Analytical applications of biological tracers

There are a number of tracers that have been used to help understand chemical reactions and interactions. Historically, development of modem tracer methods began with the pioneering work of the Hungarian physical chemist, George Charles de Hevesy, in the early 1900s. De Hevesy s work focused on the use of radioactive tracers to study chemical processes, for which he was awarded the Nobel Prize in Chemistry in 1943. Radioactive tracers, also known as radioactive labels, are based on the use of a given radioisotope. However, it is important to note that there are also isotopic tracers (or isotopic labels). Isotopes are forms of a chemical element with different atomic mass, which have nuclei with the same atomic number (i.e. number of protons) but different numbers of neutrons. Examples include H, C, and which are radioactive forms of stable elements 

Historically, the discovery of radioactivity dates back to 1896 when the French scientist Henri Becquerel believed that the afterglow observed in cathode ray tubes might be associated with phosphorescence, later realizing that this phenomenon was instead due to radiation. At first, this radiation was assumed to be similar to X-rays, but further research by Becquerel and a number of other notable scientists (including Marie Curie and Ernest Rutherford) revealed that the nature of this radiation was more complex. Subsequently, it emerged that there were three principal forms of radioactivity that result from different types of radioactive (nuclear) decay. 

During radioactive decay an unstable atomic nucleus emits radiation in the form of particular particles or electromagnetic waves. This process results in a parallel loss of energy as so-called parent nuclide(s) transform into daughter nuclide(s). The principal types of radioactive decay are alpha (a), beta (ft) and gamma (y), as described further in Table 10.1 the SI unit of radioactive decay is the Becquerel (Bq), where one Bq is one decay (or transformation disintegration) per second. 

Radioactivity can be measured by a variety of methods producing values which relate to Bq (i.e. one disintegration per second). Due to inherent limitations of certain instruments which cannot measure each and every disintegration in a sample (i.e. efficiency 100%), often the measure of decay is recorded as radioactive counts per unit time-typically counts per minute (or cpm). In most cases these instmments will additionally correct for background 

Type of radiation Penetration range in air (m) Shielding material 

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