6.1
Improving helium and neon measurements for environmental water samples
L. Palcsu, Z. Major
The analysis of dissolved noble gas concentrations in groundwater has proven to be
a reliable method to determine quantitative
palaeotemperatures and water ages. The aim
of instrumental and methodological improvements in the noble gas laboratory is to achieve
a reliable accuracy of mass spectrometric noble gas measurements for environmental water
samples.
To interpret the dissolved noble gases in
water as noble gas temperature, excess air component and T/3 He age with low error, it is necessary to achieve a very precise measurement
of noble gases. Therefore, the accuracy of each
noble gas measurements should be around 1 %
or better.
In the noble gas laboratory tritium measurements of environmental water samples
have been performed since 1998 that is based
on a VG 5400 noble gas mass spectrometer.
The VG 5400 is an all-metal, statically operated, double focused, 90◦ sector field mass
spectrometer with 57 cm extended geometry.
In 2007 a new cryosystem was installed that
makes possible the quantitative collection of
gas samples. It also contributes to the purification and separation of the different noble gases.
As a new measurement, helium and neon measurements have been developed for environmental water so far. The sample preparation
and the measurement technique were worked
out in summer 2007. Measurements are calibrated with air aliquots. Moreover, fast calibration procedure has been initiated. After
each noble gas measurement a pure helium as
well as neon aliquot is admitted to the mass
spectrometer and measured so that the instability of the mass spectrometer can be corrected for. Air equilibrated water samples have
been prepared under well know circumstances
(T = 23.5 ◦ C, p = 0.987 atm), hence the expected concentrations are 4.372·10−8 ccSTP/g
for helium and 1.779 · 10−7 ccSTP/g for neon
(ccSTP: cubic centimeter at standard temperature and pressure). Stability and reproducibil-
ity measurements for helium and neon have
been performed on ten identical water samples
taken into copper tubes. The standard deviation of the 4 He and 3 He measurements were 0.5
% and 1.2 %, whilst the measured data were
varying within the range of ±1 % and ±3 %
around the average (Fig. 1). The standard deviation of the neon measurements were 1.5 %.
Figure 1. Deviation of different noble gas measurements from the average (X-axis: number of
measurements, Y-axis: deviation in %).
These precisions are better for than expected, but seems to be too high for neon, although the accuracy is extremely good. The
average of the measured concentrations is
4.379·10−8 ccSTP/g for helium and 1.795·10−7
ccSTP/g for neon, that are tiny different from
the expected values.
We have been further working on the analytical development of noble gas measurements
for a few months. We have started developing a new measurement for the heavier noble
gases dissolved in water samples. Argon, krypton and xenon measurement will hopefully be
worked out in spring 2008.
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