Using spectral information in forensic imaging

Improved detection of forensic evidence by combining narrow band photographic images taken at a range of wavelengths is dependent on the substance of interest having a significantly different spectrum from the underlying substrate. While some natural substances such as blood have distinctive spectral features which are readily distinguished from common colorants, this is not true for visualization agents commonly used in forensic science. We now show that it is possible to select reagents with narrow spectral features that lead to increased visibility using digital cameras and computer image enhancement programs even if their coloration is much less intense to the unaided eye than traditional reagents. The concept is illustrated by visualising latent fingermarks on paper with the zinc complex of Ruhemann's Purple, cyanoacrylate-fumed fingerprints with Eu(tta)(3)(phen), and soil prints with 2,6-bis(benzimidazol-2-yl)-4-[4'-(dimethylamino)phenyl]pyridine [BBIDMAPP]. In each case background correction is performed at one or two wavelengths bracketing the narrow absorption or emission band of these compounds. However, compounds with sharp spectral features would also lead to improved detection using more advanced algorithms such as principal component analysis.     

Potential for the Remediation of Methamphetamine Contamination Using Ozone,

The deposition of methamphetamine within indoor environments due to illegal activities can pose a health risk for occupants. Current cleaning techniques are costly and inefficient, calling for the development of alternative remediation methods. In addition, the fate of methamphetamine in indoor environments is largely unknown, negatively impacting our knowledge on the health risks associated with contaminated dwellings. Under the conditions of this study, 97% of surface deposited methamphetamine on a paper substrate was consumed after 12 min of exposure to ozone, thus demonstrating potential for its use as a remediation agent. The reaction had an effective second‐order rate constant with an upper limit of 2.15 ± 1 × 10^?18 cm³/molecule/s, and the main product observed was phenyl‐2‐propanone (P2P) at 8.3% yield, as determined using GC/MS. Several products observed in this study have also been reported as by‐products of methamphetamine synthesis, including P2P—a known methamphetamine precursor, which indicates that their detection at a potential clandestine site is not necessarily evidence of manufacture.     

Background correction in forensic photography. I. Photography of blood under conditions of non-uniform illumination or variable substrate color--theoretical aspects and proof of concept

The combination of photographs taken at two or three wavelengths at and bracketing an absorbance peak indicative of a particular compound can lead to an image with enhanced visualization of the compound. This procedure works best for compounds with absorbance bands that are narrow compared with "average" chromophores. If necessary, the photographs can be taken with different exposure times to ensure that sufficient light from the substrate is detected at all three wavelengths. The combination of images is readily performed if the images are obtained with a digital camera and are then processed using an image processing program. Best results are obtained if linear images at the peak maximum, at a slightly shorter wavelength, and at a slightly longer wavelength are used. However, acceptable results can also be obtained under many conditions if non-linear photographs are used or if only two wavelengths (one of which is at the peak maximum) are combined. These latter conditions are more achievable by many "mid-range" digital cameras. Wavelength selection can either be by controlling the illumination (e.g., by using an alternate light source) or by use of narrow bandpass filters. The technique is illustrated using blood as the target analyte, using bands of light centered at 395, 415, and 435 nm. The extension of the method to detection of blood by fluorescence quenching is also described.     

Formation of Trifluoroacetylated Ephedrine During the Analysis of a Pseudoephedrine-Formaldehyde Adduct by TFAA Derivatization Followed by GC-MS

Abstract:  (+)-Pseudoephedrine reacts with formaldehyde to form (4 S ,5 S )-3,4-dimethyl-5-phenyloxazolidine. Gas chromatography–mass spectrometry (GC-MS) analysis after the reaction of this oxazolidine with excess trifluoroacetic acid anhydride (TFAA) shows predominantly N , O- bis(trifluoroacetyl)pseudoephedrine with some of the monotrifluoroacetylated derivative. In addition, variable amounts of N , O- bis(trifluoroacetyl)ephedrine were detected by GC-MS. N , O- bis(trifluoroacetyl)ephedrine was not detected upon trifluoroacetylation of the source (+)-pseudoephedrine, and nuclear magnetic resonance analysis of the (4 S ,5 S )-3,4-dimethyl-5-phenyloxazolidine showed no evidence of the (4 R ,5 S ) isomer. This suggests that the N ,O-bis(trifluoroacetyl)ephedrine is formed by epimerization during the TFAA derivatization and GC-MS analysis of the pseudoephedrine-formaldehyde adduct.     

The effect of chlorine and hydrogen chloride on latent fingermark evidence

Exposure of forensic evidence to either hydrogen chloride or chlorine can result in acidification to such an extent that enhancement of fingermarks with ninhydrin or cyanoacrylate is inhibited. Under these circumstances, pretreatment of samples with volatile bases such as triethylamine or ethanolamine prior to using these enhancement techniques can lead to successful visualization of fingermarks. Alternatively, physical enhancement techniques such as powder dusting or small particle reagent can be used on acidified non-porous substrates, although the former technique is subject to increased background under such conditions.     

Inhibition of bleach-induced luminol chemiluminescence

The luminol chemiluminescence presumptive test for blood is based on the mild peroxidase activity of hemoglobin in basic peroxide solution. However, this test is subject to interference by strong oxidants, certain transition metal ions, and true peroxidases. This paper reports methods for reducing the interference caused by hypochlorite-containing bleaches. Amines such as 1,2-diaminoethane react rapidly with hypochlorite without interfering significantly with the hemoglobin-catalyzed oxidation. Thus, addition of 0.1 mol/L 1,2-diaminoethane to a standard luminol-peroxide spray lead to almost complete inhibition of hypochlorite-induced chemiluminescence while satisfactory chemiluminescence was still observed from bloodstains. If time allows, an alternative method for reducing interference from hypochlorite bleach is to wait several days until the bloodstains have dried thoroughly, by which time the hypochlorite will have decomposed.     

Background correction in forensic photography. II. Photography of blood under conditions of non-uniform illumination or variable substrate color--practical aspects and limitations

The combination of photographs taken at wavelengths at and bracketing the peak of a narrow absorbance band can lead to enhanced visualization of the substance causing the narrow absorbance band. This concept can be used to detect putative bloodstains by division of a linear photographic image taken at or near 415 nm with an image obtained by averaging linear photographs taken at or near 395 and 435 nm. Nonlinear images can also be background corrected by substituting subtraction for the division. This paper details experimental applications and limitations of this technique, including wavelength selection of the illuminant and at the camera. Characterization of a digital camera to be used in such a study is also detailed. Detection limits for blood using the three wavelength correction method under optimum conditions have been determined to be as low as 1 in 900 dilution, although on strongly patterned substrates blood diluted more than twenty-fold is difficult to detect. Use of only the 435 nm photograph to estimate the background in the 415 nm image lead to a twofold improvement in detection limit on unpatterned substrates compared with the three wavelength method with the particular camera and lighting system used, but it gave poorer background correction on patterned substrates.