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9.9 Forensic Chemistry:  1.The job of the forensic chemist

Syllabus reference (October 2002 version)
1. The job of the forensic chemist is to identify materials and trace their origins	Students learn to: outline precautions that may be necessary to ensure accuracy and prevent contamination of samples for analysis distinguish between organic and inorganic compoundsexplain that there are different classes of carbon compounds including: hydrocarbons alkanols alkanoic acids which can be identified by distinguishing tests explain that the inorganic chemical properties of soils and other materials may be useful evidence discuss, using a recent example, how progress in analytical chemistry and changes in technology can alter the outcome of a forensic investigation	Students: solve problems and use available evidence to discuss the importance of accuracy in forensic chemistry solve problems and use available evidence to discuss ethical issues that may need to be addressed during an analytical investigation identify data, plan and perform first-hand investigations to determine a sequence of tests to distinguish between organic and inorganic compounds gather and process information from secondary sources to present information summarising a series of distinguishing tests to separate: the groups of hydrocarbons acids, bases and neutral salts in the school laboratory and in the forensic chemist’s laboratory

Extract from Chemistry Stage 6 Syllabus (Amended October 2002). ©Board of Studies, NSW.
[Edit: 7Sep06]

Prior learning: Preliminary module 8.2.1, HSC modules 9.2.1, 9.3.1.

Background: Forensic science in its broadest definition is the application of science to law. All sciences (physics – ballistics, chemistry – fingerprints, biology – DNA and geology – soil) are concerned with forensic studies. In NSW, the forensic scientist usually assists the Department of Public Prosecutions (DPP) to determine whether a crime has been committed or not, and to assist in the identification of the perpetrator of the crime.

solve problems and use available evidence to discuss the importance of accuracy in forensic chemistry

• Use available evidence, such as the Forensic Chemistry Chemistry Explained, Thompson Gale website to discuss the importance of accuracy in forensic chemistry. In your discussion use problem solving skills to identify and explain methods used to ensure accuracy and prevent contamination of samples for analysis. You should include in your discussion, exhibit collection and storage, chain of custody, examination techniques, staff training and development.
• For information on the importance of accuracy see the information in the next dot point below.

outline precautions that may be necessary to ensure accuracy and prevent contamination of samples for analysis

• To test the reliability of a method standardisation of a procedure is used. Standardisation is obtained by analysing comparison material, reference material, control sample and blank.
  
  
• Forensic science laboratories, like many analytical chemistry laboratories are governed by quality assurance. Quality assurance ensures the correct use of scientific methods despite variations in equipment and operators worldwide.
  
  
• Quality assurance in Australia is governed predominately by two bodies, The National Association of Testing Authority Australia (NATA) and The International Olympic Committee (IOC).
  
  

solve problems and use available evidence to discuss ethical issues that may need to be addressed during an analytical investigation

• Use available evidence, such as newspaper articles relating to forensic cases (e.g. Azaria Chamberlain) and a Forensic Science Society code of ethics to apply problem solving skills to identify and explain where in the forensic case ethical issues arose.
  
  
• Use the ethical issues identified to discuss the forensic case using a cause and effect relationship (what caused the ethical issue and the resulting effect). Information of this type would best be presented in the form of a table.

The Australian New Zealand Forensic Science Society click on DNA Profiling and then scroll down and click on Checks and balances for civil liberties

identify data, plan and perform first-hand investigations to determine a sequence of tests to distinguish between organic and inorganic compounds

distinguish between organic and inorganic compounds

The following information addresses the above two syllabus points at the same time.

• Organic compounds contain mainly carbon and hydrogen, but may also contain smaller quantities of oxygen, nitrogen, sulfur, phosphorus and other elements. Organic compounds mainly originate in living things, however they can also be manufactured in the laboratory.
  
  
• Inorganic compounds do not contain carbon, except for metallic carbonates, hydrogen carbonates, carbon oxides and carbides.
  
  
• Organic compounds can be easily distinguished in the laboratory. Organic compounds undergo combustion to produce carbon dioxide, carbon monoxide or carbon or a combination of these.
  
  
• Use a range of resources e.g. chemistry and biology textbooks and/or the Internet to identify common organic and inorganic compounds.
  
  
• When choosing compounds (planning) and while performing the first-hand investigation, safe work practices such as those outlined in the “Chemical safety in schools” package should be considered and applied. Your plan should also include possible issues that may arise (e.g. possibility of gases given off etc.) and strategies that will address these issues.

explain that there are different classes of carbon compounds including:

• hydrocarbons
• alkanols
• alkanoic acids which can be identified by distinguishing tests

gather and process information from secondary sources to present information summarising a series of distinguishing tests to separate:

• the groups of hydrocarbons
• acids, bases and neutral salts in the school laboratory and in the forensic chemist’s laboratory

The following information addresses the above two syllabus points at the same time.

• Gather and process information from secondary sources, such as chemistry text books, to present information in the form of a table summarising distinguishing tests to separate
  • the groups of hydrocarbons (including alkanes, alkenes, alkynes, aromatics) and also alkanols, alkanoic acids and esters (studied in 9.3)
  • acids, bases and neutral salts
    
    

• The following table shows the classes of organic (carbon) compounds, their functional group and distinguishing laboratory tests.

