hey guys.. our assessment on forensics pretty much covered every dot point on this syllabus, our class was a bit behind so we covered everything we could on the assessment.. it got a hefty 93%.. so i hope it helps...
a) Discuss the importance of accuracy in forensic chemistry
Within in a forensic investigation, the overall conclusion can determine the outcome of the case. Incorrect, sloppy or inconsistent work can result in the defence counsel to cast doubt over the validity of the whole forensic evidence because of one small fault in the evidence. The faulty evidence can lead to drastic measures being taken, such as an innocent person being convicted and forcing them to be prisoned for long periods of time. Or it could result in a person or company having to pay large amounts of fines for offences such as polluting the environment or selling allegedly contaminated products, all on the basis of inaccurate analyses.
Another side to accuracy is thoroughness and completeness, there are times when there has not been enough forensic examinations done. All existing evidence may point to one person and end cause the evidence suspect’s one person the investigation is ended, whereas further forensic tests may lead to new evidence and multiple conclusions to the investigation where it looks at more than one person being involved within the investigation taken place. Inaccurate results can cause numerous amounts of outcomes to investigations, proving to be either positive or negative for the individuals accused within the investigation.
b) discuss ethical issues that may need to be addressed during an analytical investigation
The development of ethical standards- that is the code of behaviour that governs moral decisions –has been a more issue for great forensic and philosophers of the time. The value of neutrality of science is epitomised by the vision of the scientist as the ceaseless seeker motivated only by the search for the truth. The theory of inductivism where science begins with the collection of data goes on to generalise about laws and theories and makes predictions that can be proved. But along with the scientists are the critics, the ones who believe the concepts and postulates are products not necessarily of observation but potentially unjustified anticipations, guesses, by tentative solutions to our problems, by conjectures.
Was it scientific fraud and misconduct? Conflict of interest? Or an “honest mistake”? Fraud impugns on the integrity of the research process and destroys trust on which forensic achievement is built. At the same time, intentionally fraudulent actions undermine the confidence of the society and the body politic in science and scientific injury.
During an analytical investigation, a primary issue needed to be dealt with is privacy. Evidence taken during an investigation can publicise the most intimate details of a person’s life, if led out into the public results in drastic implications for all those who are involved in the investigation and surround the people involved. Most investigations have to comply with some sort of an autopsy, leading to ethical issues concerning the way in which the body is treated during the autopsy and the procedures used to realise evidence for the investigation. Consent by guardians/family members/ parents also raise issues concerning the autopsy or any other evidence check within an investigation. Not only does this raise concerns with the victim but it also draws issues involving religious beliefs or certain morals, which the person/s may have.
Other ethical issues involved in the forensic field include financial conflicts with in the organistion and the investigation taking place, which include not only the forensic but the occupants of the court room, the court case, the witnesses and any other person involved in the investigation. Any sexual orientation becomes an issue during an investigation, the privacy of the victim and any other person involved are always concerned about confidentiality. Religion and moral usage, privacy and social control are all ethical issues plaguing the forensic industry. Government involvement also raises issues, funding the investigation and alleged information along with other problems are concerns for the government.
c) outline precautions that may be necessary to ensure accuracy and prevent contamination of samples for analysis
There are three main sources of contamination of samples in analysis.inadvertent addition of extraneous material in the collection, handling and transporting the sample before it reaches the laboratory. This is the reason for the extreme measures taken to completely seal and protect blood samples, urine samples etc... from environmental contaminations as well as spillage’s.
Secondly, an unclean laboratory such as one with old flaking paint, corroding fitting and instruments, and dust-releasing floor or walls, free flow of dirty air ventilating the labs and small amounts of stay chemicals from dirty benches or poorly maintained instruments. This can all lead to contamination of samples and alter the conclusion made to the evidence.
Thirdly, a careless analyst who does not wear suitable clothing such as gloves, lab coats, face masks etc…can result in analyst shedding contaminants such as dust, dandruff, sweat, associated dirt and breath-born impurities into samples resulting in a negative analysis.
During forensic investigation, evidence can be analysed through computer technology and the data has to be saved, not only for usage by the investigators but also for the forensic sampling the evidence and the court officials. Data can be corrupted by damage to the physical media. Physical damage can be caused by; water, fire, exposure to heat, physical breakage, magnetic fields and other possible physical causes of damage.
During wet weather, having a tent erected can allow for forensics to work in the wet weather as well as protect the evidence being analysed. The victim’s hands are usually bagged before being removed from the crime scene. Anything collected from the site must be collected with sterile instruments and kept in sterile sealable plastic bags or containers.
