tivity on the carmel faul

II-b. Laboratory protocol for the preparation of samples for
chromatographic separation of U and Th without the
insoluble/detrital matter
Preparations:
• Weighing of material.
• Dissolving the samples with twice the molar ratio of DH 3 COOH acid
in centrifugal test tubes:
CaCO 3 + 2CH 3 COOH = Ca(CH 3 COO) 2 + H 2 O + CO 2
• The insoluble material is separated with centrifuge (not more than 40 cc
per run) for 20 minutes at 4000 cycles per minute.
• The solution is separated into a Teflon container.
• After cleaning the test tubes with 5 cc of distilled water, while going
over the sides with a pipette and shaking the tube, repeating the
centrifuge procedure twice more.
• Removing the remaining solution with a 5 cc pipette to a new test tube.
• Adding and measuring spike ( 236 U- 229 Th) into the new test tube.
• Adding the contents of the new test tube to the Teflon container with
the rest of the solution, cleaning it out with distilled water.
• The solution is evaporated to complete dryness, making sure no acetic
acid remains.
• Continuing chromatographic procedure as stated above.
II-c. Measuring isotopic ratios using the ICP-MS
MC-ICP-MS: Multiple Collector Inductively Coupled Plasma Mass Spectrometer
(produced by U.K. Nu Instrument) in the Geological Survey.
The different isotopic ratios are determined by the injection of sample solutions in
Aragon Plasma (~6000 o K) which produces an ionization of nuclides which are then
accelerated in an electrical field, then separated with a magnet and are measured by
the mass spectrometer (Goldstein and Stirling, 2003). The machine contains 12
Faraday detectors and 3 Ion Counters (IC). The IC is used for very low concentrations
of 229 Th, 236 U, 230 Th, 234 U.
Measuring uranium ratios:
This is done in three cycles the following ratios are determined: 235 U/ 236 U,
235 U/ 238 U and 234 U/ 236 U.
• Cycle I - The conditions of the machine are determined by the
measuring of 235 U/ 238 U with the Faraday detector. The results are
calibrated using the known ratios which exist in nature. Then a
calibration is done for the IC0 detector.
• Cycle II - Faraday detectors measure 235 U and 238 U and the results are
calibrated for the IC1 detector.