Cyclomethicone

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 DEFINITION

Cyclomethicone is a fully methylated cyclic siloxane containing repeating units of [−(CH₃)₂SiO−]ₙ, in which n is 4, 5, 6, or a mixture of them. It contains NLT 98.0% of (C₂H₆OSi)ₙ, calculated as the sum of cyclomethicone 4, cyclomethicone 5, and cyclomethicone 6, and NLT 95.0% and NMT 105.0% of the labeled amount of any one or more of the individual cyclomethicone components.

2 IDENTIFICATION

Change to read:

A. ▲Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197S▲ (CN 1-May-2020)

Sample: Use neat liquids.

Acceptance criteria: The IR absorption spectrum 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.

3 ASSAY

Procedure

Standard solution A: USP Cyclomethicone 4 RS (neat)

Standard solution B: USP Cyclomethicone 5 RS (neat)

Standard solution C: USP Cyclomethicone 6 RS (neat)

Sample solution: Cyclomethicone (neat)

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: GC

Detector: Flame ionization

Column: 0.32-mm × 60-m fused silica; coated with a 1.0-µm ￾lm of phase G1

Temperatures

Injection port: 250°

Detector: 300°

Column: See Table 1.

Carrier gas: Helium
Flow rate: 1 mL/min
Injection volume: 1 µL
Injection type: Split ratio 1:20

System suitability

Samples: Standard solution A, Standard solution B, and Standard solution C

[Note—See Table 2.]

Suitability requirements

Relative standard deviation:
NMT 2.0% for cyclomethicone 4, Standard solution A;
NMT 2.0% for cyclomethicone 5, Standard solution B;
NMT 2.0% for cyclomethicone 6, Standard solution C

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%.

Analysis

Samples: Standard solution A, Standard solution B, Standard solution C, and Sample solution

Calculate the percentage of cyclomethicone 4 (cyclomethicone 5 or cyclomethicone 6) in the portion of Cyclomethicone taken:

Result = (rᵢ / rₜ) × 100

rᵢ = peak response of cyclomethicone 4 (cyclomethicone 5 or cyclomethicone 6)
rₜ = sum of all the peak responses

Calculate the percentage purity by adding the percentages of cyclomethicone 4, cyclomethicone 5, and cyclomethicone 6.

Acceptance criteria

Sum of cyclomethicone 4, cyclomethicone 5, and cyclomethicone 6:
NLT 98.0% of (C₂H₆OSi)ₙ

Labeled amount:
95.0%–105.0% of the labeled amount of any one or more of the individual cyclomethicone components

4 IMPURITIES

Limit of Nonvolatile Residue

Sample: 2.0 g

Analysis: Transfer the Sample into an open, tared aluminum dish, and evaporate in a circulating air oven at 150° for 2 h. Allow to cool in a desiccator, and weigh.

Acceptance criteria: NMT 3.0 mg, corresponding to NMT 0.15% (w/w)

5 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers. Avoid exposure to excessive heat.

Labeling: Label it to state, as part of the o￾cial title, the n-value of the Cyclomethicone. Where it is a mixture of two or three such cyclic siloxanes, the label states the n-value and percentage of each in the mixture.

USP Reference Standards 〈11〉

USP Cyclomethicone 4 RS
USP Cyclomethicone 5 RS
USP Cyclomethicone 6 RS