Exploiting spectroscopy and chemometrics for bloodstain dating in forensic chemistry

In a forensic context, non-destructive techniques are most desirable, due to the importance of preserving the specimens and to enable repeated analysis by all parts involved. For this reason, spectroscopies appear to be particularly attractive. By the way, the complex signals produced by those analytical techniques make it necessary to use chemometrics to extract useful information from the data. An interesting topic in which spectroscopy and chemometrics can be exploited in forensic chemistry is the determination of time since deposition of bloodstains. Bloodstains dating may, indeed, represent a crucial evidence in resolving many cases of forensic interest. Even though this topic has been investigated with limited success since the 1960s, most studies focused on ultraviolet-visible (UV-Vis) spectroscopy, while few were conducted in the infrared spectral region, with only a handful concerning near-infrared (NIR) spectroscopy. The final goal of the present work is to compare the performances of NIR spectroscopy for bloodstains dating with those of UV-Vis in the prospect of real-casework implementation. In pursuit of this goal, capillary blood was sampled and subjected to a 16-day aging, during which it was repeatedly analysed using both spectroscopic methods. Subsequently, chemometrics was applied to process the spectral data and independently assess the methods' performance. Classical preprocessing methods were used together with more targeted techniques (class centering) whose benefit was highlighted by principal component analysis (PCA). Lastly, partial least squares (PLS) regression models were computed to evaluate the effectiveness of both spectral methods in estimating the time elapsed since blood trace deposition. Comparable root mean square errors in prediction (RMSEP) were observed for both techniques, featuring an improvement with respect to the existing literature for NIR spectroscopy. Data fusion strategies for a multi-instrumental platform were also explored. Subsequently, the effects of environmental conditions, light exposure and substrate on sample ageing were studied. Controlled ageing was carried out with the use of a climatic chamber and the ageing of samples deposited on cotton, polyblend fabric, metal and glass was followed for two weeks through NIR and Raman spectroscopies. Feasibility studies were also performed applying hyperspectral imaging for a contemporary identification and dating of bloodstains. The results indicated that NIR spectroscopy integrated with adequate chemometric strategies deserves increased appreciation in forensic bloodstain dating.