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9.9 Forensic Chemistry
9.9 Forensic Chemistry
(1-i) Outline precautions that may be necessary to ensure accuracy and prevent
contamination of samples for analysis
There are three main sources of contamination
1.
2.
3.
Addition of extraneous material, during collect, handling and transport such as soil etc. Precautions
for this include using sealed containers, clean sterile utensils
State of laboratory- the cleanliness of the lab, including quality of air, presence of dust and other
contaminants
Careless analysis- not wearing protective clothing such as gloves and coat and result in the adding of
sweat, dandruff etc. By wearing protective clothing, correct storage, cleaning of equipment and taking
caution this can be avoided
(1-ii) Distinguish between organic and inorganic compounds
Organic compounds contain carbon and usually also contain hydrogen nitrogen and oxygen but exceptions
include CO2, CO and carbides) whilst inorganic compound do not contain carbon
(1-iii) Explain that there are different classes of carbon compounds including
Class
Hydrocarbons
Alkanols
General formula
Alkane CnH2n+2
Alkene CnH2n
Alkyene CnH2n-2
ROH
Example
Methane
Ethene
Elkyene
Methanol
Alkanoic Acid
RCOOH
Methanoic Acid
Test
Discolours Br2- slow
Discolours Br2- fast
Discolours Br2- fast
Dry the sample with CaCl2,add a small
piece of sodium bubbles of H2
Reacts with NaCO3 to give off CO2
(1-iv) Explain that the inorganic chemical properties of soils and other materials may be
useful evidence
The composition of soil differs from place to place, and the movement of in from a crime scene can help place
a person there if have a sample on them such as on their shoes. The compounds found within a location are
specific to that soil is rarely found elsewhere thereby scientist can test its contents, look at it microscopically to
identify parts of it or test its pH as this various from location to location. The presence of inorganic compounds
such as clay, silicon and quartz can help identify the soils original location.
Glass fragments and paint chips are another example of evidence which can be used to identify things such as
which car was involved with a hit and run. Paint contains various compounds various are used for pigments
and binders, identify these can help solve a crime, whilst contains a variety of compounds to give it different
properties including tint, therefore the identification of this can help in a crime
(1-v) Discuss using a recent example how progress in analytical chemistry and changes
in technology can alter the outcome of a forensic investigation
(1-a) Solve problems and use available evidence to discuss the importance of accuracy
in forensic chemistry
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9.9 Forensic Chemistry
It’s important to have accuracy in forensic chemistry to correctly identify the person for a particular crime
(1-b) Solve problems and use available evidence to discuss ethical issues that may need
to be addressed during an analytical investigation
Ethical issues include- invasion of privacy through the attain of a person genetic code
If accuracy isn’t obtained then the incorrect person will be imprisoned
(1-c) Identify data, plan and perform first-hand investigations to determine a sequence
of tests to distinguish between organic and inorganic compounds
The main way to distinguish between organic and inorganic compounds is that organic compounds will burn to
give off CO2 depending on the level of oxygen
(1-d) Gather and process information from secondary sources to present information
summarising a series of distinguishing tests to separate
To distinguish the different pH salts we dissolved them in water and tested their pH.
For the hydrocarbon tests see (1-iii)
(2-i) Identify that carbohydrates are composed carbon, hydrogen and oxygen according
to the formula according to the formula Cx (H20) y
Carbohydrates are compounds that contain carbon, hydrogen and oxygen only. They may be represented by
the formula Cx (H2O) y
Carbohydrates are classified into three main groups:
-
Monosaccharides (these are the basic building blocks of more complex carbohydrates). Examples
include glucose and fructose. Deoxyribose, an important part of DNA (deoxyribonucleic acid) is also a
monosaccharide.
-
Disaccharides (molecules of these contain two monosaccharide units linked together). Sucrose (cane
sugar or table sugar) is an example.
-
Polysaccharides (molecules of these are polymers made up of long chains of monosaccharide units).
