I.

Introduction

Mass Spectrometer, apparatus that converts molecules into ions and then separates the ions according to their mass-to-charge ratio. Mass spectrometers are used to identify atoms and isotopes, and determine the chemical composition of a sample.

Although many different kinds of mass spectrometers are in use today, they are all related to a device developed by the British physicist Francis William Aston in 1919. In Aston's instrument, a thin beam of positively charged ions was first deflected by an electric field and then deflected in the opposite direction by a magnetic field. The amount of deflection of the particles as registered on a photographic plate depended on their mass and velocity: the greater the mass or velocity of the ion, the less it was deflected. Aston measured the molecular weights of the isotopes of many elements as well as the relative abundance of these isotopes in nature.

All mass spectrometers have four features in common: (1) a system for introducing the substance to be analyzed into the instrument; (2) a system for ionizing the substance; (3) an accelerator that directs the ions into the measuring apparatus; and (4) a system for separating the constituent ions and recording the mass spectrum of the substance.