Forensic Analytical Chemistry – Flashcards
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| *Components of HPLC |
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| Source --> Degassing System--> Pump --> Injector --> Thermostated Column --> Detector |
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| Degassing System |
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| -HPLC -gas is compressed at the beginning and decompressed at the end and this caused bubbles to form which make the column less efficient -He gas is used because it doesn't dissolve, it picks up the gases and is swept away |
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| Low Pressure Gradient Pump |
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| -mixing chamber is located on the low pressure side of the pump -solvent A and B are mixed in chamber and then proceed to the pump |
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| High Pressure Gradient Pump |
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| -HPLC -mixing chamber is located on the high pressure side of the pump -solvent A and B go through their own pumps and then go to the mixing chamber |
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| Isocratic Solvent |
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| -one solvent mixture throughout entire analysis -used for simple mixture or polymer analysis -no equilibration time needed between runs |
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| Gradient Elution |
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| -HPLC -changing from one solvent to another mixture over time -enhances separation of early eluting components while reducing elution time for later compounds |
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| HPLC Pump |
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| -~ 60,000 psi -~ 1mL/min -sapphire, special alloys and Teflon are usually used |
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| UPLC |
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| ~ 20,000 psi less than 1 mL/min -utilizes A and B piston system (A is larger than B) - pistons move in and out, as A withdraws and pulls in solvent, B goes out and pushes solvent through |
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| *Sample Loop Injector |
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| -fixed volume is more reproducible - can do variable volume by partial filling -load sample then rotate 60 degrees to connect with mobile phase which pushes it through -need to keep length short to avoid diffusion and broadening |
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| Normal vs. Reverse Phase |
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| Normal: polar stat phase and non polar mobile phase -silica gel Reverse: nonpolar stat phase (C8 or C18) and polar mobile phase (usually water) -bonded silica (hydrocarbons chemically bonded) |
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| Spectrophotometric Detector (HPLC) |
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| -UV/Vis absorption commonly done at 254 nm -Mercury vapor lamp used -good for aromatics and conjugated molecules |
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| Diode Array Detection (HPLC) |
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| -illuminate entire sample with broad spectrum of light and then measure light absorbed, then split into wavelengths and measure with diode array -good for smokeless powders -separation = window/# of diodes |
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| Fluorescence Detector (HPLC) |
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| -monitor emission and excitation wavelengths -more sensitive than absorbance detectors (only for fluorescent compounds) -good for LSD |
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| HPLC Detectors |
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| Spectrophotometric, Diode Array, Fluorescence, IR, Refractive Index, MS |
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| Refractive Index Detection (HPLC) |
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| -one of the oldest detectors, least sensitive -as light passes through it is bent because of a change in the RI between the sample and the reference cell -negative peaks occur when the RI of an analyte is lower than the mobile phase and positive occurs when it is higher -not suitable with gradient elution because the RI of mobile phase changes -Source--> Flow Cell-->Optical Compensator-->detector-->feedback control device |
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| MS Detector (HPLC) |
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| -solvent removal is biggest problem -currently more expensive than others -allows for better identification -LCMS becoming standard for Tox |
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| HPLC Advantages and Disadvantages |
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| -more suitable for thermally unstable compounds and non-volatiles. -Generally non destructive Disadv: potentially dangerous solvents are used, more expensive interfacing to MS, samples must be particulate free and MUST be soluble |
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| Applications of HPLC |
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| -thermally unstable compounds (explosives) -non-volatiles -Tox screening -Dyes, inks, QD -Polymer analysis, fibers, plastics -drugs and drug metabolites |
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| Efficiency and Reproducibility of HPLC* |
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| -commercial columns have consistent particle size allowing for reproducibility even when switching columns -fixed volume sample loop is very reproducible |
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| Dispersive (IR) |
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| -uses a diffraction grating like UV-Vis spec -moving the angle of grating "scans" the wavelength across a single detector (takes 10-20 mins) -uses a slit to affect the resolution and to isolate a particular wavlength -earliest IR instruments had this type |
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| IR Spectra |
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| -4000-400 cm-1 -increase wavelength = increase energy -Abs=-logT |
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| FTIR |
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| -uses an interferometer to collect the whole spectrum at once -beam splitter separates original beam into 2 halves, one goes to fixed mirror, the other to the moving mirror -ion source produces an abundance of IR energy over a broad range when heated electronically -the moving mirror vibrates in and out of a # of cycles per second and the distance to the beam splitter changes - changes the pathlength -Helium laser is used as a position finder for the moving mirror -Detector measures the variation in the intensity with time |
