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cause no stable reference standard is available. This third alternative would be to specify t h a t the spec trum of the substance conform to t h a t of a reference standard of the same substance in a similar deter mination. There m a y be other pos sibilities, but these three were con sidered by the Revision Committee and the last one was chosen for use in 89 of the 90 monographs in volved.
Radiochemical Methods
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ANALYTICAL CHEMISTRY
Admission of six radioactive com pounds to U S P X V I has brought on the necessity of including assay methods for radioactivity in the procedures. Geiger-Muller or scin tillation counting assemblies are specified for the assays and a vari ety of calibration standards are used for quantitative comparison. Identification of radiocyanocobalamin solution is effected by a requirement t h a t it show a gammaray scintillation spectrum identical with t h a t of a specimen of cobalt-60 of known purity. Radiogold solu tion must show a g a m m a - r a y scin tillation spectrum identical with t h a t of gold-198 of known purity, and sodium radiochromate injection must show a g a m m a - r a y scintilla tion spectrum identical with t h a t of chromium-51 of known purity. Sodium radiophosphate solution is required to emit beta radiation having a mass absorption coefficient in aluminum within five per cent of t h a t found for a sample of phosphorus-32 of known purity. Radioiodinated serum albumin must emit beta radiation having a mass ab sorption coefficient in aluminum within five per cent of t h a t found for a sample of iodine-131 of known purity. Sodium radio-iodide is specified in both capsule and solution form, but the means of identification of course is the same in both cases. Both forms are required to emit beta radiation having a mass absorption coefficient in aluminum within five per cent of t h a t found for a sample of iodine-131 of known purity. It will be noted t h a t this requirement is similar to t h a t for radio-iodinated serum albumin. B u t additionally, paper radiochromatograms of the
sodium radio-iodides must show an iodide band having an R; within five per cent of t h a t found for an iodine-131 of known purity, and the iodate band should not have more t h a n five per cent of the activity of the iodide band. Radioactive reference standards are not provided by USPC, since these are readily available on a commercial basis.
Chromatography and Other Methods This reference to the use of radio chromatograms in identifying the radioactive compounds reflects by no means the only use of chroma tography in the Sixteenth Revision. In the assay of vitamin D , for ex ample, the methods of chromatog raphy and spectrophotometry are combined to replace a somewhat tedious and costly biological assay formerly used. In the new proce dure, interfering substances are re moved by passage of a solution of the vitamin through two chroma tographic columns, after which it is treated with antimony trichloride reagent. The latter gives a pink color which is evaluated spectrophotometrically. Another séparatory use of column chromatography is in the assay of the cardiotonic glycoside, digitoxin, where it is used to separate the pertinent glycoside from related glycosides. Chromatographic methods are used also for identification of several compounds in the new edition. Paper chromatography enables identification of another cardiotonic glycoside—digoxin—as well as of calcium leucovorin, an antidote for folic acid antagonists. I t is used also in identification of estradiol dipropionate injection and ethinyl estradiol tablets. Complexomctric titrations, too, have proved valuable in some of the USP X V I procedures. Titration of calcium ion in this way has shown itself to be much better than the oxalate precipitation used previously. Since single indicators are not always entirely satisfactory in titrations of this type, USP usually specifies the use of mixed indicators. Calcium carbonate, calcium chloride, calcium gluconate, and calcium
