Raman Spectroscopy of Pigments

Request for analysis:
A company was experiencing paint defects in their blue basecoat. The defects appeared as round dark blue spots within the blue basecoat paint.

  • To determine if the source of the contaminant was from a different pigment not specified in the basecoat formulation.

The defects were cross-sectioned and thin slices removed from the defect area. The defects were then examined using optical microscopy, energy dispersive X-ray (EDX) spectroscopy and Raman spectroscopy.
Results of the study:
An optical microscope image presented in Figure 1 shows that the defect is spherical in shape and is a more intense blue than the surrounding blue basecoat. Elemental analysis through the use of EDX spectroscopy was performed on one of the defects. This technique detected high levels of carbon, oxygen, nitrogen and iron within the defect.
Laser Raman spectroscopy was then performed and the spectrum of the blue defect material was matched to that of ferric ferrocyanide, a common blue pigment. A comparison of the defect material with a reference ferric ferrocyanide is presented in Figure 2. Ferric ferrocyanide was NOT the specified blue pigment for use in this basecoat.

Figure 1. Optical microscope image showing spherical blue defect.

Figure 2. Raman spectra showing the defect material