Advantages of MCT-B Detectors in FTIR Microscopy

Significance of the topic

The selection of infrared detectors determines the usable spectral window and sensitivity of FTIR microscopy analyses. For many materials—especially inorganic compounds, metal oxides and pigments—important diagnostic vibrational features lie at low wavenumbers. Understanding the trade-offs between extended spectral range and detector performance is essential for accurate chemical identification and imaging in research, conservation science, quality control and materials characterization.

Objectives and study overview

This technical note demonstrates the analytical advantages of a longer-wavelength Mercury Cadmium Telluride (MCT-B) detector versus a conventional MCT-A detector in an FTIR microscope. Two practical examples are presented: (1) mapping corrosion products (cuprous and cupric oxides) on a penny, and (2) differentiating titanium dioxide–containing and plain polyethylene layers in a polymer cross-section. The goal is to show how extended low-frequency coverage affects spectral detection, chemical imaging and interpretation.

Methodology

The experiments use FTIR micro-spectroscopy to collect point spectra and chemical images across small sample regions. Spectral comparison is made between two detector cutoff limits: a higher cutoff typical of MCT-A (around 650 cm-1) and an extended low-wavenumber cutoff provided by an MCT-B detector (around 450 cm-1). Peak-area imaging and correlation imaging routines are used to translate spectral differences into spatial chemical maps. Data acquisition parameters may be adjusted (notably integration/collection time) to compensate for lower detector responsivity when using the MCT-B.

Used instrumentation

The analyses were performed on a Thermo Scientific Nicolet RaptIR+ FTIR Microscope equipped with user-swappable MCT detectors. Key detector characteristics described in the study:

• MCT-A: typical spectral range up to ~4000–650 cm-1 with higher responsivity/detectivity.
• MCT-B: extended low-wavenumber response (example cutoff near 450 cm-1) at the expense of lower responsivity and detectivity (reported approximately four times lower than MCT-A for some compositions).

Practical operation includes swapping detectors depending on analytical priorities and increasing collection time to recover signal-to-noise when using lower-sensitivity detectors.

Main results and discussion

Example 1 — Copper corrosion products:

• Spatial FTIR imaging of a penny revealed regions of cuprous oxide (Cu2O) and cupric oxide (CuO). Both oxides exhibit characteristic vibrational bands at wavenumbers below 650 cm-1.
• These spectral features fall in the extended low-frequency range accessible with the MCT-B detector; an MCT-A detector with cutoff near 650 cm-1 would miss or severely truncate those bands, preventing reliable identification and mapping.

Example 2 — Layered polymer with TiO2 pigment:

• A polymer cross-section containing two polyethylene layers (one loaded with TiO2) was analyzed. The TiO2 diagnostic absorption lies partly below 650 cm-1.
• With an MCT-A detector, the TiO2 feature appears only as a shoulder at the spectral cutoff, which reduces spectral specificity. Correlation imaging based on truncated spectrum overestimates similarity between the pigment-loaded and plain polyethylene layers, producing ambiguous compositional maps.
• Using the MCT-B detector to extend coverage to ~450 cm-1 captures the full TiO2 band, markedly improving contrast and enabling unambiguous distinction between the two layers in correlation images.

Overall implications:

• Extended low-frequency coverage can be essential for detecting heavier-atom vibrational modes (metal oxides, minerals, pigments) that are otherwise invisible with standard MCT detectors.
• The MCT-B detector’s reduced sensitivity can be mitigated by longer acquisition times or optimized sampling strategies, making it suitable for targeted applications where low-wavenumber information is decisive.

Benefits and practical applications

Advantages of adopting a swappable MCT-B option include:

• Improved material identification for inorganics and pigments due to access to low-frequency bands.
• Enhanced chemical imaging contrast and more reliable correlation maps for samples with features near the long-wavelength cutoff of standard detectors.
• Flexibility in the laboratory workflow—users can switch detectors depending on whether sensitivity or spectral range is the priority.

Relevant applications: cultural heritage and conservation (corrosion/product identification), polymer and composite analysis, pigment and filler characterization, failure analysis, and any micro-FTIR studies requiring low-wavenumber diagnostics.

Future trends and potential uses

Direction for future developments and practice:

• Detector material engineering may narrow the responsivity gap while preserving extended spectral coverage, reducing the need to trade sensitivity for range.
• Advances in data processing (denoising, spectral reconstruction) could further compensate for lower detector signal and improve usable information from extended-range detectors.
• Routine workflows will likely integrate detector swapping or parallel multi-detector systems to capture both high-sensitivity and extended-range data without compromising throughput.
• Combination with complementary techniques (e.g., Raman microscopy, SEM-EDS) will strengthen multi-modal analysis of complex materials where both organic and inorganic phases coexist.

Conclusion

Extending the low-wavenumber limit of FTIR microscopy with an MCT-B detector enables detection and imaging of vibrational modes characteristic of metal oxides and some pigments that are missed by conventional MCT-A detectors. Although MCT-B detectors typically have lower responsivity, their analytical value for particular use cases—corrosion mapping, pigment identification and layered material differentiation—can outweigh the sensitivity penalty, especially when acquisition times are adjusted. The capability to swap detectors in instruments such as the Nicolet RaptIR+ provides practical flexibility for laboratories balancing sensitivity and spectral coverage.

References

Thermo Fisher Scientific. Technical note: Advantages of MCT-B detectors in FTIR microscopy. TN54676_E 10/23M. For research use only. Not for diagnostic procedures.