Trace Evidence Flashcards
Describe Locard’s exchange principle
Suggests that every contact leaves a trace
Includes things like fingerprints, blood, hair, clothes fibres, tool marks, scratches, semen etc.
Also tells us that physical evidence cannot be wrong, it can only be missed through human error
Where might you find trace evidence
At Scene
On Clothing
On Victim - and in their injuries
On Suspect - on their clothing or person
On personnel involved in scene or transfer - paramedics,
police, scientists, forensic Nurse, FME, pathologist, PF
Anyone who attends a crime scene can be involved in the transfer of trace evidence - true or false
True
Even without noticing you can contaminate the scene or be contaminated (e.g. leave a hair, pick up a fibre etc.)
Must be aware of all personal at the scene and beyond
This is why hair nets, white suits are used
At it’s simplest, which 3 objects will trace evidence be passed between at a crime scene
Evidence from the scene itself
The body/victim
And the assailant
Doesn’t happen in practice - anyone else in contact with scene may then transfer evidence
Which trace evidence would you look for on clothing
Hairs Fibres Glass fragments Paint - flakes or smears GS residues Vegetation or pollens - outdoor crime scenes
Which clothing stains may be analysed in criminal cases
Blood
Semen - sexual cases
Vomit - analysis of stomach content, especially drugs
Mud / soil - gives a lot of info about location of crime/whether they’ve been moved
How can damage to clothing be useful in criminal cases
Cuts, stabs - sharp force
Tears - blunt force
Location and extent of damage can be evidence
How can pollen analysis be used in criminal cases
Can tell you where someone has been and when they were there
Very specific shapes/sizes that can be analysed down microscope - resitant particles
Different locations will have different plants and therefore different types of pollen - can identify location
Pollen also has both diurnal and seasonal variations which indicates timings
How can paint flakes be analysed
They often have laminated (multi-layered structure) that can be viewed under a microscope
This makes it more unique and allows you to match it to a suspect source
Layer structure can be very specific for identifying cars - can sometimes date a paint chip and match to a specific vehicle (make, model, year)
Is Locard’s principle still relevant to modern forensic medicine
YES
Where are glass fragments often found
Can get tangled in the hair
Combed out by pathologist and sent for analysis
How can glass fragments be analysed
Look at their refractive index, density etc.
These properties can be matched to suspect vehicle/location etc.
Pink stained vomit suggests what
Amitriptyline overdose
Clothing searches are routine at autopsy - true or false
True
Done whether the death is natural or unnatural
Why must you be careful when searching a victims clothes
May have weapons or sharps on them
Particualrly drug abusers - risk of needlestick injury/infections
What is meant by Jigsaw evidence
When you can physically match broken pieces of evidence to the scene/substance
Anything that’s torn or broken can be physically put back together to show they had the same origin
Very powerful evidence
List examples of Jigsaw evidence (physical matching)
Plastic - RTA (number plate etc.)
Metal - from a car or tool, tip of knife (if left in body can be matched to suspect weapon)
Paper or tape when torn
Anything breakable!
