DIMETHICONE QUANTITATION USING FTIR 275 spectrum from the sample spectrum and plots the result. Solvent-subtracted spectra of the dimethicone samples were also plotted and their corresponding absorbances of interest determined. RESULTS AND DISCUSSION The infrared spectrum of dimethicone exhibits a characteristic doublet at 1097.6 and 1014.6 cm-1 due to stretching and bending of Si-O bonds in addition to a sharp band around 1260 cm-1 due to CH3-Si vibrations. The methylene chloride spectrum (Figure 1), which shows the solvent interference in these regions of interest, indicates that 4000.0 401•.0 •4OO.O •11•O.O Z•00.O 1BD0.O 1GIX].O l•OO.O ll]X].O OOO.OO 7120.O0 Figure 1. Methylene chloride spectrum. quantitative use of the 1260-cm-• band is precluded by the very strong solvent ab- sorption in that area. Even the use of solvent-subtraction is futile in that region since the solvent is, effectively, totally absorbing, an accurate subtraction is impossible. As was well noted in the work by Rihs (2), using the doublet bands for quantitative analysis affords the added advantage of determining whether interfering substances are also present. The absorption spectrum of a pure dimethicone sample should show a symmetrical doublet of essentially equal intensity. The USP dimethicone reference standard solution spectrum and the solvent-subtracted standard spectrum are given in Figures 2 and 3, respectively. Note the well resolved, almost symmetrical shape of the doublet and near equal absorption intensities in Figure 3. Next, a blank formulation (the product formulation prepared without the dimethicone) was analyzed and its spectrum plotted (Figure 4). The actual amount of interference from the blank matrix is shown by the solvent-subtracted blank spectrum (Figure 5).

276 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •400.0 •00.0 Z:i•00.0 1•00.0 1•=00.0 11•00.0 1000.O I•iO. O0 '700. O0 •0.00 400.00 Figure 2. USP dimethicone standard spectrum, 0.001 g ß ml- •' ß •oo.o :mm.o i,,m.o iem.o inm.o I m'v• o ß Figure 3. Solvent-subtracted USP dimethicone standard spectrum, 0.001 g ß ml -•