Chapter 20: Instrumentation
Major Components
Depends on Type of Experiment
AbsorptionFluorescenceEmission
SourceWavelength SelectorSampleDetectorSignal Processor
Most Common
Placement depends on experiment
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.
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