# Detection of Azo Dyes in Curry Powder Using a 1064-nm Dispersive Point-Scan Raman System
## 3 Results and Discussion  
### Spectral Characteristics of Samples  
The chemical structures of synthetic and natural yellow compounds are dissimilar and thus the vibrational modes resulting from their chemical structures will be different. Figure 2 shows the Raman spectra of metanil yellow, Sudan-I, and curry powder in the range of 400 cm\({}^{-1}\) to 1800 cm\({}^{-1}\) and Table 1 shows the assignment of Raman spectral bands. Figure 3 shows the chemical structure of metanil yellow, and Sudan-I. Metanil yellow (Figure 3a) has three unidentical aromatic rings with three nitrogen atoms (N=N and -NH) and one sulphate group (SO\({}_{3}\)\({}^{-}\)). No methylene (CH\({}_{2}\)), methyl (CH\({}_{3}\)) sites, and no organic acid (-COOH), ketone (O=C), or hydroxyl (-OH) moieties are present in the molecule. Ring I is monosubstituted, Ring II is di (1,4-)-substituted, and Ring III is di(1,3-)-substituted. Both metanil yellow (Figure 3a) and Sudan-I (Figure 3b) contain the same N=N molecular moiety and include seven conjugated C=C bonds in series which explains their similar yellow color. The three aromatic C=C double bonds in ring I in metanil yellow are conjugated only with each other and thus do not shift its visible spectrum from yellow. The two additional double bonds in the naphthene group in Sudan-I are essentially in parallel with the three conjugated ring sites and therefore also do not shift its wavelength from yellow. Curry containing turmeric has a chemical structure that includes only five conjugated double bonds and a keto-enol moiety; its color is also yellow due the added conjugation from phenolic and methoxy sites on its ring sites. Thus, the overlapping spectral bands in the visible \begin{table}
\begin{tabular}{c c c c} \hline
**Sudan-I** & **Metanil Yellow** & **Curry Powder** & **Spectral Assignments** \\ \hline \multirow{6}{*}{1593 cm\({}^{-1}\) (s)} & 1601 cm\({}^{-1}\) & \(\
u\) (C=C) stretching unconjugated & \\  & & 1599 cm\({}^{-1}\) & \(\
u\) (N-N) stretching (III) \\  & & 1599 cm\({}^{-1}\) & \(\
u\) (C=C) stretching conjugated \\  & & 1522 cm\({}^{-1}\) & \(\delta\) (Ar-O + Ar-O-R) bending \\  & & \(\
u\) (C-C) stretching on C=C-N and C-C-N \\  & & 1441 cm\({}^{-1}\) & \(\delta\) (H-C) out of plane bending \\  & & & \(\
u\) (N=N) \\
1387 cm\({}^{-1}\) & & & Aromatic ring stretching \\
1340 cm\({}^{-1}\) & & & Aromatic ring stretching _asym_ \\
1254 cm\({}^{-1}\) & & & \(\
u\) (C-O) stretching \\
1223 cm\({}^{-1}\) & & & \(\delta\) (O-H) bending _asym_ \\  & & 1188 cm\({}^{-1}\) & \(\delta\) (C-H) \\
1169 cm\({}^{-1}\) & & & \(\
u\) (C-Nazo) \(\delta\) (CH) \\  & & 1147 cm\({}^{-1}\) (s) & \(\
u\) (C-Nazo) \(\delta\)(CH) \\  & & & Ring breathing \\
1001 cm\({}^{-1}\) & & & Ring breathing (II) \\  & & & \(\delta\) (C-N=N) out of plane bending \\ \hline \end{tabular}
\end{table} 
Table 1: Assignment of Raman spectral bands of Sudan-I, metanil yellow, and curry powder [35; 36; 37; 38; 39; 40; 41]. Figure 2: Raman spectra of metanil yellow, Sudan-I, and curry powder. wavelength range cannot be used to differentiate between the natural color in spices and synthetic dyes, nor even to differentiate between the synthetic dye structures. Although different natural and synthetic dyes each have a different individual absorptivity coefficient and an individual visible spectral fingerprint, in practice deconstructing the mixture into constituent components accurately is not possible without a priori knowing precisely which chemicals and which structural analogs in the mixtures will contribute to the visible yellow spectral signal. Finally, since the visible spectra of many natural and synthetic dyes are routinely pH dependent, unless each and every reference spectra of all the compounds in a mixture and the sample are collected at the same pH, any calculations based upon the reference standard data will be uncertain and/or invalid. The spectrum range of 400 cm\({}^{-1}\) to 1800 cm\({}^{-1}\) contains vibrational modes that discern the structural components of the dyes of interest.