The police photograph the scene and sometimes take videos to ensure nothing has been moved, or tampered with. The forensic scientist must examine the evidence objectively and all scientific analyses must be accurate and reproducible. When arson is suspected the volatility of the flammable or combustible liquids, they must be packed in vapour-tight containers. Heat-sealable polyester/polyolefine bags are used since they are free of contaminants.
Extraction and isolation methods are much more sensitive today than previous methods used; even minute traces are vulnerable to misidentification. Caution such as wearing the right clothing. Using the right tools, evidence storage and debris must all be taken into consideration with a investigation for the best possible results. Even the slightest amount of debris can lead to false positive results if it has been contaminated by dirty tools or gloves.
It must be known at all times who has handled the evidence, where the evidence is, the data must be recorded every time it is handled with even if it is handled up to 100 times a day, it must be known what the evidence is stored in and where it is being kept and the method which was used to collect the evidence. Evidence that has been incorrectly handled or by unknown people may not be able to be used in court.
a) What is the basic difference between organic and inorganic compounds and how can they be distinguished from each other by experiment(s)?
Organic compounds are all compounds of carbon except such binary compounds as carbon oxides, carbides, carbon disulphide, etc.; such ternary compounds as the metallic cyanides, metallic carbonyls, phosgene (COCl2), carbonyl sulphide (COS), etc.; and the metallic carbonates, such as calcium carbonate and sodium carbonate. The total number of organic compounds is indeterminate, but some 6,000,000 have been identified and named. These fall into several structural groups such as aliphatic, alicyclic and aromatic hydrocarbons, alcohols, ethers, aldehydes, ketones, carboxylic acids, carbohydrates, amino acids, proteins, nucleic acids, etc. Inorganic compounds are generally considered to embrace all substances except hydrocarbons and their derivatives, or all substances that are not compounds of carbon, with the exception of carbon oxides, carbides and carbon disulphide
To determine the difference between inorganic and organic compounds are series of tests can be done. But the easiest one of all would be to heat a sample of the compound in air. If complete combustion occurs that is, if the sample reacts with the air by burning or decomposing fully leaving no residue, then the sample is said to be organic. During these tests all safety precautions must be taken. There are further tests to differentiate between what the organic compounds are what they inorganic compounds are. Organic compounds can be either alkanes or alkenes, alkanols and alkanoic acids. Inorganic compounds can go under categories of gases, anions, cations and complex transition metal ions.
b) Explain that the inorganic chemical properties of soils and other materials may be useful evidence.
Soils can offer important evidence in crime investigations. Inorganic chemicals are nearly impossible to decompose in soils especially over small periods of time, any traces of inorganic chemicals left behind in the soil in and surrounding the investigation area and amongst the other evidence can have traces of evidence, which can lead to the investigation being solved.
For example, in Sydney, after the drawing of the first opera house lottery the winner’s 8-year-old son was kidnapped for ransom. Several days layer, the body was found in a bushy hollow. Geologists investigating soil samples found traces of pink mortar on the boys clothes which matched mortar in a building site owned by the suspected kidnapper and this geological evidence was used in the trial of the murder.
The forensic undergoes numerous tests to determine the what inorganic evidence that maybe in the soil sample or what the inorganic material found in the crime scene can be matched to any other evidence located or suspected. The forensic scientist carries out preliminary separation using a stereoscopic microscope, large materials like plants or insects parts can be manually separated. A mineral examination by means of polarised light microscopy may be attempted. Particle size distribution may be carried out. Colour comparison of soils depends on the chemical substance. pH testing of soils can be easily determined. Groundwater tests can be used to determine hardness, sulfate or salt content. Humus content of soil can be determined by ignition loss found by heating oven-dried soils. The humus is burnt off as carbon dioxide and water. The examination of soil for pollen and other biochemical factors can reduce areas of investigation.
Discuss, using a recent example, how the progress in analytical chemistry and changes in technology can alter the outcome of a forensic investigation.
The technology change in analytical chemistry can significantly change the outcome of an investigation. Most investigation in forensic chemistry includes blood and blood stains of some sort. Until the 1970’s the analysis of bloodstains was limited to three basic types of tests:
Identifying the stain as blood by using ninhydrin
Identifying the stain of being human or animal origin
Identifying the blood group
i) The ABO system which divides blood into four different groups. Analysing protein molecules called antibodies and an antigen identifies these groups.
ii) The Rh protein group system depends son the facts that about eight in every ten people have Rh protein in their cells.
