FTIR-PAS spectra of the common types of nylon fibers appear in Fig. 17. These fibers are specimens from a fiber library assembled by the US Federal Bureau of Investigation (FBI) forensic science program. The strong absorbance bands of the FTIR-PAS spectra are truncated due to photoacoustic signal saturation but this does not detract from their value for qualitative analyses. Spectra of nylon fibers which are not in the FBI collection have been identified successfully using a FTIR-PAS spectral library of the FBI fibers for computer searching. The fibers used to produce the two top spectra of Fig. 18 are not in the FBI collection but were identified as nylon 6,6 when computer searched against the library. The lower spectrum of Fig. 18 is the nylon 6,6 spectrum from the library. The peak positions of the two upper spectra are well matched to those of the lower library spectrum. The two upper spectra also show that variations are observed in relative peak intensities and shapes due to differences in fiber diameter and cross-sectional geometry. These variations provide additional information beyond just polymer type and could be useful in establishing that two fibers in a forensic investigation are from a common source.
The FBI collection consists of approximately 50 fibers and includes all of the common types of polymers. Over 20 FTIR-PAS spectra have been searched successfully against an FTIR-PAS spectral library of the FBI fibers without any false identifications.

Fig. 17. FTIR-PAS spectra of four common types of nylon single fibers.