Chapter 18. Fundamentals of Spectrophotometry. Properties of Light

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1 Chapter 18 Fundamentals of Spectrophotometry Properties of Light Electromagnetic Radiation energy radiated in the form of a WAVE caused by an electric field interacting with a magnetic field result of the acceleration of a charged particle does not require a material medium and can travel through a vacuum 1

2 v i = ν λ i Properties of Electromagnetic Radiation where v i => velocity ν => frequency λ i => wavelength Properties of Electromagnetic Radiation in vacuum, velocity is independent of frequency, maximum value c = νλ = m/s 2

3 Properties of Electromagnetic Radiation Fig. 19-1, pg. 511 Plane-polarized electromagnetic radiation of wavelength λ, propagating along the x axis. The electric field of the plane-polarized light is confined to a single plane. Ordinary, unpolarized light has electric field components in all planes." Regions of Electromagnetic Spectrum 3

4 Line Spectrum A spectrum produced by a luminous gas or vapor and appearing as distinct lines characteristic of the various elements constituting the gas. 4

5 Emission Spectrum The spectrum of bright lines, bands, or continuous radiation characteristic of and determined by a specific emitting substance subjected to a specific kind of excitation. Ground State The state of least possible energy in a physical system, as of elementary particles. Also called ground level. 5

6 Excited State Being at an energy level higher than the ground state. 6

7 Absorption Spectrum Light shinning on a sample causes electrons to be excited from the ground state to an excited state wavelengths of that energy are removed from transmitted spectra Absorption and Emission of Light Energy Absorption Emission 7

8 Absorption Methods, Transmittance T = P/P o where T => transmittance P => power of transmitted radiation P o => power of incident radiation %T = (P/P o )*100 where %T => percent transmittance Absorption Methods, Absorbance A = - log 10 T = - log 10 (P/P o ) where A => absorbance 8

9 Relation Between Transmittance and Absorbance P/P o %T A Colors of Visible Light Wavelength Absorbed Observed violet green-yellow violet-blue yellow blue orange blue-green red green purple yellow-green violet yellow violet-blue orange blue red blue-green purple green 9

10 Components of Optical Instruments Rotating mirror Source Wavelength Selector Sample Detector P o mirror Reference mirror P Semitransparent mirror Signal Processor Absorption Spectrometer Emission Flame Photometer Flame Atomic Absorption Spectrometer Fluorescence and/or Scattering Spectrometer Absorption Methods, Beer s Law A = abc = εbc where a => absorptivity b => path length c => concentration ε => molar absorptivity 10

11 A = abc = εbc Beer s Law A c Attenuation of Light 11

12 Beer s Law A = abc Path Length Dependence, b Absorbance 0.82 Source Detector 12

13 Beer s Law A = abc Path Length Dependence, b Absorbance 0.62 b Source Detector Sample Beer s Law A = abc Path Length Dependence, b Absorbance 0.42 Source Detector Samples 13

14 Beer s Law A = abc Path Length Dependence, b Absorbance 0.22 Source Detector Samples Beer s Law A = abc Concentration Dependence, c Absorbance 0.82 Source Detector 14

15 Beer s Law A = abc Concentration Dependence, c Absorbance 0.62 b Source Detector Sample Beer s Law A = abc Concentration Dependence, c Absorbance 0.42 b Source Detector Sample 15

16 Beer s Law A = abc Wavelength Dependence, a Absorbance 0.82 Source Detector Beer s Law A = abc Wavelength Dependence, a Absorbance 0.30 b Source Detector 16

17 Beer s Law A = abc Wavelength Dependence, a Absorbance 0.80 b Source Detector 17

18 Simple Spectrometer Relationship of Transmittance and Absorpbance transmittance scale is linear absorbance scale is exponential thus, read transmittance, then calculate absorbance 18

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