Lecture 4 - Serology/Biological Evidence Flashcards
One issue with prints is that you have to match finger to finger.
The article used AI to solve this issue. Tracked similarities across different fingers that are unique to individuals and then trained the AI to match fingers regardless of which finger you have.
Biological evidence
chemical compounds or other
material derived from a living organism
Biological Evidence (2)
30 Years Ago, this would only apply to human tissue. Has now expanded into micro-organisms (bacteria, viruses) and animals (wildlife forensics)
Two conventional areas for analyzing human biological evidence
Serology and DNA Analysis
Serology
Detection and/or identification
of bodily fluids on evidence
DNA Analysis
Determination of DNA
profile from that body fluid
bodily fluids relevant to forensic investigations
blood, saliva, semen, vaginal tissue, sweat, skin, urine/fecal matter
Serological Tests (2)
Presumptive/Confirmatory
Presumptive Test
Establishes the possibility that a
specific body fluid is present
These are:
Fast (secs or mins)
-Simple (not many steps)
-Low sensitivity (high sample requirement)
-Prone to false result
-ex: light source, spray, no really involved chemistry
Confirmatory Test
Verifies and/or identifies a
specific body fluid is present
-generally slower, more work, more expensive
-few or none false results if done correctly
-holds more weight in court
-more definitive
False +
Test result indicates that a body fluid is present
when it is not present
False -
Test result indicates that no body fluid is present when it is present (maybe you didn’t spray the entire area)
ALS (P or C)
Presumptive
ALS explanation
Not very specific for which fluid is present. Can have a lot of false (+)s.
Presumptive tests (continued)
They are great tools at the scene. They can help rule out areas where bodily fluid is not present to cut down the area being searched or scrutinized. It can also help figure out what happened. It helps figure out which bodily fluids are present and where they are.
Blood
- made up of 1 WBC for every 600-700 RBCs
- WBC has DNA not RBC (DNA analysis targets WBC)
-Function of blood: provide immune response - fight infection.
Red blood cells are the target for some serology tests because they are the biggest constituent. It still indicates blood, even if you can’t get DNA.
Blood has hemoglobin (reduced iron - iron(II) ) this is the crux of most of the presumptive tests of blood. They try to detect the reduced iron in the middle of the hemoglobin complex.
Fe in hemoglobin basis for Luminol test (presumptive)
The presence of the reduced iron and the catalyst cleaves the NH-NH bond and replaces it with oxygens. This allows for resonance and delocalized electrons, which leads to fluorescence. Shows up blue.
BUT - many other substances cause fluorescence. Bleach, urine, feces.
Luminol is most common but other methods target the iron in compounds (3 P) (3 C)
P: Fluorescein, Phenolphthalein, Tetramethylbenzidine
C: Hematrace, Ouchterlony double diffusion, Teichmann
Fluorescein
super aromatic compound and then the presence of iron plus the base catalyst cleaves the ether bond, replacing it with a carboxylic acid and creates a very EN group, changes the pi bond structure and causes it to fluoresce. This shows as yellowish orange in the presence of blood.
Phenolphthalein
the presence of hemoglobin will actually convert this structure will rearrange the double bonds in the structure. colorless to bright pink in presence of hemoglobin.
Tetramethylbenzidine
yellow to dark blue. Hemoglobin and hydrogen peroxide changes the pi bond structure so now there’s a double bond between the aromatics.
Blood Confirmatory Tests
Many are based on antigens.
Hematrace Cards
Differentiate between blood from different species. The card contains human blood already. The sample is added. If the precipitation reaction is seen, it mean the blood is human. This is because antigens and antibodies are specific to a species. Only human antigens will react with the human antibodies.
Ouchterlony double diffusion
the Unknown sample goes in the center of a gelatin or agar plate. The center is surrounded by antibodies from different species. Once sample is added, it diffuses out from the center and interacts with the antibodies it matches. Whatever it matches is where a precipitation reaction will be seen.
Teichmann
One of the first confirmatory Tests - a microchemical reaction where the presence of hemoglobin will create a mineral precipitate only in the presence of blood
Saliva is important…(2)
SA cases that only involve saliva. also: saliva has so much DNA - important for other crimes.
Saliva: Primary Components
-Water, mucus
-Various digestive enzymes including
lipases (fat) and amylase (breaks down starches)
-Salts and electrolytes (Na+, Cl-, HCO3-)
What do many presumptive tests for saliva target? Why?
Amylase. It is very abundant in saliva. And it’s relatively stable. Can still get a nice sample after a couple months to a year.
Tests for Saliva.
Starch-Iodide, Phadebas Reagent
Starch-Iodide
When in the presence of iodine, starch
produces a blue solution. If amylase is present, starch is broken down and solution becomes
colorless. Proteins from blood, semen
can also react with iodine
(i.e., false positives)
Phadebas Reagent
Usually done on fabric. Spray fabric with reagent. Outline places where saliva may be. Transfer paper is added and saturated with development solution. Paper is taken off to look for color change and to see if corresponds to these suspected areas of saliva deposition. Can also be done in solution. Blue to purple color change. Amylase cleaves bond between starch and the dye, producing a color change.
Urine
may be important for sexual assault, harassment, or
mischief cases - demonstrates that someone was in a certain place
Urine Primary Components
Water
-Urea, creatinine, nitrogen waste products
- Nucleated Epithelial cells (low concentration) (Shed from urinary tract - chance for DNA profile)
-typically low pH, acidic due to nitrogen waste
Urine Tests
Most of the tests that are described for detecting urine usually involve protein or enzyme like urease breaking down urea (into NH3 and CO2, detecting the pH change with strips.
Urine Test Usefulness
Lot of false positives because many things can create a change in pH. Also, many times urine is found on fabrics which decreases the sensitivity and it’s more difficult to test as it’s embedded into porous substances. Urine also has very low DNA yield. so low chance of DNA profile.
Two applications of serological tests.
Historically, determine where DNA is to isolate and test DNA for the best chance at a profile.
However, more and more recently, the focus has shifted to where this DNA has come from and how it got there. Spitting, SA, blood.
