Quality control and stability monitoring of Vitamin C-containing products using a multiparameter measurement system

Quality control and data integrity are crucial throughout the production process in the pharmaceutical and cosmetic industries. Multiparameter measurement systems, and integration with Anton Paar’s lab execution system, AP Connect, offer a robust solution for monitoring concentration changes due to production variations or material degradation.

Vitamin C, scientifically known as L-ascorbic acid, is widely utilised in the pharmaceutical and cosmetic industries due to its antioxidant, immune-boosting, and skin-rejuvenating properties. In pharmaceuticals, vitamin C is a key component in nutritional supplements, intravenous therapies, and dermatological treatments, aiding in immune support, wound healing, and collagen synthesis.
In cosmetics, it is a key active ingredient due to its multifaceted benefits for skin health. It neutralises free radicals, which would otherwise damage cells and plays a crucial role in the synthesis of collagen, a protein that is vital for the maintenance of skin elasticity and firmness. Research shows that vitamin C serums can significantly reduce the appearance of wrinkles, improve skin texture, and enhance the skin’s natural healing process. These serums also have the ability to brighten the skin, making them effective in treating hyperpigmentation and age spots [1, 2]. Consequently, vitamin C is widely incorporated into various skin care products like serums, creams, and masks, formulations that deliver active ingredients deeply into the skin [3].
However, its high susceptibility to oxidation and degradation presents challenges in formulation stability and product efficacy [4]. Ensuring rigorous quality control and stability monitoring is essential to maintaining potency and effectiveness throughout production and shelf life.
This article explores the importance of stability assessment and advanced quality control methods for vitamin C-containing products, highlighting the capabilities of Anton Paar’s multiparameter measurement systems (Figure 1), particularly when dealing with unstable active ingredients. When combined with AP Connect, Anton Paar’s lab execution system, data management is streamlined, ensuring precise and reliable analysis as well as secure, traceable, and efficient handling of measurement data across the pharmaceutical and cosmetic industries.
Figure 1: A multiparameter measurement system with a density meter, multi-sample changer, polarimeter, refractometer, and colorimeter.

Design and operation of a configurable multiparameter measurement system

This section highlights the advantages of the modular, configurable multiparameter measurement system, designed for seamless adaption to various applications. In the described study the following samples were investigated:
• A freshly prepared, 12 % m/V L-ascorbic acid in a phosphate buffer solution (PBS; pH=6.2).
• A commercially available L-ascorbic acid-containing serum and its dilution series with bi-distilled water.
• L-ascorbic acid in both buffer and serum at 40 °C under ambient humidity conditions and a measurement period of 10 days in an impermeable container to carry out an accelerated stability study in accordance with ICH Q1 A (R2) guideline [5].
For these samples, the measurement system (Figure 1) with up to seven combinable modules was equipped with a five-digit density meter (DMA), a multi-sample changer (Xsample 530) with a 35 mL x 20 mL magazine and a sample recovery unit (SRU), a polarimeter (MCP), a refractometer (Abbemat), and a colorimeter (Lovibond PFXi-Series). Optional modules for high-precision viscosity or turbidity measurements are also available.
This system was filled automatically directly from the sample vial. A completed filling triggered the measurement start in consecutive modules by determining, in parallel, density, optical rotation (OR), refractive index (RI), and multiple colour-related quantities. The SRU accessory was used to retrieve the sample, an important feature for valuable samples or when additional measurements of the same sample are required. After each sample, thorough cleaning of the hoses and modules was performed to prevent sample carryover and cross-contamination. A comprehensive overview of the device parameters and measurement settings is available in the corresponding application report [6].
To ensure fully automated data handling, the measurement system was connected to AP Connect, which serves as a centralised platform for collecting, processing, and managing all measurement data.

