Improved Measurement of Liquid Samples Using FTIR

Significance of the Topic

Ensuring the quality and purity of liquid raw materials and products is critical in industries such as pharmaceuticals, cosmetics, chemicals and food manufacturing. FTIR spectroscopy provides rapid, non-destructive analysis, but conventional attenuated total reflectance (ATR) methods are limited by shallow penetration depth and fixed transmission cells can be difficult to use with viscous or volatile samples.

Objectives and Study Overview

This study demonstrates the advantages of Agilent’s DialPath sampling module on the Cary 630 FTIR spectrometer for liquid analysis. The goal is to show how variable fixed pathlengths from 30 to 1000 µm can simplify sample introduction, improve sensitivity for low‐level impurity detection and maintain ease of use comparable to ATR.

Methodology and Instrumentation

The Cary 630 FTIR combined with the DialPath module was used for all measurements. Key features include:

• Fixed pathlength options selected by rotating the optical head (30, 75 and 100 µm in the three-position module)
• Single-drop sample application between two horizontal IR-transparent windows; simple wiping between runs
• Data acquisition settings: 64 co-added interferograms at 4 cm⁻¹ resolution; approximately 30 seconds per spectrum
• Partial least squares (PLS) calibration models for ethylene glycol and diethylene glycol impurities

Main Results and Discussion

Identity verification of common liquid additives (propylene glycol, glycerol, triacetin, dipropylene glycol) using the 30 µm pathlength yielded spectral match scores above 0.99 against a reference library. For low‐level impurity quantification:

• Glycerol required the 75 µm pathlength to balance absorbance and sensitivity. PLS models produced R² values of 0.9895 and 0.9745 for ethylene glycol and diethylene glycol, respectively. Limits of quantitation were around 0.075 vol% and detection limits 0.04 vol%, well below the FDA threshold of 0.1 vol%
• Triacetin was best measured with the 100 µm pathlength due to its spectral transparency regions. PLS calibrations achieved R² values above 0.9997, with detection limits of 0.02–0.04 vol%

These findings underline the importance of selecting the optimal pathlength to detect low‐concentration analytes in viscous liquids, a task at which ATR alone cannot excel.

Benefits and Practical Applications

The DialPath approach offers:

• Rapid and reproducible pathlength selection without disassembly or spacers
• Improved handling of liquids with diverse viscosities and volatilities
• No consumables, simplified cleaning and faster throughput compared to conventional transmission cells
• Combined identity confirmation and quantitative impurity analysis in a single module

Future Trends and Potential Applications

Expanding this variable-pathlength concept could benefit process analytical technology and inline monitoring in pharmaceutical and petrochemical production. Integration with advanced chemometric software and automated sampling could further streamline quality control workflows for a wide range of liquid formulations.

Conclusion

The Agilent Cary 630 FTIR equipped with DialPath technology bridges the gap between traditional transmission and ATR methods. It delivers the sensitivity and flexibility required for low-level impurity quantification while retaining the ease of use and rapid analysis of ATR, making it a powerful tool for liquid sample quality assurance in diverse industries.