More recently, in America it was found that the enzyme phosphoglucomutase (PMG) could be detected in dried bloodstains. This was useful since the enzyme was polymorphic. Since this, several genetic markers have been found to be polymorphic in a specific racial group.
The advances in genetic technology provides genetic profiles of both victim and suspect, increase the discrimination potential of a specific bloodstain since it lowers the frequency of occurrence of he satin within a given population. It provides an ability to distinguish between two persons of the same blood group and in some cases, it can indicate the possible racial origin of the bloodstain.
Case: poison analysis:
Forensic chemists now make use of powerful molecular and biochemical techniques. Immunoassay techniques can e used to detect he traces of poison months or even years after a murder have been committed. Biological molecules, which are antibodies, can bind to specific proteins, which are the antigens. In radioimmunoassay (RIA), radioactivately labelled antigens that selectively react with the substance in question are added to the sample. If the substance’s antibodies are present, they attach themselves to the labelled antigens. After the antibody-antigen complexes are separated, a measure of the radioactivity of the solution is taken to see whether the substance is present.
Forensic chemists from the London Metropolitan Police Forensic Laboratory carries out radioimmunoassays and high performance liquid chromatography (HPLC) on year-old routine samples taken from the body of a landscape gardener. The samples had been preserved in formalin and a medical student noticed abnormalities. The forensic chemists were able to detect the presence of paraquat, a poisonous-water soluble ammonium compound used in some weedkillers. The gardener’s widow admitted adding weedkiller to her husbands drink and was convicted to murder.
a) State the general formula of a carbohydrate.
General formula Cn(H2O)n, hence the name carbohydrates
General molecular formula CH2O
b) Describe how to differentiate by chemical means between reducing and non-reducing sugars.
Sugars that have an OH attached to the same C as a ring O atom is attached to be reducing sugars. To determine the difference between a reducing and a non-reducing sugar, agents that can oxidise a –CHO or –CO-CH2OH group is used but this will not oxidise the many ordinary alcohols that are present in all sugars. There are three tests, which can be used to differentiate between reducing and non-reducing sugars, which are:
1) Fehling’s solution, a liquid solution of copper sulfate and potassium tartrate and sodium hydroxide that is used to test for sugar in the urine; solution turns reddish when sugar is present.
2) Benedict’s solution, a solution of sodium citrate, sodium carbonate, and copper sulfate that changes from blue to yellow or red in the presence of reducing sugars, such as glucose.
3) Tollens reagent, a solution made from silver (I) nitrate solution. You add a drop of sodium hydroxide solution to give a precipitate of silver (I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate.
c) Describe the structure of: Sucrose, Glucose, Starch and Cellulose and show using structural formulae, that these structures are formed from glucose by means of condensation reactions.
Percent composition by mass:
Molar mass = 342.30
Percentage of carbon’s in one molecule = 42.11%
Percentage of oxygen in one molecule = 51.11%
Percentage of hydrogen in one molecule = 6.48%
Percent composition by number:
Total number = 45
Carbon = 26.67%
Oxygen = 48.49%
Hydrogen = 24.45%
Fructose + glucose sucrose + H2O
Sucrose is an unusual disaccharide in that it is a non-reducing sugar. This is because both of the hydrogen atoms removed in the dehydration reaction came from OH groups that were created during ring closure. Consequently, neither of the rings is able to open.
Glycogen, another polymer of glucose, is the polysaccharide used by animals to store energy. Excess glucose is bonded together to form glycogen molecules, which the animal stores in the liver and muscle tissue as an "instant" source of energy. Both starch and glycogen are polymers of glucose, however starch is a long, straight chain of glucose units, whereas glycogen is a branched chain of glucose units. Condensation may occur with *D-glucose and *D-glucose.
Starch is the principal polysaccharide used by plants to store glucose for later use as energy. Plants often store starch in seeds or other specialised organs, for example, common sources of starch include rice, beans, wheat, corn, potatoes, etc. When humans eat starch, an enzyme that occurs in saliva and in the intestines called amylase breaks the bonds between the repeating glucose units thus allowing the sugar to be absorbed into the bloodstream.
Cellulose is yet a third polymer of the monosaccharide glucose. Cellulose differs from starch and glycogen because the glucose units form a two-dimensional structure, with hydrogen bonds holding together nearby polymers, thus giving the molecule added stability. Cellulose, also known as plant fibre, cannot be digested by human beings therefore cellulose passes through the digestive tract without being absorbed into the body. Some animals, such as cows and termites, contain bacteria in their digestive tract that help them to digest cellulose. Cellulose is a relatively stiff material, and in plants cellulose is used as a structural molecule to add support to the leaves, stem and other plant parts. Despite the fact that it cannot be used as an energy source in most animals, cellulose fibre is essential in the diet because it helps exercise the digestive track and keep it clean and healthy.
d) Distinguish between plant and animal carbohydrates’ composition in terms of the presence of: Cellulose, Starch and Glycogen.
