Forensic Material Identification Techniques, Spectroscopy - Coursework Example

Forensic science: Materials Identification Introduction That forensic science and police work cannot be separated is quite apparent if the current central role played by forensic techniques in policing and the entire criminal justice system is anything to go by. The role of forensic science in police work is categorised into the broad groups of fingerprinting, document examination, armory, photography, scene of crime, electronic crime lab and environmental science and research among other categories, depending on the departments of a given police jurisdiction (Skoog, 2007). The fingerprint section is mandated to identify invisible or hidden fingerprints and carry out examination of crime scenes and fingerprint exhibitions. The functions of this department of forensic science is to visit crime scenes, search for, enhance, collect and examine finger, palm and foot print for evidences and to make identifications via the comparison and recovering of latent prints and recognized offender prints (Skoog, 2007). In addition, this department and its officers present expert evidence at court. Whether working under document Examination, armory, photography, scene of crime, electronic crime laboratory or under environmental science and research, forensic officers operate analytical laboratories where they analyse samples related to biology (DNA), physical evidence such as glass, fibres and paint, illicit drugs, toxicology, breath/ blood alcohol and workplace drug testing among others (Skoog, 2007). This paper explores some of the numerous techniques that forensic officers in the police service apply in determining the elemental composition of the solid, liquid and gaseous materials in their hand, which could be related to a crime or a case before a court of law. The paper focuses on the each technique in isolation, discussing how each works, its weaknesses and strengths and the nature and type of sample for which each technique is ideal. The techniques will then be studied holistically then a set of recommendations will be given in the form of a flow chart or table showing which technique(s) or variants recommended for liquid, solid and paste samples. It is also worth mentioning the fact that emphasis is placed on the most modern or newest technologies and instruments that aptly address the analytical situations that criminal justice officers, especially forensic officers, face in the execution of their duties. Forensic Material Identification Techniques It is quite evident that instrumental methods have become the pillars of modern forensic identification and characterisation and analyses of materials. The current instrumental methods in forensic identification and analysis not only relies on technologies but also on sound theories on the application of forensic techniques in the analysis, identification and characterisation of toxic and nontoxic biological and chemical substances (Skoog, 2007). Generally, the operation principle of most of the analytical instruments used in modern forensic identification and analysis of materials function by changing some feature or property of the substance being tested, the analyte, into a photometric or an electronic form or signal, which a machine can detect and read (Skoog, 2007). The accuracy of forensic instruments to detect and characterize a material or its components is however hampered by factors such as fluctuating concentrations of target and intruder substances, random electronic or spurious photon transmissions. That is, these factors interfere with the accurate and reproducible measurement of the substance of interest. Among the most common techniques used to identify and characterise materials in forensic science officers in police departments is Gas Chromatography, which entails Quadrupole Mass Spectrometer (GC–QMS) and GC–MS–MS. Quadrupole Mass Spectrometer (GC–QMS) and GC–MS–MS The GC–QMS or the GC–mass selective detector (GC–MS) involves the use of analytes, which are often extracted from blood, urine, or other matrices through chemical means. In many cases, forensic scientists are forced to create derivatives of the said substances to obtain them in a volatile before they are inserted into the instrument for identification and characterisation. In its basic sense, the GC technique separates compounds and elements in a substance on the basis of their solubility in the liquid, gaseous and solid phases. In addition, elements and compounds could be separated based on their volatility. The figure below shows a Quadrupole Mass Spectrometer, a gas chromatography instrument. In this instrument, ions of a specified mass-to-charge ratio are sorted and detected in a quadrupole mass spectrometer (QMS) after which a constant AC current produces a radiofrequency field, which selects the resonant ion. Basically, in the QMS, molecules get ionized by electron ionization (EI) technique as they enter the source of ion in a sequence. As they leave the GC into the QMS, molecules are bombarded by an