41 Drug Discovery, Pharmaceuticals & Cannabis Testing
The oxidative degradation of vitamin C is visually traceable; solutions of vitamin C gradually turn yellowish or even brownish, due to the formation of degradation products such as dehydroascorbic acid (DHA). As degradation progresses, the colour can intensify to orange or brown with the formation and further decomposition of diketogulonic acid. To monitor these colour changes, four different colour scales were compared (Figure 3). Throughout the accelerated studies, CIELAB L* decreased, while CIELAB b* and the Yellowness Index (YI) increased, indicating a darkening of the system. Meanwhile, CIELAB a* initially decreased, before increasing again, refl ecting a time-dependent degradation profi le. These fi ndings suggest that tracking multiple colour values within a specifi c colour system provides deeper insights into the characteristics of the measured sample. In this study, the YI proved to be the most sensitive indicator of degradation, showing the most pronounced colour deviations during the vitamin C degradation process.
Figure 5: Correlation between water concentration and YI of diluted vitamin C serum (n=3). The mean value ± standard deviation of values is displayed in the diagram.
Figure 3: Mean value ± standard deviation of colour-related quantities (CIELAB a*, CIELAB b*, CIELAB L*, and YI) over time as a result of the degradation of L-ascorbic acid in PBS (n=3). The bar colours serve to visualize the colour of the solution after 3, 7, and 10 days.
Evaluation of vitamin C serum dilution series
To assess the robustness of the described multiparameter measurement system, a dilution series of vitamin C serum was measured. The serum was diluted, with its most abundant component, water, serving as the solvent. Across the dilution series, density, refractive index, optical rotation, and colour-related quantities were measured and plotted as a function of added bi-distilled water (Figure 4-6).
To develop a quality control method, it is advisable to start with a series of dilutions to evaluate the goodness of fi t of various measurement techniques. In this evaluation, the YI demonstrated the highest correlation among colour-related quantities, making it the preferred choice for tracking sample dilution and colour changes.
Among all measured parameters, density results showed even higher goodness of fi t in response to sample dilution (Figure 4), surpassing YI (Figure 5). RI showed a weaker correlation with concentration changes, ranking below YI (Figure 6). OR demonstrated the lowest correlation, as the MCP’s resolution was close to the observed OR variations.
Thus, density, YI, and RI are the most reliable parameters for detecting minor variations in the production process of vitamin C-containing serums, ensuring robust process control. The multiparameter measurement system effectively detected slight concentration changes in this complex sample.
Figures 4-6 illustrate the correlation between added water (% v/v) and measured quantities. A summary of the measured data along the dilution series is presented in Table 1.
Figure 6: Correlation between water concentration and RI of diluted vitamin C serum (n=3). The mean value ± standard deviation of values is displayed in the diagram.
Table 1: Mean values ± standard deviations of RI, YI, and OR along the dilution series (n=3) Added water (% v/v) 0.00
0.085
Density (g/cm³) Refractive index Yellowness index Optical rotation
0.234
1.09605 ± 0.00001 1.09596 ± 0.00001 1.09581 ± 0.00003 1.38821 ± 0.00001 1.38819 ± 0.00001 1.38813 ± 0.00001 9.53 ± 0.02
9.37 ± 0.01 1.601 ± 0.001 1.601 ± 0.001 9.16 ± 0.01 1.599 ± 0.001
Figure 7: Mean value ± standard deviation of colour-related quantities (CIELAB a*, CIELAB b*, CIELAB L*, and YI over time as a result of the degradation of L-ascorbic acid in the vitamin C serum (n=3). The bar colours serve to visualise the colour of the solution after 3, 7, and 10 days.
Measurement of the vitamin C degradation
profi le in cosmetic serum Vitamin C in serum exhibited time-dependent degradation characteristics similar to those observed in PBS. The colour-related quantities (Figure 7) and OR (Figure 8) changed, with a signifi cantly slower extent in the serum indicating a slower degradation process than in the buffered vitamin C solution (Figure 2). This behaviour may be attributed to the presence of additional antioxidants in the serum, such as ferulic acid and vitamin E, which help stabilise vitamin C.
Figure 4: Correlation between water concentration and density of diluted vitamin C serum (n=3). The mean value ± standard deviation of values is displayed in the diagram.
Among the measured parameters, colour analysis demonstrated greater sensitivity to changes compared to polarimetry. However, the sensitivity of the polarimeter can be increased by using a more sensitive version of the module (MCP 150 instead of MCP 100), allowing for improved detection of subtle degradation-related variations.
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