Mid-IR dispersion spectroscopy - A new avenue for liquid phase analysis

Area of Science:

• Analytical Chemistry
• Spectroscopy
• Physical Chemistry

Background:

• Traditional absorption spectroscopy for liquid analysis is limited by dynamic range and sensitivity.
• Mid-infrared (Mid-IR) dispersion spectroscopy offers an alternative by measuring refractive index changes.
• This technique promises improved performance over conventional methods.

Purpose of the Study:

• To experimentally and theoretically characterize Mid-IR dispersion spectroscopy for broadband liquid sample analysis.
• To demonstrate the advantages of dispersion spectroscopy over classical absorption spectroscopy.
• To highlight its potential as a robust and sensitive method for IR spectral analysis of liquids.

Main Methods:

• Development of a compact, benchtop dispersion spectroscopy setup using an external cavity quantum cascade laser (EC-QCL) and a Mach-Zehnder interferometer.
• Implementation of phase-locked interferometric detection with operation at the quadrature point and balanced detection for noise immunity.
• Analysis of ethanol-water solutions (0.5-50% v/v) to demonstrate linearity beyond the Beer-Lambert law.

Main Results:

• Dispersion spectroscopy exhibits extended dynamic range, baseline-free detection, and immunity to power fluctuations and environmental noise.
• Refractive index changes were found to be linearly related to concentration, even for highly concentrated and strongly absorbing samples.
• The optimum pathlength for liquid analysis using dispersion spectroscopy can be extended compared to absorption spectroscopy.

Conclusions:

• Mid-IR dispersion spectroscopy is a powerful technique for liquid analysis, overcoming limitations of absorption spectroscopy.
• The developed setup provides a robust and sensitive platform for broadband spectral analysis.
• Dispersion spectroscopy represents a promising advancement for future IR spectral analysis of liquid samples.