Chapter 5 167 (CysMPA) preventing oxidation and allowing better quantification of the extent of reduction.41,42 Figure 8 shows the effect 1M ammonium thioglycolate (ATG) at pH 9.4 on single head virgin hair as a function of time. Over the first 10 minutes, CysMPA is decreasing as SCMC increases. In this experiment, the decrease of cystine was first order with an exponential half–life of 123 seconds. Amino acid analysis can also be used to determine the efficiency of disulfide bond reformation during the neutralization step. Gumprecht et al.17 reported 70–90% recovery of cystine by neutralization depending on conditions. Another approach to measuring reduction kinetics in hair is chemical stress relaxation. Chemical stress relaxation methods rely on the fact that disulfide bond reduction reduces the tensile strength of the fiber. Most commonly, a hair is stress relaxed in buffer at fixed extension until a constant level of force is reached. Addition of a reducing agent causes the stress supported by the hair to decrease as disulfide bonds are broken by reduction.29,43 In the early 80s this author introduced the term “single fiber tensile kinetics” (SFTK) to describe the chemical stress relaxation methods for study of Figure 8 Effect of reacting hair with 1M ATG, pH 9.4, 23 C on SCMC and CystMPA measured by amino acid analysis. Modified from Manuszak et al. with permission of the Society of Cosmetic Chemists.42