Analytical Chemistry

which would otherwise be extremely difficult or impossible to determine. In the many recent studies of
biologically active compounds such as hormones and steroids, RIA has been extensively employed. The
monitoring of drugs and drug metabolites presents similar analytical problems, and here RIA has been
used for both medicinal and forensic investigations. There is also a growing interest in the use of RIA as
a method for screening water supplies. Table 10.1 indicates the scope of applications of RIA.

Table 10.1 A selection of compounds for which radioimmunoassays have been developed, indicating the wide applicability

of the technique

Drugs Steroids Polypeptide hormones

Amphetamines Anabolics Adrenocorticotrophic hormone (ACTH)

Barbiturates Androgens Follicle stimulating hormone (FSH)

Diazepam Corticosteroids Glucagon

Digoxin Oestrogens Growth hormone

Gentamicin Progesterones Insulin

Methadone Steroid glucuronides Luteinizing hormone (LH)

Morphine Prolactin

Nicotine

Penicillins

Prostaglandins

Tetrahydrocannabinol

Tubocurarine

The principal advantages of RIA are its sensitivity, specificity and simplicity of operation. There are
three main disadvantages. Firstly, the lengthy development periods for new methods often result from
the difficulties of producing the specific binding agent. Secondly, cross-reactions with other molecules
similar to the analyte can sometimes interfere. Thirdly, poor precision can result unless careful control
of experimental conditions is employed to ensure reproducible binding reactions. Relative precisions of
1 – 3% are typical.

Radioactivation Analysis

Nuclear bombardment reactions in which the product is radioactive constitute the basis of
radioactivation analysis (p. 456). Although in principle any bombardment-decay sequence may be used
the analyst is largely concerned with thermal neutron activation. Equation (10.13) relates the induced
activity to the amount of the parent nucleide (analyte). However, practical difficulties arise because of
flux inhomogeneities. It is common therefore to irradiate a standard with very similar characteristics
alongside the sample, e.g. for a silicate rock sample a standard solution would be evaporated on to a
similar amount of pure silica. On the assumption that identical specific activities for the analyte are then
induced in the sample and standard, the amount w 2 of analyte is readily calculated from