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

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

Procedure— Concomitantly determine the absorbances of the Standard preparationsand the Assay preparationat the selenium emission line of 196nm,with a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-scattering á851ñ)equipped with a selenium electrodeless discharge lamp and an air–acetylene flame,using water as the blank.Plot the absorbances of the Standard preparationsversus 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 mLof the Injection taken by the formula: in which Lis the labeled quantity,in mg per mL,of selenium in the Injection taken,and Dis the concentration,in µg of selenium per mL,of the Assay preparationon the basis of the labeled quantity in the Injection and the extent of dilution.