1 forensic science ( 科學鑑證 ) dr. kendrew k. w. mak department of chemistry the chinese...

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Forensic Science (科學鑑證 )

Dr. Kendrew K. W. MakDepartment of Chemistry

The Chinese University of Hong Kong

IJSO Training (Phase 3)

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Reference for slides 3 – 20:

Criminalistics – An Introduction to Forensic Sciences, 7th ed.By Richard SafersteinPrentice-Hall, NJ, c2001.

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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.

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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

The Basic Services Provided by Crime Laboratories

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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.

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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

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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

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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.

Processing the Crime Scene

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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


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Secure and isolate the crime scene:Exclude all unauthorized personnel from the scene - to prevent destruction of the important physical evidence.

Processing the Crime Scene ( 犯罪現場 )

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.

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Collect and Package Physical EvidencePrevents 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 cross-contamination.


Maintain Chain of CustodyContinuity of possession, or the chain of custody, must be established.

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Obtain ControlsThe 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.

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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.

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The Significance of Physical Evidence

Common Types of Physical Evidence

1. Blood, Semen ( 精液 ), and Saliva ( 唾液 )

2. Documents

3. Drugs

4. Explosives

5. Fibers

6. Fingerprints ( 指紋 )

7. Firearms and Ammunition

8. Glass

9. Hair

10.Impressions

11.Organs ( 器官 ) and Physiological Fluids

12.Paint

13.Petroleum Products ( 石油產品 )

14.Plastic Bags

15.Plastic, Rubber and Other Polymers

16.Powder Residues ( 殘餘物 )

17.Serial Numbers

18.Soil and Minerals ( 礦物質 )

19.Tool Marks

20.Wood and Other Vegetative Matter

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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.

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Methods of Detecting Fingerprints

Three Types of Fingerprints:

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.

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.

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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 chemicalse.g. Iodine ( 碘 ), ninhydrin ( 寧希德林 ), and silver nitrate ( 硝酸銀 ).

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Super Glue FumingSuper 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.

OCN

OCH2CH3ethyl-2-cyanoacrylate

O

O

OH

OH

ninhydrin

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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 )

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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 )

immense in eluting solvent

Original sample spot

time

solvent move up by capillary

action ( 毛細管作用 )

solvent front

separatedcomponents

Recall your experience with paper chromatography. ( 紙色層分析法 )

Recall your experience with paper chromatography. ( 紙色層分析法 )

The original spot should not be immersed in the eluent ( 洗提液 ).

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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)

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Thin-Layer Chromatography (TLC)The Adsorption Process

Adsorption (吸附作用 )

• Strictly a surface process

• Depends on electrostatic forces ( 靜電力 ) (dipole-dipole, ion-dipole and H-bonds)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)

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Thin-Layer Chromatography (TLC)The Adsorption Process

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Equipment For Thin-Layer Chromatography

TLC PlateSilica gel coated on an aluminium backing sheet

TLC Developing Chamber

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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.

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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.

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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 …… )