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| Throughput Advantage |
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| -FTIR -dispersive has a limited throughput because of the required slits which must be moved close together for resolution but FT has a much larger area of sampling because it has no slits |
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| Multiplex Advantage |
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| -the noise of the spectrum is detector limited but when the entire spectrum is spread over an entire spectrum, therefore the signal to noise ratio is much better -noise is a random function and is sometimes higher and sometimes lower, by combining scans you will decrease the noise and improve the signal |
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| Resolution of FTIR |
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| -dependent on the number of points per scan and is a function of time -increasing the number of points increases the resolution across the top of the peak, but you don't want to make more points than necessary |
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| FTIR Detectors |
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| Deuterated Triglycine Sulfate (TGS), Mercury/Cadmum Telluride (MCT) |
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| FTIR Detectors |
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| Deuterated Triglycine Sulfate (TGS), Mercury/Cadmum Telluride (MCT) |
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| Deuterated Triglycine Sulfate Detector (DTGS) |
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| -FTIR -photoelectric transducer with a wide sensitivity range and quick response -1-40 micrometers with gap in the middle -standard and most widely used |
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| Mercury/Cadmium Telluride Detector (MCT) |
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| -FTIR -photoconducting transducer -narrow sensitivity range but MORE sensitive than DTGS -1-6.5 micrometers -needs liquid nitrogen cooling (77K) to reduce thermal noise -motion of electrons is enough to cause this -common with IR microscope and GCIR -need a high sensitivity because very little light is hitting the detector -uses a semiconductor |
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| Gas Sampling for FTIR |
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| -gases are contained in a gas cell with a long path length -need a long path length to get adequate absorption, at least 10 cm -can use mirrored cells or open path systems (source is at one end and detector is at another end of the room) |
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| Liquid Sampling for FTIR |
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| -droplet is compressed between 2 NaCl or KBr discs without spacers to create film -you want a short path length (10s of microns) -most liquids are highly absorbing in IR region -fixed and variable path length cells -salt windows are sensitive to water and alcohols (can use AgCl) |
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| Window Material Ranges |
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| -NaCl= 40,000 to 600 -KBr = 43,500 to 400 -KCl = 33,000 to 400 |
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| Solid Sampling for FTIR |
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| -pellets are used if can't dissolve solid -alkali metal halide salts form glass like pellets under pressure -finely ground solid is suspended in KBr pellet - 1-2% by wt in KBr - 1-5 mg of sample is needed -pellet is 1 mm thick by 7-13 mm diameter |
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| Sampling Devices of FTIR |
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| Thin Films, Specular Reflectance, Diffuse Reflectance, Attenuated Total Reflectance, KBr Pellet |
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| Thin Film Sampling |
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| -FTIR -useful for determining composition of plastic films or thin sheets and are mounted directly on the beam -solution of material is placed on a salt plate, solvent evaporates leaving a thin film |
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| Specular Reflectance |
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| -FTIR -non destructive -IR beam is reflected off a shiny surface of the sample and you measure the reflectance -good for crystals or thicker, smooth plastic -beam penetrates slightly into the sample and you get absorbance -requires Kramer-Kronig transformation |
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| Diffuse Reflectance |
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| -FTIR -time consuming, not used often -rough surface scatters light in all directions and is compared to just KBr -intensity of the reflection is related to the molar absorptivity -sample is mixed with a non-absorbing material (~5% in KCl) on surface of silica carbide paper -requires Kubelka-Munk transform (intensities vary) -good for drugs and paint |
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| Attenuated Total Reflectance (ATR) |
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| -FTIR -beam goes from higher RI to lower RI and you want it to strike the IRE at an angle to insure total reflectance -total reflection occurs beyond the critical angle (no penetration occurs with the material) -evanescent wave penetrates the material a wee bit -a succession of several total but attenuated reflections is done -depth of penetration increases with the wavelength |
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| Forensic Applications of FTIR |
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| -identification of controlled substances -ID of auto make, model, year from paint fragments -ID of polymers |
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| Explosion |
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| A rapid build up of heat and gases that results in a violent disruption of the surrounding environment |
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| Deflagration |
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| a rapid combustion that moves through an explosive material at a velocity less than the speed of sound (<1000 m/s) -low explosives |
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| Detonation |
