Assessing non-ideal instrumental effects in high-resolution FTIR spectroscopy: instrument performance characterization

<p>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 <span class="inline-formula">+10.9 <i>%</i></span>, phase error (PE) variability of 2.11 <span class="inline-formula">×</span> 10<span class="inline-formula">⁻²</span> radians, and spectral channeling frequencies such as a persistent 2.9044 cm<span class="inline-formula">⁻¹</span>, along with emerging frequencies around 0.24 cm<span class="inline-formula">⁻¹</span> attributed to retroreflector wear and CaF<span class="inline-formula">₂</span> beamsplitter degradation. A pronounced anomaly at 40.672 cm<span class="inline-formula">⁻¹</span>, likely induced by environmental factors such as external vibrations or mechanical instability, was also identified. Implementation of a modified configuration effectively addressed these issues, reducing PE variability to 0.042 <span class="inline-formula">×</span> 10<span class="inline-formula">⁻²</span> 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 sidelobe asymmetry. Analysis of HBr transmission spectra revealed improved fitting of the P(2) 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 an increase in ILS peak height of <span class="inline-formula">&gt;14 <i>%</i></span> associated with the instrument upgrade, together with significant mean absolute error (MAE) reductions achieved by the modified configuration. In addition, a targeted retrieval case study demonstrates that explicit propagation of the empirically characterized instrumental response into the forward model reduces spectral residuals and retrieval uncertainties while increasing the retrieved total column by approximately 6 %–7 % relative to the nominal 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.</p>