Water Content Analysis in Moisturizing Skin Creams

Excellent Synergy between Karl Fischer (KF) Titration and Near-Infrared Spectroscopy (NIRS)

Water, one of the fundamental components of raw materials and finished products used in industry, must be accurately determined in the control steps of production processes. In this context, water content determination by Karl Fischer (KF) titration is one of the most widely applied analytical techniques in modern laboratories. Particularly in creams, a personal care product frequently manufactured in the cosmetics and pharmaceutical sectors, accurate determination of water content is critical for achieving the desired consistency, texture and moisturizing characteristics.

Traditional KF titration for water analysis in creams typically involves various (and challenging) pre-treatment steps, including dissolution of samples in appropriate solvent mixtures (e.g. methanol, chloroform, decanol, etc.), representing a typical workflow adopted in the industry.

On the other hand, near-infrared spectroscopy (NIRS) can contribute to quality control processes as a rapid, non-destructive and precise technique for such samples. NIRS requires no sample preparation or dissolution steps and provides a powerful technique that allows raw materials, intermediates and final products to be analysed within seconds following completion of the method development phase. Below we present details of an analytical method developed for water content analysis in moisturizing creams using NIRS technique.

Experimental Content and Workflow

This study employed a test set developed on 42 different commercial skin moisturizing cream products encountered in the global market.

Cream Production Process

In the first step, the water content of each product was determined in triplicate using Karl Fischer titration technique (See: Metrohm KF Application Note AN-K-0111). The water contents obtained ranged between 60–80% and were used as primary values in NIRS model development. For NIRS analysis, approximately 0.5 g cream sample was placed in a NIRS Slurry sampling cup and measurement was taken in transmittance-reflectance mode following application of a 1 mm gold NIRS diffuse reflector accessory to the sample. In transmittance-reflectance mode, light passes through the sample and is reflected back through a gold reflector to the detector. The Slurry sampling cup was placed in a Metrohm NIRS DS2500 Analyzer and measurements were obtained in the 400–2500 nm spectral range (See: Figure 1). Following each measurement, the gold stamp and Slurry sampling cup were cleaned with dry wipes.

Table 1: Equipment and Software Used.

The Vision Air 2.0 Complete software package was used for data collection and quantification method development processes. All instrument management, including assignment of operation procedures, was performed from the office with the aid of Vision Air Manager Network, while all actual measurements were performed in the quality control laboratory with the Vision Air 2.0 Routine interface. The obtained spectra were automatically uploaded to the database and made available directly for method development. Quantitative method development was performed using Vision Chemometrics Software.

Figure 1: Metrohm NIRS DS2500 Analyzer for spectral data collection (left), NIRS Slurry cup and 1 mm Gold Diffuse Reflector (right) were used, measurement was performed in the 400 nm–2500 nm range.

Method Development Analysis results obtained using the primary method and NIRS spectral data acquired were used together in method development. Spectral data were pre-processed using a Detrend Function combined with first derivative (2nd degree polynomial baseline correction). PLS (Partial Least Squares Regression) multivariate regression method was applied in the 1350–1500 nm and 1800–2000 nm spectral regions, which exhibit the most prominent water bands. Spectral variations resulting from changes in moisture concentration can be clearly observed (See: Figure 2).

Figure 2: Raw spectra (top trace) were pre-processed using first derivative following a Detrend function (bottom trace). Arrows indicate increasing water concentration trend.

Three samples were defined as out-of-set. These samples were found to have a wax-like structure rather than cream-like and did not overlap with the remaining sample population. The remaining samples were divided into one calibration set (31 samples) and one validation set (8 samples).

Analysis Results

Due to diverse sample matrices (each sample has different compositions), the model was fitted using 4 factors. A Standard Calibration Error (SEC) value of 0.565% was obtained, while a Standard Cross Validation Error value of 0.745% was found (see Figure 3). These values obtained from the Internal Cross Validation (leave-one-out method) process demonstrate that the model provides robust and accurate results. The defined validation set was used in external validation and recovery procedures and a Standard Prediction Error (SEP) value of 0.819% was obtained (See Table 2). These results clearly demonstrate that NIRS technique shows excellent compatibility for reliable water content analysis in skin moisturizing creams and can be used for out-of-specification detection analysis and final product inspection for quality control purposes.

Table 2: Results of quantitative method for water content in skin creams.

Conclusion Vis-NIR spectroscopy can be successfully used in moisturizing creams. This technique offers a number of unique advantages compared to conventional analytical methods: it can produce reliable results within seconds without requiring any sample preparation or chemicals and does not generate any waste. The accessories used can be easily cleaned and prepared for the next measurement immediately. As Metrohm, in addition to being global sector leader in the primary methodology (Karl Fischer Titration) underlying the analytical method, we provide fully integrated, precise and reliable solutions directly compatible with international standards through our Vis-NIRS systems and software. Nurten Uçak Titration Product Group Manager Metrohm Turkey         Pelin Kömürcü Spectroscopy Application Chemist Metrohm Turkey         Sources *1 Metrohm KF Application Note AN-K-011 Water in Moisturizing Creams (cosmetic products)