A critical evaluation of infrared analysis and mass spectrometry in forensic science
Nature of infrared analysis and nature of mass spectrometry. Summary of the uses in forensic analysis. Critical comparison of infrared analysis and spectrometry. Gathering of the information about positional isomers with the help of infrared analysis.
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“A CRITICAL EVALUATION OF INFRARED ANALYSIS AND MASS SPECTROMETRY IN FORENSIC SCIENCE”: ESSAY
Nature of Infrared Analysis
Nature of Mass Spectrometry
Summary of the uses in forensic analysis
Critical comparison of Infrared Analysis and Mass Spectrometry
In order to answer the variety of questions that is posed to the forensic investigation, it is frequently necessary to conduct studies of individual objects using methods that require deep knowledge not only in the field of forensic medicine, but sciences, including criminology. The objects of such studies are the victims, defendants, suspects, weapon injuries and fabric body, clothing, footwear, materials of criminal cases and others. Amongst the several instruments that forensic scientists use in their day-to-day use, in order to aid investigators to determine how a crime was committed, both the infrared spectrophotometer and the mass spectrometer are on top of the list. These instruments are routinely used in molecular analysis and for the determinations of chemical structures (Siegel, Saukko, & Knupfer, 2000). The methods have different value and background; however have the same power and effect in investigations. The methods provide investigation with the quick and reliable results of evidence, contributing to a rapid and successful disclosure of crime. Identification and comparison of the materials is carried out by means of spectral libraries.
Nature of Infrared Analysis
Infrared spectroscopy (IR) or Infrared Analysis is the section of the spectroscopy covering longer wavelengths (> 730 nm for visible light, a red border). Infrared spectra arise from the vibration (rotational part) of the molecules, namely - as a result of transitions between the vibration levels of the ground electronic state of molecules. IR radiation absorbs many gases, except O2, N2, H2, Cl2 and monatomic gases. Absorption occurs at a wavelength, which is characteristic for each specific gas, for CO, for example, the wavelength is 4.7 microns (P. &P. Atkins, 2009). With the help of infrared absorption spectra it can be set different molecular structure of organic (and inorganic) substances with relatively short molecules: antibiotics, enzymes, alkaloids, polymers, complex compounds. The vibration spectra of molecules of various organic (and inorganic) substances with a relatively long molecules (proteins, fats, carbohydrates, DNA, RNA, etc.) are in the terahertz range, so the structure of these molecules can be installed using radio frequency terahertz spectrometer (Mukamel, 2000). The number and position of the peaks in the IR absorption spectra can be judged from the nature of the substance (qualitative analysis), and the intensity of the absorption bands - the number of substances (quantitative analysis).
Thus, IR spectroscopy is based on the fact that irradiation of a substance is non-monochromatic infrared exists because of the vibration and electronic degrees of freedom, that is due to the absorbed incident radiation, at frequencies corresponding to the energy that appears because of the difference of the vibration and electronic levels. In the transmission spectrum appear the features that allow to judge the characteristic frequencies of molecular vibrations and their electronic properties. Spectral characteristics (position of the maxima of bands and their half-width, intensity) depend on the masses of its constituent material atoms, the geometrical structure, the characteristics of interatomic forces, the charge distribution (Hamm, Lim, Hochstrasser, 1998).