Animals produce only glycogen and not starch and cellulose where as plants produces starch and cellulose but not glycogen. In green plants starch is produced by photosynthesis; it is one of the chief forms in which plants store food. Starch obtained by animals from plants is stored in the animal body in the form of glycogen. Digestive processes in both plants and animals convert starch to glucose, a source of energy. Glycogen is a branched animal starch. Cellulose is the structural polysaccharide of cell walls.
Hence, cellulose is what makes plants, and through photosynthesis plants store food which is the starch that animals eat and that starch is changed into glucose and they glucose is stored in the animal’s body in the form of glycogen.
a) Describe the composition and the general formula of an amino acid, identify the functional groups and explain that proteins are chains of amino acids.
The -amino acids in peptides and proteins (excluding proline) consist of a carboxylic acid (-COOH) and an amino (-NH2) functional group attached to the same tetrahedral carbon atom. This carbon is the -carbon. Distinct R-groups, that distinguish one amino acid from another, also are attached to the alpha-carbon except in the case of glycine where the R-group is hydrogen. The fourth substitution on the tetrahedral a-carbon of amino acids is hydrogen.
The -COOH and -NH2 groups in amino acids are capable of ionising (as are the acidic and basic R-groups of the amino acids). As a result of their ionisability the following ionic equilibrium reactions may be written:
R-COOH <--------> R-COO- + H+
R-NH3+ <---------> R-NH2 + H+
The equilibrium reactions, as written, demonstrate that amino acids contain at least two weakly acidic groups. However, the carboxyl group is a far stronger acid than the amino group. At physiological pH (around 7.4) the carboxyl group will be unprotonated and the amino group will be protonated. An amino acid with no ionisable R-group would be electrically neutral at this pH. This species is termed a zwitterion. Like typical organic acids, the acidic strength of the carboxyl, amino and ionisable R-groups in amino acids can be defined by the association constant, Ka or more commonly the negative logarithm of Ka, the pKa. The net charge, the algebraic sum of all the charged groups present, of any amino acid, peptide or protein, will depend upon the pH of the surrounding aqueous environment. As the pH of a solution of an amino acid or protein changes so too does the net charge. This phenomenon can be observed during the titration of any amino acid or protein. When the net charge of an amino acid or protein is zero the pH will be equivalent to the isoelectric point
Proteins are macromolecules made from amino acids, some consist of one‚ long polypeptide chain while others are made from several intertwined polypeptides.
b) Describe the nature of the peptide bond, (how it is formed), and explain that proteins can be broken at different length’s in the chain by choice of enzyme.
A peptide bond is a chemical bond formed between two molecules when the carboxy group of one molecule reacts with the amino group of the other molecule, releasing a molecule of water (H2O). This is a dehydration synthesis reaction, and usually occurs between amino acids.
The resulting C-N bond is called a peptide bond, and the resulting molecule is called an amide. Polypeptides and proteins are chains of amino acids held together by peptide bonds. The backbone of PNA is also held together by peptide bonds.
The C-N bond has a partial double bond character (with the Nitrogen atom attaining a partial positive charge and the oxygen atom a partial negative charge) and the molecule can normally not rotate around this bond. The whole arrangement of the four C, O, N, H atoms as well as the two attached carbons’ in a peptide bond is planar.
A peptide bond can be broken by amide hydrolysis (the adding of water). The peptide bonds in proteins are metastable, meaning that in the presence of water they will break spontaneously, releasing about 10 kJ/mol of free energy, but this process is extremely slow. In living organisms, the process is facilitated by enzymes. Living organisms also employ enzymes to form peptide bonds; this process requires free energy. The wavelength of absorbance for a peptide bond is 220-280nm
-The substrates are compounds in the cell that are going to react chemically.
- The enzyme is the catalyst that will make it easier for the reaction to happen.
- The product is the compound that is formed by the reaction
An enzyme helps the substrates to create a product through a chemical reaction. The 'lock and key' theory explains how an enzyme's shape is responsible for helping a reaction between two substrates happen. An enzyme's surface shape is a pattern that the substrates match exactly. This allows the substrates to fit closely together as they attach themselves to the enzyme. Being so close together, the two compounds can now enter into the chemical reaction much more easily. The enzyme helps a chemical reaction to happen by bringing the substrates close together.
