The Forensics of Blood PDF

THE FORENSICS OF BLOOD Identification and Characterization of Blood and Bloodstains FORENSIC ANALYSIS OF BLOOD 1. Visual examination of evidence 2. Presumptive screening test (Is it blood?) 3. Confirmation test (Seriously, is it blood?) 4. Determine species origin (human blood?)...

THE FORENSICS OF BLOOD Identification and Characterization of Blood and Bloodstains FORENSIC ANALYSIS OF BLOOD 1. Visual examination of evidence 2. Presumptive screening test (Is it blood?) 3. Confirmation test (Seriously, is it blood?) 4. Determine species origin (human blood?) 5. Identify the blood (whose blood is it?) FORENSIC ANALYSIS OF BLOOD 1. Visual examination of evidence 2. Presumptive screening test (Is it blood?) 3. Confirmation test (Seriously, is it blood?) 4. Determine species origin (human blood?) 5. Identify the blood (whose blood is it?) FORENSIC ANALYSIS OF BLOOD 1. Visual examination of evidence 2. Presumptive screening test (Is it blood?) 3. Confirmation test (Seriously, is it blood?) 4. Determine species origin (human blood?) 5. Identify the blood (whose blood is it?)  PRESUMPTIVE SCREENING TESTS Presumptive Screening Tests – allow a field investigator to screen evidence to reduce the number of possibilities and to get a preliminary identification. e.g. Is it blood? –Negative result means the questioned stain is not likely blood –Positive result means the questioned stain is likely blood  PRESUMPTIVE SCREENING TESTS – Presumptive tests produce a color reaction or release of light – Tests rely on catalytic properties of blood (hemoglobin presence) PRESUMPTIVE SCREENING TESTS Color Tests Apply chromogen (color changing chemical) Apply oxidizing agent (hydrogen peroxide) The catalyst of the reaction is hemoglobin Rapid color change is a positive result. This means the stain is blood. PRESUMPTIVE SCREENING TESTS Remember, other non-blood substances might catalyze the reaction also. – Chemical Oxidants – Plant materials False Positive – A positive result given by a substance that is not blood. PRESUMPTIVE SCREENING TESTS 5 Types of Color Tests Benzidine Phenolphthalein O-Tolidine Tetramethylbenzidine (TMB) Leucomalachite Green (LMG) PRESUMPTIVE SCREENING TESTS Benzidine Test –Positive result = blue color Carcinogen (cancer causing) in 1974 No longer used by sane scientists PRESUMPTIVE SCREENING TESTS Phenolphthalein Test/Kastle – Meyer Test Positive result = pink Some other substance produce colors other than pink (not blood) Still used today  PRESUMPTIVE SCREENING TESTS O-Tolidine Test Positive result = blue Similar to benzidine; still carcinogenic as it can be metabolized to benzidine No longer used; gradually replaced by TMB PRESUMPTIVE SCREENING TESTS Tetramethylbenzidine (TMB) Positive result = Blue-green (+) Most common test for blood 1.Rub stain with moist swab 2. Add TMB 3. Add peroxidaseLook for quick blue color PRESUMPTIVE SCREENING TESTS Leucomalachite Green (LMG) Test Positive result = Green Not as sensitive as TMB or specific as phenolphthalein PRESUMPTIVE SCREENING TESTS Chemiluminescence and Fluorescence –Chemiluminescence – light is emitted as a product of the chemical reaction –Fluorescence – light is emitted when a substance is exposed to a shorter wavelength of light PRESUMPTIVE SCREENING TESTS Chemiluminescence and Fluorescence More sensitive than color tests May damage blood stain (no blood/DNA typing) Used to locate and define areas of blood – old blood stains – cleaned floor PRESUMPTIVE SCREENING TESTS Luminol Method similar to color test Luminol is combined with oxidant and sprayed over area thought to contain blood Emits a blue-white to yellow green glow PRESUMPTIVE SCREENING TESTS Fluorescein Fluorescein is combined with oxidant and sprayed over area thought to contain blood Fluoresces is treated with a UV light Fluorescein includes a thickener; this makes it more effective on vertical surfaces Study showed no interference with DNA analysis FORENSIC ANALYSIS OF BLOOD 1. Visual examination of evidence 2. Presumptive screening test (Is it blood?) 