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. It's this compound of Rb-Sr salts that can be attached to a special filament and placed into the mass spectrometer for analysis. Analyze the isotopic compositions of the whole rock and mineral separates on a mass spectrometer. A Mass Spectrometer is a very powerful and sophisticated instrument.
Many types exist.
Below is a simplified diagram of the electro-mechanical mass spectrometer system and a picture of a modern instrument. Understanding how a mass spectrometer functions is beyond the level of this activity. But you should know that it measures the amounts of various isotopes present in specially prepared samples of rocks and minerals as well as other materials. 4.Prepare an isochron diagramthat shows the decay curve to calculate a date. Understanding the isochron diagram is the key to determining the age of a rock using the Rb-Sr method.