Examples include celluloses, starches and glycogen
(2-ii) Identify glucose as a monomer and describe the condensation reactions which
produceGlucose (C6H12O6) is a monomer which can undergo condensation reactions to form polymers
Sucrose- is a disaccharide which formed by linking glucose and fructose together, eliminating water
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9.9 Forensic Chemistry
Polysaccharides includingCellulose- Cellulose is a naturally occurring condensation polymer formed by the polymerisation of glucose
monomers. Found in the cell walls of plants giving them their rigidity and support, it’s is a straight chained
polymer as it consists of glucose molecules with every second one been “flipped”, thus causing the molecule to
be linear. (Beta glucose molecule)
glycogen- is a polysaccharide found in animals, it consists of long chains of alpha glucose molecules, is a
branched molecule. It is more has longer chains and higher branching than starch
Starch- consists of branched chains of alpha glucose monomers and is found in plants
(2-iii) Describe the chemical difference between reducing and non-reducing sugars
Sugars which have an OH attached to the carboxyl carbon, are reducing sugars
(2-iv) Distinguish between plant and animal carbohydrates composition in terms of the
presence ofCellulose- Cellulose is a naturally occurring condensation polymer formed by the polymerisation of glucose
monomers. Found in the cell walls of plants giving them their rigidity and support, it’s is a straight chained
polymer as it consists of glucose molecules with every second one been “flipped”, thus causing the molecule to
be linear. (Beta glucose molecule)
glycogen- is a polysaccharide found in animals, it consists of long chains of alpha glucose molecules, is a
branched molecule. It is more has longer chains and higher branching than starch
Starch- consists of branched chains of alpha glucose monomers and is found in plants
(2-a) Choose equipment, plan and perform a first-hand investigation to carry out a
series of distinguishing tests for carbohydrates:
-reducing and non-reducing sugars- Heat up some of the sugar solution with some Benedict’s solution and it
will turn the solution from blue to brick red if it contains a reducing sugar
If a substance contains starch then it will turn brown iodine a deep blue/purple colour
(2-b) Use available evidence and perform first-hand investigations using molecular
model kits, computer simulations or other multimedia resources to compare the
structure of organic compounds including monosaccharides and starch
(3-i) Distinguish between protein used for structural purposes and the uses of proteins
as enzymes
Fibrous proteins are tough, stringy in appearance and are insoluble in most solvents. Globular proteins are
predominately spherical in shape and are soluble in water.
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9.9 Forensic Chemistry
Fibrous proteins form the major structural component of animal tissue. They are found in skin, hair, muscles,
tendons and supporting tissue.
Globular proteins have specialised functions as:
Oxygen carriers (in haemoglobin)
Communication agents (in nerve cells)
Defence agents (in antibodies)
Biochemical catalysts (in enzymes).
An enzyme is an organic catalyst produced by living cells that act by lowering the energy of activation required
for a reaction
(3-a) Perform first-hand investigations using molecular model kits to present
information which describes the composition and generalised structure of proteins
(3-ii) Identify the major functional groups in an amino acid
The major functional groups present in an amino acid are: amino group (-NH2). At least one amino group is
required to give the amino acid some basic (alkaline) properties. carboxylic acid group (-COOH). At least one
carboxylic group is required to give the amino acid some acidic properties
(3-iii) Describe the composition and general formula of amino acids and explain that
proteins are chains of amino acids
NH2-CH(R)-COOH is the general formula of an amino acid
Amino acids are able to link together by undergoing a condensation reaction, eliminating water. The link is
formed between the –NH2 group of one amino acid and the –COOH of the other. The bond between the two
amino acids is called a peptide bond. The linking of two amino acids is called a dipeptide, three amino acids a
tripeptide. When this process is repeated many times, a polymer is formed called a polypeptide. Proteins are
polypeptides that contain up to thousands of amino acid units joined together
(3-iv) Describe the nature of the peptide bond and explain that proteins can be broken
at different lengths in the chain by choice of enzyme
A peptide bond is the bond between to amino acids after a water molecule has been emitted. These can be
broken with the hydrolysis of protein and the use of a catalyst been an enzyme. Enzymes can break proteins
(chains of amino acids) at specific point based on the enzyme used do you the active site form in the tertiary
structure (globule)
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9.9 Forensic Chemistry
(3-v) Compare the process of chromatography and electrophoresis and identify the
properties of mixtures that allow them to be separated by either of these processes
Chromatography works based on solubility, molecular weight and actions in the stationary and mobile phases
Electrophoresis works based on polarity, isometric point and pH buffering
(3-vi) Discuss the role of electrophoresis in identifying the origins of protein and
explain how this could assist the forensic chemist
(3-b) perform a first-hand investigation and gather first hand information about a
distinguishing test for proteins.
Biuret – Add a few drop of NaOH or KOH to the sample before adding a few drops of copper sulfate solution.
