Classnotes- Ch.11 + 12 Spectrsocopy and Spectromy Chemistry PDF

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Professor James T. Spencer...

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3/25/19 Forensic Chemistry -

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Science: Atomic Theory - From a fundamental understanding of the macroscopic behavior of substances comes and understanding of the microscopic behavior of atoms and molecules - Ex: can understand baseball rules from watching baseball game - Macroscopic: substances, mixtures, physical properties and changes - Microscopic: atomic theory - Can matter be infinitely divided? - Most Greek philosophers- yes - Democritus (460 BC) and John Dalton (1800s)- No (atomos means indivisible) Matter and Mass - Law of conservation of Mass- atoms are neither created nor destroyed in chemical reactions - Total number of atoms before reaction = total number of atoms after reaction - Stoichiometry- quantitative study of chemical formulas and reactions - Chemical equations- used to describe chemical reactions in an accurate and convenient fashion - Reactants and products Forensic analysis - Combustion analysis - Mass spectrometry (MS) - Atomic Spectroscopy - Atomic Absorption (AAS) and Emission analysis (AES) - Neutron Activation analysis (NAA) - Molecular Spectroscopy - Electronic spectroscopy - Vibrational spectroscopy - Nuclear magnetic resonance (NMR or MRI) - X-ray methods - X-ray Diffraction (XRD and CAT) - Energy Dispersive X-ray Fluorescence (EDXRF) Analytical Methods - Questions to consider in choosing an analytical (chemical) method: - Quantitative or qualitative method - Sample size and sample preparation requirements - What level of analysis is required? (ex: +/- 1.0% or +/- .001%) - Detection levels and useful analytical concentration ranges - Destructive or non-destructive - Availability of instrumentation

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Admissibility (ex: are all lead pipes compositionally the same or are there sufficient variations among “known” leadpipes of the world to link two samples → useless)

Matter - Is it uniform? - Heterogenous mixture - At least 2 components - Homogenous - Can it be separated by physical methods? - No- Pure substance - Yes- homogenous mixture - Is the pure substance decomposed? - No- element - Yes- compound Isotopes - Different in mass number, but same in atomic number - All atoms of a given element have the same number of protons (and the same number of electrons) - Atoms of an element may have different numbers of neutrons Allotrope - Different chemical forms of the same element existing in the same physical state - Ex: carbon can be fullerene, diamond, or graphite - The way they’re put together gives them different properties Combustion reactions - Reaction of hydrogen (/stuff) with oxygen - Ignition temperature = 580-590 C - Explosive- the rapid release of energy into the surrounding air causes the air to very quickly expand.the explosion from pure H2 sound quieter because the air expansion is slower - Empirical formula from reaction with oxygen - Organic compounds- C to CO2 and H to H2O - Use CO2 and H2O to determine the amount of C and H in original sample - Tells us how much of each element is present in my unknown sample - Allows us to determine % of each element in the sample - % = [amount of element/ amount of sample]/100 - Compare with reference samples - Pro: easy, inexpensive, quickest - Con: destructive Mass Spectrometry - Instrumental principles and design - Spectral features - Spectral interpretation and comparison - GC-MS and LC-MS

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Most important method Very expensive Blood alcohol analysis Drug analysis- what is it Gently heats substance - creates charge particles (ions) from gas phase molecules Zap with electron beam (high energy beam) - Electron ionization (EI)- uses electron impact to ionize a molecule - Knocks of an electron - Chemical ionization (CI)- first ionizes a molecule gas which in turn ionizes the molecule of interest - A gentler method of ionization - Fast atom bombardment (FABS)- mainly for nonvolatile compounds- very harsh The MS analyzes ions to provide information about the molecular weight of the compound and its chemical structure. Magnetic field deflects lightest ions most Ions separated by mass expose film

Direct methods of measuring  (separating) mass Sample molecules are ionized by e-beam to cations (1+ by “knocking off” one electron) which are then deflected by magnetic field - for ions of the same charge, the angle of deflection is proportional to the ion’s mass