How does velocity affect blood splatter
The droplets decrease in size as the velocity increases
Can go from large droplets/pools to a fine mist
The smaller the droplets the less distance they can travel
What can cause a low velocity blood splatter
May be dripping from body, thrown off weapon splashed or spurts of arterial blood
What can cause a medium velocity blood splatter
Assault with a weapon such as a baseball bat
Striking a ‘pool’ of blood with great force
What can cause a high velocity blood splatter
Gunshots
High speed machinery
What would cause a fine mist of blood
A high velocity injury/trauma such as a gunshot wound
What can happen to blood after it has splattered
Can be transferred or smeared onto other objects
How is leuco malachite green used at crime scenes
It is a on-scene test used to indicate the presence of blood
Filter paper has LMG chemicals added and is then touched to a suspect area
If it turns green on contact it means blood is present
Non-destructive chemical test (don’t need to dissolve out the blood)
How can blood splatter distribution be analysed
Can look at the length/width of the stains - if you use this to make a ratio you can determine the angle/height it struck the surface
Will also have a direction to then - can draw lines to show their point of origin
Long and thin blood splatters suggest what
That the blood has struck the surface at a shallow angle
Can be used to estimate location of source
Short and fat blood splatters suggest what
That the blood has struck the surface at a less shallow angle
Can be used to estimate location of source
What is the typical splatter pattern of an arterial bleed
Will be pulsatile - up and down and up and down
Which non-intimate samples can be taken on arrest
Fingerprints, palm prints, other impressions
Photographs - of suspect
Which non-intimate samples can be taken on arrest as long as the inspector gives permission (i.e. reasonable force)
Body hair, other than pubic
Finger or toe-nail clippings or other material from nails
Blood or other body tissues or fluid gained by swabbing or rubbing - not a needle test
Saliva that can be swabbed from the inside of the mouth
Which non-intimate sample requires a warrant
A blood sample
Do police have the power to carry out DNA tests
Yes
Upon detention or arrest, police can carry out a DNA test
Cells scraped/swabbed from inner cheek lining
What are the 6 main benefits of the National DNA database
Early identification of linked cases (serial crimes) Early arrest of offenders Valuable intelligence Early exoneration of innocent suspects Easier identification of bodies Deterrence
What is the purpose of the National DNA Database
Holds DNA profiles from those arrested for charged with, informed they will be reported for, or convicted of a recordable offence (e.g. not a minor offence)
Also holds DNA profiles obtained from crime scenes - allows those arrested to be matched to past crimes
Who manages the National DNA database
the Forensic Science Service on behalf of the police
What happens to stored DNA, prints etc. if the suspect is convicted
They are retained on police files
What happens to stored DNA, prints etc. if the suspect is not convicted
They must be destroyed
What happens to stored DNA, prints etc. if the suspect is given an alternative to prosecution such as a fine or work order
They can be held for up to 2 years
Even 3 if it was a violent or sexual crime
After this it is destroyed
How long can police store DNA, prints etc. from a suspect in a violent or sexual crime
Up to 3 years
Even if they are acquitted or the case dropped
Police can also apply for an extension, to enable retention of DNA information for further periods of up to 2 years at a time
What is required for an intimate sample to be taken
Requires consent from the person and a Dr
Police cannot take them
List examples of intimate samples
Blood - taken via venipuncture Semen Any other tissue fluid Urine Pubic hair Dental impression Swab from body orifice other than the mouth (vagina, anus)
Can an intimate sample ever be taken without consent
Technically yes
Can be taken on the authority of an Inspector from a person in police detention if there are reasonable grounds for suspecting the involvement of the person in a recordable offence and for believing the sample will tend to confirm or disprove his/her involvement
