I. The separation of sodium from potassium in human blood serum by ion exchange chromatography.

The experiments to be described were undertaken to devise a simple and rapid technique for the separation of sodium from potassium in human blood serum in order to facilitate the simultaneous measurement of exchangeable sodium and potassium in man by the isotope dilution technique. This technique requires the intravenous administration of a known amount of radioactive sodium (Na24) and radioactive potassium (K'2), followed by the determination, after a twenty-four hour equilibration period, of the individual sodium and potassium radioactivities and concentrations in the serum. The general principles underlying the application of the isotope dilution technique to the measurement of body constituents have been outlined by Edelnan, Olney, James, Brooks, and Moore (1). The data presented here are primarily concerned with the separation of sodium from potassium in aqueous solutions and serum. The results of our measurements in man are presented in Part II of this study (2). Since the isotope dilution method for the measurement of exchangeable body sodium and potassium requires separate determinations of the radioactivity of the Na24 and K42 found in the serum, three methods for making the discrimination between sodium and potassium were considered: 1) Physical separation, which makes use of differences in the physical characteristics of the radiation from the two isotopes; 2) chemical separation by precipitation; and 3) chemical separation by ion exchange. It is difficult at present to make accurate individual measurements based on differences in physical properties of the radioactivity of Na24 and K'2 in mixtures of the two isotopes, because their half-lives, 15.06 and 12.44 hours, respectively, are very similar, and because both isotopes emit beta particles and gamma rays whose energies are not markedly different.' Chemical separation by precipitation can be carried out by a standard procedure, and James, Brooks, Edelman, and Moore (4) have developed a technique for adapting such a procedure to the requirements of the isotope dilution method. We have adopted chemical separation by ion exchange, making use of the sulfonic acid cation exchange resin, Dowex 50, in order to simplify the separation for clinical use. This procedure depends upon the greater affinity of Dowex 50 for the potassium ion than for the sodium ion so that these alkali metals may be eluted separately from the resin column with hydrochloric acid. For our purposes the separation procedure had to satisfy the following special criteria: speed and completeness of separation, relative simplicity, and acceptable reproducibility. The importance of speed is apparent from the following facts. The amount of isotope that can be safely administered to man for investigative purposes is closely limited, and consequently our dose does not exceed 0.45 rep (roentgen equivalent physical). Approximately two half-lives elapse in the 24-hour period between injection and removal of the blood sample, with a resultant four-fold decrease in radioactivity. Furthermore, the amount of radioactivity in the 50 ml. sample of blood drawn represents but a small fraction of the amount injected. Thus, it is essential to reduce to a minimum the elapsed time between drawing the blood sample and measuring