Analysis of Chemical Composition of Gases from Rock Samples by Pulsed Discharge Chromatography Combined Mass Spectrometry

Abstract

Understanding the chemical composition of gases trapped in rocks is important in a wide variety of earth science studies and gases are often extracted from rock samples and analyzed. Gas chromatography (GC) and mass spectrometry (MS) are the most frequently used analytical techniques for measuring gases extracted from rock samples, but these techniques have rarely been coupled except for gas chromatography-mass spectrometry (GC-MS) or Continuous flow isotope ratio mass spectrometry (CF-IRMS). A thermal conductivity detector (TCD) is most frequently used in GC because of its ability to detect all gases, but has limited sensitivity and thus cannot accurately detect trace gases in some rock samples. To resolve this issue, we built a new device that tests gases extracted from rock samples by GC and MS together, and employs a pulsed discharge detector (PDD) with GC analyses rather than a TCD because PDD is capable of detecting most gases and exhibits higher sensitivity than that of TCD. In this device, a rock sample can be heated or crushed in vacuum to release gases. A diaphragm gauge is then used to measure the pressure of the gases released from the rock samples. Four kinds of samples were tested using this new device: serpentinized peridotites from Inner Mongolia, volcanic rocks from Wudalianchi, shale rocks from Sichuan basin and reservoir rocks from Songliao basin. The chemical contents of H2, CH4, H2O, CO, N2, C2H4, C2H6, O2, H2S, Ar, CO2, C3H8, COS and SO2 could be measured simultaneously.