If you boil a liquid of mixture of components of same group then what happened? The volume of mixture is decreasing because of some components are vaporizing.
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Which type of components are vaporizing fast?
The answer is lower molecular weight components are vaporizing fast.
We need to identify them.
GC-MS work like this way.
Firstly it injects a very small amount of liquid mixture then make it vapour. There is a continuous innert gas flow for this the vaporized gas blow with it and ionized in ion source and detected, calculated.