However, most Drs would decline to take it as the GMC and defence unions would not back them
List potential sources of DNA
Blood Tissue Semen - one of the best Saliva -contains cheek cells Hair follicles - not just the shaft Weapon Bullet (exited) Condom Fingernail scrapings - if victim has scratched attacker Bite mark - can swab to recover perpetrator DNA Glass / bottle - drank from or inserted Cigarette - saliva on butt Stamp - if licked will have saliva/cells on it Dandruff Fingerprints Clothing, hat - cells found in these
Which part of the hair can you get DNA from
Only the follicle
Can’t get DNA from shaft only
Why is a condom such a good source of DNA
Will have victims DNA on outside (vaginal or anal cells) and assailant on inside (sperm)
Why is semen such a good source of evidence
Contains loads to DNA specific to individual - head of sperm cells are packed with paternal DNA
Also contains AP and PSA which can be used in analysis
How might you find victim’s DNA on the male suspect following a sexual assault
Vaginal cells containing victim’s DNA rubs off onto penis
This can persist for around 24hrs and even transfer into the perpetrators underwear
What are the main components of semen
Sperm - come from testes, around 20-100 million per ejaculate
Seminal fluid - mixture of fluids from the prostate and seminal vesicles
Forms a whitish fluid with a pH of 7.2-8
How much semen is produced per ejaculate
Around 2-6ml
What happens to sperm motility as time progresses
It decreases
There is rapid progression within 60 minutes of ejaculation
What are the most important constituents of semen in terms of analysis
Spermatozoa (from testes)
Acid Phosphatase (from prostate)
Prostatic Specific Antigen P30 (from prostate)
Choline, Spermine
Describe the appearance of semen immediately after ejaculation
Sticky, jelly-like liquid often forming globules
Usually white in colour, but may appear grey or even yellowish
Why might semen appear pink
Blood in the semen (hematospermia) can cause a pink or reddish colour
Describe the appearance of semen between 5-40 mins after ejaculation
It becomes more watery and liquid before finally drying in
How is semen detected at a crime scene
It will fluoresce under UV light - can scan scene with light
May also look for a dry, crusty stain - more likely than fresh sample at scene
How long do sperm survive post ejaculation
Motile sperm only live up to 4-6 hours in the vaginal canal Intact sperm (with tails) not usually found after 16 hours but have been found up to 72 hours in the GU tract Non motile sperm (no tails) normally found in upper vagina up to 3 days after intercourse, and sometimes up to 6 days or later in the cervix.
How does the pH of the vagina affect sperm motility
Acidic environment of the vagina decreases motile sperm count rapidly
How can sperm be lost
Dilution -by vaginal secretions, saliva
Drainage -under gravity, from vagina to/from anus
Degradation (seminal enzymes, loss of tails)
Defecation - flushes out anal sperm
Victim may wash or drink (dilutes)
How does semen typically drain from the vagina
Most commonly drains from the vagina down to the anus if lying on back (with gravity)
Vaginal contamination from anal semen deposits is rarer but can be seen
Why should samples be taken ASAP in sexual assault cases
Prevents the loss of semen samples
How long do sperm survive in the mouth
<6-12 hours
Swab up to 2 days just in case
How long do sperm survive in the upper vagina
Motile spermatozoa: <6-12 hours
Non-motile (tails lost): < 3 – 6 days
Acid phosphatase < 48 hours
Swab up to 7 days just in case
How long do sperm survive in the endocervical canal
Up to 7-10 days
The higher the get the better they survive - less likely to drain/be lost
How long do sperm survive in the anus
Less than 2 days
How long do sperm survive in the rectum
Less than 3 days
The higher the get the better they survive - less likely to drain/be lost
Swab up to 3 days just in case
What is involved in the examination of sexual assault victim or suspect
History - what’s happened, where
Clothing - taken for analysis (hair, semen, tears)
General Examination - injuries, evidence etc.
