## 3 Results and Discussion
### Molecular and Structural Modifications Identified by FTIR Spectroscopy
#### 3.3.1 Analysis of Cysteine Oxidation and Disulfide Bonds (~1034 cm\({}^{-1}\) Region)  
A critical indicator of oxidative hair damage is the spectral region near 1034 cm\({}^{-1}\), which corresponds to the S-O stretching vibration of cysteic acid--a product of irreversible cysteine disulfide bond oxidation [25]. The behavior of this peak varied sig \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Wavenumber (cm\({}^{-1}\))** & **Assignment** & **Component** & **Reference** \\ \hline
3279 & N–H/O–H stretching & & Keratin/bound water & [25] \\  & (Amide A) & & \\
3069 & Amide B (N–H & & \\  & stretching overone) & & Keratin & [25] \\
2964/2923/2858 & Aliphatic C–H stretches (CH\({}_{3}\) & & \\  & and CH\({}_{2}\)) & Lipids/proteins & [25] \\
1734 & C=O stretching & & \\  & (ester carbonyl) & Lipids & [26] \\
1645 & Amide I (C=O stretching) & Keratin backbone & [25] \\
1538 & Amide II (N–H bending, & Keratin & [25] \\ C-N stretching) & & & \\
1458 & C–H bending (CH\({}_{2}\)/CH\({}_{3}\)) & Lipids/proteins & [25] \\
1403 & C–H bending and amino & Proteins (alkyl side-group deformation & [25; 27] \\  & side-group deformation & chains, amino groups) & \\  & Amide III (C–N stretch, & & \\  & N–H bending) & & \\
1239 & Amide I (C–N stretch, & & \\  & N–H bending) & & \\
1170 & C–O stretching/ring & Lipids/carbohydrates & [28] \\  & (ester/carbohydrate groups) & & \\
1079 & C–O stretching/ring & Carbohydrates/proline & \\  & vibrations (e.g., proline) & in keratin & \\
1034 & S–O stretching (cysteic acid) & Oxidized keratin & \\  & (cysteine) & & \\
589 & S–S disulfide stretching & Keratin disulfide & \\  & (lower-frequency mode) & bonds & [29] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Main peaks identified in FTIR spectrum of hair. Figure 5: Fourier-transform infrared (FTIR) spectra of hair samples before and after treatment with nitrate-rich plasma-activated water (PAW-N) and peroxide-rich plasma-activated water (PAW-P) for 1 h, followed by drying. **(a)** Sample 1 (wavy hair, Type 2); **(b)** Sample 2 (straight hair, Type 1); **(c)** Sample 3 (straight hair, Type 1); and **(d)** Sample 4 (cily/kinky hair, Type 4). The spectra show transmittance as a function of wavenumber, highlighting changes in key functional groups, including the cortex (3400–3300 cm\({}^{-1}\)), medulla (2925 and 2850 cm\({}^{-1}\)), and cysteine (1040 cm\({}^{-1}\)), reflecting structural modifications in the hair matrix induced by the distinct chemical environments of PAW-N and PAW-P.  
nificantly depending on the PAW formulation and hair type, reflecting different levels of structural degradation. After exposure to PAW-N, a marked decrease in the 1034 cm\({}^{-1}\) band was observed for the wavy hair (Sample 1) and one of the straight hair samples (Sample 2), indicating cleavage of disulfide bonds and possible leaching of oxidized sulfur compounds from the keratin matrix. This spectroscopic trend aligns with the EDS results, which show a reduction in sulfur content from 3.5% to 2.8% in Sample 1, and from 4.4% to 4.1% in Sample 2 after PAW-N treatment. Such elemental losses confirm sulfur oxidation and partial removal, corroborating the SEM images that revealed cuticle erosion and fragmentation in both samples. Conversely, Sample 3 (straight hair) and Sample 4 (couily/kinky hair) showed an increase in the 1034 cm\({}^{-1}\) peak after PAW-N, suggesting localized oxidation and retention of cysteic acid. This interpretation is supported by the EDS profile of Sample 4, which maintained a high sulfur level (5.2%) after PAW-N, indicating accumulation of oxidized sulfur species rather than their loss. SEM observations further support this scenario, showing surface roughening without extensive structural damage. For all samples treated with PAW-P, a consistent and moderate increase in the 1034 cm\({}^{-1}\) peak was observed, suggesting controlled oxidation of disulfide bonds.