Improved Measurement of Liquid Samples Using FTIR

Significance of the Topic

The precise analysis of liquid samples is critical for quality control in pharmaceuticals, cosmetics, chemicals and food industries. Mid-infrared FTIR spectroscopy is a routine tool for material identification and purity assessment. Traditional transmission cells and attenuated total reflectance (ATR) each have limitations when measuring neat liquids or low-level analytes. The development of the DialPath module addresses these challenges by combining variable optical pathlengths with user-friendly sample handling.

Objectives and Study Overview

This application note evaluates the performance of the Agilent DialPath sampling module on the Cary 630 FTIR spectrometer. The main goals are to:

• Compare DialPath measurements with ATR and classic transmission approaches
• Verify identity and purity of common liquid additives: propylene glycol, glycerol, triacetin and dipropylene glycol
• Develop chemometric models for low-level impurities (ethylene glycol and diethylene glycol) in viscous matrices

Methodology and Instrumentation

Samples were analyzed using a Cary 630 FTIR equipped with the DialPath module, which provides three factory-calibrated fixed pathlengths (30, 75 and 100 μm) selected by rotating the optical head. Spectra were acquired at 4 cm⁻¹ resolution with 64 co-added scans. Partial least squares (PLS) calibration models were built for each impurity in glycerol and triacetin, using characteristic absorbance bands under Beer’s law assumptions.

Main Results and Discussion

Identity verification of the four liquid additives using the 30 μm pathlength yielded library match scores >0.99 within 30 s. For impurity quantitation:

• In glycerol, the 75 μm pathlength offered sufficient sensitivity while avoiding spectral saturation. PLS models achieved R² values of 0.9895 (ethylene glycol) and 0.9745 (diethylene glycol), with limits of detection ~0.04 vol% and quantitation ~0.075 vol% (well below the FDA threshold of 0.1 vol%).
• In triacetin, the longer 100 μm pathlength exploited transparent spectral windows (812–939 and 1100–1200 cm⁻¹). Calibration yielded R² values of 0.9997 and 0.9998 and detection limits of 0.04 vol% (ethylene glycol) and 0.02 vol% (diethylene glycol).

The ability to switch pathlengths quickly enables both gross identification and trace impurity analysis in viscous samples without cumbersome cell assembly or spacer changes.

Benefits and Practical Applications

• Rapid selection among three pathlengths without disassembly or consumables
• Elimination of leakage and fringing associated with traditional variable cells
• Accurate quantitation of volatile and non-volatile impurities in a single module
• Ease of cleaning and sample exchange comparable to ATR
• Automated data processing and color-coded reporting via MicroLab software

Future Trends and Applications

Advancements may include integration with autosamplers, expansion of pathlength options for ultra-trace analysis, and development of advanced chemometric libraries for complex mixtures. The modular design could be adapted to inline process monitoring and portable field analysis, enabling real-time quality control across multiple industries.

Conclusion

The Cary 630 FTIR combined with the DialPath module overcomes the pathlength and usability limitations of ATR and classic transmission cells. It provides a flexible, rapid and reliable solution for both high-concentration identification and low-level impurity quantitation in liquid samples, improving laboratory efficiency and confidence in quality control results.

References

No external literature citations were provided in the original application note.