Lecture 7 - Trace and Contact evidence Flashcards
(47 cards)
Forensic evidence =
evidence with a scientific basis which is used in the court of law
3 categories of forensic evidence =
Forensic evidence can be broken up into 3 categories : biological, physical and impression evidence
Fingerprints are made up of
sweat and within this sweat is DNA
Lockard’s exchange principle
With contact between two items there will be an exchange.
Every contact leaves a trace
This is the founding principle of forensic science
Important distinction should be drawn between whether the purpose of the examination is to
provide intelligence or to evaluate the strength of evidence for use in court AND whether quantitative or qualitative analysis is being done on the forensic evidence
Searching for forensic evidence summary …
first visual screening
then use the visual screening crime lite
a new technology evolving is crime-lite auto which is the future
Before using anything to search for evidence …
use your eyes
can use sunlight, white light/torch or oblique light
Visual screening - Crime lite
Crime lites use different wave lengths to identify and visualise stains, hairs, fibres, gunshot residue etc
Hand-held, high intensity light source
Filters alter wavelength allowing forensic evidence to be targeted.
Blood, semen, saliva, bone, teeth, hair, fibres, shoeprints, fingerprints, gunshot residue and accelerants.
Technology evolves - crime lite auto
In a camera, now not two separate devices i.e. video, camera and crime lite all in one
This is the type of technology that is coming to NZ
Biological evidence =
Body fluids or cells containing DNA which can be used for identification purposes.
So when you find something that is a suspect stain you need to do sampling with it to test it first so the next thing you want to do is chemical testing
Chemical testing for biological evidence - different tests based on what bodily fluid is being investigated…. Blood =
Combur, KM, Luminol
Main ones are for blood because when a crime scene is messy there is usually a lot of blood to sample from
Chemical testing for biological evidence - different tests based on what bodily fluid is being investigated…. saliva =
Phadebas
Chemical testing for biological evidence - different tests based on what bodily fluid is being investigated…. semen =
ACP
Chemical testing for blood - COMBUR
Catalytic reaction of haemoglobin on a peroxide substrate
Colour change from yellow to green
Sensitive but not specific
Tiny speck of blood can cause a reaction but it also means that a lot of other things can cause a reaction, more likely to give you a false positive
This test is carried by senior crime officers, more portable and easy to use, looks like pH sticks
Presumptive test for blood
Can react with other things such as some vegetable materials which contain the enzyme peroxidase which may give positive results
Chemical testing for blood - KASTLE MEYER
Phenolphthalein is used to detect possible presence of haemoglobin
Colour change to pink
Less sensitive but more specific
May not react with tiny amount of blood but less likely to give you a false positive which the COMBUR is likely to give you
Chemical testing for blood in general
At the scene of a crime, presumptive tests may be used to detect the presence of blood that might otherwise be overlooked, either because it occurs in minute amounts or because it merges well with its background.
The presumptive tests used for blood are based on the ability of the haemoglobin present in red blood cells to catalyse the oxidation of certain reagents.
Both testsreact to the same part of the blood, but varies in their reactions
Luminol
Luminol reacts with iron found in haemoglobin causing luminescent.
It is non-specific meaning other substances can cause a reaction including bleach, copper and horseradish.
Dead end treatment, meaning that it is the last thing done at the crime scene because once you spray it it causes damage to the sample and starts to degrade the sample
Needs to be dark in order to see the fluorescence
The image we saw in class of the footprints on the stairs treated with luminol you can say that someone with blood on their shoes was walking down the stairs, there is more blood on the left shoe than the right shoe, there is enough detail in the pattern of the shoe to give us a class match so you can say what type of shoe it is that left that footprint behind Theses kinds of photos are quite powerful to use in court
Chemical testing for saliva - PHADEBAS
Saliva is rich in the enzyme alpha- amylase (α-amylase)
The Phadebas Test reacts to α- amylase by releasing a blue dye
Presumptive test indicating saliva may be present (amylase is found in other body fluids at lower levels)
It is a sheet of paper that is placed on top of the item and then you get it wet and then have a look at it and then the area that has saliva present changes to blue in colour
Chemical testing for semen - ACP (acid phosphatase)
Acid Phosphatase (ACP) is found in unusually high concentrations in semen ACP is an enzyme that is secreted by the prostate gland, it is found in high concentrations in the seminal fluid compared with other body fluids
A suspect stain is sampled by swab or damp blotting paper and a solution (sodium alphanapthylphosphate and fast blue B) is applied producing a purple-blue colouration if positive.
Visually search with crime lite first to look for areas of fluorescent until this is made redundant
Blotting paper used and make it wet and then what was ever on that surface is transferred on to the blotting paper and then you take it off the surface and turn it over and treat it with ACP spray and then any areas where there is potentially seminal fluid turn purple and then you match it back to the surface where you take further tests from
In cases of sexual abuse and rape, the presence of semen at the crime scene provides highly important forensic evidence. As might be expected, semen can be recovered from the body of the victim in many cases. In addition, semen may also be collected from, for example, used condoms, bedding, clothes, furniture and carpets.
Blood, saliva and semen are all examples of biological materials and as such may
be used to link an individual with a specific crime scene using the technique of DNA profiling
Collecting biological evidence
Risk Assessment
Since it is a biological hazard you need to do risk assessment
For example consider hepatitis, AIDS or whether it is a COVID situation
PPE – gloves and mask (minimum)
For something more serious you are going to be fully kitted out … Eye protection, gloves, masks, disposable suit, booties, hairnet, hood
Records – photos and notes
Record how it was before you make any changes
Use sterilised/sterile equipment
To prevent yours or others DNA coming into the scene
Collecting biological evidence - blood
One wet swab using sterile water
Have swab - take it out whilst wearing gloves and a mask - put three drops of water on the end of it without touching the end (prevent contamination) - roll it through the blood stain - then back into the tube with sterilised tweezers - make a hole in it to let the air in to prevent it degrading - then in an envelope where it is sealed, signed and labelled
DNA is in the white blood cells not the red blood cells therefore you can’t see them so you roll the swab around the blood stain
Collecting biological evidence - saliva
Two swabs – one wet then one dry
Put it around the top of the bottle/can where someone has drunk from or handled it
Two swabs are counted as one sample and they go into one envelope
Collecting biological evidence - trace/touch (invisible, you can’t actually see these)
Two swabs – one wet then one dry
Can get a DNA profile from the swab of a touch/trace
Two swabs go into one envelope and are classified as one sample
