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Continuum Sources - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Indicator Electrodes, Potentiometry, Electrochemistry, Electrode Kinetics, Electrogravimetry, Polarography, Square Wave Voltammetry, Wavelength Selection, Types of Spectroscopy, Detectors, Theory of Molecular Absorption, Absorption by Organic Molecules, Applying UV-vis Spectroscopy are major topics of this course. Main points from these slides are: Continuum Sources, Thermal Radiation, Spectral Distribution Curves of a Tungsten, Incandescent Wire, Tungsten-Halogen Lamps, Gas Discharge Lamps, Hyd

Typology: Slides

2012/2013

Uploaded on 08/30/2013

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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 ~ T4therefore need constant
temperature for stability when using
incandescent sources
- Spectral distribution follows Planck’s radiation
law
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I)

1) CONTINUUM SOURCES

Thermal radiation

(^) (incandescence) –

perfect emitter, perfect absorbertheoretical “Black Body” radiation i.e.,heated solid emits radiation close to the

  • Total power ~ TBehavior of Black Body

(^4) therefore need constant

incandescent sourcestemperature for stability when using

  • Spectral distribution follows Planck’s radiation

law

Spectral Distribution Curves of a Tungsten (Black Body) Lamp

visible region requires high temp.Low temp good for IR, butshifts to shorter wavelengths. At higher temp -> maximum

UV

vis

IR

temperature.All operated at relatively low

emission.Good for IR and give some visible

goes too highOperated in air so will burn up if temp

  • Advantages

operates in air, long lifetimeNernst Glower – low power consumption,

changes with usecooled. Long lifetime, but resistanceGlower, requires more power & must beGlobar – more stable than Nernst

Visible Region

(^) sources are:

a)

operated at ~3000Glass enclosed Tungsten (W) filament - normally

(^) o K with inert atmosphere to

emission weaknm, below 350 nm glass envelope absorbs &prevent oxidation. Useful from 350 nm to 2000

b)

high as 3500Tungsten-Halogen lamps - can be operated as

(^) o K. More intense (high flux).

Fingerprints are a problem – also car headlightsalso transmits down to shorter wavelengths).quartz envelope to withstand high temps (whichkeeps filament from burning out. Requireshalide which redeposits W on filament, i.e.,Function of halogen is to form volatile tungsten-

a)

measurementsabsorptionsource for UV- most commonHydrogen Lamp

H (^2) emission is from

Line spectrum fromlimited by jacket180 nm to 370 nm

(^) 

PyrexLamp at low pressure in 100 watt Hydrogen

b)

Lamp – sameDeuterium

(^) λ

H distribution as (^2) but with

(3 to 5 times) -higher intensity

D (^2) is a heavier

High pressure Dby collisionsless loss of energyslower so there ismolecule & moves

(^2) 

with quartz jacket

d) High Pressure Mercury Lamp – can’t at very high pressures (e.g., 100 atm)with particular electronic transitions evencompletely eliminate bands associated

  • For UV-vis absorption spectrophotometry usually use H

2 for UV and tungsten for

visible region (switching mid scan)

  • Sometimes use D

2 instead of H

2

  • For fluorescence spectrophotometry use xenon arc lamp in scanning instruments
  • Hg at low pressure is used in fixed• Can use He below 200 nm wavelength (non scanning) fluorometers
  • Can use mixture of Hg and Xe

Lasers (Light Amplification by

Stimulated Emission of Radiation)

states typically using another light sourcestimulated emission and populate upperstart with material that will exhibit

Pumping source used to populate upper states can be flashlamp or another laser

Advantages of lasersOften use prism to select pumping wavelength

  1. Coherent – all radiation at same phase2) Monochromatic – very narrow band1) Intense

angle

  1. Directional – full intensity emitted as beam

Wavelength Selection

Three main approaches:

Block off unwanted radiation –

  1. optical filters

Disperse radiation & select

  1. desired band – monochromator

Modulate wavelengths at different

frequencies - interferometer

FILTERS

1)

Absorption

(^) – colored glass, colored

film, colored solutions – cheapest way