Once the reaction has occurred, the resultant product is now free to leave the enzyme. The “key and lock” theory is why proteins can be broken at different lengths in the chain by the choice of enzyme.
c) How are proteins used for structural purposes structurally different from proteins used for enzymes?
Fibrous proteins are elongated molecules in which the secondary forms the dominant structure. Fibrous proteins are insoluble, and play a structural or supportive role in the body, and are also involved in movement. One feature of fibrous tissues is that they often have regular repeating structures. Keratin, for example, which is found in hair, horns, wool, nails, and feathers, is a helix of helices and has a seven amino acid repeating structure. Silk is a fibrous protein that is composed only of b-sheets. It too has a repeating pattern: layers of glycine alternate with layers of alanine and serines in the b-sheets. Collagen, is the major protein component of connective tissue. In collagen, every third amino acid is glycine and many other of the others are proline.
Globular proteins are a highly diverse group of proteins that are soluble and form compact spheroidal molecules in water. All have tertiary structure and some have quaternary structure in addition to secondary structure. Regular secondary structures generally comprise less than half the average globular protein. Globular proteins typically consist of relatively straight runs of secondary structure joined by stretches of polypeptides that abruptly change direction. Enzymes are globular proteins as are transport proteins and receptor proteins. Myoglobin and carboxypeptidase, are globular proteins.
So what is the main difference between fibrous proteins and globular proteins? Aside from the difference in shape (elongated vs. spheroidal) and solubility (insoluble vs. soluble), fibrous proteins generally have only primary and secondary structure whereas globular proteins have tertiary and sometimes quaternary structure in addition to primary and secondary structure.
d) 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 is a technique used to separate substances, which have similar physical and chemical properties. A range of techniques includes paper, thin layer, column, gas and high-pressure liquid chromatography.
Electrophoresis is the separation of molecules by migration in an electric field. The molecules to be separated are applied to a supporting media such as agar. Most biological molecules are electrically charged so when they will move in an electric field when an electric charge is passed through them. It is a tool used to separate particles of matter based on molecular size and ionic charge. The procedure uses an electric field to move solutions through a gel, or other electrophoretic substance. Because molecules of different substances have different sizes and charges, the particles will move across the gel at different speeds. All of the like molecules will separate from the others forming a band in the gel that will slowly move toward one of the electrodes. So, if more than one electrically charged substance is put on the gel, they will separate into bands of like particles and move across the plate.
The separation properties of the components in a mixture are constant under constant conditions, and therefore once determined they can be used to identify and quantify each of the components. Such procedures are typical in chromatographic and electrophoretic analytical separations. u A mixture can be separated using the differences in physical or chemical properties of the individual components. An example is the filtering of a solid precipitate to separate it from a solution. These separations are based on the states of matter of the two components, other physical properties that are useful for separations are density and size. Some useful chemical properties by which compounds can be separated are solubility, boiling point, and vapour pressure.
e) discuss how electrophoresis can be used in identifying the origins of protein and how it can assist the forensic chemist.
Electrophoresis is an important tool, which can assist the forensic chemist to identify the origin of protein. DNA typing techniques such as the polymerise chain reaction (PCR) involve repeatedly replicating a small area of the DNA molecule so that a particular piece is present in the greater amounts. Amounts of DNA produced by the PCR are placed in a line on a gel, the DNA then travels through the gel due to the electric charge of the DNA molecules. The larger molecules travel the shorter distance because of the hindrance because of the gel. The DNA is then stained and the distance is compared to the distance that known standards have travelled.
This determines the difference between types once amplification has taken place. Electrophoresis is the basis of the newly developed technique for DNA fingerprints. It’s a method for separation and qualitative analysis of very large molecules such as proteins.
a) Outline the structure and composition of DNA.
Deoxyribonucleic acid or DNA is the compound found in al living things. It is a chemical structure that forms chromosomes. A piece of a chromosome that dictates a particular trait is called a gene.
Structurally, DNA is a double helix: two strands of genetic material spiralled around each other. Each strand contains a sequence of bases, also called nucleotides. A base is one of four chemicals, adenine, guanine, cytosine and thymine. The two strands of DNA are connected at each base. Each base will only bond with one other base, as follows: Adenine (A) will only bond with thymine (T) and guanine (G) will only bond with cytosine (C)
b) Explain why analysis of DNA allows identification of the individuals.
