Comprehensive Notes on Biological Evidence

Biological Evidence

Finding and Identifying Biological Evidence

  • Forensic scientists locate areas to sample and identify the biological evidence collected.

  • DNA testing is relatively new; serology preceded it.

  • Serology: Analysis of blood and bodily fluids (blood, saliva, semen, urine), often found degraded.

Blood

  • Screening/presumptive testing: Simple test to determine if a substance is likely present (indicative, not definitive).

  • First, a visual examination is done to locate blood, then a presumptive test is applied.

Blood Mixture

  • Blood has many functions and is a complex mixture of organic and inorganic materials like electrolytes (sodium, proteins).

  • Red color is due to hemoglobin in red blood cells (RBCs) and oxygen.

  • Hemoglobin: Substance in RBCs that carries oxygen; targeted by most presumptive tests.

  • Plasma: Serum and fibrinogen.

  • White blood cells = DNA.

Hemoglobin Molecule

  • The iron in the hemoglobin molecule is the site of oxygen binding.

Catalytic Color Tests

  • Catalytic tests employ the chemical oxidation of a chromogenic substance.

    • Oxidation: A chemical reaction.

    • Chromogenic substance: Compound that reacts to form a colored substance.

  • Blood presumptive tests use an oxidizing agent catalyzed by hemoglobin.

  • The test involves applying a chromogen solution to the suspected material/stain, followed by an oxidizing agent (often hydrogen peroxide in a 3% solution).

  • Presumptive tests are followed by confirmatory tests.

False Positives and Negatives

  • False positives (positive result from a substance other than blood) and false negatives (negative result even when blood is present) can occur.

  • Misleading results are usually attributed to:

    • Chemical oxidants (reaction before application of the peroxide).

    • Plant materials (vegetable peroxidases are thermolabile and can be destroyed by heat).

    • Materials of animal origin.

  • The age of the stain usually does not affect results.

Application Method

  • The presumptive test involves sampling a questioned stain with a clean, moistened cotton swab.

  • Add a drop of the color reagent solution, followed by a similar amount of hydrogen peroxide (H2O2).

  • Oxidation and bubbling occur with hydrogen peroxide.

  • A color reaction should occur within a few seconds.

Presumptive Tests for Blood

  • Kastle-Meyer and Hemastix are most commonly used.

  • The test consists of a plastic strip with a reagent-treated filter paper tab on one end.

Phenolphthalein

  • Phenolphthalein: A compound that changes color in the presence of blood in the Kastle-Meyer test; a weak acid/base indicator.

  • Produces a bright pink color (peroxide oxidized with hemoglobin).

Tests Using Chemiluminescence and Fluorescence

  • Blood patterns that have been cleaned up or washed away are difficult to see under normal lighting.

  • Samples are often mixed with something else and very diluted.

  • Spraying a mixture of luminol on the area.

    • Chemoluminescence (Luminol): Light is emitted as a product of a chemical reaction.

  • Fluorescence (Fluorescein): Alternate Light Source (ALS).

    • A chemical substance is exposed to a particular wavelength of light and light energy is emitted at longer wavelengths.

Seminal Fluid

  • Semen or seminal fluid: The fluid ejaculated by males that contains sperm cells.

  • Semi-fluid mixture of cells, amino acids, sugars, salts, ions, and other organic and inorganic materials.

  • Typically 2 to 6 ml: contains 100 to 150 million sperm cells per ml.

  • Spermatozoa: Sperm cell.

  • Azoospermia: Semen lacking spermatozoa.

  • Sperm count importance.

Acid Phosphate

  • Acid phosphatases: A class of enzymes that catalyze chemical reactions of certain organic phosphates used in the screening tests for SAP.

  • SAP (Seminal Acid Phosphatase) is present in high amounts in semen and is the target for forensic screening.

  • Sperm loses motility within 3 – 6 hours of ejaculation.

  • Christmas tree stain: A staining procedure used to highlight sperm cells for visualization under a microscope.

Acid Phosphatase (AP) Procedure

  • Two-step procedure:

    • Swab stain and apply reagent.

    • Reaction time:

      • 30 seconds – identification of semen.

      • 2 minutes – contaminant.

Christmas Tree Stain (Microscopic Identification)

  • Christmas tree stain:

    • Anterior = pink.

    • Posterior = dark red/purple.

    • Tails = green.

    • Midpiece = blue.

Saliva

  • A slightly alkaline secretion composed of water, mucus, proteins, salts, and enzymes found in the mouth.

  • Amylase: A family of enzymes that break down starches and sugars; they are the targeted test for saliva.

  • Can be detected up to 28 months.

Starch Iodine Test

  • In the presence of iodine, starch appears blue.

  • As amylase acts on starch, it breaks it down.

    • Starch + Iodine = BLUE

    • Amylase + Starch = Not Blue

Alternate Light Sources

  • Valuable for finding areas stained with biological fluids like semen, saliva, urine, and vaginal fluids.

  • Uses a range of wavelengths.

  • Doesn’t determine what the stain is, but indicates critical evidence at a specific location.

  • When this light interacts with compounds that can fluoresce, light is emitted at a lower energy that is then either directly visible or visible through another type of filter.

Biological Evidence Forensic scientists identify and collect biological evidence from crime scenes. Key Areas: Blood: Analyzed through serology (examining bodily fluids like blood, saliva, semen, urine); DNA testing is a more recent advancement. Presumptive tests are conducted to determine if blood is present, followed by visual examination. Blood is a complex mixture of organic and inorganic substances; its red color is due to hemoglobin. Hemoglobin: Carries oxygen and is the primary target in presumptive blood tests. Catalytic Color Tests: Employ the oxidation of a chromogenic substance during testing. Chromogen is combined with an oxidizing agent (often hydrogen peroxide) and applied to the sample. Confirmatory tests are performed to validate initial findings. False positives and false negatives can occur due to various factors. Sampling is done using a swab; a color reaction occurs within seconds if blood is present. Common tests include Kastle-Meyer and Hemastix; phenolphthalein turns pink in the presence of blood. Chemiluminescence/Fluorescence: Techniques using luminol (chemiluminescence) and fluorescein (ALS) are applied to cleaned or diluted bloodstains to enhance visibility. Seminal Fluid: Semen contains sperm cells and is identified through specific tests. SAP (Seminal Acid Phosphatase): Targeted in forensic screening to confirm the presence of semen. Christmas tree stain is used to highlight