100(WU – WW) / WU

Modifier solution— Prepare a solution of magnesium nitrate in water containing about 200 mg per mL. Just before use, transfer 1.0 mL of this solution to a 10-mL volumetric flask, dilute with 5% nitric acid to volume, and mix.

Standard solutions— Transfer 10.0 mL of Lead Nitrate Stock Solution, prepared as directed under Heavy Metals 231, to a 100-mL volumetric flask, add 40 mL of water and 5 mL of nitric acid, dilute with water to volume, and mix. Transfer 1.0 mL of this solution to a second 100-mL volumetric flask, dilute with 5% nitric acid to volume, and mix. This solution contains 0.1 µg of lead per mL. Transfer portions of this solution to four suitable containers, and dilute quantitatively, and stepwise if necessary, with 5% nitric acid to obtain Standard solutions having lead concentrations of 100 ng per mL, 50 ng per mL, 25 ng per mL, and 10 ng per mL, respectively.

Test solution— [note—Perform this procedure in a fume hood.] Transfer about 1.2 g of Corn Syrup Solids, accurately weighed, to two digestion tubes labeled Test Solution and Temperature Monitor Solution, and add 0.75 mL of nitric acid to each tube. Warm both solutions slowly to between 90 and 95 to avoid spattering. Heat until all brown vapors have dissipated and any rust-colored tint is gone from the tube labeled Test Solution (20 to 30 minutes). Cool, add 0.5 mL of 50% hydrogen peroxide dropwise to both solutions, heat to between 90 and 95 for 5 minutes, and cool. Add a second 0.5-mL portion of 50% hydrogen peroxide dropwise to each solution, and heat to between 90 and 100 until clear (5 to 10 minutes). Cool and transfer the solution labeled Test Solution to a 10-mL volumetric flask. Rinse the Test solution digestion tube with 5% nitric acid, add the rinse to the volumetric flask, dilute with 5% nitric acid to volume, and mix.

Standard blank— Use 5% nitric acid.

Test blank— Transfer 1.5 g of water to a digestion tube, and proceed as directed for the Test solution, beginning with “add 0.75 mL of nitric acid”.

Procedure— [note—Use peak area measurements for all quantitations.] Add 5 µL of the Modifier solution to 20 µL each of the Standard solutions, the Test solution, the Standard blank, and the Test blank, and mix. Separately inject equal volumes (about 20 µL) of the Standard solutions, the Test solution, the Standard blank, and the Test blank into a suitable graphite furnace atomic absorption spectrophotometer equipped with pyrolytically coated graphite tubes and adequate means of background correction. The temperature is programmed as follows. Maintain the drying temperature of the furnace at 200 for 30 seconds after a 20-second ramp time using an argon gas flow of about 300 mL per minute; maintain the ashing temperature at 750 for 40 seconds after a 40-second ramp time using an airflow of about 300 mL per minute; cool down and purge the air from the furnace for 60 seconds using a 20 set temperature and an argon gas flow of about 300 mL per minute; and maintain the atomization temperature at 1800 for 10 seconds after a 0-second ramp time with the argon gas flow stopped. [note—The temperature program may be modified to obtain optimum furnace temperatures.] Using the Standard blank to set the instrument to zero, determine the integrated absorbances of the Standard solutions at the lead emission line at 283.3 nm. Plot the integrated absorbances of the Standard solutions versus their contents of lead, in ng per mL, and draw the line best fitting the four points to determine the calibration curve. Similarly determine the integrated absorbances of the Test solution and the Test blank at the lead emission line at 283.3 nm. Correct the absorbance value of the Test solution by subtracting from it the absorbance value obtained from the Test blank. Calculate the concentration, in µg per g, of lead in the portion of Corn Syrup Solids taken by the formula: in which C is the concentration, in ng per mL, of lead in the Test solution, as determined from the calibration curve; and W is the weight, in g, of Corn Syrup Solids taken to prepare the Test solution: the limit is 0.5 µg per g.

Apparatus— Mount a ring support on a ring stand 1 to 2 inches above a gas burner, and mount a second ring 6 to 7 inches above the first. Place 6-inch open-wire gauze on the lower ring to support a 250-mL conical flask, and place a 4-inch watch glass with a center hole on the upper ring to deflect heat. Attach a 25-mL buret to the ring stand so that the tip just passes through the watch glass centered above the flask. Place an indirectly lighted white surface behind the assembly for observing the endpoint.

Standard preparation— Dissolve an accurately weighed quantity of USP Dextrose RS in water, and dilute quantitatively with water to obtain a solution having a known concentration of about 6 mg per mL.

Assay preparation— Transfer about 4 g of Corn Syrup Solids, accurately weighed, to a 500-mL volumetric flask, dilute with water to volume, and mix.

Procedure— Transfer 25.0 mL portions of alkaline cupric tartrate TS to each of two flasks, and boil. Immediately place one flask on the wire gauze of the Apparatus, and adjust the burner so that the boiling point will be reached in about 2 minutes. Titrate with the Standard preparation to within 0.5 mL of the anticipated endpoint. Heat the flask, with swirling, boil moderately for 2 minutes, and add 2 drops of methylene blue solution (1 in 100). Immediately add about 2 drops of the Standard preparation from the buret, and bring to a boil. Allow the cuprous oxide to settle slightly, and observe the color of the supernatant. Complete the titration within 1 minute by adding the Standard preparation dropwise, and boiling after each addition to the disappearance of the blue color, as determined by viewing against a white background in daylight or under equivalent illumination. If more than 0.5 mL of the titrant is required after the addition of the indicator, repeat the titration, adding the necessary volume of titrant before adding the indicator. Bring the contents of the second flask to a boil, and similarly titrate with the Assay preparation. Calculate the percentage of reducing sugars as d-glucose, calculated on the dried basis, in the portion of Corn Syrup Solids taken by the formula: in which A is the percentage of dry solids in the Corn Syrup Solids as determined in the test for Total solids; CS is the concentration, in mg per mL, of USP Dextrose RS in the Standard preparation; CU is the concentration, in mg per mL, of Corn Syrup Solids taken to prepare the Assay preparation; and VS and VU are the titrated volumes, in mL, of the Standard preparation and the Assay preparation, respectively.