Quantitative Determination of Isostearic Acid Isomers in Skin Creams by GC-MS-SIM

“Isostearic acid” is frequently listed as an ingredient of skin creams and other cosmetics. In the four skin creams analyzed, “isostearic acid” was esterified with isopropanol, as well as sorbitan or polyglycerols. “Isopropyl isostearate” was isolated by HCl treatment and saponification whereas emulsifiers (sorbitan or polyglycerol isostearates) were enriched by means of a C18-cartridge. Fatty acids in the resulting lipid fraction were transferred into methyl esters. 25:0 and 19:0 methyl esters were used as internal standards. GC-EI-MS was used to determine that “isostearic acid” was a mixture of many methyl-branched isomers of stearic acid (18:0) in all four skin creams. Thus, it may be better termed “isostearic acids”. The branched-chain nature of isostearates was verified by formation and analysis of picolinyl esters of skin cream fatty acids by GC-EI-MS. Twenty-five 18:0 isomers were detected and the main products had one methyl branch on carbons C10–C14. Two late eluting isostearic acid isomers were identified as 16-methyl heptadecanoic acid (i18:0) and 15-methyl heptadecanoic acid (a18:0). GC-EI-MS in the selected ion monitoring (SIM) mode with m/z 87 as quantification ion was used for the determination of i18:0 methyl ester. The quantities of i18:0 in the samples amounted to 10–20 mg g−1 skin cream. The contribution of i18:0 to the sum of all 18:0 isomers in the four skin cream samples was 8.5 ± 1.1%. Instead of determining all individual isostearates in a product, we suggest the quantitative determination of i18:0 followed by multiplication with factor 11.75 (~reciprocal value of 8.5%, see above) as a simple method for the quantification of isostearates in cosmetics.