Radioactivity was discovered in 1896 by Antoine Becquerel. It was found that some elements undergo spontaneous decay and that this decay occurred at a constant rate. Further investigation over the next century revealed that every atom has a small, positively charged nucleus. The nucleus is surrounded by a cloud of negatively charge electrons. The nucleus is made up of many types of particles, but for simplicity we will discuss only two types of particles, protons and neutrons. Protons have a positive charge that is equal in strength to the charge of the electrons; however, the charge of the electrons is opposite in polarity to the charge of the protons. Neutrons are slightly larger in mass than protons, but have no electrical charge.
The composition of the nucleus of an atom is represented by the formula
A = Z + N
where A is the mass number, Z is the number of protons, and N is the number of the neutrons. An isotope of an element has an equal number of electrons and protons, but has a different number of neutrons. Because neutrons are slightly larger in mass than protons, varying the number of neutrons in a nucleus (but keeping the same number of protons) results in a slightly different mass. For example, 87Sr will have a slightly larger mass than 86Sr due to an increase in the number of neutrons.
Isotopes are either stable or radioactive. Radioactive isotopes undergo spontaneous decay at a constant rate. Radioactive decay causes changes in the number of protons (Z) and the number of neutrons (N) in the parent element and leads to transformation of an atom of one element to another element, referred to as the daughter product. An example of this is the radioactive decay of 238U resulting in the daughter product 206Pb. If the daughter product itself is unstable, radioactive decay will continue until a stable atom is achieved. The long-lived radioactive isotopes decay and the daughter products accumulate in rocks and minerals, both in terrestrial and extraterrestrial material.
Through the isotope dilution method, the isotopic composition of an element in a in the sample is determined by adding a known amount of another isotope (the spike) of the same element. The ratio of these two isotopes is measured on a thermal ionization mass spectrometer (TIMS) and the composition and concentration of the unknown is determined. The mass spectrometer separates the isotopes based on their differences in mass. After the sample has been chemically purified, a salt of the element is loaded onto a filament that is mounted into the mass spectrometer. The filament is heated to a temperature hot enough to ionize the element of interest. The resulting ions are then accelerated by applying a voltage; the resulting ions are focused and the beam narrowed by the collimator. The ion beam enters a magnetic field which deflects the ions, with the heavier ions being deflected less than the lighter ones. The beams are measured by varying the magnetic field and collecting the beams one at a time (in a single collector instrument). If a multicollector mass spectrometer is being used, the collectors are spaced at the appropriate distances to collect the whole mass spectrum simultaneously.
Thermal Ionization Mass Spectrometry
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/isotope_geochron_lab/isotopes.html
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Page Last Modified: Tue 14-Feb-2006 13:01:49 MST