3. Confirmation test (Seriously, is it blood?) 4. Determine species origin (human blood?) 5. Identify the blood (whose blood is it?) CONFIRMATORY TESTS Due to the possibility of false positives with the presumptive tests, confirmatory tests are necessary –Actual proof that a stain is blood. –Establishing the presence of characteristic of blood pigment Hb or one of its derivatives. –We are actually now testing Hemoglobin which is not present in fruit and vegetables but rather only blood. Confirmatory tests involve making crystals that detect the presence of hemoglobin  CONFIRMATORY TESTS Teichmann Test and Takayama Test –Small amount of blood added to microscope Teichmann Test /Haemin Crystal Test slide –Chemical solution is added –Slide is heated to form crystals –Crystals viewed under microscope Takayama Test/Haemochromogen Crystal Test FORENSIC ANALYSIS OF BLOOD 1. Visual examination of evidence 2. Presumptive screening test (Is it blood?) 3. Confirmation test (Seriously, is it blood?) 4. Determine species origin (human blood?) 5. Identify the blood (whose blood is it?) SPECIES ORIGIN Most methods test for serum proteins Antigen = serum protein Serum proteins are found in all Antibody = produced when foreign serum animals, but are slightly different protein is detected Species ID methods based on Certain antibody will only attach to one antigen/antibody interactions species’ serum protein SPECIES ORIGIN Antibody is in antiserum Antigen (serum protein) is in blood sample Human antiserum will only attach to human blood sample Rabbit antiserum will only attach to rabbit blood sample Dog antiserum will only attach to dog blood sample SPECIES ORIGIN ◼ Blood sample (dilute) Ring Precipitin Test in top layer ◼ Antiserum in heavy bottom layer SPECIES ORIGIN Ring Precipitin Test ◼ Blood sample (dilute) in top layer Precipitate means blood and antiserum species match ◼ Antiserum in heavy bottom layer SPECIES ORIGIN Ring Precipitin Test Yes line = Yes human blood Human Antiserum FORENSIC ANALYSIS OF BLOOD 1. Visual examination of evidence 2. Presumptive screening test (Is it blood?) 3. Confirmation test (Seriously, is it blood?) 4. Determine species origin (human blood?) 5. Identify the blood (whose blood is it?) GENETIC MARKERS IN BLOOD If a stain is blood, and it is human blood, then whose is it? –Blood Group Markers –Protein/Enzyme Markers –DNA BLOOD GROUP MARKERS Many blood group markers including: – ABO Markers – Lewis System – Rhesus System  BLOOD GROUP MARKERS A person’s blood type is the result of three antigens The surface of each red blood cell is covered with proteins called antigens. BLOOD GROUP MARKERS Type A: has A antigen on surface of RBC Type B: has B antigen Type AB: has both A & B antigens Type O: has no antigens on surface BLOOD GROUP MARKERS BLOOD TYPING Blood type is detected through an agglutination test, where antibodies are added to a blood sample. Clumping, or agglutination, indicates a positive result. BLOOD TYPING ANALYSIS Blood sample mixed with 3 antibodies If blood clumps, proteins are present If no clumps, no proteins are present Anti-A antibody test Rh antibody test Anti-B antibody test BLOOD TYPING ANALYSIS ◼ A cells clump with anti-A ◼ B cells clump with anti-B ◼ AB cells clump with both ◼ O cells do not clump WHY DO BLOOD TYPING? Less expensive and quicker than DNA profiling Class evidence Can be used to link or exclude suspect to crime scene (but doesn’t prove guilt) RH FACTOR Rh protein found on RBC’s when Alexander Weiner worked with Rhesus monkeys (1940) Rh+ : 85% Rh- : 15% RH FACTOR SECRETORS 80% of the population are secretors. Their blood- type antigens are found in high concentration in their body fluids such as saliva, semen, vaginal secretions and gastric juice. GENETIC MARKERS IN BLOOD ◼ ABO blood typing and protein analysis may help eliminate a suspect. ◼ Since there are only a small number of types (ABO = 4 types), a match does not mean the stain definitely came from a certain person ◼ DNA testing can identify a person, and is becoming just as easy as the above tests