mission densitometry may be used to relate the density of 'fogging' to the amount of analyte present.
The autoradiograph may also be used as a template to mark off the positions of spots or bands.
Alternative counting procedures may then be used for final assay. Enhanced sensitivity may often be
obtained by exposure of the specimen to a scintillant or fluorescent screen.
The Statistics of Radioactive Measurements
At any one instant, only a very small proportion of the total number of unstable nuclei in a radioactive
source undergo decay. A Poisson distribution which expresses the result of a large number of
experiments in which only a small number are successful, can thus be used to describe the results
obtained from measurements on a source of constant activity. In practical terms this means that random
fluctuations will always occur, and that the estimated standard deviation, s, of a measurement can be
related to the total measurement by:
It follows that for the relative standard deviation to be reduced to 1% or less a minimum of 10 counts
must be recorded.
10.3—
Analytical Uses of Radionuclides
Although radioactive materials have been used in many different ways to solve analytical problems
there are only three groups of methods which are of major importance.
Chemical Pathway Studies
A method of analysis frequently requires the separation of the analyte from interfering materials present
within the sample matrix. Many different procedures are used to effect such separations (Chapter 4),
and the establishment of quantitative separations is a singularly important step in method development.
Careful study and control of experimental conditions are needed and a large number of experiments
may be required to establish the most suitable.
Radiotracers are uniquely well suited to such studies. The sensitivity of detection means that only very
small amounts of tracer need be added to follow the chemical pathway of the relevant species.
Furthermore, it matters little what the physical or chemical state of the tracer is, for measurements may
be made on liquids, solids or gases. Chromatography, solvent extraction and precipitation are amongst
separation methods widely studied by means of radiotracers. In the individual separation steps the
distribution of the species may be studied by simple radioactivity measurements, and subsequently the
tracer will serve as a yield indicator for the overall procedure.