Our Gas Chromatograph/Mass Spectrometer + Thermal Conductivity Detector (GC/MS+TCD) laboratory has been designed for bulk gas analytical characterization of geothermal, reservoir and volcanic gasses. The instrument is currently configured with a high vacuum inlet system that is directly attached to the gas chromatograph (Hewlett-Packard 6890 series GC) via a 10-port valve. The inlet system has been designed to handle a wide range in sample volumes/pressures and compositions. A high degree of precision has been achieved through the use of carefully known injection volumes and pressures measured using two Baratron absolute pressure transducers.
Typically, on-line calibration gasses and individual columns will be selected specifically for gasses of interest for a given project. Calibration curves and chromatographic protocols are then developed to optimize the analysis for a given gas composition. Use of the 10-port valve enables simultaneous injection of a single sample of gas onto two different columns using two detectors: 1. A Hewlett-Packard 5972 Series quadrupole mass selective detector (MS), and 2. A Gow-Mac thermal conductivity detector (TCD). Under current configuration, a sample introduced to the system is split by the 10port-valve into a 2 micro-Liters aliquot injected onto a chromatographic column to the MS detector and 1mL aliquot injected onto a second chromatographic column to the TCD. Carrier gasses for both columns are currently Helium, however the system has the capacity to easily switch to other carrier gasses (i.e. Argon, etc.) for each column as required for a given analysis. Parallel operation of both detectors exploits the high sensitivity and wide range in mass/charge characterization ability of the quadrupole mass spectrometer in addition to the high reproducibility and quantitative ability of the thermal conductivity detector. This mode of operation greatly facilitates identification of unknown peaks and optimization of conditions for quantitative analysis of given peaks using the TCD.
The high vacuum inlet system is also equipped with five transfer volumes intended for noble gas isotopic analysis using the RARGA system. By measuring an absolute abundance of Argon, for example, in a given sample using the GC/MS+TCD, and subsequently measuring high precision isotopic and abundance ratios of all five of the noble gasses using the RARGA lab, the absolute abundance of any noble gas isotope can be determined.
Additionally, the high vacuum inlet system is directly attached to a standard glass vacuum line designed for methane combustion and cryogenic sample preparation for Carbon isotope analysis. Using this feature, a single sample of gas can be analyzed for bulk gas composition using the GC/MS+TCD and carbon isotopic analysis using the PRISM (Stable Isotope Laboratory) mass spectrometer.
For Further Information Contact
B. Mack Kennedy
Center for Isotope Geochemistry, MS 70A-4418
Lawrence Berkeley National Laboratory
Berkeley, CA 94720