Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Laboratory Analysis Forensic Science
Elements and Compounds • A. Matter- anything that has mass & takes up space • B. Element- cannot be broken down into simpler substances by chemical means • C. Periodic table - chart of elements arranged in a systematic fashion • D. Atom- smallest particle of an element that can exist and still retain its identity as that element
Mixtures & Compounds Mixture – Two or more substances that are mixed together, but not chemically combined. Examples of mixtures ...Air – mixture of gasesBowl of cereal – mixture of cereal and milkSoda pop – mixture of soda syrup, water, and CO2 gasFog –water suspended in airKool-Aid – mixture of water, sugar, and flavor crystals Compounds – Two or more elements that are chemically combined. Examples of compounds ...Salt –Sodium and chlorine combined chemicallyWater –Hydrogen and oxygen combined chemically Carbon Dioxide – Carbon and oxygen combined chemically
Solutions Solutions are mixtures in which one substance is dissolved in another. Solutions have two parts: solute and solvent The solute is the substance that is dissolved. The solvent is the substance that does the dissolving Identify the solute and solvent in each solution ... Solubility - A measure of how much of a given substance will dissolve in a liquid. A substance that does not dissolve in water is called insoluble. A substance that does dissolve in water is called soluble.
The periodic table is a listing of all elements by increasing atomic number.
The vertical columns are called groups. There are 18 groups The horizontal rows are called periods. There are 7 periods.
The periodic table can be separated into metals, nonmetals, and metalloids.
Metals are shiny, malleable, ductile, and good conductors of heat and electricity.
Nonmetals are not shiny, malleable, ductile, or good conductors of heat and electricity.
Physical states • Substances change from one state to another • Phase-a uniform piece of matter, different phases are separated by definite visible boundaries • There are four phases of matter: solid, liquid, gas, and plasma.
Evaporation = Condensation = Melting = Freezing = Sublimation = Liquid -> Gas Gas -> Liquid Solid -> Liquid Liquid -> Solid Solid -> Gas Phase Changes – Physical Changes
Solids • particles vibrate but can’t move around • fixed shape • fixed volume • incompressible
Liquids • particles can move around but are still close together • variable shape • fixed volume • Virtually incompressible
Gases • particles can separate and move throughout container • variable shape • variable volume • Easily compressed • Vapor- gaseous state of a substance that is a liquid or solid at room temperature
Plasma • particles collide with enough energy to break into charged particles (+/-) • gas-like, variableshape & volume • stars, fluorescentlight bulbs, TV tubes
Selecting an Analytical Technique • Need to know whether substance is organic or inorganic • A.Organic- substance composed of carbon and hydrogen • B. Inorganic- CO2 and all substances that lack carbon • Need to consider the need for qualitative and quantitative determination
Organic Analysis • Spectrophotometry • Chromatography • Gas Chromatography (GC) • High – Performance Liquid Chromatography (HPLC) • Thin – Layer Chromatography (TLC) • Electrophoresis
Spectrophotometry • An analytical method for identifying a substance by its selective absorption of different wavelengths of light • Most applicable to organic analysis • Optimum utilization requires that a material be in relatively pure state
Chromatography • Organic mixtures are separated into their components by their attraction to a stationary phase while being propelled by a moving phase. • Useful technique for purifying substances • 1st observed in 1803 by William Henry (Henry’s Law) • One phase is always made to move continuously in one direction over a stationary or fixed phase • It’s like a race between chemical compounds. • At the beginning, all substances are mixed together. • As the race progresses, those that have preference for the moving phase will move faster and pull ahead of others. • At the end, all the substances are separated.
Gas Chromatography (GC) • separates mixtures on the basis of their distribution between a stationary liquid phase and a moving gasphase • used widely because of its ability to resolve a highly complex mixture into its components within a time period usually measured in minutes • is very sensitive • sample must be vaporized and passed through heated tube
High-Performance Liquid Chromatography (HPLC) • Different types of stationary phases (usually non- aqueous) with a liquid moving phase • can perform process at room temperature • used for organic explosives and drugs that are heat sensitive
Thin-Layer Chromatography (TLC) • incorporates solid stationary phase & liquid moving phase • because most compounds are colorless, uses UV light to reveal those that fluoresce • cannot by itself provide absolute identification; has to be used in conjunction with other procedures to prove absolute identity • powerful tool for solving analytical problems • rapid and sensitive • minimal cost and space requirements
Electrophoresis • related to TLC in that it separates materials according to their migration rates on solid phase • uses electrical current instead of moving liquid phase • characterization of proteins, enzymes, and DNA
Examples of Chromatography Liquid Chromatography Used to identify unknown plant pigments & other compounds. Gas Chromatography Used to determine the chemical composition of unknown substances, such as the different compounds in gasoline shown by each separate peak in the graph below. Thin-Layer Chromatography Uses thin plastic or glass trays to identify the composition of pigments, chemicals, and other unknown substances. Paper Chromatography Can be used to separate the components of inks, dyes, plant compounds (chlorophyll), make-up, and many other substances
Pencil Filter Paper Tape – Label with marker InkMark Paper Chromatography Lab • Obtain the supplies you’ll need. • 1 large beaker (or plastic cup) • 1 small beaker (or plastic cup) filled with water • 4 pieces of filter paper • 4 black markers for testing • 4 small pieces of masking tape • Pencil (to attach to the top of the filter paper) • Permanent marker • Timer • Write the pen number on a piece of masking tape with a permanent marker and place it at the top of the strip. • Choose one of the testing markers and draw a thick line near the bottom of the filter paper - about ¼ inch from the bottom. • Pour a small amount of water into the large cup and then hang the paper strip in the cup. Make sure the ink line does not touch the water – only the bottom of the filter paper. • Allow the water to move up the paper for 5 minutes and then remove the strip from the water. Hang it on the side of the table to dry. • Follow these directions to test the other pens.
