Mapping Measurement of Paints and Pigments with Thermoelectrically Cooled MCT Detector

Significance of the Topic

Infrared microscopy offers non-destructive, spatially resolved chemical analysis of multi-layered samples, such as automotive coatings and historical pigments. By mapping molecular distributions without liquid nitrogen, newer thermoelectrically cooled detectors simplify workflows and improve safety. This capability is critical in forensic investigations—identifying vehicle models from paint fragments—and in art conservation—revealing pigment composition and informing restoration strategies.

Objectives and Overview of the Study

This study evaluates the performance of an infrared microscopy system (IRTracer-100 with AIMsight) equipped with a thermoelectrically cooled mercury
cadmium telluride (TEC MCT) detector for chemical mapping of:

• Cross-sectioned automobile bumper coatings (transmission mode).
• Mixed yellow organic pigments (reflection mode).

Key goals include assessing spatial resolution, sensitivity, and the ability to distinguish constituents without liquid nitrogen cooling.

Methodology and Instrumentation

Samples and preparation:

• Automotive bumper: 2 cm square, sectioned by microtome into 5 µm slices.
• Organic pigments A and B: dissolved in ethanol, deposited on aluminum mirror.

Measurement conditions for coatings (Table 1):

• Instrument: IRTracer-100 with AIMsight infrared microscope.
• Detector: TEC MCT (25 × 100 µm aperture).
• Resolution: 8 cm⁻¹, scans: 100, step size: 3 µm, mapping area: 186 × 100 µm.

Measurement conditions for pigments (Table 2):

• Instrument: same as above.
• Detector: TEC MCT (50 × 50 µm aperture).
• Resolution: 8 cm⁻¹, scans: 50, step size: 50 µm, mapping area: 550 × 1 100 µm.

Data processing:

• Transmission and reflection spectra acquisition.
• Composite chemical imaging based on peak intensities, multivariate analyses (PCR/MCR), and library matching.

Key Results and Discussion

Automobile coating mapping:

• Microscopic imaging revealed three paint layers (~20 µm clear, ~20 µm color, ~30 µm undercoat) on a PP substrate.
• Infrared spectra of all three layers matched acrylic polymers with additions of polyurethane (N–H, C–O peaks) and ABS (C≡N, C=C–H peaks).
• Substrate spectrum matched polypropylene with talc additive.
• Chemical images clearly distinguished each layer by assigning colors to spectral similarity indices, visualizing layer boundaries and compositions.

Pigment mapping:

• Infrared spectra of pigment A and B showed overlapping profiles but distinct wavenumber regions.
• Composite images of the mixed deposit displayed spatial segregation: pigment A dominant on one side, pigment B on the other, with zones of overlap identified by spectral similarity scoring.

These results demonstrate that TEC MCT–based mapping achieves high spatial resolution (down to 25 µm) and chemical specificity without liquid nitrogen, facilitating analysis of microscopic heterogeneous samples.

Benefits and Practical Applications of the Method

• Elimination of liquid nitrogen reduces operational hazards and supply issues.
• High sensitivity and spatial resolution for mapping micro-layered structures.
• Rapid identification of polymer and pigment distributions.
• Applicable to forensic paint analysis, industrial quality control, and cultural heritage studies.

Future Trends and Potential Applications

Advancements may include:

• Integration of faster scanning stages and enhanced focal optics to achieve sub-10 µm resolution without cryogenics.
• Expanded multivariate and machine-learning algorithms for automated component classification and quantification.
• Portable, field-deployable infrared microscopes for on-site forensic and conservation work.
• Coupling IR mapping with complementary imaging modalities (Raman, SEM) for correlative analysis.

Conclusion

An infrared microscopy system with a thermoelectrically cooled MCT detector successfully mapped automotive paint layers and mixed pigments, matching traditional methods in sensitivity and resolution while avoiding liquid nitrogen. This approach streamlines workflows in forensic, industrial, and conservation laboratories by combining chemical specificity with spatial mapping.

References

• Application News No. 01-00826: Sensitivity Evaluation and Example Analysis of Microscopic Targets with Thermoelectrically Cooled MCT Detector.
• Application News No. 01-00822-EN: Mapping Measurement of Paints and Pigments with Thermoelectrically Cooled MCT Detector.