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| Titel |
Precise and accurate measurement of U and Th isotopes via ICP-MS using a single solution |
| VerfasserIn |
R. Mertz-Kraus, W. D. Sharp, K. R. Ludwig |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064226
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| Zusammenfassung |
| U-series isotope measurements by ICP-MS commonly utilize separate runs for U and Th and
standard-sample bracketing to determine correction factors for mass fractionation and ion
counter yields. Here we present an approach where all information necessary to calculate an
age (aside from background/baseline levels) is determined while analyzing a single solution
containing both U and Th. This internally calibrated procedure should reduce any bias caused
by distinct behavior of sample versus standard solutions during analysis and offers
advantages including simplicity of operation, calculation of preliminary ages in real time, and
simplified analysis of errors and their sources. Hellstrom (2003) developed a single-solution,
internally-calibrated technique for an ICP-MS with multiple ion counters, but to
our knowledge no such technique is available for an ICP-MS with a single ion
counter.
We use a Thermo Neptune Plus multi-collector ICP-MS with eight movable Faraday cups
and a fixed center cup/ion counter equipped with a high abundance-sensitivity filter (RPQ).
We use Faraday cups to measure all masses except 230 and 234, which are measured on
the ion counter with the RPQ detuned (i.e., Suppressor voltage = 9950 V). 238U
is maintained in a cup throughout the analysis to avoid reflections and is used to
normalize signal instabilities related to sample introduction. Each analysis has a
three-part structure, i.e. 1) background/baseline levels, 2) sample composition, and 3)
peak-tails are sequentially determined. In step 1, multiplier dark noise/Faraday baselines
plus background intensities at each mass are determined while aspirating running
solution. During sample measurement in step 2, ion counter yields for Th and U are
determined using signals of 300-400 kcps for 229Th and 233U by measuring 229Th/238U
and 233U/238U ratios first with the minor masses on the ion counter and then with
both masses in cups. Mass bias can be determined using the 233U/236U ratio of
the spike, allowing the sample’s 238U/235U ratio to be measured. In step 3, we
monitor peak-tails at half-mass positions (229.5, 231.5, 234.5) and on mass 237 while
aspirating sample solution. Tail measurement requires a distinct cup configuration to
maintain 238U in the cups; however, no sample is consumed during automated cup
reconfiguration.
We monitor the accuracy of 234U/238U ratios using CRM 145, which gives a weighted
mean atom ratio of (5.2846 ± 0.0029) Ã 10-5 (all errors 2Ïă), consistent with published and
reference values. The reproducibility of 230Th/238U ratios is monitored using the
Schwartzwalder Mine secular-equilibrium standard (SM). We detect no bias in 230Th/238U or
234U/238U ratios measured for SM at beam intensities ranging over a factor of four, consistent
with accurate correction for IC yields. Aladdin’s cave coral (AC-1) was analyzed to
check our ICP-MS method (and the preceding purification by ion exchange) on a
carbonate and yields a mean age of 125.43 ± 0.38 ka, in agreement with published
values.
We are currently applying the method to corals, speleothems, pedogenic coatings, and
tufas. |
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