Purple colour indicates the presence of proteins or peptides, but not amino acids
(3-c) Perform a first-hand investigation to carry out chromatography to separate a
mixture of organic materials such as the pigments in plants
We ground the plant material in a mortar and pestle with methylated spirits. Drew a pencil line 2 cm from the
base of the paper and add 1 cm methylated spirits into a beaker. Add a dot of the plant pigment on the line
and roll it up and put it in the beaker
(3-d) Perform a first-hand investigation and gather first hand information to identify
the range of solvents that may be used for chromatography and suggest mixtures that
may be separated and identified by the use of these solvents
From the experiment we found that substances used as solvents must be polar, but if they are
organic/inorganic they can be used to separate substances of a similar nature
(3-e) Perform a first-hand investigation to carry out the electrophoresis of an
appropriate mixture and use available evidence to identify the characteristics of the
mixture which allow it to be separated by this process
Electrophoresis is a method of separating charged substances based upon the different signs of their charges
and upon their different mobilites. The smaller molecules move faster as they receive less resistance from the
gel.
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9.9 Forensic Chemistry
(4-i) Outline the structure and composition of DNA
Deoxyribonucleic acid is double helix shaped polymer formed by a series of repeating monomers called
nucleotides which consist of a sugar molecule (Deoxyribose) attached to a phosphate and a nitrogen base,
which is attached to the sugar molecule. In DNA is there are four different nitrogen bases, which can only
joining to their complementary base which adenine joining with thymine and cytosine joining with guanine.
The alternating sugar phosphate molecules in the nucleotides form the strands which are the backbone of the
DNA which the nitrogen bases joining two strand together with hydrogen bonding between the
complementary bases.
(4-ii) Explain why analysis of DNA allows identification of individuals
DNA analysis allows the identification of individuals as a person DNA is unique. DNA consists of introns and
exons. Introns are non coding sequences which separate exons and are commonly referred to as “junk” DNA.
Exons are genes are therefore are coding sequences of DNA consisting of 200-100000 nucleotides. There are
only a small proportion of exons in DNA which as a result means it isn’t very useful in identification. Introns on
the other hand make up 95-99% of human DNA, because of this and also that the intron sequences are unique
to an individual as the structure is developed from both the parents of the individual they can be used to
identify individuals as firstly they are unique to an individual (only identical twins have identical DNA) as they
must be formed by a combination of the parents and secondly due to the multitude of introns in DNA there is
a large amount of introns that can be compared to identify individuals are some individuals can share certain
introns.
(4-iii) Describe the process used to analyse DNA and account for its use in:
- Identifying relationships between people
- Identifying individuals
DNA fingerprinting can be used to identify an individual; firstly a sample is obtained (blood saliva etc) before
the DNA is isolated from this and specific introns are removed from the sample through the use of restriction
enzymes which “cut” the DNA as a specific base sequence (the intron). Multiple copies of the selected
segments are made by polymerise chain reactions before the selected DNA sequences are then sorted by gel
electrophoresis
(4-a) Analyse information to discuss the range of uses of DNA analysis in forensic
chemistry and use available evidence in discussing the issues associated with its use in
terms of the ethics of maintenance of data banks of DNA
The storing of someone’s DNA on a data bank raises many ethical issues due to what use the information
gained from someone’s DNA is used. Defects may be able to be seen such as high risk to heart disease and if
this is used by insurance companies, employment or educational facilities the knowledge of this “faulty gene”
cause cost someone the chance to be employed in a certain field or by a certain company as well as possible
missing out on education opportunities and insurance.
(5-i) Explain what is meant by the destructive testing of material and explain why this
may be a problem in forensic investigations
Destructive testing of a material is when the original sample can’t be recovered, this may be a problem in
forensic investigation as sometimes only small samples are obtained and may need to be used for a variety of
different tests, but if the sample is not recovered then it only has limited use. The sample could also be one
which is prefer not to be alter such as painting where chromatography would ruin the paintings
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9.9 Forensic Chemistry
(5-ii) Identify, outline and assess the value of the following techniques in the analysis of
small samples
- Gas-liquid chromatography
- High performance liquid chromatography
GLC- Can only use volatile chemicals as they need to be in a gaseous state. Separation is based on the ability to
dissolve in the stationary phase. The apparatus consists of a capillary column which contains an inert liquid
inner coating heated to approximated 250 degrees, This differential solubility of compounds in the stationary
phase results in substances moving through at different rates.
