Sensitivity Evaluation and Example Analysis of Microscopic Targets with Thermoelectrically Cooled MCT Detector

Importance of the Topic

Infrared microscopy enables detailed chemical analysis of microscopic samples that are otherwise difficult to characterize, supporting quality control and failure analysis across industries. Traditional quantum detectors cooled by liquid nitrogen pose operational hazards and logistical challenges. The development of a thermoelectrically cooled mercury cadmium telluride (TEC MCT) detector addresses these issues by eliminating the need for liquid nitrogen, improving instrument safety and convenience while maintaining high sensitivity for small targets.

Study Objectives and Overview

This study compares the sensitivity and operational performance of three FTIR microscope detectors: the TEC MCT detector, the T2SL quantum detector (requiring liquid nitrogen), and the DLATGS thermal detector (room temperature). Two example analyses demonstrate the new TEC MCT detector’s capability to measure microscopic samples in transmission, reflection, and ATR modes without liquid nitrogen.

Methodology and Used Instrumentation

• Instruments: IRTracer-100 Fourier transform infrared spectrophotometer equipped with AIMsight infrared microscope and AIRsight infrared/Raman microscope in IR mode.
• Detectors compared: TEC MCT (thermoelectrically cooled), T2SL (liquid-nitrogen cooled), DLATGS (room temperature thermal detector).
• Measurement conditions (Table 1 summary): 8 cm⁻¹ resolution, SqrTriangle apodization, scans: T2SL 20 scans (7 s), TEC MCT and DLATGS 500 scans (140 s and 330 s respectively), apertures from 25×25 µm to 100×100 µm.
• Sample types: talc-filled polypropylene by transmission; substance on PCB terminal by reflection; adhesive-roller contaminant by ATR.

Key Results and Discussion

• Detector Sensitivity: TEC MCT achieved low-noise spectra from a 25×25 µm target in transmission, requiring more scans than T2SL but avoiding liquid nitrogen. DLATGS lacked sensitivity for sub-100 µm samples but provided data below 700 cm⁻¹.
• Aperture and Scan Trade-off: Increasing aperture to 50×50 µm improved signal-to-noise for TEC MCT, enabling adequate spectra with only 20 scans (7 s measurement).
• Example 1 (Reflection on PCB Terminal): Both TEC MCT and T2SL detected a solder-resist contaminant on a 30×40 µm spot without sample prep, demonstrating sub-50 µm reflection analysis without liquid nitrogen.
• Example 2 (ATR of Roller Contaminant): ATR spectra acquired by TEC MCT matched a polyethylene terephthalate library pattern, confirming polyester-based contamination using a 200×200 µm aperture.

Benefits and Practical Applications

• Eliminates liquid nitrogen handling, reducing safety risks and operational costs.
• Enables high-sensitivity analysis of 25 µm microscopic targets in transmission, reflection, and ATR modes.
• Maintains compatibility with existing microscopy workflows and diverse sample types.

Future Trends and Applications

• Further miniaturization and cooling improvements to push detection limits below 25 µm without cryogens.
• Integration of infrared and Raman modalities for multimodal microanalysis on a single platform.
• Automation and AI-driven spectral interpretation to accelerate contamination tracking and quality control.

Conclusion

The TEC MCT detector offers a practical alternative to liquid-nitrogen-cooled quantum detectors for FTIR microscopy, enabling reliable analysis of microscopic samples down to 25 µm in size across multiple measurement modes. While the T2SL detector remains preferable for sub-25 µm targets and rapid scanning, the TEC MCT enhances laboratory safety and convenience. Future advances are expected to expand sensitivity and analytical versatility further.

References

1) FTIR TALK LETTER vol.12: Using Infrared Detectors – Pyroelectric Detectors