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Chapter 20: Instrumentation
Major Components
Depends on Type of Experiment
AbsorptionFluorescenceEmission
SourceWavelength SelectorSampleDetectorSignal Processor
Most Common
Placement depends on experiment
http://www.youtube.com/watch?v=O39avevqndU
Instrument Components
Light Source
Detectable Output over wavelength region of interestContinuum Source Line Source
From Skoog etal
From Skoog, West, Holler
Common:
Visible: Tungsten filament Lamp (320-2500 nm)UV- Deuterium Lamp (160-320 nm)IR: Heated Inert Solid (Globar, Nernst Glower)AA: Hollow cathode lamp (line source)
Instrument Components
2. Wavelength Selector
Restrict radiation to a narrow band
Two TypesFilter – removes “wide” bandsMonochromator
Monochromator
A. PrismB. Grating
•Lenses/mirrors to focus the radiation•Entrance and exit silts to restrict unwanted radiation•Dispersing Element to produce the “monochromatic” radiation
Grating monochromator
Instrument Components
Bandwidth – wavelength range passed by selectorHigh Quality: ~ 0.1 nm (monochromator)Filter 50 nm
From Skoog, West, Holler
Sample Container
Holds the sampleMust be transparent in the region of interest
UV: QuartzVis: GlassIR: Salt Crystals
1. In the equation, A = bc, what quantity is represented by "" ? a) Absorptivityb) Molar absorptivityc) Path length d) Concentration
2. Can one measure a smaller amount of a compound if it has a high or low absorptivity
a) higherb) lower
Instrument Components
Radiation Detectors
StableSensitive in region of interestRapid
Examples:PhototubePhotomultiplier tube (most sensitive)Photodiode array (multichannel)CCDHeat detectors (IR)
Phototube
From Skoog, West, Holler
PMT
Why is it better then a phototubefor measuring low light levels?
Types of Instruments
Single Beam (Spectronic 20)Double Beam (separated in time or space)Multichannel
Single Beam SpectrophotometerSimple, InexpensiveWavelength range: 340 – 625 nmBandwidth: 20 nmWavelength accuracy: 2.5 nm
Spec 20
From Skoog, West, Holler
Double Beam SpectrophotometerMore complexCompensates for drifts in detector/source
Wavelength range: 195-850 nmBandwidth: 4 nmPhotometric accuracy: 0.5%T
Photodiode Array 200-1000 nm Resolution 1-2 nm Time: 0.1 s
No scanning
From Skoog, West, Holler
Advantages/Disadvantages?
An 8.64 ppm solution of [FeSCN]Cl (149 g/mol) has a T of 0.295 at 580 nm when measured in a 1.00-cm cell. Calculate the molar absorptivity coefficient for the complexat this wavelength.
Cytochrome c has a molar absorptivity of 106,000 M-1cm-1. A solution is prepared by diluting a 10.0 uM solution by a hundred. What would be the absorbance of the diluted solution if measured in a 1.00 cm cell.