Advances in Detecting Trace Evidence
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JOURNAL ISSUE NO. 249 / JULY 2003 Without a Trace? Advances in Detecting Trace Evidence hards of glass are found at the scene evidence—minute quantities of materials of a hit and run. It’s the same type of such as blood, chemicals, fibers, glass, hair, Sglass used to make most standard plant material, or plastics—are very close headlights. to being added to the law enforcement arsenal. A single hair might belong to a missing woman, but it is coated with conditioner, Connecting a person or object to a specific making microscopic analysis impossible. crime scene is often essential to proving guilt or innocence. Developing such a Investigators at the site of a plane crash link is frequently based on identifying search for minute quantities of explosives and comparing trace evidence. Because in the wreckage. trace evidence samples can look similar and the environments where they are At the scene of a rape and murder, officers found are often complex, identifying hope to find blood or semen from the unique characteristics and establishing assailant. a link can be difficult. Older techniques often cannot distinguish such evidence Currently, law enforcement has no accurate due to these challenges. way to match the glass shards or coated hair to known samples, and locating tiny particles New technologies for trace evidence may of explosive material or body fluids might help eliminate many of these obstacles, be difficult or impossible. But all that’s about allowing more trace evidence to be found to change, as new and improved techniques and identified. Here are four of the most for detecting and distinguishing trace promising new techniques. NIJ JOURNAL / ISSUE NO. 249 Distinguishing Glass Evidence making it difficult to distinguish among them. Although Dr. Almirall found an RI New and improved On a small Caribbean island, a witness called match, such a match does not weigh techniques for the police to report seeing a body on the heavily as evidence in court when it side of a road. A woman walking home from involves auto headlights. detecting and work shortly after midnight was apparently struck by a vehicle. Her death might have The lab then put the glass fragments distinguishing trace been prevented had the driver stopped through ICP–AES analysis. A quantitative to provide medical assistance instead of analysis of the fragments found that the evidence—minute leaving the scene. The accident became glass pieces recovered from the street quantities of a felony hit and run. and those from the suspect’s car were indistinguishable from one another. At materials such as A local constable was called to the scene. a preliminary hearing on the hit-and-run Among other items, he recovered nine charges, Dr. Almirall testified that the blood, chemicals, large pieces of glass that appeared to ICP–AES analysis showed strong evidence come from a car headlight. of an association between the glass frag- fibers, glass, hair, ments. Just days before the trial, the Eleven days later, local officials identified prosecutor and defense reached a plea plant material, or a suspect. No body fluids were found on agreement. plastics—are the suspect’s car, but the front fender showed signs of recent damage: a broken Dr. Almirall, now associate director of the very close to being headlight and pieces of glass lodged inside International Forensic Research Institute, the bumper. recognizes the need for highly discriminating added to the law techniques in the analysis of glass evidence. Island police shipped the evidence to He collaborates with Dr. Douglas Duckworth enforcement the Miami-Dade Police Department Crime of Lockheed Martin’s Oak Ridge National Laboratory for analysis. There Dr. José Laboratory. They have since developed an arsenal. Almirall was working on ways to analyze even better method for analyzing glass ele- glass samples using a process called ments using a process called inductively inductively coupled plasma-atomic coupled plasma-mass spectrometry emission spectroscopy (ICP–AES). He (ICP–MS). was asked to see if there was a con- nection between the glass fragments ICP–MS combines enhanced sensitivity with found at the crime scene and the broken a multielement capability. This higher level glass found on the suspect’s car. of glass analysis is a valuable tool for distin- guishing among all types of glass, including Analyzing the elements of glass specimens cookware, float glass from windows, head- helps to locate the original source of glass lights, and leaded glass. ICP–MS’s high level pieces. The elements that make up head- of sensitivity allows for the analysis of very light glass are different from those in other small fragments. glass products. ICP–AES effectively meas- ures the various elements to distinguish The two scientists are incorporating the ana- among auto headlights. lytical techniques and data generated from ICP–MS into a practical application for the Dr. Almirall first used a conventional forensic lab. They are developing a large approach, measuring and comparing the database of trace element concentrations refractive index (RI) properties of the glass using ICP–MS that will be able to rank the recovered from the crime scene with the strength of an association between known glass fragments from the suspect’s car. and questioned glass samples. Research The problem with this method—the primary continues on ICP–MS, and its use is encour- one used by crime labs—is that automobile aged through interlab validation, publication, headlights all have similar refractive indexes, and training. 