The determination of calcium in biological fluids by flame photometry.

The use of flame photometers has greatly simplified and expedited the estimation of sodium and potassium. Calcium is one of the elements which it is possible to determine by flame photometry, but little advantage has been taken of this fact in biological work for a number of reasons. The concentration of calcium in extracellular fluids is usually less than that of potassium ; a given quantity of calcium in a coal gas-air or butane-air flame emits much less light than an equivalent quantity of potassium, and the emission is more readily interfered with by the presence of other ions than is that produced by either sodium or potassium. Flame photometers differ in the extent to which effects of interference are observed, but the commercial model marketed by the Evans Electroselenium Company* is singularly free from this trouble when sodium or potassium has to be estimated in a biological fluid. The same statement, unfortunately, cannot be made in the case of calcium. The interference filter supplied for calcium estimations with the EEL flame photometer gives no reading at all for sodium if it is present in concentrations not greatly in excess of the calcium present, but in normal serum there will be about 28 equivalents of sodium for one of calcium. In such a-case the amount of sodium light passing through the filter will be sufficient to cause a serious error in the calcium reading. Moreover, so little calcium is present in serum that the maximum dilution which will give a reasonable galvanometer deflection at normal levels is about one in four. With this concentration so much protein is present that there is a serious danger of blocking the jet. The difficulty caused by clogging by protein can be avoided by precipitating the proteins as described in a method devised for use with an American instrument (Severinghaus and Ferrebee, 1950). To overcome the difficulty of sodium interference the authors recommend the use of