Instrumentation

The Noble Gas Lab consists of two magnetic sector mass spectrometers, a GVI (now Thermo) Helix SFT and a Thermo Helix MC Plus, as well as a Pfeiffer Prisma quadrupole mass spectrometer. Each instrument is accompanied by a fully automated ultra high vacuum system designed to extract noble gases from a wide variety of materials. We routinely use techniques such as conventional vacuum fusion with a double-vacuum furnace, micro-furnace heating with a Photon Machines diode laser, stepwise diffusion in custom-built diffusion cells, and in vacuo rock crushing. Additional information about each system is available below. Our extraction systems are designed to be versatile, so we can typically use any extraction system with any instrument using either existing vacuum connections or simple modifications.

The Helix SFT is a 120° magnetic sector mass spectrometer with a 35 cm radius of curvature. It is equipped with a single Faraday cup and an ion-counting electron multiplier with an ESA. The large dispersion of this instrument provides very high abundance sensitivity, better than 1 ppb from mass 4 at mass 3, and the split flight tube allows simultaneous collection of ⁴He on the Faraday cup and ³He on the multiplier. We primarily use this instrument to measure low abundance ³He and neon isotope ratios.

The MC is a multi-collecting static noble gas mass spectrometer with a 35cm 120° flight tube. Extended high-resolution electrostatic optics after the ion source allow for improved mass resolving power required to limit isobaric interference of ⁴⁴CO_2⁺⁺ on ²²Ne⁺. Equipped with five paired discrete dynode electron multipliers and Faraday detectors, the MC allows for multicollection and limits overhead to peak hopping. Four of the paired detectors are moveable without breaking vacuum, allowing us to rapidly change geometry for different isotopic measurements.

We use a Pfeiffer Prisma quadrupole mass spectrometer in conjunction with a Photon Machines diode laser to perform helium analyses for (U-Th)/He chronometry. A pure ³He spike enables us to use the isotope dilution method to overcome limitations of direct measurement of ⁴He-only. This system provides high throughput and adequate precision for thermochronology.

We maintain a number of systems to extract noble gases from different types and sizes of samples. These include the following:

• Diode lasers for precise high-temperature microfurnace heating of small samples.
• A traditional double-walled vacuum furnace for fusion of samples up to multi-gram quantities.
• Diffusion cells for stepwise degassing of small samples in precise temperature steps.
• Single-grain and automated multi-gram in vacuo mineral crushers.
• Offline splitting capabilities to prepare high-concentration gas and liquids