The accuracy of quantitative analysis of trace elements by proton-induced X-rays is examined by using solutions of known concentration of lead and uranium. Trace elements in tomato juice and well water are quantitatively analysed, referring to a calibration standard of uranium. Some of the results are compared with those obtained by usual analytical methods. As an example of impurity analyses where the atomic number of impurity is close to that of a matrix element, copper impurity in potassium ferricyanide [K 3 Fe(CN) 6 ] is studied by using the absorption edge of the Ni atom.
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