Instruments for Optical Spectroscopy

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Instruments for Optical Spectroscopy
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II. Instruments for Optical Spectroscopy

• A. Overview
• B. Sources
• 1. Continuous sources
• 2. Line sources
• C. Wavelength Selectors
• 1. Optical materials
• 2. Filters
• 3. Dispersion by a prism
• 4. Dispersion by a grating
• 5. Monochromators
• 6. Spectral purity

• D. Photon Detectors
• 1. Response
• 2. Photovoltaic cell
• 3. Photoelectric transducer
• 4. Photodiode array

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Schematic of a Spectrophotometer
Source
Mono- chromator
Sample
Dectector
Data System
Classic design for molecular absorption
spectrophotometer
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Sources for Optical Spectroscopy

• Continuous Sources
• Continuous emission over a wide range of
  wavelengths
• Examples
• Blackbody radiator (incandescent lamp for
  visible)
• Atomic/Molecular discharge (H2/D2 lamp for UV)
• Line Sources
• Emit at discrete wavelengths
• Examples
• Hollow cathode lamp for AA
• Lasers

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Black Body Radiators

• Tungsten lamp
• visible
• 350-3000 nm
• Nernst glower
• hollow cylinder of refractory metal oxide (ZrO2)
• Infrared, gt 1 ?m
• Xenon arc lamp
• high energy output in UV
• excitation source in fluorescence spectroscopy
• atomic discharge that acts much like a BBR.

Wien displacement law ?maxT 2.90 x 103 (? in
?m, T in kelvins)
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Atomic/Molecular Discharge Lamps
low-pressure gas
M(g) ? M ? M h?
High-voltage source
A commercial deuterium lamp
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Atomic/Molecular Discharge Lamps

• H2/D2 lamp
• For UV-vis spectrometers and HPLC detectors
• Useful range 175-450 nm
• Filled with H2 D2, or just D2
• D2 ? D2 ? D? D? h?
• Kinetic energy of D? and D? vary, so h? varies
  continuously
• Xe lamp
• High energy source of excitation in UV, for
  fluorescence instruments
• Spectral output depends on pressure
• High pressures continuous output overlaid with
  intense lines
• Low pressures continuous output less prominent,
  more like a line source

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Wavelength Selection

• Optics
• Filters
• Monochromators

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Optics

• Occurrence
• Lamp enclosure
• Sample cell
• Lenses
• Filters
• Mirrors

• Transparent materials for spectral regions
• UV (190-350 nm)
• fused silica
• Visible (350-1000 nm)
• borosilicate glass
• fused silica (expensive)
• plastic
• IR
• KBr for solids
• NaCl or AgCl for liquids

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Materials
Wavelength Selectors
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Transmission Curves for Optical Glasses
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Wavelength Selection

• Wavelength Dispersion Devices
• Prism
• Grating
• Filters
• Absorption filters
• Interference filters
• Monochromators
• Prism or grating, entrance and exit slits,
  mirrors and lenses
• Polychromator a monochromator without an exit
  slit

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Prisms

• Dispersion based on Snells law and change in n
  with ?
• Seldom used in modern instrumentation

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Prism MonochromatorBunsen Mount
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Transmission Grating

• Diffraction by parallel grooves on a transparent
  plate
• Not widely used in modern instrumentation
• Higher order wavelengths

m? d sin?
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Reflection Gratings

• Widely used in modern instrumentation
• inexpensive replica gratings
• compact design
• Diffraction processes fundamentally same as
  transmission grating
• multiple orders
• diffraction equation more complex
• n? d(sin i sin r)

?

• An echellette grating with blaze angle ?
• Maximum reflection at r ?
• Remove higher order ? with filter

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Czerny-Turner Grating Monochromator
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Czerny-Turner Monochromator
Concave mirrors
Focal length F
Exit slit
Entrance slit
?2
?1
Grating N grooves illuminated
Source ?1 ?2
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Linear and Nonlinear Dispersion

• Grating
• Linear dispersion
• Prism
• Nonlinear dispersion

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Dispersion and Resolving Power of a Grating
Monochromator
R mN
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Dispersion Equation for a Grating Monochromator
Angular dispersion dr/d?
dr/d? increases as d decreases
Next, we will show that the linear dispersion D
F dr/d?
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Dispersion Equation 2

• The linear dispersion increases with
• increasing F
• decreasing groove spacing d

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Dispersion Equation 3

• To maximize dispersion, we want
• small d
• large F

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Resolving Power
??d
P
?1
?2

• ??d (the diffraction-limited width) is the
  separation between two wavelengths that are just
  resolved (ca baseline resolution) when the slits
  are at the minimum useful setting (diffraction
  from slits occurs at smaller settings)
• ? (?1 ?2)/2
• m order or radiation
• N number of grooves illuminated
• Typical values are R 103 to 104 for UV-visible
  spectrophotometers

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Effective Bandwidth of a Monochromator

• Bandwith
• Span of monochromator settings to move the image
  of the entrance slit across the exit slit
• Effective Bandwidth
• Range of wavelengths transmitted by the exit slit
  at a given wavelength setting
• In diagram, width at ½ of the maximum peak
  intensity

Let w slit width
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Effective Bandwidth and Slit Width
Solution We need to resolve two lines with ??
0.6 nm. From diagram, we see that ??eff ??/2
yields baseline resolution.

