The TANGO facility consists of an all-metal low-blank high-resolution VG5400 noble gas mass spectrometer purchased from VG Fisons. Connected to the mass spectrometer is an all-metal low-blank gas extraction and purification line.
The TANGO system is designed specifically for abundance and isotopic analyses of the five stable noble gases (He, Ne, Ar, Kr, and Xe) contained in rocks, minerals, and rock and mineral fluid inclusions. Gases are extracted from samples in vacuo either by crushing which releases the gases contained in fluid inclusions or by thermal extraction using a high temperature furnace. The temperature of the sample in the furnace is computer controlled and can vary from 50 Ð 2000 ¡C. The purified noble gases extracted by either technique are adsorbed on activated charcoal and released in sequence for analysis in the mass spectrometer.
The VG5400 is equipped with a modified Nier-type electron impact ion source. The ions extracted from the ion source are measured either with an axial 17-stage Cu-Be electron multiplier for low abundance isotopes or an off-axis faraday cup for the high abundance isotopes. The electron multiplier can be operated either in analog or ion pulse-counting modes. The latter is preferred for small ion currents (1 Ð 105 ions/sec) due to the inherent low noise level of ion counting systems. The combined mass resolving power of the magnetic analyzer, source and collector exit and entrance slits is ~ 1 part in 600 amu. At this resolution 3He-HD and Ar-hydrocarbons are completely resolved and 20Ne+ and 22Ne+ are partially resolved from 40Ar++ and 44CO2++, respectively.
The sensitivity for each noble gas, defined as the number of atoms released from the sample divided by the detector signal, are as follows:
|4He||5.0 x 1014 atoms/volt||Faraday Cup, 1010 ohms|
|4He||17.4 x 105 atoms/ cps||Ion Counting (cps: count/sec)|
|22Ne||8.3 x 105 atoms/ cps||Ion Counting (cps: count/sec)|
|40Ar||1.1 x 1014 atoms/volt||Faraday Cup, 1010 ohms|
|84Kr||5.5 x 105 atoms/ cps||Ion Counting (cps: count/sec)|
|132Xe||1.2 x 105 atoms/ cps||Ion Counting (cps: count/sec)|
In ion counting mode the noise level (dark current) is ~ 2 counts per 10 seconds. Therefore, to accurately measure a 3He/4He ratio of ~10-8 would require a sample with ~ 1013 atoms of 4He.
Required sample sizes
For most measurements and sample types the recommended sample sixe is ~ 0.2 Ð 2.0 grams.
Mineral separates should be hand picked, if feasible, or magnetically separated. Samples should not be exposed to halogenated heavy liquids, HCl or HF. Samples should be cleaned ultra-sonically sequentially in methanol, acetone then ethanol and dried under a heat lamp and stored in a clean lint-free container.
For further information contact:
B. Mack Kennedy
Center for Isotope Geochemistry, MS 70A-4418
Lawrence Berkeley National Laboratory
Berkeley, CA 94720