Direct coupling of a carbon nanotube membrane to a mass spectrometer: Contrasting nanotube and capillary tube introduction systems

A carbon nanotube membrane was directly coupled to the inlet system of a mass spectrometer to evaluate its use as a novel potentially tunable membrane inlet system. Carbon nanotubes for the membrane were synthesized using the template method. Chemical vapor deposition of a hydrocarbon precursor produced nearly graphitic carbon nanotubes within the pores of an anodic aluminum oxide membrane. The selectivity of the carbon nanotube membrane was compared to that of a capillary tube. Relative to the capillary tube, the carbon nanotube membrane was preferentially transmissive to methane. Conductance of gas mixtures exhibited different dependencies on total pressure in carbon nanotube and capillary tube introduction systems. In carbon nanotubes, conductance decreased with increasing total pressure, and the extent of the decrease became progressively smaller between methane and carbon dioxide (CH4 > N2 > O2 > Ar > CO2). In the capillary tube introduction system, conductance decreased substantially only for nitrogen. The capillary tube conductance for methane was nearly independent of total pressure, and conductance increased progressively between O2, Ar and CO2.