Every individual has a different DNA pattern unless they are identical twins where their DNA sequences are the same. DNA allows indeintification of individuals due to the DNA sequence. Scientists compare the DNA from different samples to decide whether the samples came from one person, related persons or completely different persons. This helps to solve investigations especially to convict rapists or murderers and it can at the same time clear a suspect from a crime by showing that a sample could not have come from that suspect.
c) describe the process used to analyse DNA and account for its use in:
- identify relationships between people
- identifying individuals
Step 1: the double stranded DNA is dissociated into single strands by incubation at 94º.
Step 2: short pieces of purified DNA called primers are then added. By lowering the temperature, copies of the primers bond to the DNA
Step 3: a heat stable DNA polymerase is then added and the sample warmed causing the primers to synthesis completely strands of the single strands. This process is repeated for about 25 cycles to amplify the original DNA sequence.
Step 4: adding restriction enzymes then digests the amplified DNA. These cut the DNA into series of fragments of various sizes.
Step 5: the fragments are then separated by the gel electrophoresis.
Step 6: after electrophoresis, the DNA is transferred to a nylon membrane. Radioactive probes are used so that the DNA sequences to which they become attached can be tracked. A x- ray film is placed under the membrane and then later developed.
Step 7: the audiograph looks something like a bar code. The band pattern comparison shows the difference between individuals by the distribution and locations of the band patterns.
In identifying relationships, each chromosome contains nucleotides identical to those each of each parent, as well as the nucleotides that distinguish the individuality of each person. If the samples from the child and one parent are available, the nucleotides that are different from the known parents DNA must have come from the unknown parents DNA. If a sample from the suspected, but as yet unknown, parent supplies al the missing nucleotides without any superfluous nucleotides, then the suspect is the other parent.
In identification of individuals, samples of hair, skin, saliva, blood and semen containing cells can determine the DNA pattern. The DNA testing requires very small sample and heat and moisture don’t effect the DNA. Considering each person has a different DNA sequence. It is easy to identify an individual form any other person suspected.
d) discuss issues associated with the use of DNA analysis in terms of the ethics of maintenance of data banks of DNA.
Data banks raise ethical racial issues in addition to the one on civil liberties. DNA analysis can detect genes for particular inherited diseases, particularly the ones that do not develop until later in life such as Huntington disease. Some people may wish to know information and currently can choose to have such analysis’s done. But when the data already exists, its tempting to do routine screening of such DNA samples. They could provide people with information about genetic diseases that may refer not to know about. The possibility of life insurance companies demanding routine screening of such materiel and getting access to information about genetic disorders could seriously disadvantage affected people when they seek insurance.
a) Explain what is meant by the destructive testing of material and explain why this may be a problem in forensic investigations.
A large amount of forensic evidence consists of very small samples and macroscopic analysis is therefore inappropriate. When the destructive testing of material is used, for example. Determining humus in soil sample by ignition so that carbon dioxide and water are driven off, the sample is destroyed. During the forensic investigations, the defence may wish to have a sample retested by their own analyst. Some material should be set aside to cope with these inquiries.
This causes problems for forensic. Because if the evident hey had were to be so small that it could only be tested once, and when it was tested and then the sample was destroyed, this could lead to the investigation having a different outcome or suspecting some one else or clearing the suspect from something they may have committed.
b) identify, outline and assess the values of the following techniques in the analysis of small samples and the value of these technique’s to the forensic chemist:
- gas-liquid chromatography (GLC)
- high performance liquid chromatography (HPLC)
A variation of the gas chromatography process in which the chromatographic column is packed with a finely divided solid impregnated with a nonvolatile organic liquid. The sample to be analysed is injected into the inlet of the column where it is quickly and completely vaporised. The gas stream carries it into the packed section, where the vapours contact the impregnated solids. The nonvolatile liquid phase tends to condense the vapours from the gas stream, and the moving gas phase tends to evaporate the condensed sample vapours. The vapours of each compound present in the sample in effect spend a characteristic fraction of time in the condensed phase, and the remainder in the mobile gas phase. Each chemical species will tend to migrate at its own characteristic rate and will be separated from other species by the time they emerge from the column. The detector senses the emergence of each sample component and provides an electrical signal to the recorder proportional to the concentration of each component in the emergent stream.
High performance liquid chromatography:
An analytical technique used for separation of low-to-moderate molecular weight compounds of resins. The instrumentation for HPLC and size exclusion (SEC) or gel-permeation chromatographs are similar, but the columns differ. The HPLC system was developed from the column chromatography. It allows sensitive analysis of a wide range of compounds and is widely used for pharmaceutical analysis. Other uses include analysis of cosmetics, explosives, soft drinks, herbicides, and drinking water on a daily basis. When the HPLC system is equipped with a fluorescence detector, illicit drugs like amphetamines. Can be found in picogram amounts in autopsy samples. Components in HPLC can be identified by comparing their retention time with that of known standards under the same conditions. Quantitative information can be obtained from the area under the peak of the chromatogram. This is dependent on the amount of the component present and can be determined by comparison with the areas of standard.
c) Outline how a mass spectrometer operates and clarify its use for forensic chemistry.
