Advantages of a Fourier Transform Infrared Spectrometer

Significance of the topic

The transition from dispersive to Fourier transform infrared (FT-IR) spectroscopy represents a fundamental advance in vibrational analysis of organic and inorganic materials. FT-IR instruments deliver major improvements in speed, sensitivity and reproducibility compared with earlier grating-based (dispersive) designs, enabling routine, high-quality spectral analysis in academic research, quality control, forensics and industrial laboratories.

Objectives and overview of the technical note

The document compares dispersive and FT-IR spectrometers, explains the operating principles of FT-IR interferometry and the Fourier transform processing, and summarizes the principal advantages of FT-IR instruments. It also highlights practical performance characteristics using the Thermo Scientific Nicolet iS10 as an illustrative example.

Methodology and working principles

The dispersive (grating) spectrometer isolates individual wavelengths by spatially dispersing the source with a grating and selecting narrow bands with a slit while scanning. Wavelengths are measured sequentially, and spectral calibration requires an external reference.

FT-IR uses an interferometer (beamsplitter plus fixed and moving mirrors) to generate an interferogram: all wavelengths are encoded simultaneously as constructive and destructive interference while the moving mirror changes the optical path difference. A reference laser provides precise timing and an internal wavenumber standard. The detector records the composite interferogram, which is digitized and converted into a spectrum by Fourier transform. Background (single-beam) and sample single-beam acquisitions are ratioed to yield absorbance or transmittance spectra.

The technical note also outlines common spectral units (wavelength in nm, and the infrared-preferred wavenumber in cm-1) and notes that FT-IR spectra typically report in wavenumbers because they are linear in energy.

Used instrumentation

The technical note describes the general FT-IR hardware: broadband IR source, beamsplitter, fixed and moving mirrors (interferometer), a reference laser for timing and internal calibration, and one or more detectors. As a commercial example, the Thermo Scientific Nicolet iS10 FT-IR spectrometer is cited with performance characteristics including a 10,000:1 signal-to-noise ratio achieved in 5 seconds, spectral resolution better than 0.4 cm-1 and routine attenuated total reflectance (ATR) capability in a compact footprint.

Main results and discussion

The document emphasizes three main advantages of FT-IR versus dispersive instruments:

• Multiplex advantage (Fellgett advantage): FT-IR collects information from all wavelengths simultaneously. Each mirror stroke encodes the whole spectrum, enabling many rapid scans and effective signal averaging; this typically yields faster acquisitions than grating scanning, where each wavelength must be measured sequentially.
• Throughput advantage (Jacquinot advantage): FT-IR designs do not rely on a narrow entrance slit, and modern designs minimize reflective losses. More optical power reaches the sample and detector, increasing sensitivity and improving signal-to-noise ratio.
• Precision advantage (Connes advantage): The internal reference laser provides highly stable, reproducible wavenumber calibration and timing, so spectra collected across long time intervals or on different instruments are directly comparable without external wavelength calibration standards.

The note points out practical consequences: improved detection limits for weak absorptions, the ability to resolve fine spectral features (e.g., in protein amide bands), faster high-resolution scanning without severe light loss, and consistent spectral accuracy over time. It also notes that dispersive spectrometers suffer from trade-offs between resolution and throughput because closing the slit for higher resolution reduces signal dramatically.

Benefits and practical applications

Upgrading to FT-IR yields immediate advantages in spectral quality, acquisition speed, reproducibility and ease of use. Typical application areas include:

• Identification and structural probing of organic and polymeric materials.
• Quality control and QA/QC workflows requiring reproducible, high-throughput spectra.
• Forensic and environmental analysis where trace-level detection and spectral fidelity are critical.
• Biomolecular studies (proteins, lipids) where sensitivity to subtle band shapes matters.
• Routine ATR measurements, thin-film and bulk sample characterization, and laboratory workflows that benefit from rapid background/sample ratios.

Future trends and potential applications

Building on FT-IR strengths, likely directions for development and broader application include:

• Miniaturized and portable FT-IR systems for field and on-line process monitoring.
• Enhanced detectors and cooled focal-plane arrays enabling faster, more sensitive imaging and microscopy.
• Integration with chemometrics and machine learning for automated identification, quantification and spectral deconvolution.
• Hyphenated techniques such as GC-IR or AFM-IR for spatially resolved or coupled separation analyses.
• Use of quantum cascade lasers (QCLs) and tunable sources for high-brightness, narrowband applications and rapid scanning in targeted spectral regions.
• Improved ATR accessory designs and sampling interfaces for challenging matrices and in situ measurements.

Conclusion

FT-IR spectroscopy fundamentally outperforms classical dispersive infrared instruments in speed, sensitivity and spectral fidelity for most routine and advanced applications. The multiplex, throughput and precision advantages have made FT-IR the standard approach across research, industrial and forensic laboratories. Modern FT-IR instruments combine robust optics, internal laser referencing and powerful digital processing to deliver reproducible, high-resolution spectra with operational convenience.

References

1. Thermo Fisher Scientific. Technical Note 50674: Advantages of a Fourier Transform Infrared Spectrometer. 2008–2015. Thermo Scientific Nicolet iS10 specifications and application notes.