Published: 4 April, 2007

Spectrometer seperates isotopes by ionization, magnetic deflection

How does a mass spectrometer separate isotopes?

An element, the basic unit of chemistry, can exist as separate species called "isotopes" (such as those of hydrogen, 1H, 2H, and 3H, or uranium, 235U and 238U) that have different weights or "masses". These have almost the same chemical properties, but they can have far different nuclear properties. For example, tritium (3H) and 235U are radioactive. The isotopes' atoms contain the same number of protons and electrons, but they have different numbers of neutrons.

Mass is an obvious property of isotopes to employ for their separation, and mass spectrometers are such devices. If the mass of an isotope is doubled, twice the effort is required to push or pull it. Further, this push/pull requires a "handle" on the isotopic species of either a negative or positive charge, which can be formed by, for example, adding or removing an electron. Two charges on these "ions" are twice as effective as one, so that the mass spectrometer separates according to mass (m) divided by number of charges (z), or m/z. The separation of individual atoms or molecules containing the isotopes is carried out under high vacuum, as air or other gas molecules can react or interfere.

For the historically most important of the many kinds of mass spectrometers, in 1940 A. O. Nier electrostatically accelerated a beam of gaseous uranium ions, then deflected it with a magnetic field. The lighter 235U was pushed farther from a straight path and so could be collected separately from 238U. This nanogram sample (1/1000 of the smallest particle you can see) was sufficient to measure physical and chemical properties to initiate the design of the WWII atomic bomb. Then 235U for the first bombs was separated in 76 "Calutron" mass spectrometers, each three-stories high. Their high current magnets used huge solid silver conductors (better than copper) borrowed from the U.S. Treasury at Fort Knox. Although separation of isotopes for nuclear weapons now uses more efficient methods, magnetic mass spectrometers still separate stable isotopes that are valuable as tracers in chemistry and biomedicine.