Getting a Rock Sample Ready for the Mass Spectrometer

For reliable age determination, careful sample preparation is an important and often tedious process. The rock is mechanically crushed into small fragments. Fragments of the Rb-bearing minerals are then separated from the whole rock using a variety of methods, such as a magnetic separator. These materials are then used to prepare a "whole-rock" sample and several "mineral separate" samples. The whole rock sample will yield the weighted average isotopic composition of all the minerals in the rock. Each mineral separate will yield the composition of that particular mineral.

Crush the Rock
Separate the Minerals
Other Steps
There are other steps that must be carried out to prepare a sample for analysis by a mass spectrometer, such as converting the sample to a solution by dissolving the mineral separates in selected acids, using techniques of column chemistry to increase the concentration of the small amounts of Rb and Sr in the solution and then precipitating the concentrated solution as a "salt" compound. It's this compound of Rb-Sr salts that can be attached to a special filament and placed into the mass spectrometer for analysis.

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