Measurement of the vitamin C degradation profile in PBS

The degradation of vitamin C dissolved in PBS was analysed in a time series of 10 days within an accelerated stability study. To monitor the degradation, density, refractive index, optical rotation, and colour-related quantities were measured. The measurements showed that density and refractive index were not robust enough to monitor the degradation process, whereas significant changes were observed in optical rotation and colour-related quantities. The optical rotation decreased at an accelerating rate indicating a decreasing concentration of L-ascorbic acid (Figure 2).
Figure 2: Mean value ± standard deviation of the optical rotation over time as a result of L-ascorbic acid degradation in PBS (n=3).
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, reflecting a time-dependent degradation profile. These findings suggest that tracking multiple colour values within a specific 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 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 fit 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 fit 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 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.
 
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 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    0.234
Density (g/cm³)    1.09605 ± 0.00001    1.09596 ± 0.00001    1.09581 ± 0.00003
Refractive index    1.38821 ± 0.00001    1.38819 ± 0.00001    1.38813 ± 0.00001
Yellowness index    9.53 ± 0.02    9.37 ± 0.01    9.16 ± 0.01
Optical rotation    1.601 ± 0.001    1.601 ± 0.001    1.599 ± 0.001

Measurement of the vitamin C degradation profile 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 significantly 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.
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.
The measured serum exhibited a slight yellow tint in its initial state. Despite the yellow matrix and presence of additional antioxidants, significant colour changes were detected, allowing for a clear observation of degradation at each measurement point (Figure 7). This demonstrated the ability to effectively track vitamin C degradation over time, even in a complex serum matrix.

Figure 8: Mean value ± standard deviation of OR over time as a result of the L-ascorbic acid degradation in the vitamin C serum (n=3).
Once a target range for the measured parameters is established, the multiparameter measurement system can effectively detect potential degradation. By determining the correlation between the measured quantities and the extent of degradation, the level of degradation can also be monitored.

Summary

The multiparameter measurement system provides an automated and highly precise approach to quality control for multicomponent samples, leveraging interconnected high-precision measurement modules. By analysing samples of known good quality, users can define acceptance ranges for each quality attribute, ensuring consistency and reliability. The selection of modules depends on the specific characteristics of the sample being analysed.
The DMA and Abbemat modules are essential for monitoring composition changes, as both density and refractive index (RI) are composition-dependent. The MCP module is specifically designed for measuring optically active substances, while a colorimeter is used for colourful compounds or when coloured degradation products may be present. Additionally, the Xsample 530 automates the filling and cleaning process, ensuring independent, consistent, and fully automated measurements without manual intervention.
To complement this high-precision measurement setup, AP Connect provides automated data management, ensuring all results are securely stored, easily accessible, and traceable. By eliminating manual transcription and enabling seamless data integration, AP Connect enhances efficiency, compliance, and data integrity across laboratory workflows.
Figure 9 summarises the range of key parameters the selected multiparameter measurement system can monitor, enabling the detection of variations during production and throughout the shelf life of vitamin C-containing samples.

References

1. Garre, A., et al. Antiaging effects of a novel facial serum containing L-Ascorbic acid, proteoglycans, and proteoglycan-stimulating tripeptide: ex vivo skin explant studies and in vivo clinical studies in women: Clinical, cosmetic and investigational dermatology, 2018. pp. 253-263
2. Neves, J. R., et al. Efficacy of a topical serum containing L‐ascorbic acid, neohesperidin, pycnogenol, tocopherol, and hyaluronic acid in relation to skin aging signs: Journal of Cosmetic Dermatology, 2022. pp. 4462-4469
3. Boo, Y. C. Ascorbic acid (vitamin C) as a cosmeceutical to increase dermal collagen for skin antiaging purposes: emerging combination therapies:  Antioxidants, 2022. p. 1663
4. J. Agric. Degradation of Ascorbic Acid in Aqueous Solution: Food Chem. 1998. pp. 5078–5082
5. ICH Q1A (R2) Stability testing of new drug substances and drug products - Scientific guideline (Last updated: 01.08.2003)
6. C63IA012EN-A (Vitamin C) https://www.anton-paar.com/corp-en/services-support/document-finder/application-reports/quality-control-and-stability-monitoring-of-vitamin-c-containing-cosmetic-serums/