Nature of Mass Spectrometry
Mac-spectrometry is one of the most effective methods for expressing the analysis and establishing a structure of individual organic, synthetic and natural compounds and their mixtures (Price&Phil 1991). Due to its extremely high sensitivity and the possibility of using in combination with gas and liquid chromatography, this method is widely used in organic, bioorganic, biological, physical, analytical, medical chemistry, the chemistry, pharmacology, toxicology, environmental protection, forensic and in control of production. One way to establish the structure of the investigated compounds by this method is automatically registered spectrum of the comparison spectra with the bank of spectra that are entered into the computer memory. Mass spectroscopy is a method based on the research of materials by determining the mass of ions of the substance (often related to their mass ion charge) and their quantities (Fenn, Mann, Meng, Wong, Whitehouse, 1989). The sequence of values ??of the masses and their relative content (concentration) is called the mass spectrum. Mass spectroscopy uses a vacuum separation of ions of different masses under the influence of electric and magnetic fields. Therefore, the investigated substance, firstly, is subjected to the process of ionization (Price&Phil 1991). The process of ionization is excluded the study of ionic structure already ionized gases, such as electrical discharge in the ionosphere or planets. In the case of liquid and solid substances they are pre-evaporated and then ionized. Mostly the positive ions are researched, because the existing methods of ionization allow receiving them in a more direct way and in larger quantities than negative. However, in some cases tested and negative ions (Schwartz, Michael, Senko, John, Syka, 2002). The first mass spectra were obtained in the UK by JJ Thomson and F. Aston. They led to the discovery of stable isotopes. Initially, mass spectroscopy was used primarily to determine the isotopic composition of elements and precise measurement of atomic masses (March, 2000). Now mass spectrometry is one of the main method by which can be obtained the data of the masses of nuclei and atomic masses of elements (Rena, Sowell, Koeniger, Valentine, Moon, Clemmer, 2004). The variations of the isotopic composition of elements can be determined with relative error ± 10-2%, and mass nuclei - with a relative error ± 10-5% for light and ± 10-4% for heavy elements (Gothard, Busst, Branthwaite, Davies, Denison, 1980). The accuracy of quantitative molecular analysis achieves with the precision isotopic analysis, but quantitative molecular analysis often is difficult because of the equality of masses of different ions that are formed by ionization of different substances. To overcome these difficulties the "soft" ionization methods are used, which give little fragmentation of ions (Price&Phil, 1991).
To sum up, the molecular structural mass spectral analysis is based on the fact that the ionization of molecules of some substance is converted into ions, having been not destroyed, and some parts thus divided into fragments. Measurement of mass and relative content of molecular and fragmentation ions (molecular mass range) provide information not only on molecular, but also on the structural level.
Summary of the uses in forensic analysis
Both Infrared Analysis and Mass Spectrometry are widely used in forensic analysis.
First of all, photographing in infrared rays allow to detect the soot of shots in dark tissues, where it is visually hard to observe. With help of such method it can be set some details of the object that is filled with blood, without removing them. By the way, the image
Object in the infrared rays can be observed visually by means of electron-optical converter (EOC). For example, while analyzing the clothes, soaked with blood, in cases of gunshot injuries, using infrared rays can be detected many additional factors under a layer of blood shot (soot from the combustion of gunpowder) (Price&Phil,1991). The method is quite effective, does not affect the object and also allows reliably documented by photographing the result of research. Using infrared it can be easily detected the traces, suspicious for the presence of blood, semen, mineral oil (e.g. oil residues around the entrance gunshot hole or where contact with body parts of vehicles in road traffic accidents). Thus, the usage of infrared gives lots of advantages in the leading investigation. Firstly, this non-destructive method, allows to get the examples of the materials saving all the shapes and structures. Secondly, it provides the accurate measurements that do not require external calibration. Finally, it is easy in exploitation and provides the fast results, defines the things that can`t be gathered in the natural way. Mass spectrometry used a vacuum separation of ions of different masses under the influence of electric and magnetic fields. Therefore, the investigated substance is firstly subjected to the process of ionization. The process of ionization is excluded in the study of ionic structure of already ionized gases, such as electrical discharge in the ionosphere or planets. In the case of liquid and solid substances, they are pre-evaporated and then ionize. Often the positive ions are investigated, so that existing methods of ionization can receive them no more direct way and in larger quantities than negative. However, in some cases tested and negative ions. This method are used to make the expertise of the little pieces, that can`t be defined it the natural environment. The example can be the blood, pieces of skin and nails, drugs and even the pieces of dandruff.
Thus, the introduced methods give lots of advantages in the leading investigation. Firstly, these are non-destructive methods. They allow to get the examples of the materials saving all the shapes and structures. Secondly, they provide the accurate measurements that do not require external calibration. Finally, the methods are easy in exploitation and provide the fast results, define the things that can`t be gathered in the natural way.
