Abstract
Attenuated total reflection (ATR) is a widely used sampling technique in infrared (IR)
spectroscopy because minimal sample preparation is required. Since the penetration depth of
the ATR analysis beam is quite shallow, the outer layers of a laminate or 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 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 hinders library searching
because most library spectra are measured in transmission mode. Furthermore, the angle of
incidence for the internal reflection element, the refractive index of the clear coat, and
surface contamination due to inorganic contaminants can profoundly influence the quality of
the ATR spectrum obtained for automotive paints. A correction algorithm to allow ATR spectra
to be searched using IR transmission spectra of the paint data query (PDQ) automotive database
is presented. The proposed correction algorithm to convert transmission spectra from the PDQ
library to ATR spectra 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.
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