Complete the chart on your worksheet and then answer the questions. Questions: What colors did your group observe in each of the black ink samples? Do the colors occur in the same order on all the samples? Explain. Did some ink samples not work? Why?
Chromatography Challenge Work with your group to identify the pens used for each of the “Mystery Marks”. 1st – Test each of the Mystery Mark strips using the procedure from yesterday. 2nd – Compare your strips to the strips hanging in the classroom. 3rd – Write the number of the pen that you think matches each of the mystery marks in the space on your worksheet. 4th – Have your answers checked by the teacher. Keep trying until you are able to identify all 6 pens! Pen A matches # _____ Pen D matches # _____ Pen B matches # _____ Pen E matches # _____ Pen C matches # _____ Pen F matches # _____
Inorganic Analysis • Carbon does not appear among earth’s most abundant elements. • Inorganics are also encountered as physical evidence • Metals in tools, coins, weapons, scrapings • Pigments in paints and dyes • Explosive formulations • Poisons • For ID & comparison of physical evidence
The Emission Spectrum of Elements • Elements selectively absorb and emit light • When the light passes through a prism, it is separated into its component colors or frequencies (emission spectrum). • Types of emission spectrums • Continuous spectrum • shows a continuous band of colors all blending into one another • Ex – sunlight or light from incandescent bulb passes through a prism
Types of Emission Spectrum cont. • Line spectrum • 1. shows a series of lines separated by black areas • 2. Each line represents a definite wavelength or frequency • 3. Ex – light from a sodium lamp or mercury arc lamp
Emission Spectrograph • instrument used to obtain and record the line spectra of elements • Requires: • a means for vaporizing and exciting the atoms of elements so that they emit light • a means for separating light into its component frequencies • a means for recording the resultant spectrum • Uses in Forensics: Rapid comparison of the elemental composition of two or more specimens
Inductively Coupled Plasma (ICP) Emission Spectrometry • Identifies and measures elements through light emitted by excited atoms • Uses hot plasma torch instead of electrical arc to excite atoms • Has been applied in the area of identification and characterization of mutilated bullets and glass fragments
Atomic Absorption Spectrophotometer • When an atom is vaporized, it will absorb many of the same frequencies of light that it emits in an excited state. • In this technique, the specimen is heated to a temperature that is hot enough to vaporize its atoms while leaving a substantial number of atoms in an unexcited state. • Provides a determination of an element’s concentration • Useful in detecting trace amounts of elements • Drawbacks: • only one element at a time • each time the proper lamp has to be selected to match the particular element under investigation
Origin of Emission & Absorption Spectra • Subatomic particles • Proton– positive electrical charge; in nucleus • Neutron– neutral particle; in nucleus • Electron– negative charge; outside the nucleus • # of protons = # of electrons in a neutral atom
Electrons and Energy • Electrons move around the nucleus and are confined to specific electron orbitals or energy levels. • Atoms are most stable when all of the electrons are in the lowest possible energy orbitals. • When the atom absorbs energy (heat or light), its electrons are pushed into higher energy orbitals. (excited state) • Only a definite amount of energy can be absorbed when moving an electron from one level to another.
Elements are selective in the frequency of light they absorb. • Energy levels determine the selectivity. • If atoms are exposed to intense heat, energy is generated to push electrons into higher unoccupied energy levels. • Normally, an electron does not remain in this excited state for long, but falls back to its original energy level. As it falls, it releases energy in the form of light. • Because each element has its own characteristic set of energy levels, each will emit a unique set of frequency values providing a “picture” of the energy levels that surround the nucleus of each element.
Neutron Activation Analysis • Atoms of the same element have the same number of protons. • They do not always have the same # of neutrons. • Atomic mass - the # of protons + # of neutrons • Isotopes - atoms with the same # of protons but different # of neutrons • Most elements have two or more isotopes, and most are stable. • Isotopes that are unstable and decompose are considered radioactive.