The method of changing parameter in photometry has been introduced to allow the determination of a substance in the presence of a strongly interfering unknown matrix. The theory of this method is now discussed in detail. The discussion is illustrated by the case of the determination of calcium in a strongly interfering matrix of HCl and LaCl3, using an atomic absorption flame photometer. The derivations are, however, general enough to apply to any photometric method, and even to some nonphotometric methods having the same formal dependence on a changing parameter. It is shown that a changing parameter can be sensitive enough to prove the existence of interference, while not being sensitive enough to evaluate such an interference. It is also shown that even a very considerable interference can be difficult to detect and to evaluate.
Adam Shatkay, "Dilution as the Changing Parameter in Photometry," Appl. Spectrosc. 24, 121-128 (1970)