Selenium stock solution— Dissolve about 1 g of metallic selenium, accurately weighed, in a minimum volume of nitric acid. [Caution—Selenium is toxic; handle it with care.] Evaporate to dryness, add 2 mL of water, and evaporate to dryness. Repeat the addition of water and the evaporation to dryness three times. Dissolve the residue in 3 N hydrochloric acid, transfer to a 1000-mL volumetric flask, dilute with 3 N hydrochloric acid to volume, and mix. This solution contains about 1000 µg of selenium per mL.

Standard preparations— Transfer 3.0, 4.0, and 5.0 mL, respectively, of Selenium stock solution to separate 100-mL volumetric flasks, dilute the contents of each flask with water to volume, and mix. These Standard preparations contain, respectively, about 30, 40, and 50 µg of selenium per mL.

Assay preparation— Using water as the diluent, quantitatively dilute an accurately measured volume of Injection to obtain a solution containing about 40 µg of selenium per mL.

Procedure— Concomitantly determine the absorbances of the Standard preparations and the Assay preparation at the selenium emission line of 196 nm, with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering 851) equipped with a selenium electrodeless discharge lamp and an air–acetylene flame, using water as the blank. Plot the absorbances of the Standard preparations versus concentration, in µg per mL, of selenium, and draw the straight line best fitting the three plotted points. From the graph so obtained, determine the concentration C, in µg per mL, of selenium in the Assay preparation. Calculate the quantity, in mg, of selenium in each mL of the Injection taken by the formula: in which L is the labeled quantity, in mg per mL, of selenium in the Injection taken, and D is the concentration, in µg of selenium per mL, of the Assay preparation on the basis of the labeled quantity in the Injection and the extent of dilution.