On-line and off-line sample preparation of acidic herbicides and bentazone transformation products in estuarine waters

Abstract

The isolation and trace-enrichment of the acidic herbicides benazolin, bentazone, (2,4-dichlorophenoxy)acetic acid (2,4-D), (4-chloro-2-methylphenoxy)acetic acid (MCPA), 2-(4-chloro-2-methylphenoxy)propionic acid (MCPP) and the transformation products (TPs) of bentazone 6- and 8-hydroxy in estuarine waters is described. The pesticides were spiked at 0.3 gm/1 and 1–3 μ/1 (for the TPs) in estuarine waters and were preconcentrated using solid-phase extraction (SPE) methods using C 18 and styrene divinylbenzene (SDB) Empore extraction disks of 47 mm and 4.6 mm diameter in the off-line and on-line modes, respectively, followed by liquid chromatographydiode array (LC-DAD), fluorescence (FD) and/or thermospray mass spectrometric (TSP-MS) detection. In the off-line mode 5 liters of water were preconcentrated and recoveries of 40–76% were obtained except for the TPs that were not recovered at all. Blanks were investigated within a pH range of 0.7–2, thus recommending the use of SDB over C 18 Empore disks. When on-line SPE was used only 150 ml of river water were needed, with a limit of detection (LOD) varying between 5–100 ng/1 and higher recoveries than in the off-line mode, followed by LC-DAD in both cases. When on-line SPE with Empore disks was used for the determination of the acidic herbicides in real estuarine waters followed by LC-FD, only bentazone showed similar detection levels as in DAD whereas MCPA and MCPP exhibited quenching interferences from the water matrix. Confirmation of the acidic herbicides in estuarine waters was accomplished by on-line SPE-LC-TSP-MS using selected-ion monitoring (SIM) and negative-ion mode thus showing similar LOD to those achieved with on-line SPE-LC-DAD. Illustrative examples of the trace level determination of the acidic herbicides bentazone and MCPA in real estuarine water samples of the Ebro delta (Tarragona, Spain) are shown.