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1: Forensic Science

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    Forensic Science

    Forensic science is the application of analytical science to the legal process. The data produced need to be right first time, provide convincing evidence in an adversarial court system and be understandable to a jury.

    Physical Evidence

    Evidence is defined as any material that can provide, through scientific examination and analysis, information relating to a crime. The evidence may:

    1.  Prove that a crime has been committed
    2.  Provide leads to the investigators
    3.  Link the crime scene and/or the victim to the suspect
    4.  Corroborate or refute a suspect’s statement or alibi
    5.  Identify a suspect
    6.  Induce a confession from a suspect
    7.  Exonerate the innocent
    8.  Form part of expert testimony provided by a forensic scientist in court.

    Forensic samples are very varied and can be found in various locations including in or on tools, clothing, carpets, vegetation, and the human body. Weather, washing, high temperature and contamination may change analytes. When dealing with very small samples such as glass fragments, paint chips, a few fibers and blood specks, nondestructive analysis of high sensitivity is needed to permit further examination and retain valuable items.

    The Chain of Custody

    This is the single most important aspect of all forensic chemistry. The progress of the exhibit from crime scene to court must be documented rigorously at every stage. Every person handling evidence must sign and date the label and be prepared to testify in court to validate integrity of evidence. If the chain of custody is broken at any point, the evidence must be excluded from the court.

    The Crime Scene

    The access to the crime scene is limited to essential and authorized persons in order to preserve the scene. During the systematic search for evidence protective clothing must be worn to avoid contamination. Accurate records are essential. The item, its condition, the precise location with respect to a permanent position, date, and time are recorded. Evidence is photographed wherever possible in situ before moving and fingerprinting. All items must be packaged separately to avoid cross contamination. The container should be marked with the initials of the collecting officer, date, location where it was found, case number and description. Plastic envelopes are excellent for small non-biological objects. Paper envelopes are used for dry biological evidence (e.g. dried body fluids). If the sample is not dry it should be dried in the air before packing. Small containers are frequently suitable. Garments and large exhibits can be placed in bags, rolled in paper or boxes. Remember laboratory analysis is irrelevant if evidence is lost or contaminated beforehand.

    Recovery of Trace Materials

    1.  Any small piece of evidence should be collected with tweezers and put into coin envelopes (if damp) or into small plastic bags.
    2.  Shaking garments onto paper is the simplest method of collecting glass and paint fragments.
    3.  Brushing with a new clean toothbrush onto a piece of paper is useful for collecting powders and debris.
    4.  Fibres and hairs are recovered by using Sellotape sequentially on fabrics, window ledges and almost any dry surface. These are then stuck onto clear acetate sheets and stored in sealed polythene bags.
    5.  The evidence vacuum is a small powerful vacuum cleaner, equipped with filters, used for the recovery of minute particulates from firearm discharges or drug dealing. It should be performed before fingerprinting.
    6.  Swabbing using sterile cloth patches is used to collect small amounts of smeared materials, such as blood, semen etc. before blood typing or DNA profiling.
    7.  Oil, grease cosmetics etc. are extracted from fabrics in a suitable solvent.
    8.  Liquids can be pipetted, swabbed or absorbed on a suitable material. The Laboratory The procedures used in the laboratory must prevent contamination and be secure. Detailed, permanent records at every stage of examination ensure no confusion of results. Full documentation of all procedures used with checks undertaken by colleagues and the use of quality control samples is essential. The key is the competence and integrity of the scientist.

    The Laboratory

    The procedures used in the laboratory must prevent contamination and be secure. Detailed, permanent records at every stage of examination ensure no confusion of results. Full documentation of all procedures used with checks undertaken by colleagues and the use of quality control samples is essential. The key is the competence and integrity of the scientist.

    Table 1: Types of physical evidence

    Type Notes
    CONTACT TRACES  

    Fibers and Textiles

    Collecting fibres by adhesive lifts
    Glass Trace and large sections left at the scene or on suspect
    Oils, greases & cosmetic products Transferred between objects & individuals.
    Paint & paint products Paint fragment transfer from a car accident etc…
    Soil and minerals Hand picking and brushing.
    DOCUMENTS  
    Questioned documents Hand-written, typed, copied or computer generated.
    Serial number Altered or eradicated on vehicles, firearms or other objects
    FIRE/EXPLOSION  
    Explosives (bulk) Liquid or solid material
    Explosives (trace) Swabbing dry or with solvent, solvent washing, headspace analysis, vacuum sampling or adhesive lifts.
    Fire investigation Liquid, solid material and burned material.
    Firearms and ammunition Firearms, ammunition, casings etc.
    MARKS  
    Finger and palm prints Visible and latent prints lifted from various surfaces
    Tyre and footprints Prints and impressions found on surfaces.
    Tool marks Impressions or scrapes produced on surfaces.
    TOXICOLOGY  
    Body tissues Various organs from autopsy with blood, urine and stomach contents.
    Blood splatters Take photographs and measure precisely.
    Body fluids Blood, semen and saliva (liquid or dry) on clothing, fabrics or objects collected by sterile cloth patches or swabs.
    Drugs and controlled substances. Brushing, extracting or vacuuming of trace samples. Bulk sample in the form of tablets, capsules, powders and other preparations.
    Hair Collected from crime scene, victim or suspect usually by adhesive lifts.

     


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