Quantitative Aspects of Absorption - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Quantitative Aspects of Absorption - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! R O Y G B V Gamma Ray Spectroscopy X-Ray Absorption, Fluorescence UV-vis Absorption, Fluorescence Infrared Absorption Spectroscopy Microwave Absorption Spectroscopy NMR EPR Nuclear Transitions Inner Shell Electrons Outer Shell Electrons Molecular Vibrations Molecular Rotations Spin States docsity.com Quantitative Aspects of Absorption Beer-Lambert Law (or Beer’s Law) IoA = log ---- = ε b C I I T = ---- %T = T x 100 Io Io = measured source intensity I = measured intensity after absorption Intensity change does not change absorbance Absorbance Transmittance molar absorptivity concentration path length docsity.com Beer’s Law applies to all absorption processes Assumptions made in deriving Beer’s Law: 1) Only interaction between radiation (light) and the absorber (sample) is absorption. This breaks down if reflection and scattering are not compensated for. Also breaks down if the absorbed radiation is reemitted as fluorescence (not normally a problem) or if stray light in the instrument reaches detector docsity.com Assumptions made in deriving Beer’s Law: 2) Monochromatic radiation – in reality this condition is only approximated, instrument measures a narrow band of radiation ε varies with λ so the best place to measure A is at 1 where A is nearly constant with λ Measurements at 2 suffer from the variation in ε over the bandwidth docsity.com Assumptions made in deriving Beer’s Law: 3) Pathlength is the same over the volume being measured – this becomes a problem with round cells λ varies across the bandwidth so some wavelengths pass through more solution than others (b varies) The consequence is the same as for polychromatic radiation = curved response Round cell Light beam detector Different components of the incident beam are absorbent with different efficiencies docsity.com Types of Spectroscopy Absorption Atomic – AA - not covered Molecular 1) UV-vis – electronic 2) IR – vibrational 3) Microwave – rotational 4) NMR (radiowave, MHz) – nuclear spin 5) ESR/EPR (GHz) – electron spin docsity.com Types of Spectroscopy Emission Atomic – AE & AF - not covered Molecular 1) Fluorescence 2) Phosphorescence Luminescence 3) Chemiluminescence (UV-vis region) docsity.com Types of Spectroscopy ? Scattering Raman spectroscopy – infrared region Turbidimetry – UV-vis region Nephelometry – UV-vis region Index of Refraction Refractometry Optical Rotatory Dispersion docsity.com Block diagram of instrument for absorption Light Source Wavelength Selector Sample Holder Detector Signal Processing Readout Device The location of these can be reversed Range of λ’s Narrow Band of λ’s
Io Transmitted Intensity
I Here the wavelength of interest is selected first, then passed through the sample docsity.com Block diagram of instrument for absorption Light Source Wavelength Selector Sample Holder Detector Signal Processing Readout Device Range of λ’s Transmitted I at all λ’s Selected λ Band
I Here all wavelengths pass through the sample together, then the wavelength of interest is selected and detected docsity.com Emission measurements require: 1) means of exciting emission i.e., way of populating upper energy level which spontaneously emits 2) device for dispersing radiation into component wavelengths 3) a means of putting sample into the optical path, i.e., cell 4) Detector to convert the EM to an electrical signal 5) readout device or circuitry, i.e., meter, computer, recorder, integrator, etc. docsity.com Sources – important characteristics 1) Spectral distribution i.e., intensity vs. λ (continuum vs. line sources) 2) Intensity 3) Stability – short term fluctuations (noise), long term drift 4) Cost 5) Lifetime 6) Geometry – match to dispersion device docsity.com I) CONTINUUM SOURCES 1) Thermal radiation (incandescence) – heated solid emits radiation close to the theoretical “Black Body” radiation i.e., perfect emitter, perfect absorber Behavior of Black Body - Total power ~ T4 therefore need constant temperature for stability when using incandescent sources - Spectral distribution follows Planck’s radiation law docsity.com