ANALYSIS OF PARABENS BY LIQUID CHROMATOGRAPHY 385 Table II Response Factor Reproducibility Methylparaben Propylparab en Day Response Factor Response Factor 1 0.911 1.06 3 0.910 1.08 8 0.908 1.06 11 0.889 1.08 58 O.94O 1.08 308 0.945 1.08 336 0.910 1.05 353 0.935 1.06 398 0.941 1.06 400 0.936 1.04 Average 0.923 _+_ 2%a 1.07 _+_ 2% a aRelative standard deviation. •han 3% Due to the possibility of hy, l:vlysis or solvolysis on standing, re- sponse factor solutions are prepared daily. Over a long period, the relative s•andard deviation for both m?thylparaben and propylparaben response fac- tors is approximately 3% (Table II). A potential source of error in the analysis is a failure to recover quantita- tivel,• the more lipophilie parabens beyond ethyl from the lipid phase after b•'eak n•, of the emuls on. Data in Table III indicate that the addition of at least 40 ml of ethanol to every gram of sample (containing ca. 10% lipid phase) insures quantitative recovery of all the parabens in the formulations stud ed. The average results of several analyses for 7 different batches of a cosmetic emulsion, along with relative standard deviations, are presented in Tables IV and V. Data in Tab!e IV reflect the gradual hydrolysis of the para- borts s nee most of these samples were over i year old. Values in Table V were obl-ained from suspension-type samples run within a few days of produe- tion, hence the agreement between the intended level of parabens and the level f•nnd is much better. Other ehromatographie columns such as Permaphase• ETH • have been "::ed successfully, and the method appears generally applicable for the assay of p2ra-substituted benzoate esters. Although manual area measurement teeh- n:c'ues have been employed, integrators or other data acquisition systems are applicable. *E. I. du Pont de Nemours & Co., Wilmington, Del.

386 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Recovery of Parabens From an Emulsion-Type Sample as a Function of Alcohol Concentration Per Cent Per Cent Volume 95% Ethanol Methylparaben Recovered Propylparaben Recovered 20.0 102.0% 85% 35.0 100.1% 93.5% 40.0 98.5% 101.8 ___ 1.2% 40.0 a -- 100.4 ___ 2.6% apropylparaben added as a solid to paraben-free product and allo•ved to dissolve. In all other cases the parabens •vere added as alcoholic solutions, maximum volume 5.0 mi. Table IV Analytical Results on Typical Emulsion Samples Per Cent Per Cent Number Sample Methylparaben Propylparaben of Trials 1 2 3 4 5 6 7 Intended Formula Amounts 0.163 __+ 3.0% S a 0.0,88% __+ 3.7% 4 0.167 __+ 1.7% 0.101 _+ 4.3% 5 0.146 ___ 1.4% 0.08'8 __+ 4.2% 4 0.148 __+ 1.7% 0.096 __+ 2.7% 3 0.186ñ 1% 0.105 ___ 1% 3 0.157 ___ 1% 0.093 __+ 6.3% 5 0.143 ___ 1.2% 0.665 b ___ 9% 5 0.170% 0.100% aS__--- 2 (x i _ i=l n--I bExcept sample seven, where proplyparaben equals 0.075%.