| 03 Jun 2025
Assessing Non-Ideal Instrumental Effects in High-Resolution FTIR Spectroscopy: Instrument Performance Characterization
Abstract. This study investigates the impact of non-ideal instrumental effects on the performance of high-resolution Fourier Transform Infrared (FTIR) spectrometers, with a focus on the Bruker FTS 120M. Key non-idealities, including retroreflector misalignments, baseline drift, and spectral channeling, were systematically analyzed using advanced diagnostic tools such as ALIGN60 and LINEFIT. The nominal configuration exhibited significant anomalies, notably modulation efficiency (ME) deviations of up to +10.9 %, phase error (PE) variability of 2.11 × 10−2 radians, and spectral channeling frequencies such as a persistent 2.9044 cm−1, along with emerging frequencies around 0.24 cm−1 attributed to retroreflector wear and CaF2 beamsplitter degradation. A pronounced anomaly at 40.672 cm−1, likely induced by environmental factors such as external vibrations or mechanical instability, was also identified. Implementation of a modified configuration successfully mitigated these issues, reducing PE variability to 0.042 × 10−2 radians, aligning ME within the NDACC-acceptable threshold of 1.1, and achieving substantial improvements in the instrument line shape (ILS), including sharper peaks, narrower full-width at half maximum (FWHM), and reduced side-lobe asymmetry. Analysis of HBr transmission spectra revealed improved fitting of the P(6) line, characterized by lower residuals and enhanced spectral quality. Simulated Haidinger fringes near zero path difference (ZPD) highlighted alignment degradation patterns, underscoring the necessity for precise optical adjustments. Temporal trends showed a 14 % increase in ILS peak height and significant RMSE reductions in the modified configuration. Overall, this study provides a robust framework for diagnosing and correcting instrumental artifacts, ensuring the accuracy, reproducibility, and long-term stability of FTIR measurements essential for atmospheric trace gas retrievals.
The paper systematically investigates non-ideal instrumental effects (e.g., retroreflector misalignments, baseline drift, spectral channeling) in the Bruker FTS 120M spectrometer, employing diagnostic tools (ALIGN60, LINEFIT) for characterization and mitigation. This paper well highlights the importance of routine performance evaluations and maintenance schedules for long-term operational stability of FTIR measurements essential for atmospheric trace gas retrievals. While the research is technically sound and addresses a critical gap in NDACC/TCCON protocols, several issues require clarification and revision to enhance clarity, consistency, and scientific rigor.