Perchlorate is a pervasive water contaminant that has drawn national attention as a public health concern. The objective of this project was to construct a novel lab-on-a-chip (LOC) sensor to monitor perchlorate in ground and/or surface water in a sentinel mode with all the concomitant benefits of a remote, fieldable, inexpensive sensor.
Novel selective and controllable surfactant-based extraction chemistry that can segregate and concentrate perchlorate from complex environmental waters was studied. This affinity chemistry was combined with current LOC sensor design to test the ability to embed this extraction scheme within sensitive and powerful microchip electrophoretic separations coupled to electrochemical detection.
The research built on recent collaborative research performed on the analysis of perchlorate in surface water using microchip capillary electrophoresis (MCE). During preliminary studies completed on a related project, it was found that perchlorate could be resolved from interfering anions in less than 3 minutes, with detection limits at sub-ppb levels using direct injection of surface water. Tests using more complicated sample matrices such as wastewater proved more difficult due to the general increase in sample conductivity and high concentrations of interfering compounds such as chloride and nitrate. To meet the needs of real-world environmental monitoring at military ranges, new chemistry must be adapted that allows use of miniaturized MCE techniques on more complex samples. A novel solution to this problem was proposed, to integrate an extraction column for selective analyte binding using zwitterionic surfactants in the first dimension and MCE in the second dimension. A zwitterionic, surfactant-coated, stationary phase would bind perchlorate while passing common anions such as chloride and nitrate. After sample extraction, perchlorate would be eluted from the phase by changing the pH or eluting the surfactant with organic solvent.
Although perchlorate is employed as the model in this work, the system is based on platform technology that could be extended to monitor other munitions such as RDX, HMX, and TNT through the appropriate introduction of modules with desired specificity, creating an integrated multi-analyte screening device.
Two zwitterionic sulfobetaine surfactants, N-Hexadecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (HDAPS) and N-Tetradecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (TDAPS), were investigated for the selective retention of perchlorate. At concentrations of these surfactants above the critical micelle concentration, micellar interactions slow the migration of perchlorate, separating the analyte from common, higher mobility anions such as chloride, sulfate, and nitrate found in water. It was found that TDAPS provided more reproducible results than HDAPS. This novel separation chemistry was used to analyze perchlorate in drinking water samples with 99% recovery and detection limits of 5 ppb.
Further, the MCE system was improved to overcome the challenges of analyzing surface and ground water in which ionic strength is substantially higher than that of drinking water. A novel extraction method incorporating the fundamentals of electrostatic ion chromatography was proposed. Two strategies were explored for this extraction method—a reverse-phase packed bed and an in situ-generated monolith. With both strategies, a zwitterionic surfactant was physisorbed on the surface of either the packed bed or monolith. When a high ionic-strength water sample was introduced, perchlorate was retained by the surfactant while the higher concentrations of chloride, nitrate, sulfate, etc. were rinsed off the column. Perchlorate was then eluted separately and could be analyzed via MCE.
The sensor developed is capable of analyzing perchlorate over a relatively large linear range, with a detection limit of 5 ppb in drinking water that satisfies the USEPA regulatory requirement. Additionally, analysis times are approximately 15-30 times shorter than current ion chromatography techniques. While the method still faces some challenges, including issues with the concentration threshold of the packed bed platform and problems with the surfactant coating of the monolith surface, the ability to significantly reduce the concentration of competing anions in surface and ground water shows great promise for the device as a fieldable tool for perchlorate remediation sites. (Project Completed – 2012)