Dating of forensic bloodstains through spectroscopic techniques – a critical comparison towards a multiplatform approach

It is widely recognized that bloodstains dating may represent a crucial evidence in resolving many cases of forensic interest. Since the non-destructive techniques are most desirable, spectroscopies appear to be particularly attractive. Even though this topic has been investigated since the 1960s, most scientific strain focused on UV-Vis spectroscopy, while few studies were conducted in the IR spectral region. Nowadays, NIR spectroscopy has been an emerging technique thanks to the contribution of chemometrics, therefore it is likely that combination of NIR and multivariate data analysis might be determinant in forensics as well. 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. To reach this aim, capillary blood was collected from 5 healthy volunteers, deposited onto glass substrates, and stored under stable laboratory conditions in four different sessions. The aging of bloodstains was followed for 16 days measuring transflection NIR spectra and diffuse-transmittance UV-Vis spectra. The resulting data were independently analyzed by several chemometric techniques. Pre-processing of spectral profiles was conducted first with classical row pre-processing methods, such as Savitzky-Golay smoothing and standard normal variate (SNV) transform. After conducting an exploratory evaluation by principal component analysis (PCA), systematic differences between samples due to factors unrelated to aging were detected as predominant in UV-Vis spectroscopy and corrected by means of class centering. This column preprocessing was not needed for NIR spectroscopy. Subsequently, a partial least square (PLS) regression model was computed using 3 of the subjects, in order to evaluate the effectiveness of applied spectral methods for estimating time elapsed since blood trace deposition. The model was tested on the remainig two subjects, which were used as an independent test set. The two techniques gave comparable root means square errors in prediction (RMSEP) equal to about one day. In view of the creation of a multi-instrumental platform for bloodstains dating, data fusion strategies were also tested to estimate the advantages and disadvantages of low-level and mid-level approaches. The present results together with the regression models obtained from them looks promising. NIR spectroscopy is therefore seemingly worthy of increased scientific attention in the forensic field for bloodstains dating. Subsequently, the effect on blood ageing of temperature, humidity, illumination and substrate was tested with the help of an environmental camera. Preliminary results will be presented.