Genital Examination - injuries and take swabs
Blood sample
What is looked for/taken on general examination is sexual assault cases
Injuries etc. Hair (combed & Cut) Mouth Swabs Skin swabs Fingernail scrapings
What is looked for/taken on genital examination is sexual assault cases
Injuries etc. Pubic hair (combed & cut)
Vaginal swabs (vulva, low vagina, upper vagina) Anal swabs (perianal, anal canal & rectum)
Describe the double swab technique
First, moistened swab rolled over skin
This rehydrates & loosens foreign cells
Second, dry swab rolled over same area
Picks up foreign cells
Why is the double swab technique used
Has a higher recovery of cells and DNA than single swab
Where is the double swab technique used
On the skin - particularly bite marks
On vulva
Perianal skin
On penis
Which lab samples are taken from a victim of sexual assault and in which order
Clothing Head hair (combed, cut & plucked) Swabs from mouth, saliva (<2 d) Fingernail scrapings, clippings and/or swabs Skin swabs from bites (moist + dry) Pubic hair (combed & cut) Vaginal Swabs- taken up to 7 days post assault Ano-rectal swabs Blood
Are hairs plucked from living victims for analysis
Not typically
Usually comb and cut hair in the living and comb then pluck hairs from dead victims
Where are vaginal swabs taken from in sexual assault cases
Vulva (moist + dry swab)
Lower 1/3 at 3-5 cm (2 dry swabs as already moist)
Upper 2/3 after insertion of speculum (2 dry)
Endocervical canal (2 dry)
How are fingernails swabbed
A moist and then dry rolled over nail bed and edge
Use one set for each hand
Why are speculums used for high vaginal swabs
To prevent you touching the lower vaginal as this will contaminate the sample
Endocervical swabs can be taken from above post mortem - true or false
True
In life you need a speculum exam
Which intimate swab is taken first - anal or vaginal
Vaginal
From which regions are ano-rectal swabs taken
Perianal skin -3 cm radius from anus (1 moist + 1 dry)
Anal canal- gentle rotation at 2-3 cm (2 dry)
Rectum, after insertion of proctoscope to prevent anal contamination >4 cm (2 dry)
How long after assault can ano-rectal swabs be taken
Up to 3 days
How long after assault can vaginal swabs be taken
Up to 7 days for vaginal intercourse
Within 12 hours for digital penetration
Intimate swabs are taken from inside to outside - true or false
False
Taken from outside skin to internally - prevents contamination by pushing samples further in
Which swabs are taken from the penis of the suspect in sexual assault cases
Shaft (moist + dry)
Foreskin / Coronal Sulcus (moist + dry)
Glans - pull foreskin back (moist + dry)
Which swabs are taken from the suspect in sexual assault cases and in which order
Clothing Head hair (combed & cut) Fingernail scrapings, clippings &/or swabs Pubic hair (combed & cut) Swabs from penis Blood, Saliva
How long after a sexual assault can swabs be taken from the penis
Up to 3 days
How are mouth swabs taken
2 Dry swabs rubbed over & under tongue, over all sides of teeth and gums and inside cheeks
Mouth rinse with sterile water into polypot - done in the police early intervention kit
Collects cells by rinsing saliva sample
How long after oral penetration can swabs be taken
Within 2 days
Why is a mouth sample taken in the police early intervention kit
Semen only lasts a few hours in mouth so must be taken quickly
Name the vaginal swabs taken after sexual assault
Vulva
Low Vagina
High vagina
Cervical canal
Sometimes do introitus but rarer
Name the anal swabs taken after sexual assault
Perianal
Anal canal
Rectum
Also have the anal margin and internal anal but not routinely taken
How are specimens labelled as evidence in criminal cases
Name of subject Type of specimen Date & time collected Place collected - e.g. scene or mortuary Name of collector (FME or nurse) Name of witness (nurse)
Name of police officer to whom it is handed
What is the chain of custody
It is a legal term that refers to the ability to guarantee the identity and integrity of the specimen from collection through to reporting of the test results
It is the process used to maintain and document the chronological history of the specimen
Chain of custody documents should contain what information
Name or initials of the individual collecting the specimen
Each person or entity subsequently having custody of it
The date the specimen was collected or transferred
Specimen type
Patient’s name,
Brief description of the specimen
What is the benefit of a secure chain of custody
When combined with good analytic techniques in the lab it leads to the production of a legally defensible report.
