Profili Forensic Chem 9/28, 9/30
Profili Forensic Chem 9/28, 9/30 FRSC 367
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This 6 page Class Notes was uploaded by Kayli Antos on Friday October 2, 2015. The Class Notes belongs to FRSC 367 at Towson University taught by Mark Profili in Summer 2015. Since its upload, it has received 30 views. For similar materials see Forensic Chemistry in Criminology and Criminal Justice at Towson University.
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Date Created: 10/02/15
Forensic Chem Profili Fall 2015 I Gas Chromatography J J J J The sample must be vaporized before it is run through the column The mobile phase is an inert carrier gas that pushed the sample through the column The carrier gas must not react with the column or analyte GC is based on the gas flowing through the column in its gaseous phase and also being trapped within the liquid solid phase Pyrolysis is the rapid vaporization of analytes at very high temperatures so they are able to be injected into the column Can use a packed column or a capillary column The chromatograph measures the retention time of the components of the mixture GC cannot be used to identify compounds but is used to separate them for further examination The relative retention times between the same compounds will never change Some solid analytes like drugs are dissolved in methanol before they are vaporized and this will cause a methane and air peak to be observed in the chromatograph The most common detector is the flame ionization detector FID The column can only be heated to temperatures that the stationary phase can handle without decomposing which is usually below 300 C The separation of peaks can be increased by decreasing the temperature so the gases move slower Qualitative Found using the known retention times under standard conditions g Quantitative Found using the calibration curve of standards with known concentrations Factors That Determine Retention in CC The retention time is the relative time that the analyte is in the mobile phase as opposed to the stationary phase Factors like volatility the temperature of the column and polarity affect the retention time of a molecule The more volatile compounds will elute first As the temperature decreases retention time increases because the molecules are moving slower and will therefore elute slower N The Kovats Retention Index I is the retention of the analyte compared to the retention for a serious of alkanes on the same column under the same conditions Instruments For GC N 0 0 N 0 0 0 N 0 0 N 0 N 0 Carrier Gas Supply Common carrier gasses are He N and H The gas supply has pressure regulators gauges and flow meters Sample Injection System There must be enough sample injected to get good peaks but also must not be too large that the peaks are off the top of the chromatograph The whole sample must be injected quickly The most common way to inject the sample is into a flash vaporizer at the top of the column The sample is injected through a selfsealing septum Column Configurations There are packed and open tubular capillary columns The columns vary in length from less than two to over 50 meters They are usually made of stainless steel glass fused silica or Teflon They are always coiled Column Ovens The column is in an oven because the temperature must be controlled down to the tenth of a degree to correctly regulate the chromatography The best results with respect to elution time occur when the temperature is equal to or a little higher than the average boiling point of the sample Detection Systems An ideal detector should have 0 Good sensitivity 0 Results with good stability and reproducibility O The ability to maintain a linear response to solutes over a wide range The ability to handle temperatures up to 4000 A short response time independent of flow rate Reliability and be easy to use 0000 Similar responses to all solutes or be highly selective to certain classes of solutes 0 Ability to not destroy the sample very hard to find a detector that does not Detectors N 9 9 9 99999 N 9 9 Flame Ionization Detectors FID Used for drugs arson hydrocarbonsflammable compounds Most widely used and accepted detector for CC Column effluent is mixed with air and hydrogen and then ignited electrically The detector works because organic compounds produce ions and electrons when pyrolyzed and those can conduct electricity through the flame An electrical potential is applied to the ions and electrons and the resulting current is measured The FID is very sensitive and has a large range of detection The major disadvantage of FID is that is destroys the sample when it is ignited Thermal Conductivity Detectors TCD An early GC detector but still common Detects changes in heat conductivity of a stream of gas when the analyte is introduced The sensor is electrically heated and its temperature can be altered by the thermal conductivity of the gas around it The sensor may be platinum gold or tungsten The TCD is very simple and also has a long detection range It detects both organic and inorganic molecules TCD does not destroy the sample The major disadvantage is that it is not nearly as sensitive as other