Determination of sodium in bone with the aid of cation exchange chromatography.

Current methods for the determination of sodium in biological materials are subject to considerable error under certain conditions. Large amounts of Ca, P, or strong acid cause serious interference in the flame photometric method (1, 2) and coprecipitation of Na occurs with methods commonly used to remove Ca and P (3). Whereas analysis of bone for Ca, P, Mg, COz, and a number of other constituents presents no serious problem, the relative paucity of data in the literature on the Na content of bone reflects to a certain extent the difficulties encountered in the determination of Na in the presence of large amounts of interfering substances. A number of attempts have been made to circumvent the difficulties inherent in the determination of Na in bone. Bergstrom and Wallace (4) found that a multiple stage precipitation method for the removal of Ca and P gave satisfactory results. Davies, Kornberg, and Wilson (3) added Na24 to a solution of bone ash, precipitated Ca and P, and corrected their results by the amount of NaZ4 found to be occluded in the precipitate. Edelman et al. (5) constructed calibration curves for their flame photometer by analyzing solutions of known Ca and Na content, but failed to account for possible error from phosphate. Stern et al. (6), on the other hand, encountered no particular difficulties in the analysis of bone for Na by oxalate precipitation and internal standard flame photometry. The method reported herein successfully removes interfering materials from bone by cation exchange chromatography. Under the conditions employed, Na appears in the eluate entirely free from P, Mg, K, and Ca; the Na-containing fraction can then be analyzed directly by flame photometry, or indirectly by Cl titration as recommended by Beukenkamp and Rieman (7). Cation exchange resins have been used by others to effect quantitative separation of sodium from K, Rb, and Cs (8, 9), from K and Mg (7), and from Li and K in insoluble silicates (lo), to separate Na, K, Mg, and Ca in solutions of milk ash (ll), and more recently to separate Na24 from K42 in blood serum (12).