Characterization of a membrane inlet interfaced with a compact chemical ionization FT-ICR for real-time and quantitative VOC analysis in water

Abstract

A Chemical Ionization Membrane Introduction Mass Spectrometer (CI-MIMS) was evaluated, for use in the analysis of volatile organic compounds (VOCs) in aqueous solutions. The mass spectrometer was a compact high resolution FT-ICR/MS (Fourier Transform Ion Cyclotron Resonance, Mass Spectrometer) using Chemical Ionization by proton transfer from H3O+. The membrane inlet setup was based on a flat sheet PDMS membrane. Five VOCs (toluene, benzene, p-xylene, methanol and, phenol) were studied for their enrichment, response time, temperature dependence and pressure, dependence. The permeate concentrations were proportional to the feed concentrations. The enrichment factors varied from 7.7 (methanol) to 6000–9000 (xylene). The best enrichments were observed for the most hydrophobic compounds as was expected based on their octanol–water, partition coefficients. The response times were less than 1min with the exception of phenol, which exhibited a response time of 5min, due to its larger size and higher polarity compared with the other compounds.The individual membrane permeabilities of water and the analytes increased with temperature. This temperature dependence enabled the determination of the activation energies of permeation. The methanol diffusion coefficient exhibited unique temperature dependence behavior, suggesting the existence of a transition temperature in the silicone membrane for methanol. Increasing the feed, pressure from 1 to 20bar resulted in a linear increase in the enrichment factor of methanol, while the permeate pressure remained constant. Over 20bar, the permeate pressure collapsed due to membrane, compression, and the enrichment factor leveled off.A photodegradation experiment was successfully performed, and the CI-MIMS system was demonstrated to enable quantitative analysis in real-time.