by Zoe A. Grosser and Thomas P. Byron
The field of forensic science consists of evaluation of crime scene evidence to understand and attribute responsibility for a crime. The more certainty that can be attributed to the assignment of responsibility, the more likely that a trial will proceed as expected and be resolved quickly. Everyone has heard something about the ability of DNA testing to match unknown DNA from a crime scene to that of a suspect with extremely high probability. But DNA is not present at every crime scene and requires time and good reason for analysis when it is present. Other more routine analyses provide the bulk of work done in most cases. Trace evidence is the category this work generally falls into. The analysis of latent fingerprints, fibers, hair, arson evidence, gunshot residue, glass fragments, and other components present at a crime scene might be in this category.
A recent forensic survey identified the largest problem in the laboratory as being a lack of sufficient personnel to perform tests requested.
(1) More productive instrumentation would help to address backlog issues. Increased analytical capability, providing additional information in each analytical run, would improve productivity and also provide more detail to distinguish and unambiguously characterize small trace evidence samples. Two new technologies seeing wider application in forensic laboratories are infrared imaging (FT-IR) and inductively coupled plasma mass spectrometry (ICP-MS). A recent market research report indicates that each forensic laboratory in the US has approximately one FT-IR, but the number of ICP-MS systems doesn't yet register in their measurements.
(2) As applications of these instruments become more well understood and productivity simultaneously becomes documented, this will likely change. Each technique will be described and applications explored.
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| Figure 1. Comparison of an FT-IR image of authentic analgesic with 5% caffeine (top) compared with image of counterfeit product with the same composition. |
Infrared imaging
Infrared technology has been used in forensic laboratories for many years. Small samples are exposed to infrared light and the absorption monitored over a range of frequencies. The "fingerprint" spectrum provides a nondestructive way of identifying the material or characterizing it in anticipation of a match with a known material. Infrared technology was coupled with microscopy early on to allow visual inspection and identification of the sample portion for infrared analysis and to enhance the analysis of small samples. The development of imaging systems allowed more flexibility in acquiring a large frequency of views with high spatially-resolved spectra that could be individually viewed in detail or displayed in false color to show regions of chemical similarity. The ability to collect spectra in several dimensions automatically with excellent sensitivity and spatial resolution adds productivity. Several examples of how this has been found helpful will be described. Michigan State Police Forensic Science Division (MSPFSD), Lansing Laboratory, helped win a conviction by matching paint from one victim's clothing to the murderer's car.
(3) MSPFSD forensic scientists found a microscopic paint chip on the hit-and-run victim's clothing, and then associated each of its five layers to a chip taken from the van driven by the perpetrator. This evidence played a significant role in the killer's conviction for the third murder that he committed that day, a case that was otherwise puzzling because there was no known connection between killer and the third victim. The paint chip found on the victim was very small and five layers of paint had to be analyzed individually. So resolution and sensitivity in the spectral range of interest was important. The MSPFSD has progressed beyond finding an association between a paint sample and suspect vehicle to identifying vehicle model, make, year and color even in cases where there is no suspect vehicle. The Royal Canadian Mounted Police (RCMP) and the Federal Bureau of Investigation (FBI) have created a database consisting of FT-IR results from paint samples taken from a wide range of vehicles, extending the utility of this technique even further.
The detection of counterfeit pharmaceutical medicines as well as over the counter (OTC) products shows another strength of FT-IR imaging. The manufacturing, sale and distribution of counterfeit medicines are not only serious crimes, but also lead to failed treatment, disability, and even death. The World Health Organization (WHO) defines counterfeit medicines as "deliberately and fraudulently mislabeled with respect to identity and/or source" and that "counterfeiting can apply to both branded and generic products and may include products with correct ingredients, with wrong ingredients, without active ingredients, with insufficient quantity of active ingredient or with fake packaging." It is estimated that five to eight percent of the world's total pharmaceutical sales are counterfeit or of dubious quality.
