ATR is a widely used sampling technique in IR spectroscopy because minimal sample preparation is required. Using ATR, the outer layers of a multilayered paint sample can be preferentially analyzed with the entire sample intact. For this reason, forensic laboratories are taking advantage of ATR to collect IR spectra of automotive paint systems that may consist of three or more paint layers. However, the IR spectrum of a paint sample obtained by ATR will exhibit distortions, e.g., band broadening and lower relative intensities at higher wavenumbers, compared with its transmission counterpart.  This prevents library searching as most library spectra are measured in transmission mode.  A correction algorithm to allow ATR spectra to be searched using IR transmission spectra from the paint data query (PDQ) automotive database will be discussed in this presentation.  The proposed algorithm converts transmission spectra from the PDQ library to ATR spectra because it is able to address distortion issues such as the relative intensities and broadening of the bands, and the introduction of wavelength shifts at lower frequencies, which prevent library searching of ATR spectra using archived IR transmission data.  The angle of incidence for the internal reflection element, the refractive index of the clear coat, and surface contamination due to inorganic contaminants will profoundly influence the quality of the spectral library match obtained for automotive clear coats.

Simulation of attenuated total reflectance (ATR) infrared (IR) absorbance spectra from IR transmission data for library searching of forensic automotive databases

248th National Meeting (2014)

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