Abstract:

This article describes a membrane inlet-miniature time-of-flight mass spectrometer. Double silicone sheet membranes with a thickness of 50 μm was used as interface for the atmospheric pressure direct sample inlet. The experimental result indicates that the enrichment factor of membrane is increased with the increase of the flow speed of the sample, and the signal intensity is almost linear with the gas flow. There is a vacuum differential system between the double membranes and the enriched sample is pulled out quickly, there is almost no memory effect in the membrane inlet system. The response time for benzene, toluene and o-xylene is about 100 s, but it only takes 10 s to get the steady condition after opening the vacuum valve. Comparing with the capillary sample inlet, the enrichment of double membrane is quite effective, the signal intensity for 10×10^-6(volume fraction) benzene, toluene and o-xylene is enhanced 280, 370 and 600 times, when the membrane inlet was used under the same experimental conditions. Combining with vacuum ultraviolet soft ionization, the detection limit of the membrane inlet mass spectrometer is 25×10^-9 (volume fraction) for benzene, and the linear dynamic range is about three orders of magnitude. The soft ionization is fragment free and qualitative analysis quickly according to the molecular weight of compounds. The instrument was used to on-line analyze volatile organic compounds (VOCs) in mainstream cigarette smoke, more than fifty compounds of VOCs on a mass spectrum were observed. The results indicate that the membrane inlet-time-of-flight mass spectrometer will be useful for a wide range of field application, particularly for environmental monitoring.

Key words: Membrane inlet, Time-of-flight mass spectrometer, Rapid analysis, Soft ionization