Determination of sodium dodecyl sulfate in toothpaste using Vis-NIR spectroscopy

Importance of the Topic

Toothpaste requires stringent quality control to ensure consumer safety and product performance. Surfactant content, such as sodium lauryl sulfate (SLS), directly impacts cleaning efficiency, foaming behavior, and overall user experience. Traditional reference methods like HPLC demand extensive sample preparation, hazardous solvents, and significant analysis time. Vis-NIR spectroscopy offers a rapid, reagent-free alternative that dramatically reduces operational costs and turnaround time.

Objectives and Study Overview

This study aims to develop a robust Vis-NIR spectroscopic method for the quantification of sodium dodecyl sulfate (SDS) in toothpaste. A total of 37 toothpaste samples with varying SDS concentrations and additive compositions were analyzed. The goal was to correlate NIR predictions with established reference values and demonstrate the method’s suitability for routine quality control.

Methodology and Instrumentation

• Sampling: Thirty-seven commercial toothpaste samples grouped by SDS content and presence of flavoring agents.
• Spectroscopy: NIRS XDS RapidContent Analyzer operating in transflection mode across 400–2500 nm.
• Software: Vision Air 2.0 Complete for spectral acquisition, data management, and chemometric model development.
• Calibration: Partial least squares (PLS) regression with six latent factors. Spectra pretreated using a second derivative focused on specific wavelength intervals (1190–1298 nm and 1626–1804 nm) to isolate SDS features.

Main Results and Discussion

• Pretreated spectra exhibited clear absorbance variations corresponding to SDS concentration.
• The PLS model achieved an R² of 0.996, with a standard error of calibration (SEC) of 0.021 % and a standard error of cross-validation (SECV) of 0.026 % across the SDS range of 0.5–1.8 %.
• A strong linear correlation between NIR-predicted and reference SDS values confirmed the model’s accuracy and robustness.

Practical Benefits

• Reagent-free analysis eliminates the need for costly and hazardous chemicals.
• Rapid measurement cycle supports high-throughput quality control workflows.
• Method adaptable to additional analytes such as moisture, color parameters, or alternative surfactants like sodium laureth sulfate (SLES).

Instrumental Setup

• NIRS XDS RapidContent Analyzer (Metrohm catalog number 2.921.1110).
• NIRS Liquid Sample Kit in transflection mode (Metrohm catalog number 6.7400.010).
• Vision Air 2.0 Complete software package (Metrohm catalog number 6.6072.208).

Future Trends and Applications

Advancements in miniaturized NIR sensors and inline process integration will facilitate real-time monitoring of toothpaste production. Emerging machine learning algorithms and enhanced chemometric strategies promise to further improve prediction accuracy for complex formulations. Development of comprehensive spectral libraries could expand the scope to multiple oral care products and ingredients.

Conclusion

Vis-NIR spectroscopy combined with targeted spectral pretreatment and PLS modeling offers a precise, rapid, and cost-effective approach for SDS quantification in toothpaste. This method represents a sustainable alternative to traditional chromatographic techniques and is well suited for routine industrial quality control.

References

• Application Note NIR-069, Metrohm, August 2018.