Mass Spectrometer

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.

II

The Magnetic-Deflection Mass Spectrometer

In a magnetic-deflection mass spectrometer, ions with a positive unit charge are created from the sample and accelerated by an electrostatic field. A magnetic field deflects the ions according to their mass. Ions with a certain mass strike the detector. Less massive ions are deflected too much and miss the detector, and more massive ions are not deflected enough and miss the detector. The strength of the magnetic field slowly varies so the detector can measure the relative proportions of all the constituents of the sample. The detector is often connected to a computer that processes the data.

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III

Uses of Spectrometers

Mass spectrometers can provide a high degree of resolution to aid in the analysis of complex mixtures. Products of petroleum refining and processing, for example, which usually contain various closely related hydrocarbons, are difficult to separate by conventional methods of chemical analysis, but can be isolated and analyzed using a mass spectrometer. A tandem mass spectrometer, which consists of multiple mass spectrometers connected end to end, is far more sensitive than a single conventional mass spectrometer. In the field of molecular biology, a tandem mass spectrometer (consisting of two mass spectrometers) makes it possible to establish the linear sequence of the amino acids in a protein molecule in a matter of minutes. These mass spectrometers are more than a thousand times more sensitive than any single unit, making them useful for analyzing extremely small quantities of biological compounds with large molecular weights. The most sensitive devices that can be used as mass spectrometers are tandem electrostatic accelerators, which comprise a series of conventional mass spectrometers connected to a powerful electrostatic particle accelerator. These devices can separate atoms or small molecules that have masses that differ by 1 part in 100,000 or less.