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Transcript
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 1 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 2 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. 3 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) 4 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. 5 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 6 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) 7 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 8 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