Radiochemical methods quantitative analysis using

Three common quantitative applications of radiochemical methods of analysis are considered in this section the direct analysis of radioactive isotopes by measuring their rate of disintegration, neutron activation, and the use of radioactive isotopes as tracers in isotope dilution. 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

Radiochemical methods of analysis employ radioactivity, with or without chemical manipulations, to obtain qualitative or quantitative information about the composition of materials. This information may concern the nature and quantity of elements or the specific chemical form of the component of interest. For example, qualitative and quantitative determinations of elements present in river waters can be readily accomplished on the other hand, radiochemical methods can be used to determine the quantity of vitamin B12 (which contains an atom of cobalt) in a mixture of similar organic compounds. The fundamental difference between this method of analysis and all others is that, in this method, one either induces radioactivity in the sample or adds a radioactive substance to the sample. 

We have recently modified U7) one of the several radiochemical methods (U5) which have been used for surface electrochemistry investigations in order to characterize adsorption on well-defined, single crystal electrodes. Below, we will describe the technique and identify some challenging issues which we will be able to address. The proposed method is sensitive to a few percent of a monolayer at smooth surfaces, is nondestructive and simple to use. The radiochemical measurements can be made with all compounds which can be labelled with reasonably long-lived, preferably g- emitting radioisotopes. We believe this technique will fulfill the quantitative function in in situ surface analysis as Auger spectroscopy currently does in vacuum, ex situ characterization of electrodes. 

The quantitation of enzymes and substrates has long been of critical importance in clinical chemistry, since metabolic levels of a variety of species are known to be associated with certain disease states. Enzymatic methods may be used in complex matrices, such as serum or urine, due to the high selectivity of enzymes for their natural substrates. Because of this selectivity, enzymatic assays are also used in chemical and biochemical research. This chapter considers quantitative experimental methods, the biochemical species that is being measured, how the measurement is made, and how experimental data relate to concentration. This chapter assumes familiarity with the principles of spectroscopic (absorbance, fluorescence, chemi-and bioluminescence, nephelometry, and turbidimetry), electrochemical (poten-tiometry and amperometry), calorimetry, and radiochemical methods. For an excellent coverage of these topics, the student is referred to Daniel C. Harris, Quantitative Chemical Analysis (6th ed.). In addition, statistical terms and methods, such as detection limit, signal-to-noise ratio (S/N), sensitivity, relative standard deviation (RSD), and linear regression are assumed familiar Chapter 16 in this volume discusses statistical parameters. 

Direct isotope dilution analysis is applied if an amount of an analyte cannot be separated quantitatively for analytical determination. A known amount of a radioactive isotope of the element of interest is added to the sample containing the analyte. Then a portion of the analyte is isolated in high purity from the sample. This separation step need not be quantitative. The mass and activity of the isolated portion are measured and used to calculate the amount of analyte in the original sample. There are several varieties known of this radiochemical method, e.g., reverse isotopic dilution. 

The alkali metals were discovered by several special individuals using some new techniques. The daredevil showman Davy isolated potassium and sodium by electrolysis of molten salts, the young Swede Arfwedson identified lithium by quantitative analysis, Bunsen and Kirchhoff identified rubidium and cesium spectroscopically, and Perey prepared minute quantities of francium by radiochemical methods. 

---