| Blood stains, which are among the traces encountered most frequently at crime scenes, are important for potential extraction and amplification of DNA for suspect identification, as well for spatter pattern analysis to reveal a sequence of events.;Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used in detection of blood stains and age estimation because of signature absorbances in the mid-infrared region at 3300 cm-1 (Amide A), 2800-3000 cm-1 (Amide B), ∼1650 cm-1 (Amide I), ∼1540 cm-1 (Amide II) ,and 1200-1350 cm-1 (Amide III).;Position and intensity shifts for amide peaks were observed due to aging changes occurring as a result of the denaturation of blood proteins and water absorption/desorption. Partial least square (PLS) regression was used in this work to combine these changes in a multivariate calibration for blood age estimation. Calibration experiments over several months at 30°C under a variety of humidity and substrates enable prediction of blood stain age under different environmental conditions.;Amide peak intensity changes in the spectrum can be related to blood concentration. Multivariate calibrations of IR spectra of blood dilutions on four types of fabric (acrylic, nylon, polyester, and cotton) were built using PLS. Gap derivatives (GDs) were applied as a preprocessing technique to optimize the performance of calibration models. Detection limits of 0.028 microg/cm2 for acrylic, 0.020 microg/cm2 for nylon, 0.017 microg/cm2 for polyester, and 0.0027 microg/cm2 for cotton were found.;As a popular technique for rapid infrared spectra collection, ATR-FTIR requires samples to be compressed against an internal reflection element. However, depending on the accessory design, the pressure applied to the sample is not always well controlled. While collecting data from fabrics with heterogeneous coatings, we have observed systematic pressure-dependent changes in spectra that can be eliminated by reproducible pressure control. We describe a pressure sensor adapted to work with an ATR pressure tower to enable more rigorous control of pressure during ATR sampling. |
