Determination of cyclic volatile methylsiloxanes in personal care products by gas chromatography

Abstract

Objective

Organosiloxanes are prevalent in personal care products (PCPs) due to the desired properties they impart in the usage and application of such products. However, the European Chemical Agency (ECHA) has recently published restriction proposals on the amount of two cyclic siloxanes, octamethylcyclotetrasiloxane (D4) and decamethylcyclotetrasiloxane (D5) allowed in wash off products such as shampoos and conditioners which are discharged down the drain during consumer use. This legislation will require that reliable analytical methods are available for manufacturers and government agencies to use in documenting compliance with the restrictions. This paper proposes a simple analytical method to enable accurate measurement of these compounds down to the circa 0.1 weight percent level in PCPs.

Methods

While gas chromatography methods are reported in the literature for quantitation of D4 and D5 in several matrices including PCPs, the potential for generation of false positives due to contamination, co-elution, and in-situ generation of cyclic volatile methylsiloxanes (cVMS) is always present and needs to be controlled. This report demonstrates the applicability of using a combination of emulsion break, liquid-liquid extraction and silylation sample preparation followed by GC-FID analysis as a suitable means of analyzing PCPs for specific cVMS.

Results

The reliability and limitations of such methodology was demonstrated through several round robin studies conducted in the laboratories of a consortium of silicone manufacturers. In addition, this report presents examples of false positives encountered during development of the method and presents a comparative analysis between this method and a published QuEChERS sample preparation procedure to illustrate the potential for generation of false positives when an inappropriate approach is applied to determination of cVMS in personal care products.

Conclusion

This report demonstrates that an approach to determine cVMS levels in personal care products is to perform an emulsion break on the sample, isolate the nonpolar phase from the emulsion break and treat with a silylation reagent to abate potential in situ formation of cyclics during the course of GC-FID analysis. Round robin studies conducted in laboratories representing multiple siloxane manufacturers demonstrated the reliability of the GC-FID method when measuring cVMS in PCPs down to circa 0.1%.

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