ABSTRACT
RIA is a general method by which the concentration of virtually any substance can be determined. e principle on which it is based is summarized in the competing reactions shown in Figure 14.1. e concentration of Ag in the unknown sample is obtained by comparing its inhibitory eect on the binding of Ag* to a limited amount of Ab with the inhibitory eect of known standards. A typical RIA is performed by the simultaneous preparation of standard and unknown mixtures in test tubes. To these tubes are added a xed amount of Ag* and a xed amount of antiserum. Aer an appropriate reaction time, the antibody-bound (B) and -free (F) fractions of the Ag* are separated by one of the many different techniques. e B/F ratios in the standards are plotted as a function of the concentration of Ag (“standard curve”) and the concentration of Ag in the unknown sample is determined by comparing the observed B/F ratio with the standard curve (Figure 14.2). e radioactive isotopes most frequently used for labeling are 3H, 14C, 35S, 57Co, 75Se, 125I, and 131I (Table 14.1). Of these, 125I oers useful characteristics for labeling and is very widely used.
