Determination of aliphatic alcohols in alcohol mixtures using visible near-infrared spectroscopy

Significance of the Topic

Precise quantification of aliphatic alcohols such as ethanol and isopropanol is essential for quality control in antiseptic and disinfectant formulations. Rapid, non-destructive methods enable real-time monitoring of product consistency, ensuring regulatory compliance and efficient production workflows.

Study Objectives and Overview

This study demonstrates the development and validation of a visible near-infrared spectroscopy (VIS-NIRS) method for accurate determination of ethanol/isopropanol ratios in liquid mixtures. A total of 44 customer-specific samples spanning 0–100% concentration range were analyzed to establish calibration and validation models.

Methodology and Used Instrumentation

• Sample preparation: 8 mm disposable glass vials, temperature controlled at 30 °C
• Spectra acquisition: Metrohm NIRS XDS RapidLiquid Analyzer in transmission mode, spectral range 400–2500 nm
• Software: Vision Air 2.0 Complete for data handling and model development
• Pretreatment: second derivative to correct baseline variations
• Calibration: Partial Least Squares regression using three factors over selected wavelength regions

Main Results and Discussion

VIS-NIR spectra revealed concentration-dependent variations in the O–H combination band near 2100 nm and first overtone around 1460 nm. Second derivative processing enhanced spectral features and minimized baseline drift. Calibration and cross-validation metrics for ethanol showed SEC of 0.34%, SECV of 0.37%, SEP of 0.44%, and R² of 0.9998. Isopropanol models achieved SEC of 0.38%, SECV of 0.41%, SEP of 0.45%, and R² of 0.9998. Correlation plots confirmed strong agreement between predicted and reference values for both analytes.

Benefits and Practical Applications

VIS-NIRS offers real-time, solvent-free analysis of alcohol mixtures without sample destruction or extensive preparation. Its speed and accuracy make it ideal for inline or at-line quality assurance in pharmaceutical, cosmetic, and chemical manufacturing.

Future Trends and Possibilities for Application

• Integration with process analytical technology for continuous monitoring
• Expansion to multi-component analysis including water content, colorants, and additives
• Application of advanced chemometric techniques and machine learning to enhance predictive performance and reduce calibration effort

Conclusion

The developed VIS-NIRS method enables rapid, accurate, and non-destructive quantification of ethanol and isopropanol in antiseptic mixtures. With excellent calibration statistics and minimal sample handling, it supports efficient quality control and process optimization.

References

• Metrohm Application Note NIR-061: Determination of aliphatic alcohols in alcohol mixtures using visible near-infrared spectroscopy, Version 1, July 2018.