In this case the composition may be very similar between the counterfeit and the real preparation. However the distribution of the excipient or active pharmaceutical ingredient may differ and can be detected by imaging because of the spatial information gained in addition to the composition. Figure 1 shows an FT-IR image of a genuine analgesic with 5% caffeine (top image) and a synthetic "counterfeit analgesic with 5% caffeine added" prepared in the lab (bottom image). The false color image shows the distribution of caffeine is different in the counterfeit preparation when compared to the genuine preparation, indicating differences in the manufacturing process.
(4)FT-IR imaging is taken a step further in work done by the U.S. Federal Bureau of Investigation in examining latent fingerprints non-invasively for identification and examination of trace evidence that might be incorporated into the fingerprint.
(5) The fingerprint can be visualized using the ability of FT-IR to discern latent chemical information found on the fingerprint. Advanced mathematical techniques, such as second derivative calculations on a particular spectral band, can even distinguish a fingerprint deposited on an absorbent paper substrate. Small trace evidence fragments or fibers can be analyzed separately by collection of FT-IR spectra at their location within the fingerprint.
Inductively coupled plasma mass spectrometry
FT-IR can help identify molecular substances, but advances have also been made in the identification of metallic components. Metals analysis has been part of the forensic laboratory tool-kit since the introduction of commercial atomic absorption in the 1960's and used for applications such as gunshot residue detection. Increased sensitivity has been important in reducing the amount of sample that may be consumed, preserving as much of the sample as possible for further analysis. New techniques such as inductively coupled plasma mass spectrometry (ICP-MS) can provide better sensitivity, look at many elements, and provide more sophisticated interference compensation to allow a wider range of matrices to be examined accurately.
ICP-MS is often coupled with laser ablation sample introduction to allow the analysis of small samples with less consumption. A small portion of the sample is vaporized and carried into the ICP-MS with a stream of helium. This is in contrast to the more usual sample introduction which requires digestion of small samples with acid, a procedure that can be difficult and time consuming with some matrices, and even impossible if the sample is too small. A number of journal articles have described the analysis of glass using laser ablation ICP-MS with good success. A large number of elements can be examined yielding more detailed information about the composition of the glass. The more elements that can be used to distinguish glass samples, the higher the probability that a match confirms that pieces of glass originate from the same source. This might allow a better match when several types of glass are collected from a suspect or crime scene.
 Figure 2. Laser ablation ICP-MS data using GLITTER software (New Wave Research, Fremont, CA, USA) for visualization of 56Fe signal. |
A recent article summarizes consensus conditions for an internal standard (29Si) and sampling time to ensure consistent performance.
(6) Figure 2 shows a typical sample trace for glass analysis by laser ablation ICP-MS. Note that the data acquisition is divided into four time segments. This is done to account for background, instability and particle size effects in the ablated aerosol. The article indicates that the technique has potential to provide additional discrimination capability in glass analyses because of ability to provide accurate and precise analysis of major, minor and trace elements in a variety of glass types. A second article expands on LA-ICP-MS research to include the use of an advanced interference correction technique to improve results for iron determination.
(7) Iron is present in many glasses at widely different concentrations making it a good candidate for a matching marker. However, as in most analytical techniques, interferences can limit the potential of the technique for detecting low concentrations. The use of a Dynamic Reaction Cell (DRC) in conjunction with the laser ablation ICP-MS allows the interferences from ArO
+ and CaO
+ to be eliminated, improving the detection limit from 9.5 μg/g (without DRC) to 0.03 μg/g (with DRC). Good interference correction options make the technique more robust and allow a wider skill level to operate instruments productively and with good results.
Conclusions
Several new technologies are expanding into forensic science allowing additional information to be productively collected on trace evidence to distinguish individual samples. The more detailed information they provide can contribute to the certainty of matches between crime scene evidence and evidence obtained from suspects.
Tom Byron is a Senior Product Specialist for FT-IR and FT-IR imaging, and Zoe Grosser is Segment Marketing Manager for Analytical Sciences. Both authors have been with PerkinElmer for over 20 years.
More information about the methods discussed in this article is available from:
PerkinElmer, Inc.,
800-762-4000
www.perkinelmer.comReferences
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