Advantages of a Fourier Transform Infrared Spectrometer

Significance of the topic

Infrared (IR) spectroscopy is a cornerstone analytical technique for identification and characterization of organic materials across research, quality control and forensic laboratories. Advances from dispersive to Fourier transform infrared (FT-IR) instrumentation have transformed routine IR practice by delivering faster acquisition, higher sensitivity, superior reproducibility and simplified calibration, enabling measurements that were impractical or impossible with older instruments.

Objectives and overview of the note

This technical note explains the operational principles and practical advantages of FT-IR compared with classical dispersive (grating/scanning) IR spectrometers. It outlines the interferometer-based measurement approach, the key performance gains (multiplex, throughput and precision advantages), and illustrates these benefits with modern FT-IR features and typical analytical applications.

Methodology and instrumentation

The note contrasts two instrument classes and summarizes how FT-IR spectra are generated and processed:

• Dispersive spectrometers: use a source, entrance slit, diffraction grating (monochromator) and detector. The grating spatially disperses wavelengths and a narrow slit selects the wavelength that reaches the detector. Scans are performed wavelength-by-wavelength and require an external calibration source for wavelength accuracy.
• FT-IR spectrometers: use an interferometer (beamsplitter, fixed and moving mirrors) together with a reference laser and a broadband IR source. The moving mirror produces an interferogram containing the combined information of all wavelengths simultaneously. A Fourier transform of the interferogram yields the spectrum; background and sample single-beam ratios produce absorbance or transmittance spectra.

Instrumentation example highlighted in the note:

• Thermo Scientific Nicolet iS10 FT-IR: a compact research and QA/QC-grade instrument, quoted performance includes a 10,000:1 signal-to-noise ratio in 5 seconds and spectral resolution better than 0.4 cm-1, with robust ATR sampling capabilities.

Main results and discussion

The note identifies three principal FT-IR advantages over dispersive designs and explains their practical implications:

• Multiplex (Fellgett) advantage: because all wavelengths are measured simultaneously, an FT-IR can perform many rapid scans and average them to improve signal-to-noise faster than a dispersive instrument that must step through wavelengths sequentially. This yields shorter acquisition times for equivalent noise levels or improved sensitivity for the same time.
• Throughput (Jacquinot) advantage: FT-IR instruments do not use narrow entrance slits, and modern designs minimize reflective losses by reducing mirror count. The larger optical throughput increases the energy at the detector, improving sensitivity and enabling measurements on weakly absorbing or small samples.
• Precision and wavelength stability: an internal reference laser provides an intrinsic time and wavelength standard. The laser interferogram furnishes both precise timing of mirror motion and an absolute internal reference so spectra collected at different times are directly comparable without external calibration standards, improving long-term reproducibility.

Practical measurement consequences discussed include superior detectability of small absorption features (e.g., protein secondary-structure analysis by IR), high-resolution capability without severe signal loss, and consistent spectra over long timescales. The note emphasizes that performance gains have made FT-IR the de facto standard for organic compound identification since the 1980s.

Benefits and practical applications

Key benefits and uses highlighted:

• Faster data collection and the ability to average multiple scans, improving laboratory throughput and lowering per-sample measurement time.
• Higher signal-to-noise and sensitivity, enabling detection of weak bands and analysis of dilute or small samples.
• Stable, internally-referenced wavelength accuracy, simplifying QA/QC, spectral libraries usage and long-term monitoring.
• Routine compatibility with sampling accessories such as ATR (attenuated total reflectance), gas cells, and microscopy, broadening application scope in polymer analysis, pharmaceuticals, environmental monitoring, biochemistry (protein IR), and forensic identification.
• Lower maintenance relative to dispersive systems due to fewer moving optical parts and elimination of frequent external wavelength calibrations.

Used instrumentation

The note describes the general FT-IR interferometer components (source, beamsplitter, fixed and moving mirrors, reference laser, detector) and gives a specific product example (Thermo Scientific Nicolet iS10) to illustrate achievable specifications: high S/N, sub-0.4 cm-1 resolution and compact, robust construction suitable for ATR analyses.

Future trends and applications

Anticipated developments and opportunities for FT-IR include:

• Detector and source improvements that extend sensitivity and spectral range, including quantum and cooled detector technologies.
• Miniaturization and portable FT-IR systems for in-field or on-line process monitoring, enabling real-time analytics in manufacturing and environmental screening.
• Integration with microscopy, imaging and hyphenated techniques (e.g., GC-IR, LC-IR) to expand spatially resolved and complex-mixture analysis capabilities.
• Advanced chemometrics, machine learning and cloud-based spectral libraries to automate identification, deconvolution and quantitation across large datasets.
• Further automation and instrument networking to support high-throughput QA/QC workflows and remote supervision.

Conclusion

FT-IR spectroscopy, by exploiting interferometry and Fourier transformation, provides decisive advantages over dispersive IR in speed, sensitivity, resolution and reproducibility. These attributes underpin its widespread adoption across academic, industrial and forensic laboratories. Continued technological improvements and integration with computational analytics will further broaden FT-IR utility for routine and advanced spectroscopic challenges.

References

• Thermo Fisher Scientific. Technical Note 50674: Advantages of a Fourier Transform Infrared Spectrometer. Thermo Fisher Scientific; 2008–2015.