Towards Greater Sensitivity: A Brief FTIR and Infrared-Based Cavity Ring Down Spectroscopy Comparative Study

September 1, 2023

Army Combat Capabilities Development Command, Aberdeen Proving Ground, MD

The Gasmet DX4000 gas analyzer (bottom right) with PSS (left) and external laptop (top right), a portable Fourier-transform infrared (FTIR) spectroscopy system, was used by the researchers to perform this work.

A threat in the form of chemical vapor may not be visible, but rapid detection is critical for preservation of life and property. In addition, understanding the surrounding environment informs the posture that the warfighter will need to take. The field of chemical vapor detection spans far beyond the warfighter and is rich in research. A search in SciFinder for “chemical vapor detection” provides over 400,000 results with over 3,000 books, 26,000 reviews, and nearly 300,000 journal articles. The focus of this document will be with an eye towards perimeter monitoring for a wide range of gas-phase chemicals. To accomplish such sensing, compound-specific sensors should not be employed as they lack capability to detect or inform about the presence of many potential threats outside of their selected targets. A viable technique for sensing a wide range of compounds is infrared absorption as most potential threats provide an infrared absorbance spectrum which arises from each compound’s unique molecular structure.

The purpose of this article is to provide a comparison between a commercially available instrument long utilized as a standard within several defense laboratories around the globe which employs FTIR methods for detection of environmental gasses in industrial environments to a newer class of IR absorption-based detectors that use cavity ringdown to determine the absorption profile.

While there are many ways to sense gases, optical, infrared absorption-based techniques offer the capability to perform real-time, in situ analysis and direct measurement of the vibrations of a molecule (in near- and mid-IR techniques). A keen advantage is gained when using infrared absorption techniques by applying a Fourier transform to the IR signal which allows for increased throughput (known as Jacquinot’s advantage) and multiplexing (known as Felleget’s advantage) and can increase the signal-to-noise.

Detailed information on the mechanism of IR absorption is extensively documented in scientific literature. In short, gas-phase IR analysis of a chemical provides vibration-rotation spectra. For a compound to be IR-active, there must be a change in the molecule’s dipole moment with respect to the molecular vibration; this is the basis of the absorption of the IR radiation and thereby a change in the spectral absorption profile. In addition to individual bonds within a molecule, there are also larger moieties within the molecule which vibrate at characteristic frequencies which provide structural or functional information and are nearly independent from the rest of the molecule’s structure.

Cavity Ring-Down spectroscopy (CRDS) is a newer IR absorption-based technique made possible by the use of tunable pulsed lasers. Rather than using a broadband (blackbody) emitter as is done with traditional IR techniques, a pulsed laser is tuned across wavelengths into an optical cavity. Instead of measuring the absorption directly, the amount of light leaking through a highly reflective mirror within the cavity is measured with respect to time at each wavelength and a time constant for the decay can be measured.

This work was performed by Eric Languirand and Ian Pardoe for the U.S. Army Combat Capabilities Development Command (DEVCOM) Chemical Biological Center. For more information, download the Technical Support Package (free white paper) below. CBCTN-093