# Detection of Azo Dyes in Curry Powder Using a 1064-nm Dispersive Point-Scan Raman System
## 3 Results and Discussion
### Detection of Sudan-I in Curry Powder  
Figure 4a shows the raw hyperspectral Raman images of curry powder-Sudan I mixed samples across five concentration levels at 1593 cm\({}^{-1}\). At 1593 cm\({}^{-1}\), peak intensity of Sudan-I is higher than the peak intensity of curry powder. This information was used to create binary detection image for identification of Sudan-I pixels. An initial spectral intensity threshold value was set and all pixels with a spectral intensity below the threshold value were converted to background pixels (curry powder). All the pixels above the set threshold value were converted to white pixels representing Sudan-I. For a higher-intensity threshold value, some of the Sudan-I pixels had a spectral intensity lower than the threshold value. These pixels were also converted into background pixels (false negative). For a significantly lower-intensity threshold value, some of the curry powder pixels were misinterpreted as the Sudan-I pixels (false positive). To avoid the false-positive and false-negative cases, pixel-by-pixel evaluation of spectral pixels was performed. Each pixel was evaluated with its corresponding spectra to select the appropriate intensity threshold value. Based on the pixel-by-pixel evaluation, a final threshold value of 550 was set to obtain a binary detection image. All the pixels with intensities below 550 were converted to background pixels. The remaining pixels represent the Sudan-I pixels. Figure 4b shows the 1593 cm\({}^{-1}\) binary detection images of samples at five concentrations. The white pixels are scattered throughout the sample surface. The number of white pixels is low in the 1% Sudan-I image, and progressively increases in the 3%, 5%, 7%, and 10% images. A gradual increase in the white pixels from 1% to 10% indicate more Sudan-I particles were detected at increasing concentration. A total of 63, 153, 236, 344, and 515 Sudan-I pixels were detected in the 1593 cm\({}^{-1}\) binary detection images of 1%, 3%, 5%, 7%, and 10% concentration samples.  
Several lumped pixels can also be observed in Figure 4b. The lumped pixels increased from a low frequency occurrence at 1% concentration to a higher frequency occurrence at 10% concentration. This increase was because the sample surface area and sample volume were held constant (25 mm \(\times\) 25 mm \(\times\) 1 mm) for all samples, while the sample concentration increased from 1% to 10%, resulting in overlapping of Sudan-I particles at different layers within the sample depth. Figure 5 shows the Raman spectra of detected Sudan-I pixels at 1%, 3%, 5%, 7%, and 10% concentration samples. In the figure, the number of spectra of each sample corresponds to the total number of detected Sudan-I pixels in the sample.  
Figure 4: (**a**) Hyperspectral Raman images of curry powder-Sudan I mixture samples; (**b**) binary detection images at 1593 cm\({}^{-1}\).  
The progressive increase in the detected Sudan-I pixels (Figure 4b) indicates that the percentage of detected pixels and sample concentration are correlated. The 515 pixels detected in 10% sample corresponds to 5.15% of total acquired pixels. Similarly, the 63 pixels detected in 1% sample corresponds to 0.63% of total acquired pixels. The percentage of the detected pixels are linearly correlated with the Sudan-I concentration in the samples with a correlation coefficient of 0.99 (Figure 6).