Forensic analysis is a scientific study which assists criminal investigations. Whenever any crime takes place there is exchange of material between the accused and the victim such as textile fibers, blood stains, gunshot residues, broken glass, saliva, semen, etc. Chemical and biological tests on such materials can provide undisputed evidence to nail down the suspect. However, forensic analysis poses a big challenge to the analyst. Firstly, the amount of sample collected from the scene of crime is often limited and it is just not possible to replenish it. Secondly, the results of such analysis can lead to conviction so findings need to be reported with extreme care so that innocent persons do not suffer. For this reason nondestructive techniques are preferred as the sample can subsequently be tested for further confirmation.
X-ray diffraction or XRD is one such technique which is nondestructive and the sample requires minimum sample preparation prior to analysis. The only requirement is that the sample should be homogeneous in nature so as to provide uniform analysis results even if a small portion is analysed from a bulk quantity. Samples commonly received for forensic testing commonly include:
- Building materials such as cement, concrete, steel rods, bricks,etc
- Drugs of abuse and other banned substances
- Residues from site of arson such as kerosene oil, gasoline or cotton lints
- Explosive residues and splinters from sites
- Accident scene samples such as blood spots, paint chips,etc
- Assault samples such as torn garments, broken glass,cosmetic marks, semen stains,etc
- Theft and robbery site samples which include gunshot residues, tools, murder weapons, forged documents, blood residues,etc.
The samples can be analysed by XRD if it exhibits a degree of crystallinity even if the remaining content is amorphous. The technique helps establish the presence or absence of a particular material through comparison against a reference XRD data base. The present article discusses some of the XRD applications of common samples picked up from scene of crime.
Drugs and pharmaceuticals
Drug and pharmaceutical samples are commonly seized from smugglers, carriers and from raids conducted on rave parties. Such substances can pose a challenge as their identity is seldom known. It can be a pure compound or even an engineered product produced by illegal laboratories to conceal their real identity. XRD helps identify one seized material from another or to identify the seized powder with known materials such as cocaine, morphine, heroin and amphetamines or different compounds introduced to conceal the real identity. Other analytical techniques such as visible light is microscopy, GC – MS, HPLC, NMR and FT-IR also help complement the findings of XRD
Explosives are commonly recovered as powdered mixes, gels and liquids. Such materials are often recovered from hijack suspects or at post-blast sites. While it is simple to identify the pure material the analysis of residues collected from a blast site can prove to be a daunting task. It can contain a mix of both organic and inorganic material such as human blood, flesh, pellets, broken glass and residues of other damaged material. At times the concentration of explosive in the residue could be so low that analysis and detection can require separation and extraction prior to analysis.
Paints and pigments
Samples of paints and pigments are generally recovered as minute flakes from clothes of accident victims or vehicles involved in accidents. The collected specimens are compared for preliminary colour matching by optical studies followed by presence of crystalline components existing as pigments such as titanium dioxide, lead oxide or red lead. The more the number of crystalline components present the better is the degree of match. A final confirmation can be reached by ascertaining the organic composition by pyrolysis GC analysis.
It is common to recover pieces of cloth as evidence from scene of some crimes. Textile fibers are a composite of crystalline and amorphous molecules. The extent of similarity between the recovered material and accused person’s clothing can be useful evidence. XRD helps establish the crystallinity of such textile specimens. Textiles such as cotton, polyester and nylon show significant crystallinity whereas wool fibers are predominantly amorphous.
Documents examination becomes necessary in cases involving forgeries, ransom notes, counterfeit currency, hate mails and wrapping used for seized drugs. XRD provides useful information on the crystallinity,match of crystallinity of cellulose and inorganic fillers. An inexpensive copying paper generally shows absence of fillers like titanium dioxide or anatase and is predominantly cellulose.
X-ray diffraction is a useful tool in the hands of the forensic analyst as it serves to complement the findings of other analytical techniques. In recent years the technique has also found widespread application in the rapid screening of passenger baggage at major airports across the world. Specially designed screening systems help screen passenger baggage for explosives and narcotics without the need for sniffer dogs or manual searches. Such systems offer significant assistance in prevention of drug smuggling and acts of terrorism without deputing additional manpower or inconveniencing other passengers.