Critical comparison of Infrared Analysis and Mass Spectrometry
Infrared spectroscopy, to be more precise Fourier Transform Infrared Spectroscopy (FTIR), uses the mid-infrared region and this analytical tool probably yields the most information in forensic paint analysis (Chalmers & Griffiths, 2002). Despite this however, FTIR can lack in selectivity. It may not always be easy to discriminate within a particular class especially if a mixture of these is present (Siegel et al., 2000). For example FTIR is not able to discriminate between the plasticizers diisooctyl phthalate and dihexyl phthalate both separately and as a mixture (Siegel et al., 2000). FTIR is also a very sensitive technique. An infrared (IR) spectrum can be collected in a couple of seconds and with repeated scans, the signal-to-noise ratio is increased (Felgett Advantage) (Settle, 1997). This sensitivity feature of FTIR can be said to apply not only to paint analysis but also for any other forensic use. Limitations that an FTIR has are that elemental information is very limited, molecule/s must be IR active and the matrix the molecule of interest is present with must be transparent to IR (Settle, 1997). The latter limitation has a direct effect on selectivity. If the matrix is not IR transparent the IR spectrum would be a mixture and selectivity would be low (Boon & Learner, 2002). Mass spectrometry makes use of ionized particles in order to measure their mass-to-charge ratio (m/z). Despite its wide range of application and advantages in forensic science, one of the main limitations of mass spectrometry is that if the sample is a mixture, the mass spectrum would be likewise (Siegel et al., 2000). To increase selectivity, a modification to the conventional mass spectrometry has been devised: direct temperature-resolved mass spectrometry (DTMS) (Boon & Learner, 2002). Boon & learner (2002) showed in their studies that DTMS can be applied for small samples of paint made up of pigments and organic media. Compounds in paint are “physically separated” due to their different physical and chemical properties (Boon & Learner, 2002). Sensitivity of mass spectrometry can be increased by making use of the selected ion monitoring (SIM) feature of the mass spectrometer instead of the full scan mode feature (Ekman, Silberring, Westman-Brinkmalm, & Kraj, 2009). Scanning decreases sensitivity as this feature detects ion species one at time (Ekman et al., 2009). Some features that are discussed here with respect to paint analysis of both FTIR and mass spectrometry may also apply to other forensic application.
Thus, with the help of mass spectrometry can be determined the trace elements in solution. The method provides better sensitivity in comparison to graphite furnace AA. After analyzing mass spectra to ICPOES, the general solution can be made that the mass spectra is much simpler in usage. Even though, the heavy elements provide thousands of emission lines, they include near 1-10 natural isotopes in mass spectrum. Thus, the mass spectrometry has the super sensitivity (Hits, Ronald A, 1992). Infrared analysis has near 2% of the sensitivity limit. However, using the most new techniques its level can be near 0.01%. Mass spectra`s sensitivity depends mostly on the ionization method, thus, an extract with 0.1 to 100 nag may be needed for injection a sufficient amount (Karate, Francis, Ray, Clement, 1988). With the help of infrared analysis the information about positional isomers can gathered, that is not possible using the mass spectrometry method. However, IR in the comparison to MS is less sensitive for 2 to 4 orders of magnitude usually. Analyzing its selectivity, it can be said, that the mass chromatograms can be used as a strongly selective gas detector What is more, MS gives the possibility to gain the additional information on the molecular structure (Griffins, Haseth, 1986), that can`t be provided by the infrared spectra. That is why; the mass spectrometry method is more selective. According to the accuracy, MS uses isotopic internal standards and has the accuracy of ±20%. The method is leaded by the general analytical calibration. IR accuracy is ± 5% in routine analysis. However, in the preferable conditions it can be 1% (Hits, Ronald, 1992). The amount of time that is wasted to prepare the elements in MS method is between 20 and 100 min, to provide the general analysis can take from 1 to 20 hours depend on the elements. To prepare the elements for IR method can tale from 1 to 10 min and to evaluate them - 5 min maximum (Griffins, Haseth, 1986).
To conclude, both techniques are very helpful in the forensic study. On the one hand, MS is more analytical technique. It is more sensitive and selective. On the other hand, IR is more accurate and spent less time to make the analysis.
infrared spectrometry isomers
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