  Carbon compound class	Functional group	Distinguishing test
  alkane	single bond	Add drops of bromine water to sample in the presence of light; very slow reaction.
  alkene	double bond	Add drops of bromine water to sample; rapidly changes bromine from brown to colourless even in the dark.
  alkyne	triple bond	Add drops of bromine water to sample; slowly changes bromine from brown to colourless even in the dark.
  aromatic	benzene ring	When drops of bromine water are added to a sample in the presence of light there is no reaction.
  alkanol	-OH	Dry the sample with calcium chloride then add a small piece of sodium; bubbles of colourless gas form (H2).
  alkanoic acid	-COOH	Add drops of aqueous sodium carbonate to sample; bubbles of colourless gas form (CO2).
  ester	-COOR	Fruity odour; esters containing four or more carbons are water insoluble.

  
  
  

• The following table shows the distinguishing laboratory tests for acid, base and neutral salt and a forensic chemistry example for each class.

  Class	Distinguishing test	Forensic chemistry example
  acid	pH<7	Citric acid used with gold chloride to reveal fingerprints on adhesive surfaces.
  base	pH>7	Cyanoacrylate (superglue) used to reveal fingerprints on glass surfaces.
  neutral Salt	pH = 6-8	Silver nitrate (AgNO3) used to reveal fingerprints on porous substances.

explain that the inorganic chemical properties of soils and other materials may be useful evidence

• Many inorganic compounds are analysed as evidence by a forensic chemist. Analysis of the inorganic compounds present in a sample and their specific proportions can offer a positive identification. Examples of evidence that are composed of inorganic compounds include glass and soil. Inorganic compounds are important as their composition is rarely altered by bacterial action or time.
  
  
• Glass may be useful evidence in a wide variety of cases, for example, hit-and-run, burglaries and assault. Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides. Metal oxides include those of Na, Ca, K, Mg, Li, Ba and B. The metal oxides act to modify the properties of the glass. Co, Cr, Mn and Ni are used to alter the colour of the glass. The density and refractive index are used to compare glass found at a crime scene with glass fragments found on a suspect. Both methods require significant statistical treatment to determine the likelihood of the two samples originating from the same source.

  Elemental analysis of glass is performed using methods such as scanning electron microscopy/ X-ray dispersive spectroscopy, inductively coupled plasma emission spectroscopy and X-ray diffraction spectroscopy. These methods are quantitative, determining the exact proportions of each element. With modern manufacturing techniques and the homogenous nature of glass produced, a positive identification can be made.

• Soil is important when it has been moved, accidentally or deliberately, during criminal activity. The number of soil types that exist on the Earth is not known. This complexity is due to the fact that soil is a mixture of inorganic materials and organic residues, which is influenced by the climate, seasons, geographical location and geological evolution.

  The inorganic component mostly constitutes approximately 95-99% of the composition of soil. Inorganic components range in size from 1mm (sand) to <2mm (clay). Minerals present include silicates, quartz (SiO₂), feldspars (M_xAl_ySi₃O₈), pyroxenes, olivines, amphiboles, hornblendes, montmorillonite and kaolinite.

  Many methods have been developed for the forensic examination of soil. Most of these methods have been adapted from geology, soil science, chemistry and related fields.

  The following methods are generally used in the analysis of soil. In combination with each other the methods will identify the minerals present and their proportions. This information can be used to locate the original location of the soil:

  • General examination: Soil is dried and observed with a stereomacroscope. Foreign material such as fibres and glass are removed.
    
    
  • Microscopic and mineralogical examination: Crystals are identified by cross-polarisation and frequency of each crystal type determined.
    
    
  • pH measurement: Soil and distilled water mixture is centrifuged until the upper solution is clear. Upper solution is then tested for pH.
    
    
  • Colour, Munsell Color system: This is a subjective test. Samples are dried then sieved. The attributes of this system are Munsell Hue(H), Munsell Chroma(C) and Munsell Value(V) and are written in a form H V/C, which is called the Munsell Notation. Because of its perceptually uniform property, it is recognized as a standard system of color specification and has been widely used in many fields of colour science.
    
    
  • Size distribution of particles: Sample is passed through a nest of sieves. The weight of each fraction is determined.
    
    
  • Density distribution of particles: A density gradient column is used to separate known weight samples that have been previously sieved.
    
    
  • Instrumental analyses. Some examples are: emission spectroscopy, atomic absorption spectroscopy, neutron activation analysis, X-ray fluorescence, X-ray diffraction, differential thermal gravimetric analysis, thermoluminescence, Fourier transform infrared spectroscopy, pyrolysis gas chromatography and laser raman spectroscopy.
    
    
  The interpretation of soil evidence is a challenge. The usual receptor surfaces (shoe soles, tyres, floors) are regularly in contact with soil samples from different sources, therefore soil evidence is often a mixture of soils.

discuss, using a recent example, how the progress in analytical chemistry and changes in technology can alter the outcome of a forensic investigation

• The Azaria Chamberlain case (1980) is probably one of the most important in recent forensic history in Australia. It was claimed by the mother (Lindy) of Azaria that her daughter had been taken from their tent at Ayers Rock by a dingo. Inspection of the Chamberlain’s car located a stain under the passenger side dashboard.
  
  
• Forensic tests performed were found to be positive for the presence of blood. In conjunction with other evidence Lindy Chamberlain was sentenced to jail. It was revealed later however that the tests performed on the sample by the forensic scientist were only presumptive (initial) tests and further positive identification was not carried out. It was realised that the presumptive test also showed positive for other substances that could have been present in the outback dust. It was also claimed by the Chamberlains that if the stain was blood it could have been from an injured person whom they had recently helped.
  
  
• At this time DNA testing was not available and no samples were kept. The advent of DNA testing would have positively identified if the sample was blood and also could have identified the individual whom the sample had originated from.