Advantages- Sensitive, can measure micro and nanograms
- Can separate a wide variety of substances
- Can improve resolution by changing the
- Quick and easy to carry out
HPLC- The sample is in a solvent, which moves at room temperature and under pressure though a column
which is tightly packed with small silica spheres which are coated with a liquid which acts as the stationary
phases. The differential solubility of a samples molecules determines its retention time in the stationary phase
thereby altering the time it takes to pass through the column. HPLC can be used to identify a wide variety of
compounds; it isn’t limited to just volatile compounds as GLC is. HPLC is highly expensive and contamination
can easily effect the results
Advantages- Can test Non volatiles
- Good for micro samples
- Is at low temperature making it good for testing explosives
- Its quick and simple
- The column and solvent can be easily changed to improve resolution of separation
(5-iii) Outline how a mass spectrometer operates and clarify its use for forensic
chemists
Mass spectrometry is a form of analysis that separates and identifies substances on the basis of the masses of
the positive ions formed by the substances when they are bombarded by high energy particles (usually
electrons) in a high vacuum. A mass spectrometer is used to determine the mass/charge ratio of ion fragments
from a sample and can be used to identify the molecular formula of percentage abundance of components of
the sample. The sample is vaporised if necessary to produce a gaseous sample. The gaseous sample is passed
into a low-pressure ionising chamber where an electron beam ionises the sample by removing electrons from
uncharged atoms or molecules resulting in positive ions been formed. An electric field created by charged
plates accelerates the ions to high speed where they pass through a magnetic field that is perpendicular to
their pathways, causing the ions to move in a curved path. The radius of the curve depends on the mass-tocharge ratio of the ion. An ion collector traps each ion and the detector identifies its mass based on its
movement through the magnetic field, with a recorder trace been produced which shows the mass number
and relative percentage abundance of each ion fragment. This record of the ions mass-to-charge ratio is a mass
spectrum, with these been unique to particular substances allowing mass spectrometry to effectively be used
in the identification of the contents of a sample, and can be used to match a sample from a source (ie soil on
shoes to soil from a yard)
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9.9 Forensic Chemistry
(5-a) Analyse and present information from secondary sources to discuss the ways in
which analytical techniques may provide evidence about samples
(6-i) Describe the conditions under which atoms will emit light
In their normal state atoms do not emit light, but if an atom is given extra energy by either heat or electricity,
the electrons within the atom are excited into a higher energy level. When the electrons fall back down to
their ground state (lowest energy level, the atoms are normally in this state) the atom will emit light (energy).
The wavelength and frequency of light emitted is related to the energy released, with the greater the energy,
the shorter the wavelength and the greater the frequency.
(6-ii) Identify that the emission of quanta of energy as electrons move to lower every
levels may be detected by humans as a specific colour
The energy released from an electrons falls back to ground state is in the form of light which can be detected
by humans and used to form an emission spectra allowing its identification
(6-iii) Explain why excited atoms in the gas phase emit or absorb only certain
wavelengths of light
Each individual excited atom usually emits only one wavelength. Not all atoms in a sample will absorb or be
excited in exactly the same way and therefore excited electrons will travel to different energy levels (different
distances from the natural ground state). Each element produces a different wavelength of light (colour)
because each element has a unique energy level system (energy shell spacing). Therefore when an atom is
excited, electrons will travel to different shells according to the element
(6-iv) Account for the fact that each element produces its signature line emission
spectrum
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9.9 Forensic Chemistry
If a sample of an element is excited in a gas discharge tube and if the light emitted as the atoms fall back from
excited states to ground states is examined through a spectroscope we observe a series of bright coloured
lines on a black background. This is called an emission spectrum of the element (atomic emission spectrum).
The wavelength of the emitted light from one element are different from those from any other elements.
Different elements have different spectra because the transition between electron shells involve different
energy changes and so light emission are at slightly different wavelength
(6-v) discuss the use of line emission spectra to identify the presence of elements in
chemicals
Due to the atomic emission spectrum of each element been unique, it can be used to find whether a particular
substance is present in a sample and how much, with the large amount of the element producing a strong
emission spectrum. With emission spectrometers able to identify small concentrations making them useful in
identifying samples in forensics
(6-a) Identify data choose equipment plan and perform a first-hand investigation using
flame tests and/or spectroscope analysis as appropriate to identify and gather firsthand information to describe the emission spectrum of a range of elements including Na
and Hg
(6-b) Process and present information from secondary sources to analyse and identify
individual elements present in a mixed emission spectrum and use available evidence to
explain how such information can assist analysis of the origins of a mixture
9
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