detectors ElectronCapture Detectors ECD Commonly used for environmental samples because of its ability to detect halogen containing compounds with pesticides and PCBs The ECD sensor is a 3 emitter usually nickel63 which emits electrons that ionize the carrier gas resulting in the gas emitting more electrons The carrier gas will therefore emits a constant current which decreases in the presence of organic molecule which will capture the electrons before they can reach the detector ECD is used for explosives as it detects electronegative functional groups like halogens peroxides nitro groups and quinones 0 It cannot detect amines alcohols and hydrocarbons 0 Commonly used to detect chlorinated insecticides GC Columns Open Tubular Capillary Columns 0 There are two basic types WCOT wall coated open tubular and SCOT support coated open tubular 0 The inside of WCOT capillary tubes is coated with a thin layer of the stationary phase 0 The inside of SCOT tubes are lined with a thin film of a support material which hold much more stationary phase as WCOT and can handle more sample Packed Columns 0 The columns themselves are typically made of glass metal or Teflon 0 They re about two to three meters long and have an inner diameter of two to four nanometers 0 Instead of having the stationary phase in a thin coating these tubes are packed with packing material that is coated with the stationary phase 0 These tubes are coiled Solid Support Materials 0 Most support material is made from diatomaceous earth which consists of the skeletons of many different diatoms that once lived in ancient bodies of water 0 The diatoms used pores for nutrient intake and waste release so their remains are well suited for use as a support material The Stationary Phase 0 The liquid stationary phase in a GC column should have 0 Low volatility 0 Stability at a range of temperatures 0 No interactions with the analyte ltgt Solvent characteristics so k and or fall within a suitable range 0 For the column to work the analyte should have some interaction with the stationary phase so the components of the mixture are retained somewhat This is based on polarity I Mass Spectrometry Expensive but reproducible reliable and able to identify compounds MS can be a detector for CC Gives a mass spectrum Basic Principle N If an object already in motion is subjected to a sideways force it will move along a curved path The more massive a particle is the less it will move It s possible to find the mass of the particle if you know its initial speed the magnitude for the sideways force and the curved path that the particle travels What Happens In MS N N N Atoms are able to be deflected by a magnetic field if they are ionized Electrons are knocked off of the atoms to turn them into cations This ionization happens in the ionization chamber where the molecule is bombarded with high energy electrons Acceleration And Deflection N The ions are then accelerated to the point where they all have the same kinetic energy Once they are up to speed they are then subjected to the magnetic field which deflects them This amount to which they are deflected is reliant on the mass of the ion as well as the charge called the mass to charge or mz ratio Once the ions are accelerated they pass through three small slits which work to concentrate the beam of ions Detection N There is a vacuum to pump out air molecules so the ions don t collide with them and also to remove some of the ions as they do not all need to be measured The beam of ions that remains is then detected electronically After the ions are deflected by the magnetic field they go to the detector Not all ions will reach the detector Some are not deflected the correct amount and will end up hitting one of the magnetic sides of the MS and become neutralized These ions are then removed via vacuum Ionization N When the vaporized sample enters the ionization chamber it becomes ionized by the electrons that are emitted by a metal coil which is heated electronically The electrons are drawn up to a positively charged plate called an electron trap When the vaporized molecules pass between the coil and the electron trap they are bombarded with electrons This results in their ionization and also fragmentation as the resulting molecular ion is unstable These ions are then pushed through the MS by a positively charged metal plate which repels them Uses N N N Can provide clues to the structure of the original molecule based on ionization and how it fragmented Will also provide information of the molecular weight This is due to the fact that the molecules will fragment in predicable ways Molecular Ion M N N N The whole original molecule only missing a single electron Usually the highest mass peak on the spectrum unless there are isotopes The weight of the ion is the molecular weight of the initial compound Isotopes N The same elements sometimes have slightly different masses Like for example 12C and 13C which occur in 989 and 11 of carbon atoms respectively Isotopes will have their own peaks so for a fragment with carbon there will be the large 12C peak with a much smaller 13C peak one mz unit to the right This isotope peak is denoted M1 for carbon