Spectrograph
Forensic analysis of a wide and diversified range of samples seized at crime scenes, accidents, fire debris, explosions, and autopsies requires the use of several analytical methods and tools, such as gas chromatography/mass spectrometry (GC/MS), atomic absorption spectroscopy (AAS), inductively coupled plasma spectroscopy (ICP), infrared spectroscopy (IRS), nuclear magnetic resonance (NMR) spectroscopy, and high performance liquid chromatography (HPLC). Spectrography and spectroscopy are basically synonyms, referring to "a picture of a spectrum." The terms spectroscopy and spectroscope are more commonly used because they are older and easier to pronounce than the denominations spectrography and spectrograph. Spectrography is also known as spectral imaging techniques.
Spectrographs or spectroscopes are optical instruments that measure wavelengths and energy levels, radiated from atomic bonds between elements in molecules, or from other light sources such as stars. Spectrographs and spectroscopes disperse light into wave patterns known as a spectral image. The first spectroscope was developed at the beginning of the nineteenth century and consisted of three metallic tubes containing lenses disposed with converging axes and a flint glass prism, which dispersed light originated from a light source or the radiant energy emitted by chemical compounds into a wave spectrum. The spectral image allowed the quantitative analysis of chemical elements, index of refraction, wavelengths, and mass as well as the composition of chemical molecules. Its first application was in astronomy (telescopes) and chemistry (analytical spectroscopy) to determine the composition of chemical elements present in nebulas, stars, and in unknown chemical compounds.
With the advent of photography, spectroscopes were renamed spectrographs because a camera was coupled to the device instead of a telescope, allowing the development of the resulting spectral image into a photographic picture. During the twentieth century, with the advances in physics and electronic technology, the photographic camera was substituted by a photomultiplier that permitted instantaneous spectrographic analyses. A variety of spectral imaging technologies are presently available that are supported by computer software. Examples of forensic applications for these technologies include: isolating trace residues on surfaces; identifying fibers and micro particles; detection and quantification of organic and inorganic contaminants in food, water, and air; identifying semivolatile and volatile (explosive) fuel residues; and analyzing paint fragments.
Infrared (IR) spectroscopy uses infrared light to identify substances, due to chemical bonds vibrating in different frequencies, absorbing different amounts of infrared wavelengths, and emitting specific quanta of radiation (e.g., energy at known wavelengths). The device registers the absorbed wavelengths and produces a graph that is compared to those of known substances, which are recorded in a computer database. Each peak in the spectrum represents a different chemical element with unique properties. Each chemical molecule gives a unique spectrum, known as a fingerprint region. Forensic experts may use this method to identify types of drugs in a sample or paint chips from a car. Forensic analysts can gather
physical evidence to support claims of sexual assault by testing samples of blood or urine from the victim with infrared spectroscopy. If Rohypnol or other "date-rape" drugs are found, investigators have not only physical evidence of the crime, but also information about what investigators should search for in the suspect's house.
Gas chromatography/mass spectroscopy is a combined method used in forensics to identify residual fuels, such as accelerants and chemical residues, in the debris of a fire scene in order to determine whether the fire was accidental or was caused intentionally. These methods are also used to verify the purity of chemical products and the presence of contaminants in cosmetics, hygiene products, and food products. High performance liquid chromatography is another forensic method for identification of food and cosmetic contaminants.
Mass spectrometry is used to measure the masses of chemical isotopes (e.g., molecular mass) and to detect impurities in materials. Beams of ionized gas molecules are accelerated in the mass spectrograph, passing through a combined electric and magnetic selector that deflects them, before entering into a vacuum chamber. The amount of deflection is given by the mass/charge ratio, with each molecule being fragmented into smaller particles. In the vacuum chamber, a magnetic field interferes with the beam trajectory creating a spectrum on a photographic plate. Each peak in the spectrum represents a specific mass/charge ratio of a charged fragment and the largest mass/charge ratio indicates the molecular ion used to determine the molecular mass.
Another application of spectrometry is in the forensic analysis of questioned documents. Imaging spectrometers equipped with spectral scanners permit the detection of slight differences in inks and paper surfaces, as well as the presence of erased or added lines in numbers or letters. These optical instruments scan the document point by point through absorption, reflection, and fluorescence of materials, forming a spectral image where existing adulterations become evident to the naked eye. Spectral imaging is a convenient forensic method because it does not destroy evidence during analysis.
Atomic absorption spectroscopy (AAS) allows the precise quantification of inorganic elements in paints, water, air, or soils. AAS can, for instance, determine environmental contamination of water by mercury or other heavy metals. However, when multiple inorganic elements need to be simultaneously analyzed in a sample, inductively coupled plasma (ICP) spectroscopy is the method utilized. The ICP method can detect multiple metals in a solid matrix, in welding fumes, or in water or paints. Another analytical method used in forensics is Raman spectroscopy, especially when the preservation of samples is important as with court exhibits. This method can identify drugs, chemicals, fibers, and paints through spectral microscopy.
Determining the postmortem interval (PMI) or the time elapsed since death is crucial for investigators of a murder, especially when the body was subjected to environmental influences such as water, soil, or insects. In these cases, postmortem metabolic changes can be assessed through high-resolution magnetic resonance spectroscopy (H-MRS). It also provides additional valuable information to other traditional forensic methods used to determine PMI. In one study, decomposing brain tissue was used in H-MRS to identify metabolites and gases that helped to determine the time elapsed since death. The brain metabolites showed expected decreased concentrations that correlated with the estimated PMI of known samples.
In spite of the great utility of analytical instruments in forensic investigations, it is important to keep in mind that nothing substitutes human scientific and technical competence along with the exchange of information when interpreting data, especially when lives are at stake in a criminal court. One example of this was given by a scientist in a 2004 report alerting that bullet matching based on chemical analysis has sometimes been biased by errors in analysts' interpretation of data. In the report, "Forensic Analysis: Weighting Bullet Lead Evidence," the limitations of lead content analysis as a tool for matching evidence and evidence validation were described. Chemical analysis through ICP spectroscopy detects minute amounts of trace elements in bullet fragments such as arsenic, antimony, copper, silver, cadmium, tin, and bismuth, which are present in less than 1% of bullet lead alloys. Although the resulting bullet characteristics are accurate, the way data is interpreted may be misleading. It was long assumed that if two bullets are chemically identical, they originated from the same smelting source or were manufactured at the same day at the same factory. FBI examiners even assumed in courts that they came from the same bullet box. The report featured evidence from forensic chemists that even in a single lead smelting pot, sometimes the composition varied from one batch of bullets to the next batch, whereas the composition of different pots matched, implying that bullets made from different pots, by different manufacturers, sometimes matched.
Another forensic analytical chemist at the Committee of the National Academies discovered that bullets made of lead from different sources can get mixed into the supply and manufacturing processes, which can lead to the same ammunition box containing bullets with different elemental compositions. The National Academies Committee concluded in their report that it is impossible to determine with absolute certainty that a bullet from a crime scene came from a specific box of bullets, or that two bullets were manufactured on the same day by the same manufacturer. In the face of these and other gathered data generated from spectrography and shown in the report, the committee has asked the FBI to revise its rules on the interpretation of results from bullet chemical analysis.
