Innovative New Method Speeds Up Correction of ATR Infrared Spectra

Representation of frequency variation in spectral peaks: © bobby - stock.adobe.com

Infrared spectroscopy is a vital analytical tool used across scientific disciplines, but interpreting data from attenuated total reflection (ATR) measurements has long presented challenges. Traditional correction methods are time-consuming and can compromise accuracy, limiting the technique's potential. However, a recent breakthrough by researchers at the Leibniz Institute of Photonic Technology (IPHT) promises to revolutionize this process, offering a rapid and precise solution (1).

Led by Thomas G. Mayerhöfer, William D. P. Costa, and Jürgen Popp, the team focused on optimizing ATR correction for mid-infrared (MIR) spectra, which are often plagued by refractive index variations affecting peak accuracy. Their study, published in the Journal Applied Spectroscopy, introduces a novel approach that significantly accelerates and enhances the correction process. The team evaluated their correction methodology for multiple ATR crystal materials and accessories (1).

The conventional ATR measurement configuration utilizes an angle of incidence of 45° and lacks a polarizer, resulting in incomplete spectral information, particularly for complex samples. Mayerhöfer and his colleagues addressed this limitation by leveraging both the unpolarized 45° ATR spectrum and the polarization angle, enabling the determination of the ATR s-polarized spectrum. This critical insight paved the way for an improved non-iterative Kramers–Kronig analysis, facilitating the rapid calculation of the complex refractive index function (1).

This method achieves remarkable speed, running approximately two orders of magnitude faster than iterative approaches and completing within seconds on standard office PCs. The team validated their technique using various ATR crystals, including diamond, Zinc Selenide (ZnSe), and Germanium (Ge), along with different ATR accessories. Furthermore, they demonstrated its efficacy in correcting spectra of octadecane, highlighting its versatility across diverse samples (1).

The researchers also conducted rigorous simulations to assess potential sources of error, such as incidence angle spread and polarization angle dispersion (1). By addressing these factors, they ensured the reliability and accuracy of their correction scheme, positioning it as a superior alternative to existing methods.

Traditional methods of correcting infrared attenuated total reflection (ATR) spectra to resemble transmission spectra have long been hindered by several challenges (1–3). A key issue lies in the discrepancy between ATR and transmission spectra, where ATR introduces alterations in peak positions, peak intensities, and peak shapes, particularly pronounced at lower wavenumbers. This discrepancy stems from differences in effective thickness and penetration depth, influenced by the refractive index, leading to shifts in absorbance band maxima and significant changes in infrared spectral band shapes. While high-index ATR crystal materials and increased angles of incidence can mitigate some of these effects, they often sacrifice sensitivity and fail to fully eliminate ATR distortions. Additionally, the critical angle requirement for ATR spectroscopy imposes limitations on the types of samples that can be analyzed, further complicating the correction process (1–3).

Read More: Multicomponent Search and Advanced ATR Correction

This innovation achieved by Mayerhöfer, Costa, and Popp represents a significant advancement in ATR correction methodology. By streamlining the process and eliminating iterative complexities, their approach not only enhances efficiency but also preserves precision, crucial for reliable spectral analysis. With its wide applicability and robust performance, this advanced method is poised to provide a new tool for infrared spectroscopy, opening potentially new applications for ATR-based research, discovery, and routine laboratory and process analysis.

Through rigorous experimentation and thorough error analysis, the researchers have laid a solid theoretical and practical foundation for future advancements in spectroscopic techniques. Their work underscores the importance of continual innovation in analytical chemistry, driving progress towards more efficient, sustainable, and accurate methods for general analysis and scientific inquiry.

References

(1) Mayerhöfer, T. G.; Costa, W. D. P.; Popp, J. Sophisticated Attenuated Total Reflection Correction Within Seconds for Unpolarized Incident Light at 45°. Appl. Spectrosc. 2024, 78 (3), 321–328. DOI: 10.1177/00037028231219528

(2) Sobieski, B.; Chase, B.; Noda, I.; Rabolt, J. Artifact Correction in Temperature-Dependent Attenuated Total Reflection Infrared (ATR-IR) Spectra. Appl. Spectros. 2017, 71 (8), 1868–1875. DOI: 10.1177/0003702817690408

(3) Perera, U. D. N.; Nishikida, K.; Lavine, B. K. Development of Infrared Library Search Prefilters for Automotive Clear Coats from Simulated Attenuated Total Reflection (ATR) Spectra. Appl. Spectrosc. 2018, 72 (6), 886–895. DOI: 10.1177/0003702818759664