Simultaneous determination of multiple quality parameters of polyols using Vis-NIR spectroscopy

Importance of the topic

Near-infrared spectroscopy (Vis-NIR) offers a rapid, reagent-free approach to quality control of polyols, replacing time-consuming titration methods and reducing laboratory costs and waste. Its ability to measure multiple parameters simultaneously aligns with industry demands for efficient raw material verification and regulatory acceptance for hydroxyl value determination.

Objectives and Study Overview

This study aimed to develop and validate Vis-NIR methods for simultaneous quantification of acid value, hydroxyl value and moisture content in polyol samples. A total of 140 samples with known reference values were used to build calibration models and a 25 % subset was reserved for external validation.

Instrumental Setup

The following equipment and software were employed:

• NIRS XDS RapidLiquid Analyzer (Metrohm 2.921.1410) for spectral acquisition in transmission mode over 400–2500 nm.
• Disposable glass vials, 8 mm diameter (Metrohm 6.7402.000), for sample containment.
• Vision Air 2.0 Complete (Metrohm 6.6072.208) for data acquisition, management and model development.

Methodology

Samples were pipetted into disposable vials and equilibrated at 30 °C for 30 s before scanning. Spectra were pre-treated using a second derivative algorithm to remove baseline effects and enhance analyte-specific features. Partial Least Squares (PLS) regression models were created, selecting optimal wavelength ranges for each parameter.

Main Results and Discussion

High correlation was observed between Vis-NIR predictions and reference methods:

• Acid value (0–8 mg KOH/g): R² = 0.989, SEC = 0.23, SECV = 0.33, SEP = 0.38 mg KOH/g.
• Hydroxyl value (70–125 mg KOH/g): R² = 0.999, SEC = 0.46, SECV = 0.50, SEP = 0.68 mg KOH/g.
• Moisture content (75–400 ppm): R² = 0.902, SEC = 25, SECV = 30, SEP = 52 ppm.

The prediction errors are on par with conventional titration, confirming the suitability of Vis-NIR for accurate quality assessment.

Benefits and Practical Applications

Vis-NIR enables near-instantaneous, multi-parameter analysis without chemicals or waste disposal. The approach streamlines QC workflows, lowers per-sample cost and supports high-throughput screening of polyol batches in industrial and research settings.

Future Trends and Possibilities

Extensions of this methodology could include detection of solvent or solid content, differentiation of primary and secondary hydroxyl groups and integration into inline process monitoring. Advances in chemometric algorithms and miniaturized spectrometers will further enhance portability and real-time analysis capabilities.

Conclusion

Vis-NIR spectroscopy provides a robust, efficient alternative to classical titration for simultaneous determination of key quality parameters in polyols. The validated methods deliver comparable accuracy with substantial savings in time, cost and environmental impact.

References

Metrohm Application Note NIR-065. Simultaneous determination of multiple quality parameters of polyols using Vis-NIR spectroscopy. Version 1, first published August 2018.