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| a reaction that moves through an explosive material at a velocity greater than the speed of sound (>1000 m/s) -high explosives |
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| Types of Detonation |
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| Low Order: an incomplete reaction of the explosive material (due to air pockets, wrong type of initiator, deterioration over time, poor contact, or inadequate containment of low explosive) High Order: a complete reaction of the explosive material |
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| Explosive Types |
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| Explosive Mixture: a mechanical blending of an oxidizer and a fuel to produce a mixture that can produce an explosion -usually low explosives (except for ANFO) Explosive Compound: a chemical compound whose molecules can produce an explosion without the addition of a fuel or oxidizer -TNT, NG -oxidizer and fuel are chemical bonded together |
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| Low Explosive |
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| -must be contained -deflagrate -Smokeless powder, black powder, pyrotechniques |
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| Smokeless Powder |
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| -low explosive -used in ammunition, pistols, rifles, shotguns -Single base: NC -Double base: NC+NG -Triple base: NC+NG+ nitroguanidine/TNT -used for battleships -diphenylamine is a stabilizer used becuase NC loses nitro groups over time and it scavenges them |
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| Smokeless Powder Shapes |
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| ball- like pringles Disk-may or may not be perforated Rod or Tube Lamelli=flat cut shape (trapezoidal) T-shaped-new |
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| Black Powder |
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| -low explosive -charcoal + KNO3 + sulfur -pyrodex has chlorate instead of S -muzzle loaders and fireworks -hasn't changed much since middle ages |
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| Pyrotechniques |
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| -mainly black powder with various metal salts and metal powders for color and effect -may contain flash powder |
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| High Explosives |
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| detonate primary, secondary, or tertiary |
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| Primary Explosive |
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| -do not require a detonator -friction sensitive, shock sensitive, temperature sensitive -NG, EGDN, HMTP, TATP, Azides, fulminates, pulcrites, DDPN |
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| Nitroglycerin |
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| -primary explosive -6500 m/s -used in dynamite, double based smokeless powders, and some military formulations -modern ones might have EGDN-less sensitive -can seep out of old TNT and that is bad -cold crystals of NG can rub together and that's enough to go off |
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| EGDN |
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| -ethylene glycol dinitrate -primary explosive, high |
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| HMDP |
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| -hexamethylene triperoxideamine -primary explosive, high -peroxide based -5100 m/s -extremely sensitive -popular with some extremist groups -found occassionally in meth labs |
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| TATP |
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| -Tetraacetone triperoxide -primary explosive, high -peroxide based -easily made from things you buy at the store -extremely sensitive |
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| Secondary Explosives |
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| -high explosive -booster charges -requires energy from primaries -commercial: TNT, Dynamite, PETN, Tovex -military: RDX, HMX |
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| TNT |
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| -secondary explosive, high, commercial -trinitrotoluene -6500 m/s |
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| Dynamite |
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| -secondary, high, commercial -mixture of NG, EDG, NC and/or TNT with sawdust or diatomaceous earth -not shock sensitive, requires blasting cap -components can be added to make it more stable |
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| PETN |
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| -secondary, high, -pentaerythrite tetranitrate -8300 m/s -used in detonating cord and shaped charges -commercial, but can be military |
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| RDX |
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| -secondary, high, military -8360 m/s -used in C4 -military |
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| HMX |
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| -secondary, high, military |
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| Tertiary Explosives |
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| -high -ammonium nitrate based -requires energy from secondary and primary -ANFO, water gels |
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| ANFO |
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| -tertiary, high -ammonium nitrate/ fuel oil -commercial-used in mining operations with booster charges -requires a blasting cap -was used in OK city bombing -AN fertilizer is readily available |
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| IED |
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| -improvised explosive device -a device placed or fabricated in an improvised manner incorporating destructive, lethal, noxious, bla bla bla -contains a detonator or fusing mechanism, a timer, remote, trigger, arming mechanism, explosive, containment, and concealment -LOW: poor man's C4, flash mix, armstrong's mix, homemade black powder -High: TATP, HMTD -Pipe bombs |
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| "Poor Man's C4" |
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| -low IED -postassium chlorate and vaseline (90/10) |
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| Flash Mixes |
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| -low IED -potassium chlorate and alluminum/sulfur |
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| Armstrong's Mixture |
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| -low IED -potassium chlorate and red phosphorus -particularly nasty |