Needs to be a clean, concise process
What is demonstrated by the chain of custody
Collection of the correct item From the correct person Transported correctly Stored correctly No interference No addition No loss No contamination
Give an example of a typical chain of custody
Samples from the victim/suspect is take by a FME or a forensic nurse
This is then handed to the police who transfer it to the scientist in the forensic lab
Results are given back to the police and eventually court
How are vaginal swabs processed in the lab
The material on the bud of the swab extracted by soaking in 1ml of distilled water
Centrifuge
The supernatant removed to another tube
The cell pellet re-suspended in more distilled water and then dropped on a slide
It is heat fixed and can be stained or tested for various compounds - sperm, AP etc
How is sperm detected in the lab
Microscopy is diagnostic
Look for sperm themselves in the sample
How is acid phosphatase analysed in the lab
There are chemical tests that allow it’s detection
Is acid phosphatase specific for semen
No - it is not specific
Gives false positives
How is prostate specific antigen analysed in the lab
There are immunological tests for it
Can be done with antibodies from rabbits, by gel electrophoresis with AB-Ag precipitation or by more modern dye tests which are more specific
What is acid phosphatase
Enzyme from prostate gland
Found at high concentrations in the semen (400x higher than elsewhere)
Low concentration in vaginal secretions & other bodily fluids
For how long is acid phosphatase detectable
Up to 48hrs
How can you test for acid phosphatase
There is a colour test - can be done as a field test (filter paper with chemicals)
If it turns purple in under 30s it is positive
Can be done on clothing and other material
What is Prostatic Specific Antigen
It is a glycoprotein that is specific to the prostate gland in males - produced there
It is secreted into the seminal fluid - one of the major proteins
What is the function of PSA
To liquefy the seminal fluid
High conc. PSA a useful forensic marker for the detection of seminal fluid - true or false
True
Useful for even small amounts
What are the advantages of testing for PSA
Accepted marker for semen in criminal cases.
PSA test is not presumptive like Acid Phosphatase
Detection possible in cases where sperm cannot be found
Can be recovered from samples up to 30 years old
Can be detected even if semen is very dilute
Detectable in post-ejaculate urine
Why might sperm not be found in a semen sample
If the man has had a vasectomy - cut vas deferens (will also miss the seminal vesicle fluids)
If they are azoospermic
Will also see AP
Can you identify an individual from AP or PSA
No
Not identifiable
Just tells you it’s semen
When is a vasectomy considered successful
Once the sperm count is zero
Checked for with follow up sperm count tests
How would you test for semen in men who have had vasectomies
Cant test for sperm so have to employ different tests such as AP and PSA
Where might you get mud/soil samples from
Clothing
Footwear
Tyres
May also leave print in mud that can be matched (2 way transfer)
How can mud/soil samples be analysed
Look at the colour when dried
Assess particle size/density
Mineral Composition
Biological composition (plants, pollens, microbiology -bacteria etc)
Can help identify location
Mud/soil samples may be important parts of sexual crime scenes - true or false
True
If victim was dragged across ground or pinned down
What is palynology
The analysis of plant pollens
Fairly new science but becoming more important in forensics
Where can pollen samples be taken from
Soil
Cars: tyres, wheel arches (thrown up in mud), upholstery (from person)
Hair, clothes and shoes
Under fingernails
Nasal cavities, gut - inhaled or swallowed
Firearms
Other substances: drug packages, explosives
What is the most common type of trace evidence encountered
Fibres
Found in may crimes and can provide critical evidence
Can you individualize fibres
Not usually
Especially due to mass production
Describe primary, secondary and tertiary fibre transfer
Primary - from scene to person (A-B)
Secondary (A- C via B) - e.g. sit somewhere transfer fibres from different surfaces
Tertiary - fibres transferred from scene to person to another person via a common surface
which natural fibres may you find at a crime scene
Animal fibres (protein):
Silk (fibroin)
Wool (keratin)
Hair (keratin)
Vegetable fibres (cellulose):
Seed fibres
Stem fibres
Leaf fibres
Mineral fibres (asbestos)
which artificial fibres may you find at a crime scene
Regenerated fibres which are manufactured from natural raw materials
- e.g. rayon, acetate
Synthetic fibres which are produced from synthetic chemicals
