Paper outline
Introduction
Analytical Approaches Used
Challenges Faced when using Gas Chromatography
Current Solutions
Conclusion
Reference List
The Role of GC within Forensic Applications
Introduction
Gas chromatography (GC) is a technique used to separate volatile components of a complex mixture. The method employs a column made of a flow-through narrow tube in which a sample containing various chemical components passes in a gas stream. The gas stream in chromatography is referred to as a mobile phase.The different components in the gas stream interact with a specific column filling referred to as a stationary phase depending on their chemical and physical characteristics. The individual chemicals are then detected and identified electronically as they exit the column. Different parameters are employed to ensure successful separation of exiting gases at different retention times. These include the nature of the stationary phase, carrier gas flow rate, temperature and column length. The use of gas chromatography in forensic applications has gained popularity mainly due to the versatility, reliability and sensitivity of the technique. In the crime laboratory particularly, gas chromatography in combination with othertechniques such as mass spectrometry and infrared spectroscopy have helped solve unique crime related problems (Amirav, Gordin, Poliak, MAlon&Fialkov, 2008). Its ease of use, simple instrumentation and automation that allows for easy interpretation of data means the technique will continue to be used in a long time to come. This essay will discuss the role of gas chromatography within forensic applications. The essay will elaborate on the analytical approaches used, the challenges faced and some of the current solutions.
Forensic science encompasses many fields of science and creates a connection between their applications with law. In most cases it entails using chemistry and related sciences principles to the examination of physical evidence obtained from analysing samples mainly collected from crime scenes (Houck & Siegel, 2010). The information is then interpreted by forensic experts and can be used as part of evidence in a court of law. Gas chromatography is however not applicable in all forensic analyses. The most common areas of application include toxicological analyses in blood and drug abuse. The technique is also widely applied in criminalistics which include analysis of explosives, trace evidence of fibres, paints and other polymers and debris from fire scenes (Bogusz, 2008).
Analytical Approaches Used
In almost all forensic applications of gas chromatography, the sample under study is prepared through dissolving it in a suitable solvent. The formed solution is then introduced into the chromatograph instrument by use of a syringe. The analysis of a suspected compound is carried out by use of a comparative substance known as a standard. For instance, to analyse a sample suspected to be a drug such as hashish, a small ration is dissolved in an appropriate solvent such as methanol, chloroform or methylene chloride (Barry &Grob, 2004). The sample is then introduced into the heated injector port of the chromatogram instrument. The carrier gas which in most cases is helium, nitrogen or hydrogen constituting the mobile phase is also introduced into the injector port. Due to the high temperature in the port the liquid sample introduced is volatilized. The carrier gas carries with it the volatilized sample and introduces it into the stationary phase where chromatographic separation occurs. The sample under analysis; hashish emerges from the column in a certain retention time. This is then compared with the known standard of the sample under analysis. By studying the retention time and the information recorded on the electronic detector, one can positively identify the compound under study if it is part of the introduced sample (Ciba Foundation Symposium, 2009).
If the subject compound of analysis is solid in nature and can not easily dissolve in conventional solvents, an alternative approach known as pyrolysis is employed.Here, the solid sample is introduced to high temperatures until it decomposes into a gaseous substance. This is most commonly used for products such as polymeric fibres and paint chips collected from crime scenes. The gaseous state of the compound can now then be introduced into a gaseous chromatograph column. The gas chromatography employs the use of various detectors. However, the most commonly used detector in the field of forensic science is mass spectrometry, nitrogen-phosphorus and flame ionization. The ability of the mass spectrometry detector to give a more definitive identity of compounds under study as well as accurately determine the sample quantity has made it the detector of choice in majority of forensic applications. Flame ionization detector is mostly applicable in determining samples related to arson cases mainly due to its increased sensitivity to hydrocarbons. In almost all incidents of arson cases the accelerants have hydrocarbons as the primary constituent (Muller, Levy &Shelef, 2011). On the other hand, Nitrogen-Phosphorus Detectors are mostly used in toxicological tests and drug analysis. The gas chromatography technique is preferred over other methods in forensic applications as it can accurately provide information on actual identity of the compound if the appropriate standard is used. This qualitative information is vital particularly in cases of complex criminal investigations. The capability of the technique to provide quantitative data concerning a sample is vital especially in toxicological studies where such information can be used by physicians to make appropriate recommendations on an intoxicated patient. This is also important in drug analysis studies where gas chromatography helps accurately determine the quantities of the different components making up a sample. In such cases, the technique is used to detect whether the compound under analysis is pure for instance in drug syndicate incidents and this can provide vital evidence for use in a criminal case. Gas chromatography can facilitate the formulation of a chemical identity or signature of the compound under study by utilizing minute sample collected as evidence in a criminal case (Savchuk, Barsegyan, Barsegyan&Kolesov, 2008).