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| Homemade Black Powder |
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| -low IED -charcoal, S and KNO3 -grains of S and charcoal (From wood) indicate homemade |
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| Pipe Bomb |
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| -IED -0.75 to 1 in diamter and 6-8 in long -can glue metal on outside for shrapnel -needs an initiation device like an electrical match or a blasting cap -simple to make, materials available |
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| Shaped Charge Effect |
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| the particular shape of the explosive with a Cu sheeting around it focuses the explosive downword |
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| Blast Pressure |
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| blow air and lower the pressure so much that you get a partial vacuum that sucks lighter material back in |
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| Where to look for debris |
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| -in or near crater: explosion creates presure wave out leaving low pressure at site of explosion that pulls in light material Away from crater: distance dependent on force of the explosion and obstruction and heavier debris is carried by force of explosion |
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| TWGFEX methods |
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| 1/2: GCMS, IR, EDX, XRD, LCMS 3: GC, LC, IC, CE 4: burn, flame, MP, Spot tests |
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| Post Blast Chemical Analysis |
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| -GC/MS primary -HPLC and CE as alternative methods -SEC for plastic fragments -TLC still used sometimes for screening for SB and DB powders |
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| Pre-Blast Device Detection |
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| -IMS detection of vapors from explosives -canines (NC, black powder, ANFO) -X-ray |
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| Taggents |
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| Marking Agents: semi-volatile materials added to less volatile high explosives -DMNB added to C4 -not really taggants Identification Tags: taggents, small color-coded particles added to high explosives which will survive blast -identify manufacturer |
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| Forensic Applications of ink analysis |
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| -detection of counterfeit currency -authentication of historical documents -written materials relative to another crime -ink stain comparison |
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| Carbon Based Ink/India Ink |
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| -some of oldest known -used by ancient chinese -mixture of carbon black and glue made into cakes (made from soot from candles) -cake is ground with water to make a suspension -today shellac and borax are used with wetting agents instead of glue -waterproof -used in drawing and printing ink and fine art prints |
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| Fountain Pen Ink |
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| Iron Gallo-tannates: made from Fe salts and gallo-tannic acid from tree galls - iron salts tend to be acidic and can destroy paper when exposed to enough moisture in the air -declaration of independence and constitution - initially nearly colorless and exposure to air turns it brown-black - modern inks contain synthetic dyes synthetic dyes in aq. solutions: brilliant colors but not as stable as blue-black |
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| Ballpoint Inks |
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| -most frequently used instrument for writing -ink is a viscous mixture of synthetic dyes in various solvents (many are proprietary) -dyes: crystal violet, methyl violet, tetramethyl para rosaniline, and victoria blue -solvents: glycols and glycerin, benzyl alcohol |
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| Rolling Ball Marker Inks |
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| -more similar to marker ink than ballpoint -similar inks to those used in textile industry -not water fast -less dye than BP, more than fountain |
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| Different Types of Inks |
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| India/Carbon, Fountain (Iron Gallotannate and Synthetic), Ballpoint, Rolling Ball, Fiber Tip, Gel |
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| Fiber Tip Pen Inks |
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| -inks contain dyes and additives similar to fluid inks -water based or xylene based (water resistant and permanent) -metal chelated dyes are relatively light fast (Cu thiocyanate) |
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| Gel Pen Inks |
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| -water based, pigmented gel ink -black and colored pigments rather than dyes -special pigments, resin and additives to give high static viscosity -thixotrpic (formulated to become thinner when used) -easily recognized by TLC -made for space! |
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| Spectroscopic Techniques (Inks) |
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| -illumination with UV-IR -visual comparator gives fluorescence -microspectrophotometer -color and brightness of fluoresence -PIXE and Xray for metal content of pigments or dyes -micro-FTIR: can take reflectance spectra through microscope of an individual line and compare it to other parts of the check -difficult to remove contribution of paper components because of its heterogeneous nature -diffuse FTIR of ink extracts |
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| Chromatographic Tehcniques (Inks) |
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| TLC, Spectral Reflectance, Micro Punch |
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| TLC analysis of Inks |
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| -most commonly used -small sample size, quick screening -micro-plugs taken with blunt hypodermic needle -extract with pyridine (BP), or ethanol-water(others) -can look at Rf, color, fluorescence for differentiation -can show different amounts of different components |
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| Spectral Reflectance of Inks |
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| -chromatographic technique -measurement of reflectance at various wavlengths can be used to compute the RGB profile -used in paint matching devices at paint stores |
