A method is described for using multiple standard additions to determine sodium and potassium in samples of digested but unpurified blood of oceanic tuna fish whose flame emission responses are nonlinear. An exponential curve is fit in a rational manner to signals of the unknown and two or more spiked unknowns. Evidences are given suggesting that higher precision can be obtained in determination of these and similar elements in biological materials in a direct and simple manner.
T. R. Folsom, N. Hansen, W. E. Weitz, and G. J. Parks, "Determination of Sodium and Potassium in Blood of Oceanic Fish by Emission Flame Spectrometry Using an Exponential Extrapolation with Multiple Standard Additions," Appl. Spectrosc. 29, 404-407 (1975)