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



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