The RARGA facility consists of an all-metal high-resolution noble gas mass spectrometer constructed in-house. Connected to the mass spectrometer is an all-metal gas extraction and purification line.
The RARGA system is designed specifically for abundance and isotope analyses of the five stable noble gases (He, Ne, Ar, Kr, and Xe) contained in bulk fluids, e.g. gases and liquids. The gas or liquid samples are loaded directly onto the sample purification line. The non-condensable gas, such as the noble gases, exsolved from liquid samples under vacuum are processed in the same manner as bulk gas samples. Purification consists of exposure to a series of cold traps and metal chemical getters. The purified noble gas fraction is adsorbed on activated charcoal and released in sequence for analysis in the mass spectrometer.
The RARGA mass spectrometer has a split flight tube with two collectors positioned for simultaneous detection of the 3He and 4He ion beams. The collector used for the low abundance 3He ion beam, and most other measurements, is a venetian blind style electron multiplier operated in analog mode. The second collector is a faraday cup. RARGA is equipped with a Signer-Baur electron impact ion source (GS-60). The magnetic analyzer has a 90o and 25.4 cm radius geometry. The exit and entrance slits of the ion source and multiplier collector, respectively, are adjustable providing variable mass resolution up to ~ 1 part in 550 amu, adequate for resolving 3He+ from HD+.
A typical sensitivity for 36Ar, defined as the number of atoms released from a sample divided by the detector signal, is ~ 2.3 x 1014 atoms/volt (1010 ohm).
For most measurements and sample types the recommended sample sizes are: 10 ml of liquid or ~10-20 cc-atm of gas. We prefer to have samples collected in Cu-tubes that can be sealed with refrigeration clamps. Our sealed Cu-tube sample containers provide 9.8 cc of fluid.
The collection technique used depends on the sample type. Below is an example of Geothermal Well sampling:
RARGA Sampling Techniques: Geothermal Wells
For further information contact:
B. Mack Kennedy
Center for Isotope Geochemistry, MS 70A-4418
Lawrence Berkeley National Laboratory
Berkeley, CA. 94720
e-mail: [email protected]