FIU CHM 4130 - CHAPTER 7A_Xiao_Components_of_Optical_Instruments_2018 (1) (58 pages)

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CHAPTER 7A_Xiao_Components_of_Optical_Instruments_2018 (1)



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Topics You Know By Now From Physics From Chemistry EM radiation and its Photoelectric effect properties Electromagnetic Diffraction spectrum Refraction Beer s Law etc Reflection Quantized states in Coherent and atoms incoherent radiation lead to line spectra Polarization of radiation Quantized states in molecules Scattering of radiation lead to broad or continuum spectra Components of Optical Instruments Chapter 7 2 Components of Optical Instruments The generic spectrometer n Sources n Sample Holders n Wavelength Separators and Slits n Detectors n Signal processor and readout Components of Optical Instruments The generic spectrometer n Sources n Sample Holders n Wavelength Separators and Slits n Detectors n Signal processor and readout Absorption measurements Fluorescence Phosphorescence measurements Emission Chemiluminescence measurements 5 Generated beam with sufficient stable radiant power 6 Spectroscopic Sources Al Mg Continuum Source Line Source Continuum Line Source Ingle and Crouch Spectrochemical Analysis 7 Application Continuum sources absorption and fluorescence spectroscopy Line sources atomic absorption spectroscopy atomic and molecular fluorescence spectroscopy Raman spectroscopy Laser sources Raman spectroscopy molecular absorption spectroscopy emission spectroscopy as part of instruments for Fourier transform infrared spectroscopy 8 Optical Continuum Source Characteristics Ingle and Crouch Spectrochemical Analysis 9 Tungsten Filament Heated to 2870 K Useful Range 350 2500nm 10 Tungsten Halogen 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 11 Intensity Spectrum of the Tungsten Halogen Lamp Weak intensity in UV range Good intensity in visible range Very low noise Low drift 12 Deuterium D2 Lamps Discharge of excited deuterium gas Continuum of UV light 185 400 nm Ultraviolet source for UV Vis and HPLC 13 Arc Lamps Electrical discharge is sustained through a gas or metal vapor Continuous emission due to rotational vibrational energy levels and pressure broadening An mercury arc lamp An krypton long arc lamp An electric arc lamp 14 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 15 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 16 LASER Light Amplification by Stimulated Emission of Radiation Emits very intense monochromatic light at high power intensity Coherent nature of their outputs all waves in phase unique and parallel Narrow bandwidths High resolution spectroscopy Raman spectroscopy molecular absorption emission spectroscopy Fourier transform infrared spectroscopy Not useful for scanning wavelengths Laser Setup Mechanism of Laser action 1 Pumping 2 Spontaneous emission 3 Stimulated emission 4 Adsorption Pumping Spontaneous emission Spontaneous Incoherent radiation Differs in direction and phase Stimulated emission 1 2 Stimulated emission coherent radiation Same direction and phase 3 Stimulated Emission A photon incident on an excited state species causes emission of a second photon of the same frequency which travels in exactly the same direction and is precisely in phase with the first photo Excited level Ground level Absorption Population Inversion is Necessary for Light Amplification Population inversions are 2 obtained by pumping 3 2 3 Why four level laser systems are more efficient as compared to three level lasers Easy population inversion Completely empty to start with Advantages of Lasers High intensities Small dot Nearly Monochromatic narrow bandwidth Coherent constructive interference Types of Lasers Solid state lasers Ruby a three level device Nd YAG a four level laser source Gas lasers lines w specific s in UV vis IR Neutral atom He Ne Ion lasers Ar Kr Molecular lasers CO2 N2 eximers XeF Dye lasers Solution of organic compounds limited tunability in the visible Semiconductor diode lasers Light emitting diodes with various emitted limited tunability in the red and IR Sample Holders Cells Must Examples contain the sample without chemical interaction be more or less transparent to the wavelengths of light in use be readily cleaned for reuse be designed for the specific instrument of interest quartz is good from about 190 3000 nm glass is a less expensive alternative from about 300 900 nm Plastic containers for visible region NaCl and KBr are good for cell window in the IR region Cells can be constructed to transmit light absorbed at 180 degrees to the incident light allow emitted light to exit at 90 degrees from the incident light contain gases lower concentrations and have long path lengths 1 0 and 10 0 cm cells are most common Absorbance usually in a matched pair Fluorescence Phosphorescence Chemiluminescence Spectrophotometer cells Cells for flow through measurements Sub Micro fluorescence cells 10 160 microliter Disposable cuvettes for UV and VIS applications BRAND UV cuvettes 220 900 nm spectral range Wavelength Selectors Used to select the wavelength or wavelength range of light that either impinges on the sample fluorescence and phosphorescence is transmitted through the sample absorption and emission This selected wavelength then strikes the detector the ability to select the wavelength helps you to discriminated between phenomena caused by your analyte and that caused by interfering or non relevant species Are often combined with a set of SLITS discussed later Various types based on filters CHEAP COLORED GLASS based on prisms LIMITED APPLICATIONS based on gratings GREAT STUFF Wavelength Selectors 1 Filters Simple rugged no moving parts in general Relatively inexpensive Can select some broad range of wavelengths Most often used in field instruments simpler instruments instruments dedicated to monitoring a single wavelength range Types of filters Absorption filters absorb specific wavelength ranges of light cheaper more common suitable for visible region Interference filters depend on destructive interference of the impinging light to allow a limited range of wavelengths to pass through them more expensive available for ultraviolet visible and infrared region Absorption Filters Absorp on Filters


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