Analysis of Siloxane Content in Commercial Shampoos: An Undergraduate qNMR Laboratory Experiment

Nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for the structural determination of organic compounds. Every student majoring in chemistry should be introduced to NMR starting in the organic chemistry sequence and additionally in other upper division courses and laboratories (such as instrumental chemistry, analytical chemistry, and physical chemistry). However, many undergraduate curricula often lack appropriate experiments needed to expose students to techniques such as quantitative NMR (qNMR), which is common in industrial and medicinal chemistry. In this experiment, we use quantitative NMR spectroscopy to determine polydimethylsiloxane (PDMS) content in a commercial shampoo (though this experiment could be employed on almost any commercial material containing siloxanes). Students at Berea College enrolled in Advanced Laboratory classes (junior and senior level, CHM 371 and CHM 470) analyzed samples from a commercial shampoo during a two-week period (two 4 h class sessions). During Fall 2024 and Spring 2025, students analyzed samples of two shampoos (shampoo A and shampoo B) with an unknown amount of PDMS (hypothesized to be between 0.0 and 2.0 weight%) using dimethyl maleate (DMM) as an internal standard. Student analysis finds the average weight% PDMS content of shampoo A to be 0.492% with a standard deviation of 0.051% and shampoo B to be 0.409% with standard deviation of 0.017%. This experiment allows students to analyze and interpret 1H NMR spectra to determine the PDMS content of a shampoo, gain hands-on experience with NMR spectroscopy, and calculate error and error propagation in a real-world example.