Forensic Science (科學鑑證)
IJSO Training (Phase 3)
Dr. Kendrew K. W. Mak
Department of Chemistry
The Chinese University of Hong Kong
1
Reference for slides 3 – 20:
Criminalistics – An Introduction to Forensic Sciences, 7th ed.
By Richard Saferstein
Prentice-Hall, NJ, c2001.
2
Definition and Scope of Forensic
Science
Forensic science (科學鑑證):
The application of science to law.
Apply the principles and techniques of sciences to the analysis of
evidence recovered during crime investigation.
Provide expert court testimony (證據).
The procedures and techniques used must:
• have a firm scientific foundation
• be admissible (可採納的) to the courts.
3
The Basic Services Provided by
Crime Laboratories
Basic Services Provided by Full-Service Crime Laboratories
1.
Physical Science Unit
2.
Biology Unit
3.
Firearms Unit
4.
Document Examination Unit
5.
Photography Unit
4
The Basic Services Provided by
Crime Laboratories
Physical Science Unit:
Use scientific tests and modern analytical instruments to identify and
compare crime-scene evidence.
E.g. drugs, glass, paints, explosives and soil……
Biology Unit:
Identifying and DNA profiling of dried bloodstain (血跡) and other
body fluids.
Comparison of hair and fibers.
Identification and comparison of plants.
Firearm Unit:
Examine firearms, discharged bullets, cartridge cases (子彈殼),
shotgun shells, and ammunition (彈藥) of all types.
Detect firearm discharge residues on garments and other objects.
Estimate the distance from which a weapon was fired.
5
The Basic Services Provided by
Crime Laboratories
Document Examination Unit:
Study handwriting and typewriting on questioned documents to
establish authenticity and/or source.
Analysis of paper and ink, erasures, and burned or charred
documents.
Photography Unit:
Examine and record physical evidence using special photographic
techniques to make invisible information becomes visible.
E.g. digital imaging, infrared (紅外線), ultraviolet (紫外線), and X-ray
photography
6
Optional Services Provided by
Crime Laboratories
Optional Services Provided by Crime Laboratories
1.
Toxicology (毒理學) Unit
2.
Latent Fingerprint (潛伏指紋) Unit
3.
Polygraph (測謊) Unit
4.
Voiceprint Analysis (聲紋分析) Unit
5.
Evidence-Collection Unit
7
Processing the Crime Scene
Physical Evidence:
Establishing that a crime has been committed
Providing a link between a crime and its victim or a crime and its
perpetrator.
Collected by experts who are very knowledgeable in crime
laboratory’s techniques, capabilities, and limitations.
Forensic science begins at the crime scene.
If a physical evidence was not properly collected and preserved, it
is useless.
8
Processing the Crime Scene
The Key Steps:
1. Secure and isolate the crime scene
2. Record the scene – by photography, sketches and notes
3. Conduct a systematic search for evidence
4. Collect and package physical evidence
5. Maintain chain of custody (保管鏈 / 保管流程)
6. Obtain controls
7. Submit evidence to the laboratory
8. Crime-scene safety
9
Processing the Crime Scene
(犯罪現場)
Secure and isolate the crime scene:
Exclude all unauthorized personnel from the scene - to prevent
destruction of the important physical evidence.
Record the scene:
Permanently record the scene in its original state –
to document the condition of the crime site and to record the
location of physical evidence.
photography, sketches, and notes
Conduct a systematic search for evidence:
The search for physical evidence at a crime scene must be thorough
and systematic – to collect as much relevant evidence as possible.
10
Processing the Crime Scene
Collect and Package Physical Evidence
Prevents any change from taking place between the time it is
removed from the crime scene and the time it is received by the
crime laboratory.
Avoid contamination (污染), breakage, evaporation, accidental
scratching or bending, or loss through improper or careless
packaging.
All items must be packed separately to prevents damages and crosscontamination.
Maintain Chain of Custody
Continuity of possession, or the chain of custody, must be established.
11
Processing the Crime Scene
Obtain Controls
The examination of evidence often requires comparison with a known
standard or control (對照). Such materials may be obtained from the
victim, a suspect, or other sources.
Crime-scene Safety
•
Forensic and crime-scene personnel may encounter potentially
infectious materials that collected from the scenes (e.g. blood)
•
Alert to sharp objects, knives, hypodermic syringes (皮下注射針
筒), razor blades (剃刀刀片), etc.
•
Avoid contamination of pens, pencils, notebook, paper, etc.
12
The Significance of Physical
Evidence
Examination of physical evidence for identification or comparison.
Identification
To determine the physical or chemical identity of a substance using
modern analytical techniques (分析技術).
Comparison
Perform the same tests and examinations to the suspect and control
specimens to determine whether they have a common origin.
13
The Significance of Physical
Evidence
Common Types of Physical Evidence
1. Blood, Semen (精液), and
Saliva (唾液)
11. Organs (器官) and Physiological
Fluids
2. Documents
12. Paint
3. Drugs
13. Petroleum Products (石油產品)
4. Explosives
14. Plastic Bags
5. Fibers
15. Plastic, Rubber and Other Polymers
6. Fingerprints (指紋)
16. Powder Residues (殘餘物)
7. Firearms and Ammunition
17. Serial Numbers
8. Glass
18. Soil and Minerals (礦物質)
9. Hair
19. Tool Marks
10. Impressions
20. Wood and Other Vegetative Matter
14
Fundamental Principles of
Fingerprints
1st Principle:
A fingerprint is an individual characteristics.
Theoretical calculation – 64 billion different fingerprints.
