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

Significance of the topic

Water content in moisturizing skin creams is a critical quality attribute affecting texture, stability and consumer experience. Traditional Karl Fischer titration provides accurate reference values but involves sample dissolution, solvent use and time-consuming steps. Implementing near-infrared spectroscopy (NIRS) offers rapid, non-destructive, and environmentally friendly at-line analysis, enhancing quality control efficiency in personal care and pharmaceutical production.

Aims and overview

This study aimed to develop and validate a NIRS-based quantification model for water content in commercially available skin creams. Using Karl Fischer titration results as primary reference, the work sought to enable fast and reliable at-line measurements for incoming materials, intermediates and final products.

Methodology

Samples from 42 different moisturizing creams were analyzed. Water content was determined in triplicate by Karl Fischer titration according to AN-K-011, yielding values between 60% and 80% w/w. NIR spectra were collected over 400–2500 nm using diffuse reflectance in a slurry cup with a 1 mm gold reflector. Spectral preprocessing included second-order polynomial detrending and first derivative treatment, focusing the chemometric model on key water absorption bands at 1350–1500 nm and 1800–2000 nm. Partial least squares regression (PLS) with four latent factors was used for calibration and validation.

Used instrumentation

• Metrohm NIRS DS2500 Analyzer
• NIRS Slurry Cup
• Gold diffuse reflector (1 mm)
• Vision Air 2.0 Complete software for method development
• Vision Air Manager and Vision Air 2.0 Routine for data acquisition and instrument control

Main results and discussion

The PLS model demonstrated strong correlation (R2 = 0.992) between NIRS-predicted and Karl Fischer reference values. Calibration error (SEC) was 0.565% w/w, cross-validation error (SECV) 0.745% w/w, and external validation error (SEP) 0.819% w/w. Three wax-like outliers were excluded prior to model splitting (31 samples for calibration, 8 for validation). The robust performance confirms NIRS suitability for precise water determination in diverse cream matrices.

Benefits and practical applications

• Rapid results within seconds without sample preparation or chemicals
• Non-destructive, minimal waste and easy-to-clean accessories
• At-line implementation for real-time quality control and out-of-specification analysis
• Flexible method adaptable to various cream formulations

Future trends and opportunities

• Extending full spectral analysis to monitor additional ingredients such as UV filters
• Integration into process analytical technology (PAT) for continuous manufacturing
• Development of portable NIRS solutions for in-field or point-of-care applications
• Advanced chemometric strategies to handle complex matrices and detect degradation or contaminants

Conclusion

Near-infrared spectroscopy provides an efficient and sustainable alternative to Karl Fischer titration for water content analysis in moisturizing creams. The developed chemometric model achieves high precision and robustness, enabling rapid at-line quality control without reagents or extensive sample preparation.

References

• [1] Karl Fischer Application Note AN-K-011, Water in moisturizing creams (cosmetic products).