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| Micro Punch |
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| -used for sampling documents -punch tiny plugs out of paper and then push them all out into a microcentrifuge tube to do extaction |
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| Age Dating Inks |
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| -look at solvent changes, solvent evaporates over time -Dye changes: strong bonding of dye and paper fiber occurs over time and it becomes less soluble |
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| C-H, Aromatic C-H, O-H, amine, CdoubleO, nitrile, CdoubleC, C-C |
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| CH alkane: 3000-3850 CH alkene 3100-3000 CH aromatic: 3150-3050 C-C alkane: not useful CdoubleC, alkene: 1680-1600 CdoubleO aldehyde: 1740-1720 ketone: 1725-1705 COOH: 1725-1700 Ester: 1750-1730 Amide: 1680-1630 O-H: free 3650-3600 h-bonded 3400-3200 N-H, amine: 3500-3100 C-N, amines 1350-1000 CtripN, nitrile 2260-2240 |
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| Detection Methods of TLC |
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| -physical: fluorescence quenching, fluorescence, absorbance, optomechanical scanning, electronic scanning -Microchemical Detection: pre and post chromatographic derivitization, universal and specific spray reagents -Microbiological detection; bioautography |
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| Fluorescence Detection of TLC |
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| -compounds that fluoresce absorb UV light and fluoresce back in the visible -a number of dyes are fluorescent -use mercury vapor lamp at UV source -254 for quench and 365 for fluoresce -quenching: analyte absorbs UV leaving a dark spot behind behind on a light background |
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| Chemical Visualization Spray Reagents |
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| Universal: Iodine vapor, concentrated sulfuric acid Specific: ninhydrin, iodoplatinate, fast blue b, erlich, diphenylcarbizone |
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| Iodine Vapor Visualization of TLC |
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| -universal spray reagent -must have double bonds -gives brown spots -often reversible |
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| Concentrated Sulfuric Acid Visualization |
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| -universal reagent -brown-black -messy, dangerous, no other tests possible, chars the plate -always last one done -requires heat |
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| Ninhydrin Visualizaiton |
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| -group specific, amines -blue -often 1st spray done -many drugs, drug metabolites |
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| Iodoplatinate Visualization |
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| -various colors -many drugs, particularly opiates |
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| Fast Blue B Visualization |
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group specific TLC Spray Reagent cannibinoids |
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| Erlich's Reagent Visualization |
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| -LSD |
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| Diphenylcarbizone Visualization |
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| barbituates |
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| On Plate Quantitation |
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| -fluorescence intensity -UV/Vis absorbance with densitometer -Diffuse reflectance (silica is good diffuse surface) -laser desorption |
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| Off Plate Quantitation |
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| -used with limited amount of material -scrape off and elute with appropriate solvent -do UV/Vis, GCMS, HPLC |
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| Forensic Applications of TLC |
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| -drug screening, Tox, -ink analysis, QD -Explosives |
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| GSR |
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| the total residues resulting from the discharge of a firearm, it includes both gunpowder and primer residues, plus metallic residues from projectiles, fouling |
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| Primer Residue |
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| -a particle composed of PbBaSb fused together in a single unit, which exhibits the correct morphology |
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| GSR Dispersion |
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| -when the weapon is discharged, products of combustion, unburned powder and particles from primer are discharged fromt he muzzle, breach and other openings |
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| Why Test? |
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| -inidcates possible firing of weapon recently -can infer distance from shooter to target |
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| Modified Griess Test |
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| 1) confirm nitrite sensitivity of test paper using a nitrite test swab saturated with 15% acetic acid solution and dab at corners 2) mark reference points of evidence on test paper by placing evidence face down on emulsion coated side of test paper and marking seams, buttons, etc with paper 3) create layers of reaction items by soaking cheesecloth in acetic acid, ringing, placing on top of evidence on top of photosensitive paper 4) heat reaction layers with an iron and acetic acid steam is forced through the layers causing the color-producing reaction 5) examine and interpret results, discard the cheesecloth and separate evidence from test paper, any orange on the paper are a result of the reaction 6) distance determination by comparing pattern from victim vs. test shots at standard distances contact, 3, 6, 9, 12, 18, 24 |
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| Other GSR Test |
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| Sodium Rhodizonate, Dithiooxamide Test, Primer GSR ID |
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| Sodium Rhodizonate Test |
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| -GSR -test for lead with the evidence revolver -blue violet is positive -can be used to detect bullet wipe |
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| Dithiooxamide Test |
