Determination of Water in Moisturizing Skin Creams

Karl Fischer (KF) Titration and Near-Infrared Spectroscopy (NIRS) in Perfect Synergy

As being one of the fundamental components of raw materials and finished products in many industries, it is highly important to accurately determine the content of water in the control steps of production processes. In this context, water determination by Karl Fischer Titration (KF) is one of the most frequently applied analysis techniques in today’s laboratories. Particularly in skin creams, one of the most frequently produced personal care products in the cosmetics and pharmaceutical sectors, the correct determination of the water content is vital in order to bring the consistency, texture and moistening characteristics to the desired level. Various (and relatively difficult) sample preparation steps for creams involving dissolution steps in suitable solvent mixtures (ex. methanol, chloroform, decanol, etc.) prior to conventional KF titration define the workflows in the industry. In contrast, near-infrared spectroscopy (NIRS) offers a rapid, non-destructive and sensitive technique for such samples, and can adequately complement quality control processes. In NIRS, there is no sample prepa-ration/ dissolution step needed. Once a method is developed, the quality of incoming materials, intermediates and final products can be determined with a strong and robust technique in a matter of seconds without sample preparation. Below we present you the details of an analysis method developed for water analysis in moisturizing skin creams via NIRS technique.

Experimental Setup&Workflow

The set of samples used in this study consisted of 42 different commercially available moisturizing skin creams available in the global market.

Cream Production

As a first step, the water content was determined by Karl-Fischer titration in triplicate (See Metrohm Karl- Fischer Application Note AN-K-0111). The water content was determined to range from 60% to 80% and these values were used as primary values in the NIRS model development. In order to analyze the samples with NIRS, approximately 0.5 g cream were applied on the NIRS Slurry Cup, the NIRS Diffuse Reflectors, Gold 1 mm was pressed on the sample and creams were analyzed in transflection mode. The transfection mode works in such way that the light that passes through the sample reflects via the gold reflector back into the detector. The Slurry Cup was placed in the Metrohm NIRS DS2500 Analyzer and the spectra were acquired over the spectral range from 400 nm to 2500 nm (See Fig. 1). After the measurement, the gold stamp and the Slurry Cup were carefully cleaned using dry paper towels.

Table 1: Used Equipment&Software.

For data acquisition and developing the quantification method, the software package Vision Air 2.0 Complete was used. The instrument management, such as the assignment of the operating procedures was conducted in the office using Vision Air Manager Network, whereas the actual measurements took place in the quality control laboratory using Vision Air 2.0 Routine. The acquired spectra were automatically uploaded to the database, where they were accessible for method development.

The quantitative method development was performed in the Vision Chemometric Software.

Figure 1: The Metrohm NIRS DS2500 Analyzer was used for spectral data acquisition over the full range from 400 nm to 2500 nm (left), and the used NIRS Slurry Cup and 1mm Gold Diffuse Reflector (right).

Method Development

The analysis results obtained by the primary method (Karl Fischer titration) in combination with the NIRS spectra were used in the subsequent method development. The spectra were pre-treated using a Detrend function (2nd order polynomial baseline correction) in combination with a 1st derivative (See Fig. 3). The multivariate regression method Partial Least Squares Regression (PLS) was performed over the spectral regions 1350–1500 nm and 1800–2000 nm, which represent the two most prominent water bands. The spectral variations arising due to concentration changes of moisture were apparent (See Fig. 2).

Figure 2: The raw spectra (upper trace) were pre-treated using a Detrend function with a subsequent 1st derivative (lower trace). The arrows indicate the trend to higher water concentration.

Three samples were identified as outliers. Those samples were more wax-like than creams and did not fit to the other sample population. The remaining samples were split into a calibration set (31 samples) and a validation set (8 samples).

Analysis Results

Because of the varying sample matrices (each sample is a unique product with different ingredients), the model was fitted using 4 factors. A Standard Error of Calibration (SEC) of 0.565% and a Standard Error of Cross Validation (SECV) of 0.745% was obtained (see Figure 3). These Figures of Merits of the internal cross validation (leaveone-out method) indicate the model is robust and provides accurate results. The defined validation set was used for external validation, yielding. A Standard Error of Prediction (SEP) of 0.819% was obtained (See Tab. 2). These results show clearly that NIRS is excellently suited to reliably determine the water content in moisturizing creams and it can be used in quality control for outof- specification analysis and final product inspection1.

Figure 3: Correlation plot of the moisture concentration determined by Karl Fischer titration and predicted by NIRS. The blue marks stand for samples used in the calibration set, the turquois marks are samples used in the validation set.

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

Conclusion

Vis-NIR spectroscopy can be successfully used for quantification of water content in moisturizing creams. Vis-NIR spectroscopy offers a number of unique advantages over traditional analysis methods: It generates reliable results within seconds without the need for any sample preparation or chemicals, and does not generate any waste. The accessories are cleaned easily, and are readily available for the next measurement. As Metrohm, being the global industry leader in the primary methodology (Karl Fischer Titration) which forms the basis of this analysis technique, we also offer fully integrated, sensitive and reliable Vis-NIRS system/software solutions which are compliant with all international standards. Nurten Uçak Titration Product Manager Metrohm Turkey       Pelin Kömürcü Spectroscopy Application Chemist Metrohm Turkey         References: *1 Metrohm KF Uygulama Notu AN-K-011 Water in Moisturizing Creams (cosmetic products)