Standard stock solution— Dissolve about 160 mg of lead nitrate, accurately weighed, in 100 mL of water containing 1 mL of nitric acid. Dilute with water to 1000 mL, and mix.

Standard solutions— [NOTE—Prepare these solutions on the day of use.] Transfer 10.0 mL of Standard stock solution to a 100-mL volumetric flask, dilute with water to volume, and mix. Each mL of this solution contains the equivalent of about 10 µg of lead. Dilute accurately measured volumes of the diluted Standard stock solution with water to obtain solutions having known concentrations of about 1 µg, 2 µg, and 5 µg of lead per mL.

Test solution— Transfer about 5 g of Myristic Acid, accurately weighed, to an evaporating dish. Add 5 mL of a 25% sulfuric acid solution, and distribute the sulfuric acid uniformly through the sample. Within a hood, place the dish on a steam bath to evaporate most of the water. Place the dish on a burner, and slowly pre-ash the sample by expelling most of the sulfuric acid. Place the dish in a muffle furnace that has been set at 525, and ash the sample until the residue appears free from carbon. Prepare a blank by ashing 5 mL of a 25% sulfuric acid solution. Cool, and cautiously wash down the inside of each evaporation dish with water. Treat both the sample and the blank as follows. Add 5 mL of 1 N hydrochloric acid. Place each dish on a steam bath, and evaporate to dryness. To each dish add 1.0 mL of 3 N hydrochloric acid and approximately 5 mL of water, and heat briefly on a steam bath to dissolve any residue. Transfer each solution quantitatively to a 10-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Using a suitable atomic absorption spectrophotometer (see Spectrophotometry and Light-Scattering 851) equipped with a lead electrodeless discharge lamp, an air–acetylene flame, and a suitable burner head, perform a blank determination with water, following the manufacturer's operating instructions. Concomitantly determine the absorbances of the blank, the Standard solutions, and the Test solution at the lead emission line of 283.3 nm, using a slit-width of 0.7 nm. Determine the corrected absorbance values by subtracting the absorbance of the blank from the absorbance of each of the Standard solutions and from the absorbance of the Test solution. Prepare a standard curve by plotting the corrected absorbance values of the Standard solutions versus their corresponding concentration, in µg per mL. From the calibration curve, determine the lead concentration in the Test solution. Calculate the lead content, in µg per g, in the portion of Myristic Acid taken by the formula: in which C is the concentration, in µg per mL, of lead from the standard curve; and WS is the weight, in g, of Myristic Acid taken: not more than 2 µg per g is found.

(Official January 1, 2007)

Resolution solution— Proceed as directed for the System Suitability Solution in Fatty Acid Composition under Fats and Fixed Oils 401, except that only stearic acid and palmitic acid are used.

Standard preparation— Prepare as directed for the Assay preparation using 100 mg of USP Myristic Acid RS instead of the substance to be examined.

Assay preparation— Proceed as directed for the Test Solution in Fatty Acid Composition under Fats and Fixed Oils 401.

Chromatographic system (see Chromatography 621)— Prepare as directed for Fatty Acid Composition under Fats and Fixed Oils 401. Chromatograph the Resolution solution, and record the peak responses as directed for Procedure: the resolution, R, between methyl stearate and methyl palmitate is not less than 1.5.

Procedure— Separately inject equal volumes (about 1 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and identify the methyl myristate peak in the chromatogram obtained from the Assay preparation by comparing the retention times of the peaks in that chromatogram with those in the chromatogram obtained from the Standard preparation. Measure the responses for all of the peaks in the chromatogram obtained from the Assay preparation, excluding the solvent peak, and calculate the percentage of C14H28O2 in the portion of Myristic Acid taken by the formula: in which A is the the methyl myristate peak response; and B is the sum of the responses of all the peaks in the chromatogram, except the solvent peak.