Novel approaches to real-time monitoring of volatile organic compounds

Abstract

Clean water and air are essential to human survival. Contaminations of these resources have occurred from both natural and human source. In the last century, a variety of anthropogenic activities have increased the rate of these contaminations. Governments all over the world have regulations ip place to protect water and resources by monitoring water quality, air emission, and the disposal of chemicals/wastes. Water and air polluted from many sources. Volatile organic compounds (VOCs) comprise of small organic molecules with high vapor pressure. The presence of VOCs in water and air at even trace level pose a threat to public health, since many of them are toxic and lead to the formation of ozone photochemical smog. So the measurement of these trace level contaminants is significant importance. The conventional approach to VOCs analysis is to bring the sample to the laboratory for analysis. Water samples are analyzed by head space or purge and trap, which are excellent laboratory techniques. The disadvantage is that they can not beused for continuous, on-line monitoring of a water stream. In this research, a novel an on-line purge and trap device for continuous monitoring of VOCs was developed. The purge chamber was designed for continuous extraction of VOCs from water with nitrogen. The analytes were preconcentrated on a microtrap prior to analysis by gas chromatography flame ionization detector (GC-FID). The microtrap served as a fast injection device for carrying out analysis at high frequency. Continuous monitoring was done by making injections at fixed intervals. This system showed high sensitivity, high precision, detection limit in the ppb level, and stable response over long periods of continuous operation. Factors affecting system performance were studied. A predictive model based on gas-liquid partitioning is also presented. The microtrap used in this study was previous developed at New Jersey Institute of Technology. It is a versatile technology that has been used in both air and water monitoring. Efforts were made to improve the performance of the microtrap using nanoscale carbon sorbents, called carbon nanotubes (CNTs). The CNTs have been the subject of intense research because of their novel mechanical and electrical properties. However, serious challenges still remain with their implementation in real world devices. Chemical vapor deposition (CVD) appears to be a convenient self-assembly method for CNTs. In thes project, we studied the application of self-assembled CNTs in a steel capillary to fabricate a microtrap for the nanoscale adsorption/desorption of VOCs molecules. The CNTs were self-assembled by CVD as a thin-film in the walls of a micro capillary using both CO and C₂H₄ as the carbon source. Trace level VOCs such as hexane and toluene were adsorbed and then rapidly desorbed from the CNT film inside the capillary. The desorption pulse served as an concentrated injection for the detector, and also as an injection for GC separation. The sorption of toluene was found to be much stronger than hexane, which was attribuled to the ᴫ-ᴫ interaction between the CNT and the aromatic ring.