Mass Spectrometry

Mass spectrometry is an instrumental method of obtaining structure and mass information about either molecules or atoms by generating ionized particles and then accelerating them in a curved path through a magnetic field. Heavier particles are more difficult for the magnetic field to deflect around the curve, and thus travel in a straighter path than lighter particles. Consequently, by the time the particles reach the detector, a mixture of ions will have separated into groups by mass (or more specifically the mass-to-charge ratio of the individually weighted ions.) The ions are produced from neutral molecules and atoms by stressing them with some form of energy to knock off electrons. In the case of molecules, fragmentation as well as ionization usually occurs. Each type of molecule breaks up in a characteristic manner, so a skilled observer can interpret a mass spectrum much like an archaeologist can reconstruct an entire skeleton from bone fragments. A mass spectrum can help establish values for ionization energy (the amount of energy it takes to remove an electron from a neutral atom or molecule) and molecular or atomic mass for unknown substances. The extremely high sensitivity of mass spectrometry makes it indispensable for analyzing trace quantities of substances, so it is widely used in environmental, pharmaceutical, forensic, flavor, and fragrance analysis. The petroleum industry has used mass spectrometry for decades to analyze hydrocarbons.

The basic principle underlying mass spectrometry was formulated by J. J. Thomson (the discoverer of the electron) early in the century. Working with cathode ray tubes, he was able to separate two types of particles, each with a slightly different mass, from a beam of neon ions, thereby proving the existence of isotopes. (Isotopes are atoms of the same element that have slightly different atomic masses due to the presence of differing numbers of neutrons in the nucleus.) The first mass spectrometers were built in 1919 by F. W. Aston and A. J. Dempster.

There are five major parts to a mass spectrometer: the inlet, the ionization chamber, the mass analyzer, the detector, and the electronic readout device. The sample to be analyzed enters the instrument through the inlet, usually as a gas, although a solid can be analyzed if it is sufficiently volatile to give off at least some gaseous molecules. In the ionization chamber, the sample is ionized and fragmented. This can be accomplished in many ways—electron bombardment, chemical ionization, laser ionization, electric field ionization—and the choice is usually based on how much the analyst wants the molecule to fragment. A milder ionization (lower electric field strength, less vigorous chemical reaction) will leave many more molecules intact, whereas a stronger ionization will produce more fragments. In the mass analyzer, the particles are separated into groups by mass, and then the detector measures the mass-to-charge ratio for each group of fragments. Finally, a readout device—usually a computer—records the data.

Mass spectrometers are often used in combination with other instruments. Since a mass spectrometer is an identification instrument, it is often paired with a separation instrument like a chromatograph. Sometimes two mass spectrometers are paired, so that a mild ionization method can be followed by a more vigorous ionization of the individual fragments.