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Chapter 3 Forensic Laboratory Techniques

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1 Chapter 3 Forensic Laboratory Techniques
© 2016, 2012 Cengage Learning. All Rights Reserved

2 Introduction—Objectives
Distinguish between physical and chemical properties. Describe presumptive and confirmatory tests. Compare and contrast different types of microscopes. Explain how qualitative analysis differs from quantitative analysis. Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

3 Introduction—Objectives
Differentiate between thin-layer chromatography, gas chromatography, and high-performance liquid chromatography. Calculate R (retention factor). List and describe three types of spectroscopy. Compare and contrast techniques for visualizing fingerprints. Describe the structure of DNA. Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

4 Introduction—Vocabulary
chemical property - property of a substance that describes how it reacts in the presence of other substances chromatography - any of several processes used to separate a mixture into its individual components based on their attraction to a stationary liquid or solid confirmatory test - test done to establish with certainty the characteristics of a substance frequency - the number of waves that pass a specific point within a given time; usually expressed in cycles per second or hertz (Hz) Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

5 Introduction—Vocabulary
physical property - property of a substance that can be observed or measured without changing the chemical identity of the substance presumptive test - a test to screen evidence and narrow down the possible type of the substance Rf value - retention factor; in paper and thin-layer chromatography, ratio of the distance a substance traveled to the distance the solvent traveled wavelength - the distance between crests, or peaks, of two consecutive waves Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

6 Where is Laci? Laci Peterson — 27 and pregnant, disappeared from her home Dec. 24, 2002 Grid pattern search along Dry Creek revealed no evidence FBI joined the investigation Concrete powder found in Peterson’s boat Hair samples tested Laci’s blood found on truck door 6 Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

7 Introduction Physical properties are properties that can be measured without changing the identity of the evidence Chemical properties determine how a substance behaves in the presence of other substances. Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

8 Presumptive & Confirmatory Tests
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) Confirmatory tests are used to make a more specific identification. Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

9 Microscopy— Compound Light Microscope
Most widely used microscope Light source and multiple lenses Magnifcation between 40x to 1000x View hair, fibers, and cells Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

10 Microscopy—Stereomicroscope
Sometimes called a dissecting microscope Light is reflected from the specimen surface Produces a three-dimensional image useful for dissecting Better surface details Inspect insect larvae, paint chips, and other small items Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

11 Microscopy— Comparison Microscope
Two microscopes connected to one eyepiece Two samples are visible side by side Useful for comparing bullet striations, fibers, and hair Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

12 Microscopy— Electron Microscopes
Uses beams of electrons to form images Magnification up to 500,000x Black and white Transmission electron microscope (TEM) passes electrons through the sample to produce images of internal structures Scanning electron microscope (SEM) passes electrons over the surface to produce a three-dimensional image Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

13 Analytical Techniques
Quantitative — analysis results in a measurable amount Qualitative — analysis results in a description or identification of the components Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

14 Chromatography—Paper
Separates substances within a mixture based on their physical properties Substances adhere to solid surfaces or dissolve in a solvent differently Compare a known substance to an unknown one Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

15 Chromatogram—Rf value
Distance substance traveled Distance solvent traveled Rf = Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

16 Chromatography—Other Types
Passes liquid or gas through a column or tube packed with a porous solid material Thin-layer chromatography (TLC) Gas chromatography (GC) High-performance liquid chromatography (HPLC) Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

17 Electromagnetic Radiation
Light travels in electromagnetic waves Crest — highest point Wavelength — distance between two consecutive crests Frequency — how many waves pass through a point within a given time period Search for latent fingerprints Locate trace evidence Determine the structure of a molecule Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

18 Electromagnetic Spectrum
Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

19 Spectroscopy Substances, when heated, emit light at a specific wavelength Spectroscopy analyzes the light emissions to determine what elements are present Detects accelerants and explosive residues Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

20 Spectroscopy Mass spectroscopy — combined with gas chromatography to identify atoms and molecules by their masses Atomic absorption spectroscopy (AAS) — measures the amount of light of a specific wavelength absorbed by atoms of a substance Ultraviolet (UV) spectrscopy — measures wavelengths of light; used to determine the concentration of elements in a substance Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

21 Fingerprint-Developing Techniques
Fingerprint Databases — FBI Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

22 DNA Analysis Deoxyribonucleic acid — found in the cells of all organisms DNA fingerprint is a DNA pattern that distinguishes one individual from another Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

23 Chapter Summary Physical properties are characteristics that can be observed and measured without changing the chemical identity of the substance. Chemical properties are characteristics that determine the way a substance interacts with other substances. Presumptive tests screen evidence for possible identification. Confirmatory tests determine the identity of evidence. Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

24 Chapter Summary The comparison microscope is one of the most useful tools in forensic investigations because it allows for the side-by-side comparison of samples. Quantitative analysis will always produce a number or a proportion. Qualitative analysis will always provide a description or statement. Chromatography is used to separate a mixture into its individual components. Wavelength is the distance between peaks of consecutive waves. Forensic Science II: Forensic Laboratory Techniques, Chapter 3 © 2012 Cengage Learning. All Rights Reserved

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