Unit 4 - Forensic Investigation Procedures in Practice
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Unit 4 - Aim B - Use analytical techniques to examine forensic evidence collected from a simulated crime scene
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Unit 4 - Forensic Investigation Procedures in Practice
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PEARSON (PEARSON)
This is an in depth assessment discussing the analytical techniques used to examine forensic evidence collected from a crime scene. This includes biological evidence, chemical evidence and physical evidence.
Unit 4 - Forensic Investigation Procedures in Practice
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Unit 4 – Aim B - Use analytical techniques to examine forensic evidence
collected from a simulated crime scene.
Biological evidence:
Blood -
Luminol presumptive test –
At a crime scene the presence of blood is usually found by close visual examination however
sometimes the blood may be invisible to the naked eye. This could be due to the blood being
cleaned up before the crime scene team arrives or that the amount of blood is too little to
be seen. This is where the luminol test comes in. This test aims to find the blood invisible to
the naked eye and is considered to be one of the most important and well-known methods
in forensic science. The forensic scientists and investigators at a crime scene will spray the
luminol onto surfaces at the scene to detect any blood stains or drops. The solution will
react with the blood and change the structure of the molecules, temporarily adding energy,
this energy causes the luminol to glow a blue colour. This is easy to see and can show
forensic scientists where the blood is or was.
However, the reaction is not specific to just blood so a follow up presumptive test, like the
phenolphthalein test is usually carried out as well.
The luminescent pattern observed in the test can also show other things such as the route of
exit from the crime scene, drag marks in blood or the attempt to clean up any blood.
Hemastix presumptive test –
Hemastix are reagent strips initially designed for testing blood in urine, however they can
now be used quickly and easily by forensic scientists to find the presence of blood on a
surface. The forensic scientist will apply one drop of deionised water onto the pad end of
the test strip and rub the end onto a suspected blood stain. Within 1 minute the colour will
change. A colour change from orange to green indicates a positive result. No colour change
means a negative result. A negative result means there is no blood present, or the amount is
below the limit of detection. Very high concentrations of blood can cause the colour to turn
blue.
Presumptive Phenolphthalein Test/ The KM test –
A phenolphthalein test is a presumptive test that reacts with the haemoglobin molecule
found in the red blood cells in blood. A positive reaction will show a pink colour. This test is
carried out on a suspected bloodstain and although a positive reaction means blood is
present, a false positive is also possible. The intensity of the pink colour indicates how much
haemoglobin there is, therefore showing that blood is present.
The forensic scientist will dampen a sterile swab with sterile water and rub the tip of the
swab onto the suspected stain. They will then add 1 drop of alcohol reagent onto the
surface. After this 1 drop of phenolphthalein reagent is added, as well as one drop of
,hydrogen peroxide reagent. An immediate colour change to pink indicates a positive
reaction. No colour change means a negative reaction.
Presumptive Leucomalachite Green –
This presumptive test for blood is a catalytic test based on the activity of the haemoglobin in
the blood.
The forensic scientists will swab a suspected bloodstain with a swab which has been
moistened by deionised water. 1 or 2 drops of the leucomalachite green reagent will be
added and if there is no colour change the test can continue. If there is a colour change at
this point it means the results are inconclusive. Next 1 to 2 drops of hydrogen peroxide is
added and an immediate colour change of blue-green indicates a positive result. No colour
change means a negative result. A negative result means that either no blood is present, or
the amount is too little so can’t be detected.
Blood typing:
Blood typing is a method which tells you what type of blood you have. The blood type is
based on the certain proteins in the blood cells, called antigens. Your blood type depends on
what type your parents are as it is passed down to you. There are 4 major blood types, type
A, B, AB, and O. To begin with a blood sample is needed. This test determines your blood
group or type. The blood sample is combined with antibodies of type A and B blood. The
sample is then checked to see if the blood cells stick together. If they do stick together, it
means the blood reacted with one of the antibodies. The next phase is blood typing.
Genetic:
Genetic testing also known as DNA profiling is a technique used to compare a suspects DNA
to DNA found at the scene or on the victim. This gives proof about their involvement in the
crime. DNA can either prove someone’s involvement in a crime or dismiss someone from
having any involvement. Everyone has their own unique DNA with no one having the same
meaning it is easy to identify if someone’s DNA left a crime scene is theirs or not. DNA is
very precise and due to everyone having unique DNA it is a very reliable technique used in
forensics. However, if the sample of DNA doesn’t match with someone on the database as
the person who committed the crime has never been arrested before and their DNA is not
stored on the database, it is difficult to find whose it is, meaning other evidence will need to
be used alongside. This is done by using either the saliva, blood, hair, and semen from the
suspect or if a person’s DNA is already on the database a sample won’t be needed. The
forensic scientists will use and examine this sample in the lab to extract the DNA and make a
conclusion on whose it is.
