Silicon dioxide and nonsilicon dioxide residue on ignition—
Weigh accurately about 1 g into a previously ignited, tared 50-mL platinum crucible. Add 4 drops of sulfuric acid. Heat carefully on a hot plate until the specimen is thoroughly charred and fumes no longer are evolved. Ignite the crucible at 700 ± 25
(see
Residue on Ignition 
281
) to constant weight. Wet the residue carefully with 1 mL of water, and slowly add 20 drops of hydrofluoric acid.
[Caution—Hydrofluoric acid is an extremely hazardous chemical. When handling it wear a face shield, arm protection, and rubber gloves, and perform the operation in a hood.
] Evaporate slowly on a hot plate to dryness, then ignite at 700 ± 25
for 10 minutes, cool to room temperature in a desiccator, and weigh accurately. Repeat the addition of hydrofluoric acid, evaporation, and ignition, to constant weight. Calculate the percentage of silicon dioxide from the difference between the net weights before and after the hydrofluoric acid treatment: it is not more than 3.0%. Calculate the percentage of nonsilicon dioxide residue on ignition from the final net weight: it is not more than 2.0%.
Limit of free ethylene oxide—
Standard stock solution—
[Caution—Ethylene oxide is toxic and flammable. Prepare solutions of it in a well-ventilated fume hood.
] Using the special handling described in the following, complete the preparation. Ethylene oxide is a gas at room temperature. It is usually stored in a lecture-type gas cylinder or small metal pressure bomb. Chill the cylinder in a refrigerator before use. Transfer about 5 mL of the liquid ethylene oxide to a cold, 10-mL serum vial. Seal the vial, and store in a refrigerator. Transfer about 40 g of acetone, accurately weighed, to a tared 50-mL serum vial that is capable of being tightly sealed with a polytef-lined septum and a metallic crimp cap. Seal the vial, and accurately weigh it. Using a gas-tight gas chromatographic syringe that has been chilled in a refrigerator, transfer about 60 µL of the liquefied ethylene oxide to the same vial. Weigh the vial, and determine the amount added by weight difference. This Standard stock solution contains about 1 µg of ethylene oxide per µL. [NOTE—This solution may be kept for 1 week in the crimp-sealed serum vial, stored in a freezer.]
Standard preparations—
To four separate, tared 50-mL serum vials that are capable of being tightly sealed with polytef-lined septa and metallic crimp caps, transfer 1.0 g of the Polyethylene Oxide under test. Seal the vials. To the separate vials, transfer 2.0, 4.0, 6.0, and 8.0 µL of the
Standard stock solution, and mix. These vials contain about 2, 4, 6, and 8 µg of ethylene oxide, respectively, from the
Standard stock solution. Heat the vials at 100
for 30 minutes, and cool to room temperature.
Test preparation—
Transfer about 1 g of the Polyethylene Oxide under test, accurately weighed, to a tared 50-mL serum vial that is capable of being tightly sealed with a polytef-lined septum and a metallic crimp cap. Weigh the vial, and determine the amount of Polyethylene Oxide added by weight difference. Seal the vial, heat at 100
for 30 minutes, and cool to room temperature.
Chromatographic system (see Chromatography 621)—
The gas chromatograph is equipped with a flame-ionization detector, a 0.53-mm × 10-m capillary column bonded with a 20-µm layer of phase G45, and a split injection system. The carrier gas is helium, flowing at the rate of about 15 mL per minute. The makeup gas is also helium, with a split flow rate of about 15 mL per minute. The injection port and detector temperatures are maintained at about 200
and 250
, respectively. The column temperature is maintained at about 70
for 5 minutes after injection, then programmed to increase at a rate of 10
per minute to about 200
and to maintain this temperature for 5 minutes. Inject 300 µL of the gaseous headspace from the vial of
Standard preparation that contains about 6 µg of ethylene oxide from the
Standard stock solution into the gas chromatograph, and record the peak responses as directed for
Procedure: the relative standard deviation for replicate injections is not more than 5%.
Procedure—
[NOTE—A headspace apparatus that automatically transfers the measured amount of gaseous headspace may be used to perform the injections.
] Using a gas-tight syringe, separately inject equal volumes (about 300 µL) of the gaseous headspace of each of the
Standard preparations and the
Test preparation into the gas chromatograph, record the chromatograms, and measure the areas of the peak responses. Determine by a retention time comparison whether ethylene oxide is detected in the
Test preparation. Plot the responses of the
Standard preparations versus the content, in µg, of ethylene oxide in each vial, as furnished by the
Standard stock solution; draw the straight line best fitting the four points; and calculate the correlation coefficient for the line. A suitable system is one that yields a line having a correlation coefficient of not less than 0.99. Extrapolate the line until it intercepts the content axis on the negative side. From the intercept, determine the total amount,
TU, in µg, of ethylene oxide in the
Test preparation. Calculate the percentage of ethylene oxide in the portion of Polyethylene Oxide taken by the formula:
100(TU / W)
in which
W is the weight, in µg, of Polyethylene Oxide taken to prepare the
Test preparation: the limit is 0.001%.
