Chemical analysis, with radioactive isotopes

Even nowadays the application of radioactive isotopes is the most sensitive method for the analysis of biomolecules or their reaction products. Besides the low detection limits, the replacement of a naturally overbalancing stable isotope by its radioactive analogue does not interfere with the physical or chemical properties of the enzyme (with some exceptions for hydrogens). Figure 6 lists some frequently used radioactive isotopes and their half-life periods. 

The number of protons is unique to the element but most elements can exist with two or more different numbers of neutrons in their nucleus, giving rise to different isotopes of the same element. Some isotopes are stable, but some (numerically the majority) have nuclei which change spontaneously - that is, they are radioactive. Following the discovery of naturally radioactive isotopes around 1900 (see Section 10.3) it was soon found that many elements could be artificially induced to become radioactive by irradiating with neutrons (activation analysis). This observation led to the development of a precise and sensitive method for chemical analysis. 

While he was investigating radioactive isotopes with Ernest Rutherford in 1913, George de Hevesy had an idea. Nuclear scientists were commonly forced to work with only tiny quantities of radioactive material, which would be very difficult to see using standard techniques of chemical analysis. But every single atom of a radioisotope advertised its presence when it decayed, since the radiation could be detected with a Geiger counter. So, if a... 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

The presence of radium in biological materials or environmental samples is generally determined by virtue of its radioactivity. Except in the laboratory where radium compounds have been isolated and determined for a certain purpose, determination of radium compounds in biological and environmental samples is relatively rare. As a Group IIA alkaline earth element, radium is similar in its chemical behavior to other members of that group, especially its nearest neighbor, barium. For example, radium tends to precipitate as the sulfate, which is the basis for its isolation for chemical analysis by coprecipitation with barium sulfate. Furthermore, radium associates with calcium in living systems and accumulates in bone. The determination of radium compounds or specific isotopes is usually accomplished by a separation procedure, followed by quantitative analysis of total radium based on its radioactivity. 

Sometimes the nucleus can be changed by bombarding it with another type of particle. This is referred to as induced radioactivity. In 1934, Irene Curie, the daughter of Pierre and Marie Curie, and her husband, Frederic Joliot, announced the first synthesis of an artificial radioactive isotope. They bombarded a thin piece of aluminum foil with ot-particles produced by the decay of polonium and found that the aluminum target became radioactive. Chemical analysis showed that the product of this reaction was an isotope of phosphorus. 

Isotopes are ideal tools for use in analysis a single atom can be detected when using radioactive isotopes, as compared to chemical methods in which the detection limit of an element is enhanced a million times. Stable isotopes can also be detected with great accuracy nowadays although not quite with the same sensitivity as radiation-emitting isotopes. 

To insure that a statistical average behavior is observed in the chemical experiments with No and Lr, it has been necessary to make repeated measurements for each data point. Indeed, the determination of the distribution coefficients for Lr in a solvent extraction experiment required over 200 experiments to define the behavior of about 150 atoms of Lr (JL). Experiments of this kind are exceptionally difficult and computer-controlled equipment has been devised to perform either a portion or all of operations needed for the chemical tests and the analysis of samples. Computer automation, although requiring a larger effort to implement, permits an experiment to be repeated many times in rapid sequence with the added advantage of doing each quickly before the complete decay of the radioactive atoms of a shortlived isotope. 

Neutron activation analysis (NAA) is a nondestructive method based upon the conversion of stable isotopes of chemical elements to vmstable radioactive isotopes by irradiation with thermal neutrons within a nuclear reactor. If the ensuing decay of irradiated nuclides occurs via y-radiation, a y-ray spectrum can be obtained on a y-spectrometeq which is characteristic of a specific element. 

Of special interest is the discovery of rare earths, noble (or inert) gases, and, finally, the elements predicted by 0. I. Mendeleev on the basis of the periodic system. Although these elements were discovered by means of chemical analysis and spectroscopic method, the histories of the above groups of elements are in many respects highly individual and separate chapters have been devoted to their presentation (Chapters 7, 8 and 9). No less peculiar is the history of the two stable elements which proved to be the last to be discovered on Earth—hafnium and rhenium (Chapter 10). The first part of the book ends with the history of radioactive elements (Chapter 11), which introduces the reader to the world of radioactivity, the world of unstable elements and isotopes the most of which were obtained artificially by means of nuclear reactions. 

Water is poorly soluble in hydrocarbons and determination of its solubility by ordinary chemical analysis requires a large amount of hydrocarbon samples. The solubility of water in benzene (Joris and Taylor 1948) and other hydrocarbons (Black et al. 1948) was determined with the use of tritium ( H, T) as tracer. These studies give an example where correction for the mass difference between an element of natural isotopic composition and its radioactive tracer is necessary. Water labeled with T was prepared by bombardment of heavy water (D2O) with... 

Use of a radioactive tracer to determine a chemical yield is part of a broad suite of techniques known as isotope dilution The analyst wishes to measure the amount of a stable element X in a sample from which a pure chemical fraction of X can be only incompletely separated. A tracer aliquot containing a known mass of X (Mx), labeled with a radioactive isotope of X characterized by radioactivity of a known intensity (C), is added to the sample. A separation is performed to obtain a pure sample of X of mass Ms and a measured radioactive intensity of C which is less than C due to losses in the separation procedure Ms is determined by any suitable standard quantitative-analysis method (e.g., gravimetry of a stoichiometric compound of X). The specific activity of the chemical fraction, C /Ms, is equal to the specific activity of the element X in the mixture after tracing but prior to... 

The allowable concentrations of sodium in silica gel catalysts are so low that chemical methods of analysis may not be sufliciently sensitive. Acitivation analysis by neutron flux converts sodium to a radioactive isotope so that 3 ppm can be determined with a standard deviation of 0.2 ppm (353). 

Several authors have reported the application of radioactive substances and TLC in clinical analysis and diagnosis [190, 724, 730]. Others have tested and compared the accuracy of routine chemical methods with isotope techniques [399, 419, 634, 728]. 

In the late 1940s and in the first half of the 1950s, many investigators began to use radioactive isotopes to determine the rate and site of synthesis of cell components, and some combined chemical analysis of labeled components with radioautography. The technique was also called autoradiography for some reason. 

Mendeleev s definition put into play a tacit distinction between the chemical order and the physical order, a distinction attacked by Urbain, who, along with Paneth, was one of the chemists behind lUPAC s (the International Union of Pure and Applied Chemistry) new definition of the element. Urbain was opposed to the idea that when Rutherford bombarded nitrogen or phosphorous with alpha rays he generated only a physical phenomenon, and so Urbain rejected the approach that would place radioactive isotopes in the same box in the periodic table as the non-radioactive ones. Furthermore, the phenomenon of radioactive decay demonstrated that even the idea of a simple body that lay behind Lavoisier s understanding of an element was no longer valid. Urbain interpreted radioactive decay as a form of analysis, and because simple bodies did not survive the bombardment with alpha particles, they could not be considered truly simple. Nevertheless, Mendeleev s concept of the element managed to escape this particular line of attack. Because it was a conceptual notion, an abstraction, as we have explained above, Mendeleev s element was able to resist the attacks of the most powerful instruments of modern atomic physics. 

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