FIU CHM 4130 - CHAPTER 13_Xiao_Molecular Spectrometry_2018 (48 pages)

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CHAPTER 13_Xiao_Molecular Spectrometry_2018



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UV VIS Molecular Spectroscopy Chapter 13 From 190 to 800 nm Reflection and Scattering Losses The solution Power of radiation after passing through the sample solution T A A a b c Power of radiation before passing through the sample solution Psolution P Psolvent P0 P log T log P0 abc kc absorptivity pathlength concentration Beer Lambert Law Absorption Variables Beer s law and mixtures The total absorbance of a solution at a given wavelength is equal to the sum of the absorbances of the individual components present Each analyte present in the solution absorbs light The species do not interact The magnitude of the absorption depends on its A total A1 A2 An A total 1bc1 2bc2 nbcn If 1 2 n then simultaneous determination is impossible Assumptions of the absorption law The incident beam is monochromatic The absorbers absorb independently of each other Incident radiation consists of parallel rays perpendicular to the surface of the absorbing medium Path length traversed is uniform over the cross section of the beam Absorbing medium is homogenous and does not scatter the radiation The incident beam is not large enough to cause saturation effects Deviations from Beer s Law Real limita ons Chemical factors Instrumental factors Deviations from Beer s Law Real limita ons 1 Deviations in absorptivity coefficients at high concentrations 0 01 M due to electrostatic interactions between molecules in close proximity 2 Low absorber concentration but high electrolyte ion conc 3 Change in refractive index of the medium when concentration changes 4 Scattering of light due to particulates in the sample 5 Fluorescence or phosphorescence of the sample Successful at low analyte concentrations 0 01M Deviations from Beer s Law Chemical factors 1 Shifts in the position of a chemical or physical equilibrium involving the absorbing species 2 Deviations can only be observed when concentrations are changed 3 A common example of this behavior is found with acid base indicators Chemical Equilibria Consider the equilibrium A C AC If is different for A and AC then the absorbance depends on the equilibrium A and AC depend on A total A plot of absorbance vs A total will not be linear Chemical deviation of acid and base forms of phenol Deviations from Beer s Law Instrumental factors nega ve absorbance errors Unsatisfactory performance of an instrument may be caused by fluctuations in the power supply voltage an unstable light source or a non linear response of the detector amplifier system Polychroma c radia on Stray radia on Instrumental deviation with polychromatic radiation Ideal measurements with monochromatic source radiation In practice polychromatic sources a grating or with a filter A nearly symmetric band of wavelengths surrounding the wavelength to be employed Radiation of dichromatic beam Deviation of polychromatic radiation B Instrumental deviation with stray radiation PS stray light T P PS P0 PS A log T P PS A log P0 PS A abc kc Deviations from Beer s Law Mismatched cells Path length and optical characteristics of cells are not equal solutions Using carefully matched cells Using a linear regression procedure Using single beam instruments Instrument Noise Uncertainties Noise associated with the instrument instrumental noise Slit width Scattered radiation Concentration uncertainty and transmittance The uncertainty in concentra on as a func on of the uncertainty in transmi ance can be sta s cally represented as sc 0 434sT c T logT Rela ve standard devia on of concentra on measurement Absolute standard devia on of transmi ance measurement Best precision absorbance value in the range from 0 2 0 7 Instrument Noise Johnson Thermal noise Shot noise Flicker noise Case I thermal detector dark current and amplifier noise Johnson thermal nose Case II photo type detectors such as photomultiplier tube shot noise Case III source the slow drift in the radiant output of the source source flicker noise Instrument Noise Johnson Thermal noise Shot noise Flicker noise RelaBve concentraBon uncertainBes arising from various categories of instrumental noise Instrument Noise Slit width Instrument Noise Sca ered radia on Useful range from 340 780 nm be careful with absorbance below 380 nm Components of instrumentation Sources Wavelength selector Sample Containers Radiation transducers Signal processors and readout devices Components of instrumentation Sources Agron Xenon Deuteriun or Tungsten lamps Monochromators Quarts prisms and all gratings Sample Containers Quartz Borosilicate Plastic Transducers Photomultipliers Deuterium D2 Lamps Discharge of excited deuterium gas Continuum of UV light 190 400 nm Ultraviolet source for UV Vis and HPLC 29 Deuterium and hydrogen lamps a continuum spectrum in the ultraviolet region D2 Ee D2 D D h Excited deuterium molecule with fixed quantized energy Dissociated into two deuterium atoms with different kinetic energies Ee ED2 ED ED hv Ee is the electrical energy absorbed by the molecule ED2 is the fixed quantized energy of D2 ED and ED are kinetic energy of the two deuterium atoms Tungsten Filament Heated to 2870 K Useful Range 350 2500nm 31 Tungsten halogen lamps 350 2500 nm Blackbody type temperature dependent Why add I2 in the lamps W I2 WI2 Weak intensity in UV range Good intensity in visible range Very low noise Low drift Glass envelop Low limit 350 nm Source Tungsten Halogen why Iodine added Reacts with gaseous W near the quartz wall to form WI2 W is redeposited on the filament Smooth spectral curve stable output and little UV radiation Gives longer lifetimes Allows higher temperatures 3500 K 33 Intensity Spectrum of the Xenon Arc Lamp High intensity smooth continuum from UV Vis range Small radiating arc region point source Closely match the sun s spectrum Absorbance and fluorescence applications 34 Hg Arc Lamp 1 Continuum Line Source strong UV output with discrete lines 2 High Power Source 3 Often used in photoluminescence simultaneous activation of several fluorochromes 4 Expensive and a short lifetime 35 Sample Containers Quartz or fused silica for work below 350 nm Silicate glasses employed between 350 nm and 2000 nm Plastic containers used in the visible region KCl and NaCl used in IR Cell washer single cell and double cell Types of Instruments Single beam Double beam in space Double beam in time Multichannel spectrometer Single Beam Simple but requires a stabilized voltage supply Single Beam Visible Photometer Single Beam UV Vis Absorption Spectrophotometer Double Beam Space


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