MODEL RELEASED. Mass spectrometer being used by a forensic scientist. He is preparing a sample to be placed in the machine. The display screen (centre left) will show the sample's chemical composition. This allows small traces of both known and unknown materials to be detected. For example, drugs can be detected in hair samples. A mass spectrometer accurately detects the mass of molecules of a sample, even if it is only present in very small amounts. It does this by ionising the sample and then measuring the deflection of these ions as they pass through a magnetic field. This allows a calculation of the charge to mass ratio of the ion.