(sye'' kloe meth' i kone).
» Cyclomethicone is a fully methylated cyclic siloxane containing repeating units of the formula:
in which n is 4, 5, or 6, or a mixture of them. It contains not less than 98.0 percent of (C2H6OSi)n, calculated as the sum of cyclomethicone 4, cyclomethicone 5, and cyclomethicone 6, and not less than 95.0 percent and not more than 105.0 percent of the labeled amount of any one or more of the individual cyclomethicone components.
Packaging and storage Preserve in tight containers.
Labeling Label it to state, as part of the official title, the n-value of the Cyclomethicone. Where it is a mixture of 2 or 3 such cyclic siloxanes, the label states the n-value and percentage of each in the mixture.
USP Reference standards 11
Identification Proceed as directed under 197S, except to use neat liquids. The IR absorption spectrum, determined in a 0.1-mm cell, exhibits maxima only at the same wavelengths as that of a similar preparation of USP Cyclomethicone 4 RS, USP Cyclomethicone 5 RS, or USP Cyclomethicone 6 RS.
Limit of nonvolatile residue Evaporate 2.0 g in an open, tared aluminum dish in a circulating air oven at 150 for 2 hours, allow to cool in a desiccator, and weigh: the weight of the residue so obtained does not exceed 3.0 mg, corresponding to not more than 0.15% (w/w).
Assay The gas chromatograph is equipped with a thermal conductivity detector and a suitable recorder, and contains a 3.66-m × 3-mm column packed with 20% liquid phase G1 on 60- to 80-mesh packing S1A (see Gas Chromatography under Chromatography 621). The column is temperature-programmed at a rate of about 8 per minute from 125 to 320, the injection port is maintained at a temperature of about 300, and the detector block is maintained at a temperature of about 350. Helium is used as the carrier gas, flowing at a rate of about 20 mL per minute. Separately inject about 1 µL of USP Cyclomethicone 4 RS, USP Cyclomethicone 5 RS, and USP Cyclomethicone 6 RS into the gas chromatograph, record the chromatograms, and note the retention times of the peaks. Similarly inject about 1 µL of Cyclomethicone, record the chromatogram, and measure the responses of the major peaks. Calculate the percentage of cyclomethicone 4, cyclomethicone 5, and cyclomethicone 6 by dividing 100 times the response of each peak at the retention time of the corresponding reference standard by the sum of all of the responses in the chromatogram. The percentages obtained from duplicate injections agree to within 1.0%. Calculate the percentage purity by adding the percentages of cyclomethicone 4, cyclomethicone 5, and cyclomethicone 6.
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USP35NF30 Page 1778Pharmacopeial Forum: Volume No. 31(4) Page 1140