A critical component of improving the quality of the nation's water is improving the collection and assessment of data. As the indicators are improved, it will be possible to more precisely track changes in water quality. The resulting status and trends indicator data will be invaluable for targeting resources and for managing and improving key water quality programs that protect and enhance public health and the environment.
The objective of this SERDP Exploratory Development (SEED) project was to build a miniaturized prototype of an explosives sensor based on amplifying fluorescent polymer (AFP) detection technology and evaluate its ability to detect trace amounts of TNT and related explosive compounds in groundwater.
A miniaturized prototype sensor was constructed, assembled, tested and debugged. This prototype sensor was engineered using technology from two other Nomadics sensors: an underwater TNT sensor (SeaDog) and the Fido® TNT vapor sensor (Fido). Both sensors employ patented AFP technology. The vapor sensor is capable of detecting TNT at the parts-per-quadrillion level, and the aqueous phase sensor can detect TNT in water at low parts-per-billion (ppb) levels. Data logging capability from the Fido sensor also was implemented to record data while sampling, which allows users to analyze the traces for TNT presence at a later time.
The new prototype TNT sensor consisted of a photodiode, two bandpass filters, a sapphire separation window, a sapphire substrate coated with AFP, and an excitation filter and light emitting diode (LED). To test the sensor, the minimum detection limit (MDL) for TNT was determined, interferent screening was performed, the effects of pH and ionic sensitivity were examined, and the longevity of the AFP-coated substrate was established.
The results showed that the sensor was capable of deployment in a two-inch groundwater monitoring well and that its performance was comparable to that of the previous laboratory- scale prototype. The sensor’s MDL for TNT was found to be 2.7 ppb. Other groundwater contaminants (trichloroethene [TCE] and potassium perchlorate [KClO4]) were detected by the sensor, but easily differentiated from nitroaromatics such as TNT. At very high concentrations, however, the contaminants could mask low levels of nitroaromatics, and it was established that further testing against potential interferents would be necessary. The baseline response of the sensor was only affected by very high or very low pH, and increasing ionic strength resulted in an increase in emission intensity. Finally, after exposing the AFP-coated substrate to seven days of extreme testing, the photon counts dropped below the usable threshold.
Use of a miniaturized probe such as the one developed in this project could offer virtually real-time sensing of explosives in groundwater with essentially no waste production at a greatly reduced life cycle cost over what is currently available. Successful development of additional AFP materials could expand the capability of the probe to detect other explosives such as RDX and HMX, further increasing the technology’s utility for environmental monitoring applications.