Chapter 3 Forensic Laboratory Techniques
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Chapter 3 Forensic Laboratory Techniques © 2012 Cengage Learning. All Rights Reserved
Transcript of Chapter 3 Forensic Laboratory Techniques
Chapter 3
Forensic Laboratory Techniques
© 2012 Cengage Learning. All Rights Reserved
2 Forensic Science II: Forensic Laboratory Techniques, Chapter 3
© 2012 Cengage Learning. All Rights Reserved
Introduction—Objectives
1. Distinguish between physical and chemical properties.
2. Describe presumptive and confirmatory tests.
3. Compare and contrast different types of microscopes.
4. Explain how qualitative analysis differs from quantitative analysis.
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Introduction—Objectives
1. Differentiate between thin-layer chromatography, gas chromatography, and high-performance liquid chromatography.
2. Calculate R (retention factor). 3. List and describe three types of
spectroscopy. 4. Compare and contrast techniques for
visualizing fingerprints. 5. Describe the structure of DNA.
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Introduction—Vocabulary
1. chemical property - property of a substance that describes how it reacts in the presence of other substances
2. chromatography - any of several processes used to separate a mixture into its individual components based on their attraction to a stationary liquid or solid
3. confirmatory test - test done to establish with certainty the characteristics of a substance
4. frequency - the number of waves that pass a specific point within a given time; usually expressed in cycles per second or hertz (Hz)
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Where is Laci?
1. Laci Peterson — 27 and pregnant, disappeared from her home Dec. 24, 2002
2. Grid pattern search along Dry Creek revealed no evidence
3. FBI joined the investigation
4. Concrete powder found in Peterson’s boat
5. Hair samples tested
6. Laci’s blood found on truck door
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Introduction
o Physical properties are properties that can be measured without changing the identity of the evidence
o Chemical properties determine how a substance behaves in the presence of other substances.
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Presumptive & Confirmatory Tests
o Presumptive tests allow a field investigator to screen evidence to reduce the number of possibilities and to get a preliminary identification. (saliva, semen, blood, urine, and vaginal secretions)
o Confirmatory tests are used to make a more specific identification.
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Microscopy— Compound Light Microscope
o Most widely used microscope
o Light source and multiple lenses
o Magnifcation between 40x to 1000x
o View hair, fibers, and cells
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Microscopy—Stereomicroscope
o Sometimes called a dissecting microscope
o Light is reflected from the specimen surface
o Produces a three-dimensional image useful for dissecting
o Better surface details
o Inspect insect larvae, paint chips, and other small items
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Microscopy— Comparison Microscope
o Two microscopes connected to one eyepiece
o Two samples are visible side by side
o Useful for comparing bullet striations, fibers, and hair
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Microscopy— Electron Microscopes
o Uses beams of electrons to form images o Magnification up to 500,000x o Black and white o Transmission electron microscope (TEM)
passes electrons through the sample to produce images of internal structures
o Scanning electron microscope (SEM) passes electrons over the surface to produce a three-dimensional image
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Analytical Techniques
o Quantitative — analysis results in a measurable amount
o Qualitative — analysis results in a description or identification of the components
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Chromatography—Paper
o Separates substances within a mixture based on their physical properties
o Substances adhere to solid surfaces or dissolve in a solvent differently
o Compare a known substance to an unknown one
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Chromatogram—Rf value
Distance substance traveled
Distance solvent traveled Rf =
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Chromatography—Other Types
o Passes liquid or gas through a column or tube packed with a porous solid material
o Thin-layer chromatography (TLC)
o Gas chromatography (GC)
o High-performance liquid chromatography (HPLC)
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Electromagnetic Radiation
o Light travels in electromagnetic waves • Crest — highest point
o Wavelength — distance between two consecutive crests
o Frequency — how many waves pass through a point within a given time period
o Search for latent fingerprints o Locate trace evidence o Determine the structure of a molecule
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Electromagnetic Spectrum
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Spectroscopy
o Substances, when heated, emit light at a specific wavelength
o Spectroscopy analyzes the light emissions to determine what elements are present
o Detects accelerants and explosive residues
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Spectroscopy
o Mass spectroscopy — combined with gas chromatography to identify atoms and molecules by their masses
o Atomic absorption spectroscopy (AAS) — measures the amount of light of a specific wavelength absorbed by atoms of a substance
o Ultraviolet (UV) spectrscopy — measures wavelengths of light; used to determine the concentration of elements in a substance
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Fingerprint-Developing Techniques
Fingerprint Databases — FBI
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DNA Analysis
o Deoxyribonucleic acid — found in the cells of all organisms
o DNA fingerprint is a DNA pattern that distinguishes one individual from another
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Chapter Summary
o Physical properties are characteristics that can be observed and measured without changing the chemical identity of the substance.
o Chemical properties are characteristics that determine the way a substance interacts with other substances.
o Presumptive tests screen evidence for possible identification. Confirmatory tests determine the identity of evidence.
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Chapter Summary
o The comparison microscope is one of the most useful tools in forensic investigations because it allows for the side-by-side comparison of samples.
o Quantitative analysis will always produce a number or a proportion. Qualitative analysis will always provide a description or statement.
o Chromatography is used to separate a mixture into its individual components.
o Wavelength is the distance between peaks of consecutive waves.