Fingerprints –
,Ninhydrin –
There are different tests to be carried out to find and collect fingerprints at a crime scene.
The first one being the Ninhydrin test. Ninhydrin is a chemical powder that can be absorbed
in ethanol at room temperature. A solution of ninhydrin is sprayed onto fingerprints using a
spray bottle. This method is used on fingerprints found on absorbent surfaces like cardboard
and paper. The ninhydrin reacts with the amino acids present in the oils from the fingers.
The reaction causes a strong blue-purple stain pattern called Ruhemann’s purple. Ninhydrin
prints can take longer than 24 hours to develop. In order to see the prints, they must be
viewed under a light source or a laser. An advantage of this method is the ability to see any
weak fingerprints at a crime scene which can’t be seen by the naked eye. The bright
fluorescent colour is easy to spot and provides a greater contrast to background colours.
When carrying out this method the temperature, acidity and humidity must be controlled to
ensure optimum results are found. These prints which are visible as blue-purple stains are
usually photographed under white light.
Silver nitrate –
Silver nitrate has been used for many years as a way of detecting latent fingerprints. It was
one of the few reagents suitable to use on absorbent surfaces until ninhydrin was
introduced in forensic science. It is usually used for fingerprints on paper but can also be
used on cardboard and plastic. It isn’t useful on items which have been exposed to water.
The silver nitrate solution is sprayed onto the latent prints and left to dry for 30 minutes.
After drying the forensic scientist will expose the material to a UV lamp. Within an hour
black/grey prints are seen. The method reacts with silver ions with chlorides in the
fingerprints, which are seen as these black/grey prints when exposed to the UV light. The
salt content in the fingerprint makes the technique possible. The developed prints are
photographed immediately as the reaction will soon fill the background. The salt content in
the fingerprint makes the technique possible.
Iodine –
To find latent fingerprints iodine can be used to develop them. The residue left in the
fingerprints react with iodine to reveal temporary brown impressions. The prints are
developed on a paper surface using iodine fumes which are then sprayed with polar and
non-polar solvents. The fingerprints developed are clear and permanent. This method is
easy, and the amount of training is minimal so makes it very adaptable. This method is
helpful when dealing with prints on rough and porous surfaces, like paper.
Amido black –
Amido black is a dye that stains the protein molecules found in blood blue-black. It is not
just specific to blood, however. The amido black solution can be methanol or water based.
, Methanol is stronger at staining compared to a water-based formula, however, is more
dangerous and toxic. This method is good for prints found in blood at a crime scene. The
results of blue/black stains enhances the visibility of pattern, ridges and impressions
meaning documentation and identification is easier.
Cyanoacrylate fuming –
Cyanoacrylate fuming is also called super glue fuming and it aims to detect latent
fingerprints on non-porous surfaces such as glass and plastic. The deposition of polymerized
cyanoacrylate ester on the residue found on latent fingerprints makes this method possible.
The method develops clear white fingerprints which can then be enhanced with post and
pre-treatment procedures. This method is efficient and non-destructive.
Hair:
Hair left at a crime scene either from the victim or offender provides valuable information
that can help the investigation to identify the person. Trichology is the study of human hair
and the structure and function of it. Hair is one of the most resistant pieces of evidence to
decay compared to other body tissues and fluids. Meaning it remains on the body the
longest after a death. This means hair is one of the most regularly found pieces of evidence
found at a crime scene. There are 3 major tests done to analyse hair. The first one is testing
the hair for drugs or deficiencies in the persons system, the second test is viewing the hair
sample under a microscope to determine whether the hair was forcefully removed, cut or
diseased. The last test is analysing the DNA from the root of the hair. Extracting the DNA
and analysing it helps to narrow down who was involved in a crime. Each hair is different,
and each have different characteristics like colour and shape. This variability helps forensic
scientists to determine age or gender and even race from the hair sample as well as
determining where on the body the hair is from.
Hair falls out very easily and can be easily transferred during physical contact. Making it
evidence in a crime where physical or violent acts are committed, like sexual assault,
aggravated assault, and murder. Crimes with less physical contact like drug possession or
dealing and anything online won’t have any hair evidence as no physical contact was made.
Skeletal:
Forensic anthropology is the study of human remains. It is the analysis of human remains in
cases where human remains are buried and found. People who specialise in this are called
forensic anthropologists. They recover bones and examine them to determine who died and
how and also how long ago they died. The forensic anthropologist can analyse the skeleton
evidence and can determine different factors. The growth of the bones and teeth tells the
anthropologist if the person is a child or adult. The pelvic bone shape can provide evidence
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