A Mass Spectrometer is used to Measure Isotopic Ratios

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.

In this case the Rb-Sr rich salt sample prepared from the rock to be age-dated is placed into a mass spectrometer and then converted into charged molecules. A stream of these charged molecules is directed across a large magnet. The magnetic field deflects the path of the charged molecules. The amount of deflection is greatest for lighter molecules and least for heavier molecules. Molecules containing isotopes of different masses therefore follow different paths after passing by the magnet. Detectors are strategically placed in the paths of the different molecules to measure the relative amounts of the different isotopes.

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