electron beam after which the electrons are removed from the molecules. The result is an unstable positive ion, also referred to as molecule ion, which break into numerous unstable fragments. The charged fragments then move with carrier gas molecules into the mass detector at lower pressure. Upon being formed in the ionization chamber, analyte identification is achieved by the detection of a unique fingerprint of ion fragments. Spectroscopy Spectroscopy is the other commonly used identification and characterisation method in forensic science. A common instrument in this technique is the Fourier Transform infrared spectrophotometer (FTIR) in which the analyte is bombarded with infrared radiation. It has been established that the polar bonds in organic compounds have a natural frequency of vibration similar to the frequency of infrared radiation. This similarity implies that once the frequency of the infrared radiation matches the bond’s natural frequency, the amplitude of the vibration proportionally increases. At this point, the infrared is absorbed. The reading of an infrared spectrophotometer is in the form of the amount of light absorbed versus the wavelength, often given in the units of percent transmission and wavenumbers (cm-1). The type of bond in the analyte determines not only the frequency but also the intensity of absorption. For a skilled chemist, it is rather easy to determine the functional groups by an examination of the infrared spectrum. An advantage of the FTIR and the gas chromatography is that the obtained spectrum can be compared with that of a known sample. Hence, both methods have the advantage of using available evidence to support their identification and characterisation. Spectroscopy is highly recommended for the determination and characterisation of materials used in failed products such as polymers, additives and fillers. From these materials, samples can be taken via dissolution or by cutting them into slices using a microtome from specimen. For testing certain drugs of abuse, a type of spectroscopy known as the ultraviolet-visible-near infrared spectroscopy is highly recommended. For microscopic samples, UV-visible-NIR micro-spectrophotometers are advised. For trace evidences of fibres and paint chips, the UV-visible-NIR micro-spectrophotometer is often used. The above spectroscopic techniques are also often used in the analysis of inks and papers of contentious documents and for measuring the color of microscopic glass fragments. It is worth noting that these techniques do not change the nature of samples, implying that UV-visible-NIR micro-spectroscopy is non-destructive. Characterization techniques such as ultraviolet–visible spectroscopy, infra-red spectroscopy, nuclear magnetic resonance spectroscopy and an environmental scanning electron microscope are also useful in the analysis of thermoplastics. However, samples that cannot be characterised directly are dissolved in a suitable solvent and examined directly by UV, IR and NMR spectroscopy. In addition, infra-red spectroscopy is quite effective in determining and characterizing polymers, be they in paste, liquid or solid form. Whereas the Gas Chromatograph/Mass Spectrometer is used in forensics to measures the molecular weights of substances and compare them with a predetermined library or database, the Infrared and ultraviolet spectrometers are quite effective in determining certain organic compounds. Although these are the two commonest forensic instruments, the atomic absorption spectrometer, which is mainly used to determine and distinguish certain metals within a compound, is also quite useful in the forensic determination and characterisation of materials. The Atomic Absorption Spectrometer The images below show an atomic absorption spectrometer and a flow chart of its operations respectively. The atomic absorption spectroscopy (AAS) is the most ideal instrumental and spectro-analytical technique for the quantitative determination of chemical elements. It applies the principle of the absorption of optical radiation by free atoms in the gaseous state (Welz et al., 2005). In particular, the technique is ideal for determining the concentration of elements in a sample. In fact, the method has been used to determine the concentration of more than 70 elements in solution and solid forms. Its use extends beyond forensics into toxicology, pharmacology and biophysics studies (Welz et al., 2005). Specifically, forensic scientists use the method to analyse metals in biological fluids and tissues such as urine, plasma and blood (Welz et al., 2005). Other tissues and fluids in which AAS has been extensively used to determine and characterise metals are muscle tissue, semen, saliva, brain tissue and liver. AAS is also used to conduct water analysis for any traces of metals. Thus, AAS is the most appropriate tool for liquid sample that require the accurate determination of the amount of a single dissolved element such as lead or other metals. It is also ideal for analysing liquid samples in which rapid determination and