This article presents a correction algorithm to enable attenuated total reflection (ATR) spectra to be searched using infrared (IR) transmission spectra of the paint data query (PDQ) automotive database.
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. The current article proposes a correction algorithm to convert transmission spectra from the PDQ library to ATR spectra, thus addressing distortion issues such as the relative intensities and broadening of the bands, as well as the introduction of wavelength shifts at lower frequencies, which prevent library searching of ATR spectra using archived IR transmission data. (publisher abstract modified)