No two have even been found to be identical.
2nd Principle:
A fingerprint will remain unchanged during an individual’s lifetime.
3rd Principle:
Fingerprints have general ridge (脊) patterns that permit them to be
systematically classified.
15
Methods of Detecting Fingerprints
Three Types of Fingerprints:
Visible prints:
Made by fingers touching a surface after the ridges have been
in contact with a colored material such as blood, paints,
grease (油脂), or ink.
Plastic prints:
Ridge impressions left on a soft material such as putty (油灰),
wax, soap, or dust.
Latent prints:
Or invisible prints. Impressions caused by the transfer of body
perspiration (汗水) or oils present on finger ridges to the
surface of an object.
16
Methods of Detecting Fingerprints
The method used for visualizing a latent print depends on the type of
surface that is being examined.
Hard and nonabsorbent surfaces (e.g. glass, mirror, tile, and
painted wood)
Developed by the application of a powder (adhere to perspiration
residues and/or body oils left on the surface) or treatment with Super
Glue®
Soft and porous (多孔) surfaces (e.g., papers, cardboard, and cloth)
Treatment with one or more chemicals
e.g. Iodine (碘), ninhydrin (寧希德林), and silver nitrate (硝酸銀).
17
Methods of Detecting Fingerprints
Super Glue Fuming
Super glue consists of almost pure ethyl-2-cyanoacrylate.
The invisible fingerprints residues react
with the CA (cyanoacrylate - 氰基丙烯酸
酯) fumes and atmospheric moisture to
form a white polymer (polycyanoacrylate)
on the fingerprint ridges and become
visible to the naked eyes.
Spray with Ninhydrin Solution
Ninhydrin reacts with the residual amino
acids (氨基酸) present in trace amounts
of perspiration and forming visible
purple-blue colored prints.
CN
O
OCH 2 CH 3
ethyl-2-cyanoacrylate
O
OH
OH
O
ninhydrin
18
Different Types of Chromatography
(色層分析法)
Thin-layer chromatography
(薄層色層分析法)
Column chromatography
(柱色層分析法)
19
Thin-Layer Chromatography
A very important technique for rapid and qualitative analysis (定性
分析) of small amounts of material.
Suitable for the analysis of mixtures in small and microscale
experiments, without the need for expensive instrument.
TLC sheet
containing
a thin
layer of
adsorbent
(吸附劑)
Original
sample spot
immense in
eluting solvent
solvent front
solvent move up
by capillary
action
(毛細管作用)
separated
components
time
The original spot should
not be immersed in the
eluent (洗提液).
Recall your experience with
paper chromatography.
(紙色層分析法)
20
Chromatographic Method
•
Chromatography - the separation technique based on the selective
distribution of chemicals between a stationary phase (固定相)
and a mobile phase (流動相)
•
Due to the selective interaction of chemicals with the stationary
and mobile phases, separation is achieved after a certain period of
time.
Types of Chromatography
• Thin-Layer Chromatography (TLC)
•
•
•
(a more effective version of paper chromatography)
Column Chromatography
Instrumental Technique
Gas Chromatography (氣相色層分析法 ) (GC or GLC)
High-Performance Liquid Chromatography (高效能液相色譜法)
(HPLC)
21
Thin-Layer Chromatography (TLC)
The Adsorption Process
Adsorption (吸附作用)
•
•
Strictly a surface process
Depends on electrostatic
forces (靜電力) (dipoledipole, ion-dipole and Hbonds)
Common adsorbents for TLC
•
Silica gel (硅膠) (SiO2):
has polarized (極化) Si-O and O-H bonds that interact with dipole
(偶極) in the solutes (溶質)
•
Alumina (氧化鋁) (Al2O3)
(The activity of the stationary phase partially depends on its water content)
22
Thin-Layer Chromatography (TLC)
The Adsorption Process
23
Equipment For Thin-Layer
Chromatography
TLC Plate
Silica gel coated on an
aluminium backing sheet
TLC Developing Chamber
24
Procedure For Thin-Layer
Chromatographic Analysis
•
Carefully apply a small drop of the sample
solution to a chromatographic plate near the
edge of a narrow end.
(Handle the strip only by the sides to avoid
contaminating the plate with oils from your fingers.)
•
Prepare the developing chamber.
•
Place the TLC plate with spotted end down in
a closed jar.
(The level of the eluent should be just below the sample
spot)
•
When the solvent front has advanced nearly
to the top of the TLC plate, the development
of the chromatogram (色譜展現) is complete,
and the plate is withdrawn from the
developing chamber.
25
Visualizing The Developed Plate
(Fluorescent Indicator熒光指示劑)
If the compounds in the sample are colorless, they must be made
visible by some visualizing reagents or methods.
Visualization with fluorescent indicator
Many commercially available pre-made TLC plates are coated with
fluorescent indicator (熒光指示劑). When a developed TLC plate is
held under ultraviolet (UV) irradiation, spots that either quench or
enhance fluorescence can be seen.
26
Visualizing The Developed Plate
(Fluorescent Indicator)
TLC adsorbents come with zinc sulfide (硫化鋅)
(will fluoresce green under 254 nm wavelength light)
A dark spot appear where there is an eluted substance which
quenches this fluorescence.
Applications
Compounds that absorb UV at 254 nm
• Aromatic compounds (芳香族化合物)
• Conjugated alkenes (共軛烯烴)
• Conjugated carbonyl compounds (共軛羰基化合物)
(ketones 酮 , aldehydes 醛 , esters 酯 ……)
27