One of the detectors commonly used in the laboratory is the mass spectrometer.  Inside the mass spectrometer is a vacuum chamber where, as sample molecules are streaming in, they are bombarded with high energy. This energy causes the molecules to break apart into fragments. 

The separation is based on mass and relative strengths of their molecular bonds.  Each chemical we analyze will have its own distinguishing pattern of masses and mass abundances called a mass spectrum.  Natural atomic isotopes cause clustered responses.  For example, about 1/4 of chlorine atoms found will have an atomic weight of 37, the remainder will be 35.