The chemical analysis of blood or urine samples
for alcohol is used to decide whether or not a person is above the legal
limit for driving impairment. This procedure can also be used to determine
the alcohol content of beverages.
Alcohol technical defence cases involve calculations
to establish a person's alcohol level at times prior to the provision
of a sample to test for impairment. These cases may involve:
. Laced drinks
. The consumption of alcohol after driving (so-called post-incident consumption
or "hip flask" defences)
. Cases where data is required to show when an individual would have been
under the statutory limit to drive
In alcohol technical defence cases we provide
a questionnaire to obtain the information we need to carry out the calculations.
The forensic analysis of audio, CCTV and
video data can provide information to identify people or to determine
the sequence of events during an incident. The data is enhanced and analysed
digitally, using specialist computer software.
The analysis of digital data is a relatively
new area of forensic science, where standards are still evolving. As a
consequence, the reliability of the interpretation of the results is highly
dependent on the training and experience of the analyst.
The strength of the evidence that can be
obtained from digital techniques depends critically on the quality of
the original data. As with all techniques that are based on the computer
enhancement of data, the principle is 'rubbish in, rubbish out'.
The distribution, size and shape of bloodstains on a victim, on a suspect
or at a crime scene can be crucial to the interpretation and the reconstruction
of the events that produced the bloodstains. For example, in an alleged
kicking case the distribution and nature of blood on footwear is vital
for the proper interpretation of the incident.
Blood pattern analysis involves the careful recording and measurement
of bloodstains at the scene of crime or on items examined at the laboratory.
It may also involve the reconstruction of an incident, to decide between
alternative versions of events.
The strength of evidence that can be obtained depends upon the different
versions of events. While it may be possible to sometimes exclude a particular
explanation of an incident, it is more likely that the results of blood
pattern analysis will only suggest that one version of events is more
probable than another.
During the forensic investigation of an alleged assault or rape, the
examination of items for body fluids can provide information to identify
the attacker. It can also help to corroborate or refute different versions
of events.
The intimate swabs and clothing of the victim and suspect are examined
for body fluids that may have been produced and transferred during the
incident. These include :
. Blood
. Semen
. Vaginal material
. Saliva
. Urine
. Faeces
Any body fluids that are detected may be subjected to DNA profiling to
identify from whom they originated.
The factors involved in the production, transfer and persistence of body
fluids are very complex. They require careful interpretation that takes
into account both the client's and the prosecution's versions of events.
Using Cell-Site Analysis it is possible to
infer the geographical location where mobile telephone calls may have
been made. This can assist in placing individuals at a specific location
or tracing their movements. Cell-site analysis requires access to call
records from the service provider and may also require a site survey to
locate the position and coverage of the relevant phone masts.
With specialist computer software it is possible
to recover a huge amount of stored data from mobile phones and other digital
devices such as PDAs and SatNavs. The data includes text messages, the
call register, audio and video files, photos and the user's contact list.
Instrumental chemical analysis is used to identify and quantify a range
of chemicals and materials that may be encountered in criminal investigation.
For example lachrymators (tear-gas) and noxious substances, such as acids
and ammonia, are sometimes encountered in assault cases and CS gas is
routinely used by Police Forces to help control individuals without the
need for lethal force.
Also it is not unusual for food, drinks or
household and commercial goods to be contaminated with foreign materials
that need to be identified to decide whether the contamination is accidental
or deliberate.
The examination of computer hard-drives and other storage media, including
hand-held computers and PDAs, using specialist software can yield information
to support or refute a particular version of events.
Damage to clothing and other items may provide
evidence to support or refute a particular sequence of events.
The examination involves the careful recording
and measurement of the damage. It may also involve the reconstruction
of an incident, to decide between alternative versions of events.
The strength of evidence that can be obtained
depends upon the different versions of events. While it may be possible
to sometimes exclude a particular explanation of an incident, it is more
likely that the results of damage analysis will only suggest that one
version of events is more probable than another.
DNA profiling is of enormous value in criminal
investigation because it has the potential to uniquely identify an individual
from traces of body fluids left at the scene of a crime or recovered from
items relating to the incident.
Almost all of the body's cells contain a
complete DNA sequence that is unique to an individual. The DNA can be
extracted and profiled in the laboratory and compared with other profiles
obtained in the case, or with profiles retained on the National DNA Database.
DNA profiling can provide very powerful evidence
in many cases; however in others it is not so effective. For example,
when a DNA profile contains a mixture of possible contributors, or when
a profile is obtained from an unidentified body fluid then the results
may be open to a number of possible explanations.
Chemical analysis is used to identify controlled
drugs and to determine their weight and purity. Links between samples
of drugs from different locations are investigated using both chemical
techniques and physical methods, such as packaging comparison. Drugs examiners
also investigate premises suspected of being used for cannabis cultivation,
to assess the viability of the set-up and to estimate the likely yield.
Most of the laboratory methods used for analysis
have been validated and are well-established. However, sometimes drugs
are identified using screening tests or field tests which do not give
such reliable results and which are subject to interferences from other,
non-drug compounds.
