Determination of water content in moisturizing skin creams using near-infrared spectroscopy

Significance of the Topic

Moisturizing skin creams are widely used in personal care and pharmaceutical applications, where water content critically influences texture, stability, and skin hydration. Accurate and efficient moisture measurement is essential for quality control, product consistency, and compliance with industry standards.

Objectives and Overview of the Study

This study aimed to establish a rapid, non-destructive near-infrared spectroscopy (NIRS) method for quantifying water content in commercial moisturizing creams. Karl Fischer titration served as the reference technique to develop and validate the NIRS calibration model for at-line and final product inspection.

Methodology and Instrumentation

Spectrum acquisition was performed using a Metrohm NIRS DS2500 Analyzer in transflection mode over the range 400–2500 nm. Approximately 0.5 g of cream was placed in a NIRS slurry cup, covered with a gold diffuse reflector (1 mm), and measured without sample preparation. Spectral pre-treatment included second-order polynomial detrending and first derivative transformation. Partial least squares (PLS) regression was applied to the 1350–1500 nm and 1800–2000 nm regions, targeting prominent water absorption bands.

Main Results and Discussion

The PLS model employed four latent factors to capture matrix variability across 42 commercial samples. Calibration yielded an R2 of 0.992, a standard error of calibration (SEC) of 0.565 %, and cross-validation error (SECV) of 0.745 %. External validation on an independent set produced a standard error of prediction (SEP) of 0.819 %. Three wax-rich outliers were excluded due to their atypical spectral features.

Benefits and Practical Applications

• Non-destructive, reagent-free analysis generating no chemical waste
• No sample dissolution or solvents required
• Results in seconds, enabling high-throughput at-line quality control
• Capability to detect additional formulation components (e.g., UV filters)

Future Trends and Opportunities

Advancements in NIR detector technology and chemometric algorithms may extend applicability to broader cosmetic and pharmaceutical matrices. Integration with process analytical technology (PAT) frameworks can facilitate real-time monitoring. Emerging machine learning approaches could further improve model robustness and transferability across production lines.

Conclusion

Vis-NIR spectroscopy provides a fast, reliable, and eco-friendly alternative to Karl Fischer titration for water determination in skin creams. Its minimal sample handling, absence of reagents, and rapid throughput make it ideal for routine quality assurance in the cosmetics industry.

Instrumentation Used

• Metrohm NIRS DS2500 Analyzer
• NIRS Slurry Cup (6.7490.430)
• NIRS Diffuse Reflectors, Gold 1 mm (6.7420.000)
• Vision Air 2.0 Complete software

References

• [1] Karl Fischer Application Note K-11: Water in moisturizing creams (cosmetic products).