Microtrapping of volatile organic compounds with carbon nanotubes

Micro-sorbent traps referred to as microtraps serve as integrated concentration-injection devices for continuous monitoring in gas phase streams. The application of carbon nanotubes as unique sorbents for the fabrication of microtraps for the nano-scale adsorption/desorption of volatile organic molecules is presented in this paper. The microtrap application requires high adsorption capacity as well as easy desorbability; the latter being critical for injection mode of these integrated devices. The micro-sorbent characteristics of single and multi-walled carbon nanotubes for gas phase adsorption/desorption of several volatile organic compounds like DCM, ethanol and benzene etc. has been studied. The nonporous nature of carbon nanotubes (CNTs) eliminates the mass transfer resistance related to diffusion into pore structures, thus allowing easy desorbability. At the same time, their high aspect ratios lead to large breakthrough volumes. As compared to a commercial sorbent carbopack, the breakthrough volume was as much as an order of magnitude higher in the CNTs, while the higher rate of desorption measured as the peak width at half height of the desorption band was found nearly eight times lower (i.e., 0.26 seconds with SWNT and 1.89 seconds with carbopack). The trapping and desorption characteristics of single and multi walled nanotubes were found to be comparable. We also found that the presence of disordered carbon impurities, which could be removed by controlled oxidative annealing, could greatly degrade the performance of CNTs. This research has suggested that CNTs can be used in micro-sorbent traps and surprisingly enhance the efficiency of the integrated concentration-injection devices. Consequently, this will open the doors to the application of high-capacity, CNTs-based sorbents as a better alternative to conventional sorbent in continuous monitoring devices.