The significance of possible links between
drug samples needs to be interpreted with care. It must take into account
the characteristics of other seizures of the same type of drug.
The detection of explosives traces can, amongst
other things, provide information about whether an explosion was accidental
or deliberate, whether a person has been in contact with explosives or
whether explosives have been stored in a place.
There are many different types of explosives.
These include military materials such as C4, commercial materials such
as gelignites and improvised explosives that may be made from common household
chemicals such as hair dye. Analytical chemistry is used to detect and
characterise explosive traces.
The significance of any findings depends
on many factors including the type of explosive detected, the legitimate
activities of the suspect and the risk of secondary transfer of traces
from someone else.
Fibres evidence can provide information about
whether there has been physical contact between people, or whether someone
was present at the scene of a crime.
Physical contact between people may result
in fibres being transferred from the clothing of one person to another.
In addition, fibres from someone's clothing may be left at a scene of
crime, for example adhering to woodwork or glass at the point of entry
in a burglary.
The recovery of fibres must take place under
carefully controlled conditions to avoid inadvertent transfer of fibres
between the suspect's and the victim's clothing. Fibres are identified
and compared using microscopy and chemical analysis.
The strength of evidence depends upon the
number and type of fibres recovered.
The examination of firearms may involve:
. The classification of firearms and ammunition
according to current legislation
. Assessing the mechanical condition of firearms
including damage, modifications, deactivation and health and safety issues.
. The comparison of cartridge cases and missiles
from a scene or deceased/injured person with those test-fired from a suspected
crime firearm
. The analysis and reconstruction of complex
shooting incidents by consideration of the capability of particular weapons
and ammunition, the interpretation of wounds and the damage caused by
the discharge of firearms, and the determination of the range and trajectory
of shots fired
. Assessing the functionality of weapons
not classed as firearms, such as crossbows and harpoon guns.
For details please consult our Handwriting and Documents division at
Document Evidence
(our sister company).
Fire investigation involves determining where,
when and how a fire started by examining the scene of a fire, reviewing
the physical evidence and taking into account background information from
witnesses and emergency service personnel.
Analytical chemistry is often used to examine
samples from a fire scene for the presence of flammable liquids that may
have been deliberately applied to accelerate a fire and that can indicate
the cause of a fire as arson.
Footwear marks are often found at crime scenes, or they may be found
on the skin in violent assaults. Comparison of the scene marks with control
marks made by footwear taken from the suspect can uniquely link or eliminate
the suspect.
The scene mark and control marks from the footwear are examined for the
presence or absence of features such as manufacturing defects and random
damage.
In the absence of any unique features in the mark, the examiner will
decide on the strength of evidence based on the quality of the mark and
the commonness of the shoe.
A window may often be broken at the point of entry in a burglary, or
as a result of criminal damage. Similarly, glass objects may be broken
during an assault. If someone is close to breaking glass then tiny fragments
of glass are transferred onto their skin, hair and clothing.
These glass fragments can be recovered at the laboratory and compared
by physical and chemical tests to the control glass from the crime scene.
If the recovered glass fragments match the control glass, then the examiner
will decide on the strength of evidence based upon the number of matching
fragments and the frequency of occurrence of the control glass.
When a gun is fired, microscopic particles of material from the ammunition
are ejected from the gun and these may be deposited on the firer or anyone
in the close vicinity.
This gunshot residue (also known as firearm discharge residue) can be
recovered and identified in the laboratory using sensitive analytical
chemistry techniques.
The significance of the findings depends on a number of factors including
the number and type of particles that were detected, the time interval
between the shooting and the examination, and whether other legitimate
activities might explain the results. It is also critical to consider
the risk of inadvertent contamination from, for example, armed officers
who may have been involved in the arrest.
Physical contact between people may result in hairs (head, body or pubic
hair) being transferred from one person to another. Hairs may also be
found at a scene of crime, for example adhering to woodwork or glass at
the point of entry in a burglary.
Hairs are identified and compared using microscopy. If the root and sheath
material is still adhering to the hairs, they can be subject to DNA profiling
and compared to a reference sample from a subject.
If a DNA profile is obtained from hair then it may uniquely identify
a person. Otherwise, it is more likely that hair comparison will only
suggest that the hair is more probably from one person rather than another.
Minute fragments of paint, plastic or other building materials may be
recovered from the clothing or tools of someone who has forced entry into
a building or vehicle.
The layer sequence of fragments of recovered paint can be determined
by microscopy and compared to that of the control paint from the scene.
In addition, the physical and chemical properties of the paint or other
particulates can be measured and compared to the control materials from
the scene.
The significance of the evidence will depend to a large extent on how
unusual is the building material that has been transferred to the suspect
during the incident.
Toolmarks
The presence of damage on a tool or weapon used in the commission
of an offence can provide a unique link between that instrument
and the mark it has left at the scene of the crime or upon the victim.
The scene marks and control marks made with the instrument in question
are examined for the presence or absence of features such as manufacturing
defects and random damage.
In the absence of any unique features in the mark, the examiner will
decide on the strength of evidence based on the quality of the mark and the
commonness of the instrument.
Toxicology
For details please consult our toxicology partner Roar Forensics.
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