L. Kinsley, G. Mortimer, T. Esat and M.T.
We have determined the operating characteristics
and the precision obtainable from the newly acquired Finnigan
multi-collector inductively-coupled mass spectrometer 'NEPTUNE'
in measuring uranium isotopes. Such a measurement places high
demands on instrument performance as the isotope ratio of interest
is small ~5x10-5 and susceptible to small amounts of interfering
molecular beams. It involves combined use of Faraday-cups and
an 'electron-multiplier' and the accounting for, or elimination
of tailing from intense 238U beams. In addition, there appears
to be a discrepancy in the U isotopic composition between modern
corals and sea-water. We have determined the U isotopic composition
of sea-water to help resolve the difference.
Data collection consisted of 30 2-second simultaneous
integrations of 234U, 235U 238U, a peak jump, and 10 2-second
integrations of 235U and 238U. Each experiment included 10 cycles
of 6 blocks
and lasted for 1_ hours. The 235U/238U ratio
with 235U measured in a Faraday cup and in the electron multiplier
(SEM) provided a direct calibration for the SEM gain, including
any non-linearities, except for a correction in dead-time between
different count-rates of 234U and 235U. The use of 235U for SEM
calibration restricts the usable signal intensity to about 2
volts 238U (1011 Ohm) so as not to overload the SEM.
The present results show that the isotopic
composition of uranium can be measured to a precision better
than 1 with Neptune SEM and retardation lense in a reproducible
fashion. The instrument, and the particular measurement technique
used, can achieve this independent of beam intensity. The measured
234U/238U ratio for the standard is identical to the secular
equilibrium value (5.467x10-5). Indicating that standard 'ALH'
is in secular equilibrium and also that the instrument can provide
absolute values of this ratio when appropriately calibrated.
Uranium isotopic composition of 'modern' sea-water is indistinguishable
from uranium isotopic composition of 'modern' corals within the
The previously determined mean sea-water value
of 144±2 (Chen et al., EPSL 80(1986)241)
should be revised to ~148.4±0.5 in close agreement with
the modern coral value of ~148-149 .
Chen, J.H., Edwards, R.L. and Wasserburg, G.J., 238U, 234U and
232Th in seawater, Earth and Planetary Science Letters,