quantification of the presence a variety of elements is required (Welz et al., 2005). Surface Analysis Techniques There are also many forensic techniques that are appropriate for surface analysis, thus ideal for analysing solid sample from which a determination of surface or sub-surface elemental composition is required rapidly. One such method is the X-ray photoelectron spectroscopy (XPS), which is undoubtedly, one of the renowned techniques for surface and sub surface analysis (Ray & Shard, 2011). Some key features of XPS include high surface sensitivity, analysis depths of about 6-8 nm, high vacuum (P ~ 10-8 millibar), ultra-high vacuum (UHV; P < 10-9 millibar) conditions and pressures of a few tens of millibar. X-ray photoelectron spectroscopy (XPS) is technique, a quantitative spectroscopic technique, which is quite surface-sensitive (Ray & Shard, 2011). It determines and characterizes the elements that compose the surface of an analyte in units of parts per thousand range, empirical formula, chemical state and electronic state of the elements in a material. In this technique, spectra are achieve through the irradiation of a material with an X-ray beam with the concurrent measurement of kinetic energy and the amount of electrons that escape from the surface of the material in question (Turner & Jobory, 1992). That is, only electrons emanating from the top 0 to 10 nm of the material are measured. XPS, a chemical analysis technique, analyses the surface chemistry of a material either in its original state or in a treated form. The treatments to be done on a sample could include fracturing, cutting or scraping, all geared at furthering the exposure of the bulk chemistry. The other common treatment for samples to be analysed by XPS is ion beam etching, which is applied to help in cleaning off surface contaminations. In addition, ion bean etching helps expose the inner layers of a sample (Turner & Jobory, 1992). A type of XPS known as in-depth profiling is used in analyses that seek to identify and characterise substances on the basis of their reaction on exposure to heat, reactive gases or liquids/solutions, ultraviolet light or ion beam implant. XPS finds applications in areas such as surface analysis of metals, inorganic compounds, polymers, semiconductors, glasses and ceramics (Turner & Jobory, 1992). Other areas of XPS application are chemical phase identification, quantitative elemental identification, depth profiling, and determination of surface contaminants, coating analysis, corrosion analysis, and failure analysis and interface characterization (Turner & Jobory, 1992). Cyclic Voltammetry (CV) and Pulsed Electrochemical Detectors (PED) CV and PED are the other two instrumental techniques used in forensics to determine and characterise materials. In cyclic voltammetry systems, which are also liquid chromatography detectors, drugs can be effectively determined, especially in biological fluids. Example of a metabolite found in toxic substances that forensic scientists test are the glucuronides (Boone et al., 2001). These substances are susceptible to electrochemical detection. This type of detection measures the electric current as a function of the potential difference between electrodes to which an analyte is connected. Since an element’s reduction potential is comparatively unique, when an increasing potential difference across electrodes in solution reaches this voltage, current proportionally increases as the element gets reduced at the electrode's surface, presenting forensic scientists with a method of detection (Boone et al., 2001). Conclusion The police department is faced with myriad situations in which evidences have to be presented and proved to be connected to the crime in questions. Thus, the role of forensic scientists and techniques is rather apparent. Importantly, the various instrumental techniques that forensic experts may use to determine and characterise materials prior to their presentation as evidence are quite integral. Past and current technological advances have availed several techniques for forensic experts to determine and characterise materials. These techniques include Cyclic Voltammetry and (PED), Surface Analysis Techniques (XPS), Quadrupole Mass Spectrometer (GC–QMS) and GC–MS–MS and spectroscopic techniques. References Boone, C. M., Douma, J. W., and Franke, J. P. (2001) “Screening for the Presence of Drugs in Serum and Urine Using Different Separation Modes of Capillary Electrophoresis.” Forensic Science International, 121: 89–96. Ray, S., and Shard, A.G. (2011) “Quantitative Analysis of Adsorbed Proteins by X-ray Photoelectron Spectroscopy.” Analytical Chemistry, 83(22): 865. Skoog, D. (2007) Principles of instrumental analysis, sixth edition. Canada: Thomson Brooks/Cole. Turner, D. W., and Jobory, M. I. (1992) “Determination of Ionization Potentials by Photoelectron Energy Measurement.” The Journal of Chemical Physics, 37(12): 300. Welz, H. B., Becker-Ross, S., and Florek, U. H. (2005) High-resolution continuum source AAS. Weinheim, Germany: Wiley-VCH. Read More