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Ionization produces singly charged ions. The intact charged molecule is the molecular ion. Energy from the electron impact and instability in a molecular ion can cause that ion to break into smaller pieces (fragments) - The methanol ion may fragment in various ways,with one fragment carrying the charge and one fragment remaining uncharged. Chromatography - Used to separate mixtures of compounds (most things in nature come as mixtures) - Based upon the differing interactions between the components of a mixture and another compound - Chromatographic systems have a stationary phase (which can be solid or liquid) and a mobile phase (usually liquid or gas) - The mixture to be separated is placed at the beginning of the chromatographic system (stationary phase) - The mobile phase then “pushes” the components of the mixture through the system - Each component absorbed on the stationary phase with a different strength (stronger means moves more slowly through the system) - Each component comes out the end of the system at a different time (retention time) Hydrogen emission - A Swiss schoolteacher in 1885 (J. Balmer) derived a simple formula to calculate the wavelengths of the emission lines (purely a mathematical feat with no understanding of why this formula worked) WHY!!!!! Bohr’s model - Electrons in around nucleus with quantized (only allowed certain values; ie. football game score) energy states - When in a state, no energy is radiated but when it changes states, energy is emitted or gained equal to the energy difference between the states - Emission from higher to lower - Absorption from lower to higher Microscopic properties - Light energy may behave as waves or as small particles (photons) - Particles may also behave as waves or as small particles - Both matter and energy (light) occur only in discrete units (quantized) Emission - Heated gas gives off light - Not just emission- atoms and molecules can also absorb energy Spectroscopy - When electromagnetic radiation passes through a substance, it can either be absorbed or transmitted, depending upon the structure of the substance.

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When a molecule absorbs radiation it gains energy as it undergoes a quantum transition from one energy state (Einitial) to another (EFinal). Atomic spectroscopy - Ground state- the lowest energy state of an atom or molecule (most stable state) - Excited state- results when ground state electrons are excited by energy or higher energy states - Emission- When an electric current is passed through a gas, the gas emits light. Due to the change of energy of the gas. The electrons in the atoms of the gas become excited to a higher energy state (the “excited state”) and when they return to the original, low-energy state (“the ground state”), the atoms of the gas emit the excess energy as light. - Absorption- This is due to the change of energy of the gas. The electrons in the atoms of the gas become excited by absorbing energy (light). Atomic emission - Uses quantitative measurement of the optical emission from excited atoms to determine concentration Atomic absorption - uses the absorption of light to measure the concentration of gas-phase atoms. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Neutron Activation analysis (NAA) - Neutrons interact with a target nucleus to form a compound nucleus in an excited state. The compound nucleus will decay into a more stable configuration through emission of one or more gamma rays. This new configuration may yields a radioactive nucleus which also decays by emission of delayed gamma rays, but at a much slower rate according to the unique half-life of the radioactive nucleus. - About 70% of the elements have properties suitable for measurement by NAA. - Parts per billion or better - Used to figure out of Napoleon died - Arsenic in hair - Died of stomach bleeding

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Green wallpaper coloring was made from an arsenic containing compound

-Electron spectroscopy - When white light passes through or is reflected by a colored substance, a characteristic portion of the total wavelength is absorbed -The remaining light will assume the complementary color to the wavelengths absorbed -UV-visible -Molecules are exposed to light having an energy that matches an electronic transition within the molecule, energy will be absorbed and the electron is promoted to a higher energy orbital. A spectrometer records the wavelengths at which absorption occurs. -

Vibrational spectroscopy - Different types of bonds have characteristic regions of the spectrum where they absorb - Infrared and Raman spectroscopy - Forensic application - Uses of computer databases of IR’s of known compounds - Analyzing alcohol- the breath is tested with a mechanism similar to a breathalyzer (chemical oxidation) but uses the infrared absorptions of alcohol. - Analyzing drugs- the drugs’ various chemical components absorb infrared light. The absorptions are compared to known samples using a database. - Analyzing fibers - Analyzing paints...