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9.9 Forensic Chemistry: 5. Small samples and sensitive analytical techniques
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5. Much forensic evidence consists of very small
samples and sensitive analytical techniques are
required
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Students learn to:
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Students: |
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit: 28 Jul 09]
explain what is meant by the destructive testing of material and explain why this may be a problem in forensic investigations
- Almost any technical and scientific technique can be applied in forensic science. Techniques can be classified as non-destructive and destructive techniques.
- Non-destructive techniques include optical methods (naked eye, microscopy, photography) and spectroscopic techniques (ultra-violet, Fourier transform infrared, X-ray fluorescence). Destructive techniques include chemical reactions (screening tests for explosives, drugs, blood) spectroscopic techniques (mass spectrometry, inductively coupled plasma/ mass spectrometry) and analytical separation (thin layer chromatography, gas-liquid chromatography, high performance liquid chromatography).
- Techniques are considered destructive in forensic science if they modify the sample in some way. This includes reversible reactions and separations where the individual components have not been modified. It is important to note that the integrity of a sample in its original form is paramount in a criminal trial. This allows the sample to be re-analysed and results reproduced by an independent analyst.
- All possible non-destructive techniques must be performed on a sample prior to the use of a destructive technique.
identify, outline and assess the value of the following techniques in the analysis of small samples:
- gas-liquid chromatography
- high performance liquid chromatography
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Gather information from secondary
sources such as the manufacturing company web
sites below to describe how the following instruments
operate:
- gas-liquid chromatography
- high performance liquid chromatography.
The following web sites are useful for the collection of secondary source information. Note that gas-liquid chromatography (GLC) often used to be abbreviated to gas chromatography (GC).
Gas chromatography 
– specifically gas-liquid chromatography, Faculty of
Health and Wellbeing, Sheffield Hallam University, Sheffield, UK
High Performance Liquid Chromatography (HPLC), Office of
Biotechnology, Iowa State University, Ames, Iowa, USA.
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Information collected from a web site prepared by a
manufacturer may be biased towards their instrument.
Process information from at least two
different companies to evaluate the validity of their
claims about the instrument. Some considerations in your
assessment might include:
- limitations of the instrument
- required sample size
- reliability.
- Using the information collected and the outline below process the information and assess the value of GLC and HPLC to a forensic chemist.
- In your assessment use your problem solving skills to include difficulties which the forensic chemist could face using techniques which analyse such small quantities.
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Examples of application of GLC and HPLC include:
- drunk driver blood and urine samples
- analysis for poisons in autopsy samples
- analysis of inks used in forged banknotes.
analyse and present information from secondary sources to discuss the ways in which analysis techniques may provide evidence about samples
outline how a mass spectrometer operates and clarify its use for forensic chemists
The following information addresses the above two syllabus points at the same time.
- Present information collected from secondary sources such as the manufacturing company web sites below to describe how a mass spectrometer operates. Information can be presented in the form of a flow chart.
- Analyse the information collected from the web sites to make and justify generalisations concerning the relative molecular mass, range and relative amounts of elemental isotopes present in a sample. To justify your generalisation at least two sources of information must be consulted.
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Information collected from a web site prepared by a
manufacturer may be biased towards their instrument.
Compare at least two different companies to
analyse the validity of their claims
about the instrument. Some considerations in your
assessment might include:
- reliability
- limitations of the instrument
- required sample size
The following websites are useful for the collection of secondary source information. Instrument manufacturers maintain these web sites.
Mass Spectrometry
Mass Spectrometry International, UK
An example of an instrument that uses mass
spectroscopy , Evans Analytical Group, Sunnyvale,
California, USA.
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