METHOD FOR SILICONES ON HAIR 387 recovered from the sample. The total weight of silicon, divided by the hair sample weight, gave the concentration of silicon on the hair. The quantity of siloxane polymer treatment on the hair was determined by multiplying the silicon value by the proper gravimetric factor for the specific polymer. A carefully aligned and adjusted atomic absorption spectrometer typically records an absorbance of 0.050 absorbance units for 10 •xg of silicon per milliliter (10 ppm) in wet MIBK. For AAS, sensitivity is generally defined as that quantity of analyte per milli- liter which will produce an absorption signal of one percent, or an absorbance of 0.0044 absorbance units. The sensitivity for organosilicon in wet MIBK is 0.88 •xg of silicone per milliliter. The instrument should be expected to reproduce an absorbance measure- ment of 0.050 within _+ 0.002 absorbance units. The absolute detection limit for silicone for these experiments was 0.002 absorbance units. An absorbance of 0.002 units may not be readily discernible from the normal background noise with instru- ments equipped only with light-emitting diode array (LED) readouts, but may be seen on a stripchart recorder. The detection limit for silicon, based on a total solvent volume of 7 ml, is about 3 •xg, with a precision of -+ 3 }xg. If the spectrometer features scale expansion capability, moderate scale expansion of 2 X or 3 X may provide some sensi- tivity improvement. Scale expansion is an aid when one is attempting to discern small differences in absorbance signals between samples. RESULTS AND DISCUSSION METHOD SELECTION Siloxanes, when placed in contact with porous surfaces such as hair, tend to be adsorbed into the substrate and can only be partially extracted with organic solvents. Treated hair must be decomposed prior to extraction. Several reagents were tried for preparing treated hair samples for AAS assay, including caustic (KOH and NaOH), phosphoric acid, and tetramethylguanidine. While the hair protein structure could be decomposed with sodium or potassium hydroxides, those materials were not selected, as they caused cleavage of the organosiloxane polymer into cyclics, silanols, and silicates. Those reac- tion products were either volatile or non-extractable in organic solvents. Phosphoric acid and tetramethylguanidine did not sufficiently decompose the hair at room temper- ature, even after several weeks, and when heated also caused siloxane polymer degrada- tion and generation of non-extractable species. As an alternative, an enzyme digestion method was discovered in a literature search (2) that employed papain (an enzyme found in papaya). This enzyme is a proteolytic en- zyme that is active with free sulfhydryl groups. An aqueous solution containing 0.13% papain and 2% sodium sulfite as a catalyst (with the pH adjusted to 6.8) was heated with a hair sample at 65øC for three days. Samples prepared by this method were further extracted with methylisobutyl ketone (MIBK) and a small quantity of hydro- chloric acid before AAS analysis. The HCI served to precipitate otherwise soluble pro- teins and aided in phase separation later in the procedure. The extraction was completed by shaking the vial and contents on a Burrell mechanical shaker, and completing phase separation in a centrifuge. The top (solvent) layer was drawn off and the silicon content was assayed by atomic absorption spectroscopy utilizing a nitrous oxide-acetylene flame.

388 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS METHOD ACCURACY DETERMINATION The silicon content of organosilicon materials was determined by AAS after dilution of the siloxane of interest in an organic solvent. However, the silicon absorption signal, or signal intensity, can vary, depending on the chemical structure of the siloxane. Calibra- tion error is minimized if a siloxane of known purity and having the same chemical structure and vapor pressure as the material to be assayed is found from which to pre- pare calibration standards. In actual practice, it was more convenient to compare the signal intensity of the compounds of interest to calibrating solutions prepared from known purity standard materials and then to decide if the signal response differences between the materials introduced an acceptable level of error. If the error was only a percent or so relative to the actual silicon content of the material, that error is negli- gible when measuring silicon levels of a few parts per million. Normal instrumental signal-to-noise ratios will impart far greater measurement error, especially with readings close to the detection limit. Standard deviation of the measurement is given in all of the data tables. To test the accuracy of the method for detecting the proper ratio of Si polymer emul- sion, several samples were assayed. Table I contains the results from samples of three different aminofunctional siloxane emulsions: amodimethicone and two trimethylsilyl- amodimethicone (TSA) polymers [see Figure 1] with different polymer chain lengths and percent amine functionality. The assayed Si values are within about 10% of the calculated values. The calibrating standard used in the previous example was a well-characterized di- methyl fluid with a viscosity of 1000 centistokes. To check the correlation of various neat organofunctional siloxane fluids with this standard, several samples of different functionality siloxane fluids were analyzed. Those data are reported in Table II. The Si assay of these siloxane polymers matched very closely to the calculated % Si values also, except for one polymer (siloxane E) which contained a very long dimethyl siloxane chain, and which because of its high molecular weight is less soluble in the organic solvent. As a result of these tests, it was decided to use the 1000-centistoke dimethyl fluid as the primary standard for all of the future silicon determinations. PRECISION OF METHOD The method was tested for reproducibility (precision) by analyzing duplicate samples Table I Quantitation of % Si in Aminofunctional Siloxane Fluids Calculated Calculated Calculated Sample emulsion molecular wt % Si % Si in % Si (35% silicone solids) of polymer in polymer emulsion by AAS (+ SD)* A TSA, x = 96, y = 2 7624 36.7 12.9 12.4 _ .2 B Amodimethicone 38038 36.8 12.9 12.1 --- .2 C TSA, x = 45.8 3939 35.5 12.8 11.5 --- .2 y= 2.2 * Means represent averages of two determinations, and standard deviations (SD) represent only instru- mental variation.