Step 1: the sample is vaporised, if necessary, before entering the main body of the mass spectrometer.
Step 2: the vaporised samples passes through a small inlet into ionising chamber, which is at low pressure. Here an electron beam ionises part of the sample by knocking out electrons from the neutral atoms or molecules to form mainly ions with a single positive charge.
Step 3: the positive ions are accelerated to high speeds by an electric field.
Step 4: ions then pass through a perpendicular magnetic field. This field causes the ions to move in curved paths with a radius dependant on the mass-to-charge ratio of the ions. For the singly charged ions, their radius depends on their mass.
Step 5: only ions with particular radius reach the collector. By changing the electric or magnetic field, different masses can reach the collector.
Step 6: the detector identifies the mass of each particle from its path. The data are recorded as a mass spectrum.
Mass spectrometers are sensitive detectors of isotopes based on their masses. They are used in carbon dating and other radioactive dating processes. The combination of a mass spectrometer and a gas chromatograph makes a powerful tool for the detection of trace quantities of contaminants or toxins. A number of satellites and spacecraft have mass spectrometers for the identification of the small numbers of particles intercepted in space
Mass spectra of compounds are unique and can be used as “fingerprints” for identifying the compounds. If the library of mass spectra is available, then the mass spectrum of an unknown sample can be compared with those in the library and if an exact match is obtained. Then the sample can be confidentially identified. Computers can be used to match unknowns with standard if the spectra are stored electronically.
a) Describe the conditions under which the atom will emit light.
Light is emitted when an electron in an allowed high energy or excited orbital falls into a lower energy or ground state orbital. The light emitted is in a single whole quantum of energy or a photon of light. The greater the amount of energy to be released, the shorter the wavelength of the radiation emitted. Big jumps in energy release u.v light, medium jumps in energy release visible light and small jumps in energy levels release radiation.
b) Explain how line emission spectra are formed and why a particular element shows specific lines in its spectrum.
In 1913, it was Neils Bohr who solved many of the problems at the time by proposing that the electron revolves around the nucleus of the atom with a definite fixed energy in a fixed path, without emitting or absorbing energy. The electron in the hydrogen atom exists only in certain definite energy levels. These energy levels are called Principal Quantum Levels, denoted by the Principal Quantum Number, n. Principal Quantum Level n = 1 is closest to the nucleus of the atom and of lowest energy. When the electron occupies the energy level of lowest energy the atom is said to be in its ground state. An atom can have only one ground state. If the electron occupies one of the higher energy levels then the atom is in an excited state. An atom has many excited states.
When a gaseous hydrogen atom in its ground state is excited by an input of energy, its electron is 'promoted' from the lowest energy level to one of higher energy. The atom does not remain excited but re-emits energy as electromagnetic radiation. This is as a result of an electron 'falling' from a higher energy level to one of lower energy. This electron transition results in the release of a photon from the atom of an amount of energy (E = hn) equal to the difference in energy of the electronic energy levels involved in the transition. In a sample of gaseous hydrogen where there are many trillions of atoms all of the possible electron transitions from higher to lower energy levels will take place many times. A prism can now be used to separate the emitted electromagnetic radiation into its component frequencies (wavelengths or energies). These are then represented as spectral lines along an increasing frequency scale to form an atomic emission spectrum.
c) Discuss the use of line emission spectra to identify the presence of elements in chemicals.
Atomic emission spectroscopy was developed towards the end of the 19th century, but was mainly a tool for scientists for a couple of decades. As the 20th century progressed analytical chemists began to use it for qualitative analysis of samples. One of its earliest applied uses was in the steel industry to monitor compositions of steel as they were being made, it was particularly good for measuring amounts of various metals in such materials.
Forensic chemists have used the atomic emission spectroscopy routinely. It was widely used for soil analysis, both for agricultural and for forensic purposes. Forensic regularly use emission spectra to identify small samples found at crime scenes. For soils the detection of an unusual element in a sample can often pinpoint the location from which the soil sample came from. This technique has greatly increased their ability to identify the source of quite small samples.
Research the use of DNA evidence in one famous court case. List the resources you used for this research. Show in your write-up:
O.J. Simpson's trial for the brutal murder of his wife and her companion lasted nearly nine months and divided the country.
