Determination of Chemical Warfare Agent Degradation Products at Low-Part-per-Billion Levels in Aqueous Samples and Sub-Part-per-Million Levels in Soils Using Capillary Electrophoresis

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Abstract
A significant enhancement in the method detection limits is observed in the analysis of chemical warfare agent (CWA) degradation products in environmental samples by capillary electrophoresis (CE) using electrokinetic injection. The CE method uses indirect UV detection of the nonderivitized acidic analyte and a cationic surfactant, didodecyldimethylammonium hydroxide, for reversal of the electroosmotic flow. Analytes studied include the dibasic acid methylphosphonic acid (MPA) and its monoacid/monoalkyl esters, RMPA, where R = ethyl, isopropyl, and pinacolyl (2-(3,3-dimethylbutyl)). The CE method uses an attractive buffer system which is highly stable and inexpensive, and, in addition to reversing the electroosmotic flow, provides excellent separation efficiencies within a 3-min run. This CE method is also free from interference caused by carbonate, humic acids, and fluoride. Compared to pressure injection, electrokinetic injection with this CE buffer system provided substantially lower detection limits, up to 100-fold lower for samples in reagent water. However, to best realize the benefits of the electrokinetic injection enhancement for environmental samples, a prior cleanup of the sample using standard ion-exchange cartridges is necessary. This cleanup step uses sequential cartridges to remove sulfate (barium cartridge), chloride (silver cartridge), and cations (H+ cartridge). Using this approach, detection limits for these four acids were as low as 1−2 μg/L for water samples and 25−50 μg/L for aqueous leachates of soil samples (10 mL of leachate/1.5 g of soil). The utility of this method for separation of CWA degradation products by CE is discussed in terms of pressure injection versus electokinetic injection. The effects of voltage and time of injection on the separation were investigated. Results from three types of soils and four types of water (groundwater, artificial seawater, tap water, bay water) indicated that the method has potential for environmental monitoring. Quantitative CE analysis with electrokinetic injection enhancement of detection limits of these types of environmental samples requires the use of an appropriate internal standard approach. The data presented here indicate that an internal standard-based approach could be expected to give analysis results in the sub-part-per-million concentration range of 90−110% of the true value.