Forensic discrimination of photocopy and printer toners I. The development of an infrared spectral library

Abstract

Microscopical reflection-absorption by infrared spectroscopy (R-A IR) was shown as a viable technique for analyzing the polymer resins contained in dry, black photocopy and printer toners. The sampling technique involves a heat transfer of the toner from a document to the reflective surface of aluminum foil followed by analysis by R-A IR. The technique is simple, fast, and readily available to most forensic laboratories. A searchable spectral library was created that contains 807 toner samples analyzed by R-A IR. Ninety-eight groups were established based on spectral characteristics, and a flowchart was developed to assist with group assignments. A blind study was conducted to compare twenty photocopied documents each paired to a test document to determine if the pair could have been produced from the same copier. The analyst obtained 100% correct results in this study. Tests on thirty samples with the spectral library produced 90% first hits for the correct group. The three remaining samples were correctly determined by visual comparison of spectra for the top three hits. An actual case study was conducted where the investigation was narrowed from 400 possible machines to eight based on a comparative study of the photocopy toners.

Química forense: perspectivas sobre novos métodos analíticos aplicados à documentoscopia, balística e drogas de abuso

“…Embora a química forense seja um tema muito importante e que desperte cada vez mais interesse perante a sociedade científica, a sua aplicação no campo da criminalística ainda constitui uma nova linha de pesquisa no Brasil. A Figura 1 mostra a distribuição dos recentes métodos analíticos que estão sendo aplicados em documentoscopia, balística forense e drogas de abuso, nos períodos de (1a) 1990-2000 e (1b-c) 2001-2010. Na documentoscopia, até 2000, as técnicas de espectroscopia molecular (infravermelho, [1][2][3][4] Raman, 5,6 fluorescência molecular 7 ) e de separação (cromatografia [8][9][10] e eletroforese [11][12][13][14][15] ) foram as mais empregadas, apresentando juntas, 219 citações (Figura 1a). A partir de 2001, com o surgimento de novas técnicas em espectrometria de massas (MS -mass spectrometry), problemas complexos que existiam até o momento tornaram-se solucionáveis.…”

Forensic discrimination of photocopy and printer toners. III. Multivariate statistics applied to scanning electron microscopy and pyrolysis gas chromatography/mass spectrometry

“…The median autoscaled data set was analyzed by PCA to reduce the data dimensionality to a manageable level and to see if any major trends or clusters were evident in plots of the first few principal components [33,34]. In projections of the data into the space of the first few PCs, toner samples were labeled according to the reflection-absorption infrared (R-A IR) spectra grouping scheme previously established [4]. Agglomerative hierarchical cluster analysis [35,36] was used in an exploratory fashion to group the samples analyzed by SEM without regard to their R-A IR classification.…”

“…The ease of sample preparation and the discriminating information on polymer and organic components produced by IR spectroscopy of toners has been demonstrated. Merrill et al [4] have developed a library of reflection-absorption (R-A) IR spectra from dry toners and have separated the spectra into distinguishable groups. Egan et al [5] have verified the grouping of copy toners in this library by achieving 96% correct classification into the previously defined groups using multivariate discriminant analysis on 430 selected toner spectra.…”

“…Because of the large number of manufacturers producing photocopy machines and toners, identifying the possible source of photocopied documents is not an easy task for the forensic scientist. Our two previous papers on the forensic analysis of photocopy and printer toners dealt with discrimination achievable by reflection-absorption (R-A) IR spectroscopy [4,5].…”

Hyperspectral imaging for non-contact analysis of forensic traces

“…Gebhardt et al showed that this change in measurement setup results in a disparity between measured spectra, which is dependent on the optical properties of the specimen and the optical path length [56]. Spectral data bases created for the identification of forensic traces like fibers or printer toners [57,58] may therefore need to be adapted to HSI applications.…”