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

Significance of the Topic

The chemical composition of alcohol mixtures used in antiseptic products is a critical quality parameter.
Rapid, nondestructive, real-time analytical techniques help ensure product efficacy and regulatory compliance.

Aims and Overview of the Study

This work demonstrates a fast method to quantify ethanol and isopropanol in binary mixtures using visible near-infrared spectroscopy (VIS-NIRS).
The goal is to enable simultaneous determination of both alcohols in under a minute, suitable for routine quality control.

Methodology and Instrumentation

44 custom liquid samples with ethanol/isopropanol ratios from 0 to 100% were prepared.
Spectra were acquired in transmission mode over 400–2500 nm at a controlled temperature of 30 °C.
Chemometric modeling employed second-derivative pretreatment and partial least squares (PLS) regression.
Calibration used 33 samples, and validation used an independent set of 11 samples.

Used Instrumentation

• Metrohm NIRS XDS RapidLiquid Analyzer (Part No. 2.921.1410): spectral range 400–2500 nm
• 8 mm disposable glass vials (Part No. 6.7402.000)
• Vision Air 2.0 Complete software (Part No. 6.6072.208) for data acquisition and method development

Main Results and Discussion

Excellent correlation between VIS-NIR predictions and reference values was achieved for both analytes.

• Ethanol model: R² = 0.9998; SEC = 0.34 %; SECV = 0.37 %; SEP = 0.44 %
• Isopropanol model: R² = 0.9998; SEC = 0.38 %; SECV = 0.41 %; SEP = 0.45 %

Spectral features near 1460 nm (first O-H overtone) and around 2100 nm (O-H combination band) varied systematically with concentration.
Second-derivative spectra compensated for baseline shifts and improved model robustness.

Benefits and Practical Applications

• Analysis time under one minute per sample
• No chemical reagents or sample preparation required
• Simultaneous quantification of multiple alcohols in a single measurement
• Real-time feedback for manufacturing and quality assurance

Future Trends and Possibilities

• Implementation of inline VIS-NIR sensors for continuous process monitoring
• Expansion of methods to additional formulation components (moisture, additives, colorants)
• Development of advanced multivariate algorithms for complex mixtures
• Integration with Industry 4.0 data systems for automated QC workflows

Conclusion

Visible near-infrared spectroscopy using the Metrohm XDS RapidLiquid Analyzer provides a reliable, rapid, and nondestructive approach to quantify ethanol and isopropanol in antiseptic formulations.
The validated chemometric models deliver high accuracy and precision, supporting efficient quality control in industrial settings.

References

• NIR Application Note NIR-061, Metrohm AG, Version 1, July 2018