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| -GSR -for Cu (green), Ni (blue/pink), and acetone extracts (blue) -performed after Griess |
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| Primer GSR ID |
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| -usually performed using SEM/EDX -use a tape lift or sticky stubs to pick up particles |
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| Paint |
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| a suspension of pigment in a film former (also called a vehicle) |
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| Film Former/Coating |
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| -usually organic polymers with the purpose of protecting the surface and holding the pigments -Varnish: commonly polyurethane dissolved in solvent which normally doesn't contain a stain, protects -Stain: mix of organic dyes dissolved in solvent, made to penetrate wood and stain, not protect -Enamel: a glossy, thermosetting paint but now any paint which dries glossy, solvent based, requires heat to complete |
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| Solvent Drying Mechanisms |
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| -Solvent Evap: used for rust proof paints (rustoleum and varnish) -Heating/Thermosetting: chemical reaction is accelerated, used for automotives -Oxidation: double bonds react with oxygen in air and you get a large 3D polymer |
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| Paint Binders |
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| -Acrylic Lacquers: long chain, few crosslinking, soften when heated, used for GM topcoats -Acrylic Enamels: polymer that crosslinks extensively when heated, hard surface and resistant to attack by solvents, freq with cars -Alkyd Enamels: also extensively crosslinked when heated, used on imported cars -Alkyd Resins: drying vegetable oils (linseed), air oxidation=crosslinking -Polyurethane Resins: condensation polymer of isocyanates + activated alcohol compounds, used for floor coatings, resistant to water/alcohol -Vinyl resins: vinyl chlorate or vinyl acetate, common house paint |
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| Paint Chips |
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| pieces of paint have come off surface and usually contain layers |
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| Paint Smear |
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| the top layer of paint, which has loosened due to being wet or air oxidation, smears onto another surface after brushing contact, layer structure not present |
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| General Analysis of Paint |
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| -color layer analysis -same number/order of layers -relative thickness -additives |
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| Paint Primer |
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| -1st layer -coats metal and helps prevent rust and provides a good binding surface for the next layers on top of it |
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| Surfacer |
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| -also a primer, provides a smooth surface for next layers |
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| Base Coat |
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| -gives actual color to car, subject to fading with UV |
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| Effect Coat |
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| -metallic layer, clear binder with something suspended in it like mica to give metallic appearance |
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| Clear Coat |
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| -protects lower layers and provides depth of color to car -can be purchased locally |
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| Microchemical Test of Paint |
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| -solubility: in acetone, pyridine, dichloromethane -Pyrolysis GC: anlaysis of film former -FTIR: individual layers, primarily binders, some pigments |
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| Classes of Compounds of Forensic Interest (UV/Vis) |
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| -Aromatics: drugs/metabolites, explosives, organic dyes, amphetamines -Nitro Compounds: TNT, DNT, nitroaromatics, NG, NC, generally below 210 -Transition Metals: inorganic dyes/pigments |
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| Parts of UV Spectrophotometer |
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| -Light source: combo of tungsten and deuterium lamps -switch to tungsten at 350 for less background -Monochrometer: quartz or fused silica, glass or plastic for visible -Cuvette: responds to intensity of light |
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| Single Beam (UV) |
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| -slit allows only desired wavelength to go through and grating changes the light going through the exit slit -filter cuts out stray light and focuses it -typically measure the intensity of light as it goes through the reference and compare to the sample |
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| Double Beam (UV/Vis) |
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| Source-->Mono-->mirror 1--> sample or reference-->2nd mirror--->photomultipler-->detector -uses a rotational mirror (aka chopper) to split beam to go to reference and sample and measures the ratio of the top to the bottom |
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| Photodiode Array (UV/Vis) |
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| -all light goes through the sample and then grating does the separation on to the diode -computer reads the charge off of each diode which translates to the intensity of light -as you widen slits, you allow more light from adjacent wavelengths which increases sensitivity but decreases selectivity |
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| Sources of Error for UV/Vis |
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| -round cuvettes don't have uniform pathlength -any particles can scatter light in cuvette -deviations at high and low (on calibration curve) due to light satter and self-absorbance |
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| Isobestic Point |
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| -point where both states have same absorptivity at some wavelength -usually interested in total concentration of drug in sample and you use Iso point for this can change pH and do series of measurements to find isobestic point |