-The names of the accused
-When/where the case took place
-If the accused was/were declared guilty or not guilty based on DNA evidence
O.J. Simpson: Week-by-week
April 3 - 7, 1995
APRIL 3 - The next phase of the trial began as prosecutors started to present the scientific evidence that they say links O.J. Simpson to the murders of Nicole Brown Simpson and Ronald Goldman.
Criminalist Dennis Fung started testifying about the painstaking process of collecting blood stains and other evidence. He explained for the jury how the evidence is collected, catalogued and stored.
The prosecution has said that DNA evidence ties Simpson to the murders. The evidence includes blood stains in Simpson's Bronco and on a pair of socks in his bedroom. Prosecutors say the stains match the blood of Nicole Simpson. They also say tests found blood from Goldman in the Bronco and on the glove found beside Simpson's house.
The defense is expected to challenge the reliability of the evidence
O.J. Simpson: Week-by-week
May 15 - 19, 1995
MAY 15 - The defense continued its attack against damaging DNA test results by again suggesting blood stains were contaminated and by questioning the competency of the testing laboratory.
Dr. Robin Cotton of Cellmark Diagnostics conceded that her laboratory in the past had twice made false matches in DNA testing due to sample handling error or cross-contamination, in which blood from one sample was inadvertently mixed with blood from another.
Testifying for a sixth day, Cotton also disclosed that some data used for comparisons in the Simpson case included tests on only two other blacks. Cotton explained that in calculating the rarity of Simpson's genetic profile, she relied on five genetic markers or "loci."
"And so in your data base, the number of people that you have typed like Mr. Simpson across all five loci is just two people, isn't that right?" defense attorney Peter Neufeld asked.
O.J. Simpson: Week-by-week
May 22 - 26, 1995
MAY 22 - In a series of complex hypothetical questions, defense lawyer Barry Scheck tried to prove that DNA testing can be imprecise and that the results are open to interpretation.
Scheck asked Gary Sims, a forensic chemist with the California Justice Department, to interpret the number of blue dots on different DNA tests to show that the DNA in the prosecution's blood evidence varies greatly.
Scheck argued that divergent DNA levels suggest that police work was sloppy and that some of the evidence may have been planted.
The greatest amount of DNA was found on a blood drop labeled evidence item 52, Sims said. The presence of one blue dot -- which designated a portion of Ronald Goldman's DNA -- was so unusually high that the lab repeated the test.
O.J. Simpson: Week-by-week
Oct. 2 - 3, 1995
OCTOBER 2 - After deliberating for less than four hours, the jury announced that it has reached a verdict.
The quick verdict surprised many legal analysts, who had predicted it would take anywhere from two days to two months.
When Judge Lance Ito called the jury of 10 women and two men back into court, he said: "You buzzed three times and indicated that after receiving the verdict form that you have reached a verdict in this case. Is that correct, Madam Forewoman?"
"Yes," replied the 51-year-old black woman who was elected by the jurors to lead the panel.
Earlier in the day, jurors asked to re-hear testimony from the limousine driver who drove Simpson to the airport shortly after the murders.
The request for a readback of Allan Park's testimony suggested the jurors were looking at the critical issue of whether Simpson had enough time to kill Nicole Brown Simpson and Ronald Goldman.
Park, who picked Simpson up for a trip to the airport, was considered one of the most important time-line witnesses, joining houseguest Brian "Kato" Kaelin, the last person known to see Simpson before the murders. Their testimony created a 78-minute window of opportunity for Simpson to commit the murders.
Simpson contended he was at home preparing for a trip to Chicago, but he presented no alibi testimony.
Park said he arrived at Simpson's house at 10:22 pm the night of the murders and didn't see Simpson's Bronco parked outside when he was searching the curb for street numbers. Park testified that at 10:55 pm he saw a large, shadowy figure of an African-American person at the front door of Simpson's Rockingham estate. Moments later, Simpson answered the intercom that Park had been sounding for 15 minutes.
OCTOBER 3 - The verdict is announced — not guilty.
Very thorough; and the only small thing I can think of is that its probably a good idea to say that it is the non-coding bits (introns) of DNA that is unique.
But other than that, awesome. And you weren't joking when you said you covered everything you could in that. Looks like she basically handed you the entire syllabus as a assignment.
I've got two classes at my school, Forensics is my teachers speciality but the other class complained [since almost all of them have tutoring] saying that their tutors didn't have any notes for this topic. It worked out they're doing industrial which plays to their teachers strengths.
I'm liking it, apart from the fact that some of the information I'm having to source from legal studies [ethics on DNA databases] and Physics [emission spectra].