), and 2,4-DB (70%) when a 500-mL aqueous sample of pH 9.5 was percolated through the tandem-cartridge system. The effectiveness of the system in removing the long carbon chain fatty acids as well as the basic and neutral organic interfering substances from the sample was also demonstrated. Acidic herbicides are mainly chlorophenoxy acids and related compounds which are widely used in agriculture and forestry for weed control. When released to the global environment, they may generate certain toxicological effects on human beings and aquatic life. EPA method 515.1 for acidic herbicides in drinking water makes use of liquid-liquid extraction (LLE) followed by gas chromatography-electron capture detector (GC-ECD). 1 Solid-phase extraction (SPE) with adsorbent materials of ion exchangers, 2-6 reversed-phase bonded silica, 7-12 polymers, 13-16 and graphitized carbon black (GCB), 17-20 has been widely investigated for sample preparation of the acidic herbicides in water prior to chromato- graphic determination. Of the numerous organic pollutants, the acidic herbicides are among those most compatible to SPE instead of conventional LLE as the sample preparation technique for various environmental waters. This is because, in common water matrixes, those acidic herbicides are ionized and show a low tendency of adsorption on the surface of glassware. 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