421-821-chapter-6-7

# 421-821-chapter-6-7 - Absorption Spectroscopy Introduction...

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Absorption Spectroscopy Introduction A.) Absorption : electromagnetic (light) energy is transferred to atoms, ions, or molecules in the sample. Results in a transition to a higher energy state. - Transition can be change in electronic levels, vibrations, rotations, translation, etc. - Concentrate on Molecular Spectrum in UV/Vis (electronic transition) - Power (P) : energy of a beam that reaches a given area per second - Intensity (I) : power per unit solid angle - P and I related to amplitude 2 E o E 1 h ν Energy required of photon to give this transition: (excited state) (ground state)

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B.) Terms : 1.) Beer’s Law: A = ε bc The amount of light absorbed (A) by a sample is dependent on the path length (b), concentration of the sample (c) and a proportionality constant ( ε molar absorptivity ) Amount of light absorbed is dependent on frequency ( λ ) c Increasing Fe +2 concentration
B.) Terms : 1.) Beer’s Law: A = ε bc Transmittance (T) = P/P o %Transmittance = %T = 100T Absorbance (A) = log 10 P o /P No light absorbed- % transmittance is 100% absorbance is 0 All light absorbed- % transmittance is 0% absorbance is infinite

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Relationship Described in Terms of Beer’s Law A = Absorbance = ε bc = -log(%T/100) ε = molar absorptivity : constant for a compound at a given frequency ( λ ) units of L mol -1 cm -1 b = path length: cell distance in cm c = concentration: sample concentration in moles per liter. Therefore, by measuring absorbance or percent transmittance at a given frequency can get information related to the amount of sample (c) present with an identified ε and λ . Note: law does not hold at high concentrations, when A > 1
Example 4: A solution that was 3.78x10 -3 M in X had a transmittance of 0.212 when measured in a 2.00-cm cell. What concentration of X would be required for the transmittance to be increased by a factor of 3 when a 2.00-cm cell was used?

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C.) Components of an Instrument for UV/Vis Absorbance Measurements : 1.) Basic Design: Hitachi Instruments U-3010 Light Source, λ selector, Sample cell holder, Detector (amplifier, recorder)
a) Desired Properties of Components of UV/Vis: Light Source λ Selector Creates Proper λ Narrow Bandpass: Stable: Selects Desired λ Constant P Large Light Throughput: Good Precision Increase P Intense: Increase P Easier to See Absorbance Sample Cell Holder Detector Fixed Geometry: Stable Constant b Sensitive to λ of Interest Transmits λ of Interest: Increase P

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b) Light Sources UV/Vis (~ 200 – 800 nm): 1. Deuterium & Hydrogen Lamps (UV range) - continuous source, broad range of frequencies - based on electric excitation of H 2 or D 2 at Low pressure 40V Electric Arc Electrode Filament D or H Gas 2 2 Sealed Quartz Tube In presence of arc, some of the electrical energy is absorbed by D 2 (or H 2 ) which results in the disassociation of the gas and release of light D 2 + E elect D * 2 D + D ’’ + h ν (light produced) Excited state
- h ν will vary continuously from ~ 160nm up to 375nm (UV range) due

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## This note was uploaded on 05/09/2010 for the course CHEM 221 taught by Professor Dr.robertpowers during the Fall '07 term at San Diego.

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421-821-chapter-6-7 - Absorption Spectroscopy Introduction...

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