NEAR IR SURFACTANT ANALYSIS 453 Table VI Some Characteristic Functional Group Frequencies in NIR (11) Aliphatic Hydrocarbons Aromatic Hydrocarbons Carboxylic Acids 1600-1800 nm 1100-1250 nm 2000-2400 nm 1300-1450 nm 1685 nm 1143 nm 2150, 2460 nm 1420-1450 nm 1450 nm 1900 nm 2100 nm Ammonium Ion 1540 nm 1900-2300 nm Water 1330-1550 nm 1850-2090 nm First overtone C-H stretch Second overtone C-H stretch Combination Combination First overtone C-H stretch Second overtone C-H stretch Combination Combination First overtone O-H stretch Second overtone C-O stretch Combination O-H stretch/bend First overtone N-H stretch Combination N-H stretch/bend First overtone O-H stretch Combination O-H stretch/bend and 6 are NIR spectra of reagent grade SLS, dried ALS raw material, and reagent grade benzoic acid. (The physical properties of viscosity and pH of themselves have no near infrared spectrum but are suspected of altering hydrogen bonding and thus effecting the water bands.) Characteristic functional group frequencies for these constituents are listed in Table ¾I. Based on the frequency ranges of Table ¾I and the near infrared spectrum of Figures 5 and 6, the absorption bands for ammonium lauryl sulfate, so- dium lauryl sulfate, and benzoic acid can be assigned and are listed in Table VII. t.aO0' t.000' .800' .600' .400' .200' I I I I I I I 1200 t400 1600 1800 2000 2200 2400 HAVELENGTH Figure 5. Nearin•ared reflectancespectra •rreagent grade SLS and dried ALS raw material. AL•q •L•q

454 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1. •oo, ß 800' • ß 500' • lEO0 1400 1600 t800 EOO0 P. EO0 E400 WAVELENGTH (nat) Figure 6. Near infrared reflectance spectrum of benzoic acid. A comparison of the wavelengths chosen by multiple linear regression analysis (Tables IV and V) for ALS and SLS and the wavelength assignments (Table VII) indicates that there is a pattern to wavelength selection. The wavelengths selected for active detergent Table VII Frequency Assignments for ALS, SLS, and Benzoic Acid ALS SLS Benzoic Acid 24O4 24O4 2352 2352 2312 2312 2272 2272 2144 2040 1920 1760 1760 1728 1728 1568 1412 1412 1208 1208 2140, 2160 2464 1664 1132 Combination aliphatic C-H Combination aromatic C-H Combination N-H Combination O-H First overtone aliphatic C-H First overtone aromatic C-H First overtone N-H Combination aliphatic C-H Second overtone aliphatic C-H Second overtone aromatic C-H