Interpreting the data from the gas chromatography analysis is one of the most important steps. This is why the gas chromatography instrument is connected to the detector. The connection between gas chromatography and a mass spectrometer is the most common of these combinations. The mass spectrometer facilitates the performance of a full scan or a selected ion monitoring with modern GC-MS instruments performing such functions concurrently or separately. The spectrum generated offers a guide on the concentration of the sample under analysis. The spectrum gives analytical information in both original and comparative formats (de Vos et al, 2011). The information can be relayed in two ways. In one way, the spectrum generated is compared with a library of earlier studied spectra that serve as standards.Computers are nowadays used to give accurate information by displaying the data on the spectrum scales even through easily readable digital displays. Computer programs have been developed that can simultaneously correlate data such as between retention times. The other method involves measuring the peaks in relation to another. The peaks are assigned different values and are associated with different chemical or physical characteristics of the different compounds. Selective ion monitoring entails monitoring only the peaks that are closely associated with the suspected analyte. This is based on the knowledge that related ions such as isotopes will have almost similar or closely related retention times (Mühlen& Marriott, 2012). The methods have been in use in forensic applications and the results obtainedfound reliable and efficient.
An example of application of gas chromatography is in analysis of body fluids. This is attributed to the ability of the technique to give accurate results when analysing non-polar compounds. In analysing blood samples such as serum or plasma, the technique can easily detect psychopharmaceuticals found in blood in detectable amounts (Varlet, Lagroy De, Croutte, Augsburger&Mangin, 2012). For urine to be analysed through gas chromatography, it requires to undergo derivatization since it contains myriad of polar metabolites. In such analysis, a comprehensive screening protocol is required to ensure the sample is well prepared. For the product such as the urine sample, derivatization improves the quality of detection especially for compounds containing hydroxyl and amino end groups such as phenols, amines and imines (Halket, &Zaikin, 2006).Use of solvent extraction methods that utilize compounds such as butyl acetate and butyl chloride help in fractionating the basic, neutral and acidic drug compounds. These can then be readily introduced into the gas chromatography equipment for analysis. With such preparation procedure, the technique can record high levels of accuracy in detection (Casale, Colley &LeGatt, 2012).The application of gas chromatography in criminal forensics has gained such interest that in America, there is an American Society for Testing Materials (ASTM) which carries out analysis on fire debris in arson suspect scenes and utilizes gas chromatography technique for detection and identification. Detection of illegal narcotics and poison detection in toxicological analysis in biological specimens today largely employs gas chromatography owing to its high level of accuracy. Security systems in many nations are employing GC-MS technology in detecting explosives especially in airports (Cook, LaPuma, Hook &Eckenrode, 2010). Medical detectives employ GC-MS techniques to analyse metabolic and congenital disorders in a myriad screening tests as well as in crime scene analyses (Watterson &Desrosiers, 2011).
Diagram showing a typical spectrum of human plasma analysis by gas chromatography
Source: Berthet, Bouchard, Schüpfer, Vernez, Danuser& Huynh (2011)
Diagram showing a gas chromatogram spectrum representation of PI and THPI metabolites in a human urine sample.
Source: Berthet, Bouchard, Schüpfer, Vernez, Danuser& Huynh, 2011)
Challenges Faced when using Gas Chromatography
The major shortcomings associated with gas chromatography include poor performance by the GC screen. Since the instrument requires heat to allow volatilization of analytical compounds, the method may not be applicable in analysis of some components that are thermal labile. The temperature of the heat must also be within acceptable ranges as it can affect the performance of column packing. In other cases, some analytes are adsorbed irreversibly into the chromatographic system. This makes analysis a complex and time consuming process. With the ability of the system to analyse complex mixtures comes a shortcoming in the ability to ensure the system is functioning optimally and generating accurate results (Stauffer, Dolan & Newman, 2008). When the gas chromatography uses a Nitrogen Phosphorus detector, it can not give accurate results when putrid analytes are introduced. These products in most cases containorganic bases which when in high concentrations are known to affect the performance of a Nitrogen-Phosphorus detector rendering the technique ineffective. The gas chromatography machine is costly making it inaccessible in many settings. Furthermore, its operation requires expertise training increasing the cost of its use. The system is also slower in sample analysis as compared to other chromatography techniques (Burleson, Gonzalez, Simons& Yu, 2009).
Current Solutions
The challenges encountered when working with the gas chromatography technique in forensic applications emanate from sample preparation methodology, the physical and functional parameters of the GC equipment and interpretation of results.To overcome the problems in the usually error prone sample preparation stage, highly selective methods targeting one or few analytes are in more application today (Moroni et al, 2010). To increase efficiency, new solvents, membranes and columns have been designed. The use of on-line preparation stage allows automation and system integration to ensure the preparation of the analyte and result generation is matched (Koning, Janssen & Brinkman, 2009). The GC instrument parts have been undergoing modifications in line with modern day inventions to increase efficiency. One part targetedis the column with the length and breadth undergoing changes to decrease time for analysis. Shorter columns have been found to reduce analysis time and in turn increase efficiency. Adjustments in column temperature based on theoretical research data have informed development of more efficient GC equipment. The gas chromatography equipment in use today has undergone many changes including automation. The computer controlled equipment generates real time accurate data displayed on computer screens recording a major shift from earlier models that depended on the visual acuity of the expert to interpret data. Of course this method was prone to deviations due to human and parallax errors (Masˇtovska´ &Lehotay, 2005).
