A homogeneous sampling membrane inlet photoelectron ionization miniature time-of-flight mass spectrometer for on-line determination of ethane

Ethane is the second largest component among natural gas, and the detection of ethane is an effective method for rapid identification of the leakage of the natural gas pipelines. In this work, a homogeneous sampling membrane inlet was developed and coupled with the homemade photoelectron ionization miniature time-of-flight mass spectrometer (PEI-mini-TOFMS) for in situ, on-line and highly sensitive ethane detection. The membrane area of the homogeneous sampling membrane inlet was increased from 490 mm2 to 1256 mm2, gaseous sample is injected from the top port and flowed through the membrane surface and out of the bottom two ports, with the three ports arranged in a triangular shape. The highest average flow velocity of the gas on the surface of the membrane reached 0.4 m s 1, and the optimal gas pressure in the PEI source was enhanced from 2.2 Pa to 4.0 Pa with this new design. The new design improved the comprehensive sensitivity of ethane by a factor of 3.0 compared with that of the traditional two-hole membrane inlet with the membrane area of 490 mm2. The semiconductor cold trap controlled the sample relative humidity (RH) at 10-12%, enabling direct sampling for highly sensitive analysis with RH as high as 70% and temperature from 7 degrees C to 40 degrees C. The quantitative range was 1-50 ppmv with a limit of detection (LOD, S/N = 3) lowered to 420 ppbv within 1 min, and zero humidity quantitative calibration with cold trap further reduced the relative standard deviation (RSD) of the signal intensities to 2.84%. The performance of the novel method developed in this work demonstrated a potential application on the above-ground natural gas pipelines leakage monitoring.