Profili Forensic Chemistry 10/5, 10/7
Profili Forensic Chemistry 10/5, 10/7 FRSC 367
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This 4 page Class Notes was uploaded by Kayli Antos on Friday October 9, 2015. The Class Notes belongs to FRSC 367 at Towson University taught by Mark Profili in Summer 2015. Since its upload, it has received 21 views. For similar materials see Forensic Chemistry in Criminology and Criminal Justice at Towson University.
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Date Created: 10/09/15
Forensic Chem Profili Fall 2015 9 Mass Spectrometry N N 22 22 IR N N 22 22 Fragmentation 0 The impact from the electrons can lead to fragmentation of the molecule 0 We know that the same type of molecule will fragment in the same way under the same conditions Background 0 The mass spectrum is the output graph which compares the mass charge ratio of particles to their relative abundance O The abundance that is assigned is based on the most abundant particle which is set as 100 and other particles are percentages of the base peak Fragmentation Patterns 0 A molecule will usually break into a cation and a radical 0 The bonds of the molecule will break so as to form the most stable canon 0 The stability of the radical is not important to fragmentation Rule Of Thirteen 0 Used to determine the molecular formula of a hydrocarbon of formula CnHm 0 Step 1 n M m only use the integer leave the remainder Step 2 m n remainder from step l 0 If there is an atom of a different element in the structure that elements atomic mass should be subtracted from M first and then add the heteroatom into the final formula ltgt What Is IR Light 0 A Form of electromagnetic radiation whose wave region is longer than visible light and shorter than radio waves 0 It has a low energy What Is The IR Region 0 The wavelength region is from 25 to 25 pm 0 cm391 is the unit used for wavenumbers which range from 400 to 4000 0 The wavenumber is proportional to energy and frequency and reciprocal to wavelength IR Spectrum 0 Near IR 0 Middle IR 22 22 22 22 0 Far IR Principles 0 When certain molecules are hit with IR light they will absorb the energy and the atoms will vibrate 0 The frequency of vibration depends on the amount of atoms in the molecule and how long and strong the individual bond is 0 There are different kinds of vibration that can occur like stretching rocking scissoring wagging and twisting Energy Trends 0 The energy follows the vibration frequency of the atoms 0 Lighter atoms vibrate more rapidly than heavier atoms C H N H O H bonds vibrate at 2800 cm 1 Double and triple bonds vibrate more rapidly than single bonds CEC vibrates at 2100 2200 cm391 CEC vibrates at 2240 2280 cm 1 CO vibrates at 1680 1750 cm391 CC vibrates at 1620 1680 cm 1 C O vibrates at 1025 1200 cm 1 This can be used to measure different functional groups in a molecule Intensity Trends 0 These are the changes in the dipoles that result from the atoms vibrating 0 Polar bonds the strong dipoles will absorb a lot of energy O H CO CN C O are some strong dipoles ltgt Nonpolar bonds will absorb weaker than the polar bonds CC CEC are some nondipole bonds What Kind Of Sample Can Absorb IR 0 We can only measure vibration that alters a dipole moment and absorbs within the IR region 0 Diatomic gasses don t have a dipole moment so the IR cannot take a measurement of them 0 Every functional group have a specific IR absorption 0 More complex molecules yield more complex IRs 0 IR can be used to identify a molecule IR Spectrum Distribution 0 Bonds to hydrogen 4000 2500 cm 1 0 Triple bonds 2500 2000 cm1 0 Double bonds 2000 1500 cm 1 0 Fingerprint region 1500 400 cm391 Fingerprint Region 0 Characterized by many bands and much overlap ltgtltgtltgtltgtltgt 0 The overall pattern shows the whole structure and is like the molecules fingerprint 0 Should have an idea of the simple structure of the molecule before trying to interpret the fingerprint region z Procedure 0 IR light passes through a sample and the energy recorded by the sample is measured 0 The monochromator selects the IR light needed before it hits the sample An interferometer can be used instead to measure all wavelengths of light in one run 0 The data is processed and placed into a graph of wavelength vs transmission 0 This graph can then be compared to known references Instrumentation 0 The conventional method is to select a single frequency of radiation with the monochromator to expose the sample to each time 0 FTIR Fourier Transform IR replaces the monochromator with an interferometer which is capable of running different frequencies in one run IR Components 0 IR Resources 0 Nernst glower quartz halogen o Globar silicon carbide 0 Sample 0 Solids 0 Mortar And Pestle Sample is ground to give a very fine powder A small amount is mixed with liquid paraffin to yield a paste Several drops of the paste are placed between two plates made of sodium chloride and then placed in the instrument 0 Potassium Bromide Disk A miniscule amount of the solid is added to pure KBr and finely ground Placed in a small die and mechanically put under pressure for several minutes to form the disk This disk is then ready to be placed in the machine 0 Thin Films 22 22 0 The film can be placed in a holder and placed directly into the machine 0 This is often used to check the calibration through the use of polystyrene sample 0 Liguids 0 A drop is placed between two KBr or NaCl plates and then placed in the machine 0 Gases 0 Must be placed in a special cylindrical cell with windows at each end 0 Monochromator Interferometer 0 Used to select certain wave regions to hit the sample with 0 Detector 0 Charge coupled device COD 0 Indium Gallium Arsenide InGaAs 0 Lead sulfide PbS 0 Display 0 Show the data that was collected 0 Translate that data into a graph of transmission vs frequency wavenumber ltgt Microscope 0 IR can be attached to a microscope 0 Use microscope to find sample and then hit it with light from IR 0 Cumbersome and expensive Advantages 0 Provides a graph that is unique to the compound and identifiable 0 When attached to a computer can manually check spectrum against library of graphs Disadvantages 0 Usually cannot be used to quantify a substance 0 Sample preparation can be difficult
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