Methods

Noble Gas-Isotopes Analysis

Our department runs a mass spectrometer with a sample preparation unit for helium and neon isotope analysis in sea and fresh water. The system can be operated fully automatic. This enables us to measure up to 3000 samples of seawater per year. All gases of a water sample are extracted and transfered to a glass ampoule. The ampoules are connected to the mass spectrometer and opened unter high vacuum conditions. The sample gases are transfered to multi step cryogenic system. Here all gases except helium and neon are removed. The sample is analysed for ⁴He, ²⁰Ne, and ²²Ne in a quadrupol mass spectrometer and for ³He and ⁴He in a sector field mass spectrometer (MAP 215-50). The system is calibrated by the use of measurements of defined amounts of air. Additionally measurements for blanks and linearity are performed. The difficulty of precise noble gas isotope analysis consists in first: a good resolution between ³He and HD peaks (difference 0.006 amu), second a warranty of extreme low background and third the linearity of the system. The accuracy for sea water samples is 0.4% for ³He/⁴He- and He/Ne-ratios and 0.5% for concentrations.

Tritium-Analysis

We detect Tritium by mass spectrometric measurement of its decay product ³He. Our lab runs a mass spectrometer which garanties a resolution of 3He and HD (mass difference 0.006 amu). The system operates fully automatic. This enables us to perform up to 3000 seawater samples a year. The sample will be extraced of all gases and subsequently stored for about 6 month in glas containers. Then the glas bulbs are connected to the sample inlet system and unsealt under high-vaccum conditions. Accumulated ³He (half-life time of tritium 12.32 years) is separated from water and analysed for ³He and ⁴He with a sectorfield mass-spectrometer (MAP 215-50).

The system is calibrated by definded amounts of atmospheric air. Additionally blank and linearity checks are executed. The measurement of ⁴He serves to quantify contaminations of the sample by air. With this system we are able to detect 0.01 TU (1TU = T/H-ratio 1E-18). This is equivalent to 0.6mBq/l water!

This low detection limit is required to investigate changes of water masses in the oceans of the southern hemisphere. The contamination of samples with low concentrations generates tremendous problems.

Difficulties of the exact tritium analysis consists of a good resolution between ³He and HD peaks, a stable and extreme low background and the linearity of the systems.