Characterization of a sheet membrane interface for sample introduction into a time-of-flight mass spectrometer

In the present study, we constructed a membrane inlet assembly for selective permeation of volatile organic compounds in air into a time-of-flight mass spectrometer. Temporal evolution of analyte through a 127-μm thick polydimethylsiloxane membrane was measured by monitoring the ion signal after a step change in the sample concentration. The results were well fitted by a non-steady-state permeation equation. The diffusion coefficient, response time, and sensitivity were determined experimentally for a range of polar (halogenated) and nonpolar (aromatic) compounds. We found that the response times for various volatile organic compounds were greatly influenced by the alkyl chain length as well as the number of substituted halogen atoms. The detection limits for toluene and o-xylene were 0.06 and 0.2 ppm by volume, with linear dynamic ranges greater than three orders of magnitude. These results indicate that the membrane inlet/time-of-flight mass spectrometer will be useful for a wide range of field applications, especially for real-time environmental monitoring.