Mapping Measurement of Paints and Pigments with Thermoelectrically Cooled MCT Detector

Significance of the Topic

Infrared microscopy coupled with thermoelectrically cooled MCT detectors offers non-cryogenic, high-resolution chemical mapping of microscopic multilayer samples. This capability is critical in forensic investigations of automotive paint fragments and in heritage science for non-destructive pigment analysis, enabling identification and spatial distribution of polymers and organic compounds without liquid nitrogen handling risks.

Objectives and Study Overview

This application note demonstrates mapping measurements of:

• Automotive bumper paint layers to identify polymer composition and layer structure.
• Binary mixtures of yellow organic pigments to resolve spatial distribution.

The aim is to evaluate a thermoelectrically cooled MCT detector’s sensitivity and mapping performance compared to traditional liquid-nitrogen-cooled detectors.

Methodology and Instrumentation

Samples were sectioned or deposited and analyzed in transmission or reflection mode. Two experiments used distinct measurement settings:

• Automotive Paint Mapping
  
  • Instrument: IRTracer-100 with AIMsight microscope
  • Detector: Thermoelectrically cooled MCT
  • Resolution: 8 cm⁻¹, 100 scans, SqrTriangle apodization
  • Aperture: 25 × 100 µm, step size 3 µm, mapping area 186 × 100 µm
• Organic Pigment Mapping
  
  • Instrument: IRTracer-100 with AIMsight microscope
  • Detector: Thermoelectrically cooled MCT
  • Resolution: 8 cm⁻¹, 50 scans, SqrTriangle apodization
  • Aperture: 50 × 50 µm, step size 50 µm, mapping area 550 × 1,100 µm

Main Results and Discussion

• Automotive Coating
  
  • Three paint layers (~20 µm clear, ~20 µm color, ~30 µm undercoat) all exhibited infrared bands of acrylic polymers, polyurethane (N-H, C-O), and traces of ABS (C≡N, C=C-H).
  • Bumper substrate matched polypropylene with talc filler.
  • Chemical images constructed from spectral similarity revealed precise layer boundaries and thicknesses not evident in optical micrographs.
• Organic Pigments
  
  • Infrared spectra of pigments A and B overlapped but specific wavelength regions allowed differentiation.
  • Composite chemical maps showed spatial segregation: pigment A predominated on one side, pigment B on the opposite, with intermixing zones detected.

Benefits and Practical Applications

• Eliminates need for liquid nitrogen, reducing operational hazards and supply issues.
• Enables high-resolution chemical mapping of layers and heterogeneous samples down to 25 µm in transmission mode.
• Supports forensic paint matching, quality control in coating processes, and heritage science investigations.

Future Trends and Applications

• Integration of multivariate data analysis (PCR/MCR) to enhance component discrimination in complex matrices.
• Automated workflows for rapid mapping over larger areas and three-dimensional reconstructions.
• Expanded detector technologies for mid-IR imaging with improved sensitivity and spatial resolution.

Conclusion

A thermoelectrically cooled MCT detector integrated with infrared microscopy enables sensitive, non-cryogenic mapping of automotive coatings and organic pigment mixtures. The approach accurately resolves layer compositions and spatial distributions, offering a safer, more accessible alternative to liquid-nitrogen-based systems, with broad applications in forensic, industrial, and cultural heritage analyses.

References

• Shimadzu Corporation. Application News No. 01-00822-EN. Jan. 2025.
• Shimadzu Corporation. Application News No. 01-00826. Sensitivity Evaluation and Microscopic Targets Analysis.