- e.g. nylons, polyesters, acrylics
becoming more common
Describe the different mechanisms of fibre transfer
Whole or fragmented fibres that lie loosely on the surface may be shed
Whole or fragmented fibres may be embedded in material and may be pulled out or shed
Fibre may break due to stress and a piece of the fibre may then be shed
How are fibres examined in the lab
Microscopy - normal light, polarised, UV, comparison
IR Spectroscopy
Dye type (chromatography)
Incidence (how common is it) and number analysed
Newer, synthetic fibres are more useful as easier to identify
The more fibre tests you can do the more sure you can be that they are a match - true or false
True
Need to make sure that no test can distinguish them
As soon as they become distinguishable they are no longer a match
What can you ascertain from a hair sample
Species
Sex
Site
Identify the individual via DNA if you have the follicle
Where are hair fibres often found
On weapons
On windscreens following a RTA
Often stuck in blood - good glue for fibres
Hair fibres are common evidence in which types of crime
Assault
Sexual assault
Describe the structure of the hair follicle
Forms the base of the hair - root is attached here initially
Has blood supply, muscles and sebaceous glands
Describe the phases of hair development
Anagen phase - growth phase, present 80-90% of time
Catagen phase - slow growth/resting
Telogen phase - hair pushed out of follicle then shed
Describe the structure of the hair cuticle
Overlapping scales of keratin
Pattern indicates species
Describe the structure of the hair cortex
Has parallel spindle cells in regular parallel array
Also has pigment granules (colour, shape & distribution important)
Describe the structure of the hair medulla
Central cylindrical canal
Pigment pattern can be continuous, Interrupted, Fragmented, Absent
In humans they are absent or interrupted but can be inter/intra individual variation - used to identify species and some other useful factors such as race
What is the medullary index and why is it useful in hair analysis
It is the ratio of medulla to cortex in a hair strand
Can be used to help identify source - e.g. if less than 1/3 it is likely human
What can the morphology of hair fibre scales, cortex, medulla tell us
The species Body area Racial origin Age Sex Whether they have bleached/dyed their hair
How can you tell if the hair has been forcibly removed
Sheath cells will also be present in the sample
Can you identify a suspect through hair sample comparison
Not reliably
How can you tell which direction glass was broken from
You analyse the stress marks (Wallner Lines) along the broken edges
Radial cracks have Ridges at Right angles to the Rear face
Concentric cracks have ridges at right angles to front face
What is a radial glass fracture
Cracks which radiate out from the point of impact on glass
Will have Ridges at Right angles to the Rear face
What is a concentric glass fracture
Form a spider web pattern around a circular centre
Will have ridges at right angles to front face
How does tempered glass break
It tends to shatter on impact
Designed this way for safety - e.g. side windows of cars (if an occupant impacts it the energy is dispersed)
Describe the glass structure of car windscreens
They are usually laminated - have plastic in between 2 layers of tempered glass
Will shatter on impact to disperse energy but the plastic will keep occupants in the car
What happens to glass when struck by a bullet
Creates a massive fragmentation pattern - both forwards and backwards
Will have fragments on both sides - present on victim and shooter
What is meant by a toolmark
Any impression, gouge, cut, scrape made by hard object into a softer surface
Will leave behind damage
What is a 3D toolmark called
Impression
What is a 2D toolmark called
Striation
Caused by linear scratching of surface
What identifiable features may you get from a toolmark
Class characteristics - general grouping such as type, size or shape
Individual characteristics - unique marks due to random wear or damage of individual tool
Can be transient and change or may be permanent
In which substances would footwear or tyres leave 3D impressions
Soft materials such as mud, sand or snow
In which substances would footwear or tyres leave 2D prints
Left on harder surfaces in substances like blood, dirt or dust
What identifiable features may you get from footwear or tyre marks
Class Characteristics - general grouping such as manufacturer, type, size, shape etc.
Individual Characteristics - Unique identifying features due to random wear & damage
E.g. glass, nails, stones trapped in tread or a wear pattern
Transient or permanent