Conclusion
The use of gas chromatography in forensic applications has greatly transformed the field of forensic sciences. The accuracy and efficiency of the technique has informed its widespread use in applications such as toxicological tests, analysis of explosives, trace evidence of fibres, paints and other polymers and debris from fire scenes. The technique however faces some technical challenges that may affect accuracy of results. Nevertheless, advancements in research and technology have enabled continued adjustments to increase efficiency and accuracy of the technique and as evidence shows, the role of GC within forensic applications remains relevant.
Reference List
Amirav, A.; Gordin, A. Poliak, M. Alon, T. and Fialkov, A. B. (2008).”Gas Chromatography Mass Spectrometry with Supersonic Molecular Beams”.Journal of Mass Spectrometry 43: 141–163.
Barry, E. F., &Grob, R. L. (2004).Modern practice of gas chromatography. Hoboken, NJ: Wiley-Interscience.
Bell, S., Fisher, B. A. J., &Shaler, R. C. (2008).Encyclopedia of forensic science. New York, NY: Facts On File.
Berthet, A., Bouchard, M., Schüpfer, P., Vernez, D., Danuser, B., & Huynh, C, 2011, Gas chromatography-tandem mass spectrometry (GC/APCI-MS/MS) methods for the quantification of captan and folpetphthalimide metabolites in human plasma and urine.Analytical &Bioanalytical Chemistry, 399(6), 2243-2255.
Bogusz, M. J. (2008). Forensic science. Amsterdam: Elsevier.
Burleson, G, Gonzalez, B, Simons, K, & Yu, J 2009, ‘Forensic analysis of a single particle of partially burnt gunpowder by solid phase micro-extraction–gas chromatography-nitrogen phosphorus detector’, Journal Of Chromatography A, 1216, 22, pp. 4679-4683
Casale, J, Colley, V, &LeGatt, D 2012, ‘Determination of Phenyltetrahydroimidazothiazole Enantiomers (Levamisole/Dexamisole) in Illicit Cocaine Seizures and in the Urine of Cocaine Abusers via Chiral Capillary Gas Chromatography-Flame-Ionization Detection: Clinical and Forensic Perspectives’, Journal Of Analytical Toxicology, 36, 2, pp. 130-135.
Ciba Foundation Symposium.(2009). Gas Chromatography in Biology and Medicine.John Wiley & Sons.
Cook, G, LaPuma, P, Hook, G, & Eckenrode, B 2010, ‘Using Gas Chromatography with Ion Mobility Spectrometry to Resolve Explosive Compounds in the Presence of Interferents. Journal Of Forensic Sciences ,55, 6, pp. 1582-1591.
de Vos J, et al,2011, ‘Comprehensive two-dimensional gas chromatography time of flight mass spectrometry (GC×GC-TOFMS) for environmental forensic investigations in developing countries’, Chemosphere, 82, 9, pp. 1230-1239.
Halket JM, Zaikin VG (2006). “Derivatization in mass spectrometry –7.On-line derivatisation/degradation”.European Journal of Mass Spectrometry 12 (1): 1–13.
Koning S, Janssen H & Brinkman U, 2009, Modern Methods of Sample Preparation for GC Analysis.Chromatographia Supplement.
Masˇtovska´K&Lehotay S, 2005, Practical approaches to fast gas chromatography–mass spectrometry. Journal of Chromatography.
Houck, M. M., & Siegel, J. A. (2010).Fundamentals of forensic science. Burlington, MA: Academic Press
Mühlen, C, & Marriott, P 2012, ‘Retention indices in comprehensive two-dimensional gas chromatography’, Analytical &Bioanalytical Chemistry, 401, 8, pp. 2351-2360.
Moroni, R, et al, 2010, ‘Statistical modelling of measurement errors in gas chromatographic analyses of blood alcohol content’, Forensic Science International, 202, 1-3, pp. 71-74.
Muller, D, Levy, A, &Shelef, R 2011, ‘Detection of gasoline on arson suspects’ hands’, Forensic Science International, 206, 1-3, pp. 150-154.
Savchuk, S, Barsegyan, S, Barsegyan, I, &Kolesov, G 2008, ‘Chromatographic study of expert and biological samples containing desomorphine’, Journal Of Analytical Chemistry, 63, 4, pp. 361-370.Varlet, V, Lagroy De Croutte, E, Augsburger, M, &Mangin, P 2012, ‘Accuracy profile validation of a new method for carbon monoxide measurement in the human blood using headspace-gas chromatography–mass spectrometry (HS-GC–MS)’, Journal Of Chromatography B: Analytical Technologies In The Biomedical & Life Sciences, 880, pp. 125-131.
Stauffer, E., Dolan, J. A., & Newman, R. (2008).Fire debris analysis. Boston, Mass: Academic Press.
Watterson, J, &Desrosiers, N 2011, ‘Examination of the effect of dose-death interval on detection of meperidine exposure in decomposed skeletal tissues using microwave-assisted extraction’, Forensic Science International, 207, 1-3, pp. 40-45.
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