3 NIJ JOURNAL / ISSUE NO. 249 Because trace HOW DOES STATIC SIMS WORK? evidence samples Secondary ion mass spectrometry (SIMS) can be divided into two operational types: can look similar dynamic and static. The semiconductor industry has used dynamic SIMS for years, mainly for analyzing bulk metals. Static SIMS provides information about organic and the environ- compounds “adsorbed” onto a surface. (Adsorption is the binding of a substance ments where on the surface of another and is distinguishable from total absorption.) The principle behind static SIMS is simple: the trace sample is bombarded with a they are found high-energy atom. The term “static” indicates that the degree of surface bombard- ment is low enough so the chemical composition of the surface is not changed. Intact are often complex, molecules, their fragments, and atoms are “sputtered” into a gaseous state from the surface. Some fraction of these particles are charged, or ionized, and can then be identifying unique measured using a mass spectrometric detector. The detected masses help to identify characteristics and the surface chemistry of the trace evidence. For example, an ion at mass 550 indicates a hair conditioner chemical and is easily differentiated from an ion having a mass of establishing a link 270, which is derived from heroin. can be difficult. Older techniques Identifying Chemical Composition surface of extremely small trace evidence often cannot samples—as small as 1/10,000 of an inch. Forensic scientists continue to search for This method generates atomic and molecu- distinguish such new ways to find chemical residues on lar information from only the top-most clothing, fingernail, hair, and skin samples. molecular layer of the sample, leaving it evidence due to Such residues may provide a link between largely intact for further analyses. a suspect and a chemical weapon or agent. these challenges. INEEL scientists conducted tests using Many chemicals are designed to endure static SIMS in combination with pattern and to absorb into substances, but detection recognition techniques. They were able can still be difficult. Research conducted by to differentiate a wide range of coating scientists at the Idaho National Engineering samples by manufacturer, and often by and Environmental Laboratory (INEEL) focus- specific coating product. Although the es on the persistent nature of chemicals. samples looked similar, the chemical Static secondary ion mass spectrometry makeups of their various coatings were (static SIMS) is used to distinguish trace considerably different. chemicals and residue on various materials. The goal is to find links between suspected Static SIMS shows real potential for distin- sites and possible offenders. Static SIMS guishing chemicals in forensic samples may possibly change future methods for well beyond current analytical approaches. detecting chemical residues. This technique differentiates and identifies specific samples of physical trace evidence, Chemical characterization of trace evidence including coating materials, fingernail polish, is not always successful. Conventional analy- and paint. For example, it provides a wealth sis attempts to break down the sample into of information about chemicals found on hair separate chemical entities—simplifying iden- and fiber samples. tification, but destroying the sample in the process. With this method, the samples SIMS and related techniques may be tend to be small and, therefore, analyses used more frequently once small, easy- are often not precise enough to detect the to-use SIMS instruments are developed. chemicals involved. Static SIMS may be applied more widely in the near future as the cost of analysis Static SIMS uses a different approach. It decreases and the technique becomes identifies the chemical composition of the simpler to use. 4 NIJ JOURNAL / ISSUE NO. 249 DETECTING AND ANALYZING CHEMICAL CONTAMINANTS ON HAIR AND FIBER Idaho National Engineering and Environmental Laboratory (INEEL) researchers did studies using various static secondary ion mass spectrometry (SIMS) instruments. They used ion trap SIMS to distinguish trace hair samples using consumer chemicals as identifiers. Chemicals found in hair conditioning products produce distinctive chemical signatures, allowing the identification of hair samples based on the product used. SIMS is unaffected by the presence of hair dyes, which complicate microscopic techniques. Scientists typically characterize forensic human hair samples using a microscope. The presence of colorants and chemicals commonly present on human hair defeats this method. SIMS takes advantage of the presence of these chemicals to improve identification.