DIRECT ANALYSIS OF DIMETHICONE IN AQUEOUS EMULSIONS 213 for 1 h or until all the water evaporated. After cooling to room temperature, 5.0 mL of anhydrous ethanol was added and mixed well. The open jars were returned to the oven and heated again at 115°C to evaporate the ethanol and any remaining water by azeotropic evaporation. Following evaporation, the jars were cooled to room temperature and 25.00 mL of hexane was pipetted into each jar. The jars were then capped and sonicated for 1 min. After settling for 2 h, 15–20 mL of the upper solution was transferred to clean, capped vials and stored at room temperature until analyzed by FTIR in transmission mode. METHOD 2: FTIR-ATR Tw o different commerc ially available dimethicone emulsions were compared. Both DM 5102E and DM 5700E contained 50% dimethicone but different emulsifying surfactants. No differences were observed in the FTIR-ATR absorption spectra of these two different emulsions at any dilution. DM 5102E was used as the primary standard for calibration. After its concentration was verifi ed by Method 1 (transmission FTIR of the hexane ex- tract), portions of this DM 5102E emulsion were diluted by mixing with distilled water to prepare a series of secondary calibration standards in the range of 0–35.0% (g/100 g). These aqueous emulsion standards were analyzed directly by FTIR-ATR. Liquids from fi ve different retail skin-protection wipe products were squeezed from the wipes and ana- lyzed directly by FTIR-ATR without any further preparation. Dimethicone concentra- tions of each sample were calculated by comparing the integrated peak areas with the primary calibration standards. METHOD 3: FTIR-ATR ST ANDARD ADDITION Each of the retail sk in protection wipe products was analyzed again by Method 2 follow- ing standard addition. The primary standard was added to each sample, increasing its dimethicone concentration by 50%. After mixing to ensure homogeneity, each spiked sample was again analyzed directly by FTIR-ATR without any further preparation. The increased absorbance of each sample was used to calculate its original dimethicone con- centration, rather than the external calibration in Method 2. POTENTIAL INTERFERENC ES To examine potential matrix interference, a series of 3% dimethicone emulsions containing various surfactants and preservatives were prepared and analyzed. A 20% dimethicone emulsion was prepared by blending 200 g of pure dimethicone oil (Belsil® DM 350), 10 g of Tween 20, and 10 g of Tween 80. Then, 780 g of DI water was added slowly with vig- orous stirring at 80°C. The resulting mixture w as homogenized with a Silverson homogenizer for 10 min at medium speed. The resulting 20% stock solution was then mixed with deionized water and individual surfactants or preservatives at levels typically present in personal care products, such that the fi nal dimethicone of each sample was 3.00%. Triplicate samples of each surfactant and preservative were prepared and analyzed by all three methods.

JOURNAL OF COSMETIC SCIENCE 214 RESULTS Calibration of FTIR-ATR for aqueous emulsifi cations of dimethicone yielded outstand- ing linearity for a 12-level series of standards up to 35% (r2 = 0.9997) as shown in Figure 2. The excellent precision of the FTIR-ATR method was demonstrated by repeated anal- ysis of selected calibration standards, yielding relative standard deviations of less than 0.3% (Table II). Five different retail O TC wipe products with label claims in the range of 3–5% emulsi- fi ed dimethicone were analyzed. Analytical results from all three methods (Table III) compared favorably with label claims and with each other. The agreement between stan- dard addition and external calibration (Methods 2 and 3) for fi ve different wipe products is a strong indication that there is no matrix interference with this method. This was further verifi ed by the dimethicone test results in the presence of various surfactants and preservatives shown in Table IV. No statistically signifi cant interference was observed for any of the ingredients tested. CONCLUSIONS Direct test ing of dimet hicone in aqueous emulsions by FTIR-ATR is a viable method that has been ignored in the past. Dimethicone is a rare analyte that may be analyzed by FTIR-ATR in the presence of large quantities of water because of its unique absorption frequency at 1,260 cm-1. It may, in fact, be accurately measured across a wide range of Table II Replicate FTIR-ATR Assays of Standards Dimethicone conc. (%) Average (n = 6) RSD (%) 3.00 3.01% 0.27 12.00 12.00% 0.15 21.00 21.00% 0.18 30.00 30.03% 0.26 Table III Results of Three Analytical Methods—% Dimethicone (g/100 g) Wipe samples Method 1 (n = 6) Method 2 (n = 6) Method 3 (n = 6) Product Claim (%) Average (%) RSD (%) Average (%) RSD (%) Average (%) RSD (%) A 3.2 3.31 3.29 3.26 0.30 3.12 11.7 B 3.6 3.65 4.71 3.70 0.37 3.74 9.26 C 3.0 3.19 5.85 3.05 0.27 3.09 15.7 D 3.0 3.25 5.17 3.14 0.24 3.30 13.1 E 5.0 5.20 3.71 5.23 0.23 5.50 7.08 Six separate replicates of each product were tested by the three methods: Method 1: Extraction in hexane FTIR transmission . Method 2: Emulsion (no extraction), FTIR-ATR, external calibration . Method 3: Emulsion (no extraction), FTIR-ATR, standard addition .