Criminalistics Study Guide
Criminalistics: An Introduction to Forensic Science - Chapter 14: Metals, Paint, and Soil
Trace Elements
Definition: Trace elements are small quantities of elements found in manufactured products and natural materials, present in concentrations of less than 1%.
Relevance to Forensics: For criminalists, the presence of these trace elements serves as "invisible" markers, aiding in establishing the source of a material or providing additional points for comparison.
Emission Spectrograph
Function: An emission spectrograph vaporizes and heats samples to a high temperature, exciting the atoms present in the material.
Process:
The excited atoms emit light, which can be separated into its components, producing a line spectrum.
Each element can be identified by its characteristic line frequencies.
Application: Emission spectra can be matched line for line in comparisons between samples.
Origin of Spectra (1 of 4)
Atomic Structure: An atom comprises a nucleus containing protons and neutrons, with electrons located in orbitals surrounding the nucleus.
Energy Levels: The orbitals are associated with definite amounts of energy, termed energy levels.
Origin of Spectra (2 of 4)
Characteristic Energy Levels: Each element has its own set of unique energy levels at varying distances from the nucleus.
Energy Absorption: Atoms absorb energy only in fixed values (via heat or light), pushing electrons into higher energy orbitals, placing atoms in an excited state.
Origin of Spectra (3 of 4)
Excited State to Emission: Electrons remain in the excited state briefly before returning to their original levels, releasing energy in the form of light emission.
Emission Spectroscopy: This method collects and measures the emitted light energies.
Origin of Spectra (4 of 4)
Relation of Energy and Frequency: The specific frequency of absorbed or emitted light is given by the formula:
Where:
E = energy difference between two orbitals
h = Planck's constant
f = frequency
Characteristic Frequencies: Each element emits a unique set of frequencies due to its distinct energy levels.
Inductively Coupled Plasma (ICP)
Concept: ICP is a form of emission spectrometry where the sample aerosol is introduced into a hot plasma, generating charged particles that emit light at characteristic wavelengths for element identification.
Applications: Commonly used in forensic casework for identifying and characterizing materials such as mutilated bullets and glass fragments.
Neutron Activation (1 of 4)
Definition: A nuclear chemistry technique that utilizes a nuclear reactor as a neutron source to bombard atoms.
Result: Captured neutrons produce radioactive isotopes (isotopes have the same number of protons but different neutron counts).
Neutron Activation (2 of 4)
Gamma Ray Measurement: Identifying the radioactive isotope requires measuring the energy of emitted gamma rays resulting from radioactivity.
Neutron Activation (3 of 4)
Analysis Method: Neutron activation analysis measures gamma-ray frequencies from specimens bombarded with neutrons, allowing for sensitive and nondestructive analysis of 20 to 30 trace elements simultaneously.
Neutron Activation (4 of 4)
Forensic Application: Used for trace elements analysis in various materials, including metals, drugs, paint, and soil.
Limitation: Requires access to a nuclear reactor, limiting its practical forensic application.
Paint Evidence (1 of 2)
Composition: Paints consist of pigments and additives suspended in a binder, drying into a hard film when applied to surfaces.
Context in Forensics: Frequently encountered evidence in hit-and-run and burglary cases.
Paint Evidence (2 of 2)
Automobile Coatings: Various coatings are applied by manufacturers, including electrocoat primer, primer surfacer, basecoat, and clearcoat.
Methods for Paint Comparison (1 of 3)
Significance: Due to the diversity of automotive paint, paint comparisons hold forensic importance.
Comparison Technique: Side-by-side analysis under a stereomicroscope to assess color, texture, and layer sequence.
Individualization: Only feasible when paints possess a sufficiently complex layer structure.
Methods for Paint Comparison (2 of 3)
Distinctions Techniques: Pyrolysis gas chromatography and infrared spectrophotometry help identify paint binder formulations, enhancing forensic comparisons.
Methods for Paint Comparison (3 of 3)
High-temperature Pyrolysis: Solid materials like paint can be heated to decompose into gaseous products for analysis via gas chromatography (GC).
Database Use: Crime labs may reference color charts and the Paint Data Query (PDQ) database to identify vehicle make/model from small paint samples.
Paint Collection and Preservation (1 of 2)
Recovery Protocol: Paint chips are commonly found on persons or objects in hit-and-run situations.
Reference Samples: Uncontaminated reference paint must be collected from undamaged areas of the suspect vehicle nearest to affected areas.
Paint Collection and Preservation (2 of 2)
Packaging: Use druggist folds or glass/plastic vials for paint samples.
Whole Item Submission: Items embedded with paint should be collected whole for laboratory analysis.
Tool Evidence: Tools used in crimes should also be gathered, along with reference paint samples.
Soil Evidence
Value in Forensics: Soil is valuable due to its omnipresence at crime scenes and transfer potential to criminals.
Differentiation: Most soils can be differentiated by visual appearance; comparison provides a sensitive method for distinction based on color and texture.
Soil Comparison Techniques
Characterization: Forensic geologists analyze mineral content in soils, utilizing variety for differentiating samples.
Soil Collection
Collection Protocol: Standard and reference samples should be gathered from various areas within 100 feet of the crime scene as well as the crime location.
Evidence on Suspects: Soil clinging to suspects should remain intact, wrapped individually for laboratory analysis.