• Problem
• Let D-1 1.2 nm/mm for some monochromator. What
  slit width w is needed to resolve the two sodium
  D lines at 589.0 and 589.6 nm?

??/2
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Slit Width and Resolution
??eff wD-1
??eff ??
??eff 0.75 ??
??eff 0.5 ??
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Effect of Bandwidth on Benzene Vapor Spectra
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Filters

• Types
• Absorption
• Interference
• Application
• Wavelength selection for inexpensive photometers
• Removal of higher order wavelengths in grating
  monochromators

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Output of a Typical Wavelength Selector
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Absorption (Transmission) Filters

• Colored glasses
• Silicate glasses with metal oxides
• Large bandpass
• Types
• Transmission
• Cutoff
• Neutral density filters
• to reduce radiative power at all wavelengths

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Secondary Calibration Standards for UV-Visible
Absorption Spectrophotometry
http//www.hellma-worldwide.de/calib.htm
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Interference Filters

• Construction Two layers of transparent glass
  separated by a dielectric
• Select a narrow portion of spectrum based on
  thickness of the dielectric
• Typical effective bandwith 10 nm.

Bandwidths shown in diagram are not consistent
with abscissa.
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Transmission by an Interference Filter
For small ?, condition for constructive
interference is n? 2d
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Absorption vs Interference Filters
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Photon Detectors

• Types
• UV-visible
• Photovoltaic cell (photocell)
• Photomultiplier tube (phototube)
• Photodiodes and phototransistors
• IR
• Thermocouple
• Bolometer

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Relative Response of Some Photon Detectors
Photomultiplier tube (A)
GaAs photovoltaic cell (C)
Silicon photodiode (F)
Thermocouple (H)
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Photovoltaic Cell

• Radiant energy generates current at interface of
  a semiconductor layer (Se) and a metal (Ag or Au)
• Response characteristics
• 350-750 nm
• fast response, lt 1 ?s
• current ? incident power
• 10 to 100 ?A
• moderate sensitivity
• Rugged, inexpensive detector for portable
  instruments

Photocurrent generated at Ag/Se interface
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Phototube or Photomultiplier
Cathode has photoemissive surface (easily
oxidized metal on inert support)
Photoelectric effect!
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Photomultiplier Tube

• Phototube with about 9 stages (dynodes)
• Highly sensitive for UV-vis
• Gain 105-107
• Fast response time, lt 1 ?s

Gain ? nN n of electrons emitted per
incident photon or electron N number of stages
(cathode dynodes)
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Silicon Photodiodes

• Reversed bias pn junction on a silicon chip
• With reverse bias, depletion layer forms, no
  current flows
• Photon causes formation of holes and electrons,
  resulting in current flow
• Response
• 190-1100 nm
• more sensitive than phototube (but less than PMT)
• Component in diode array detector

pn silicon diode
pn silicon diode with reverse bias voltage applied
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e-
-

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Photodiode Arrays
Grating

• i ? P
• monitor i at each position with microprocessor
• Detector for a polychromator
• no exit slit
• Advantages
• fast recording of spectra (? 100 ms)
• monitor all ? simultaneously
• HPLC detector

white light
Silicon chip with multiple photodiodes (e.g.,
1024)
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Optical Diagram for a UV-Visible Diode Array
Spectrophotometer
Agilent Model 8453
http//www.chem.agilent.com/scripts/PDS.asp?lPage
298

• Deuterium and tungsten lamps for UV and visible
• Shutter provides correction for stray light
• Polychromator design with photodiode array
  detector
• All ? are passed through sample and detected
  simultaneously
• No exit slit

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Specifications for Agilent 8453 UV-Visible
Diode Array Spectrophotometer

http//www.chem.agilent.com/scripts/generic.asp?lP
age299indcolNprodcolY
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Primary Calibration Standards for UV-Visible
Spectrophotometry
National Institute of Standards
Technology Certificate of Analysis Standard
Reference Material 935a Crystalline Potassium
Dichromate for Use as an Ultraviolet Absorbance
Standard This Standard Reference Material (SRM)
is intended for use as a reference standard for
the verification of the accuracy and linearity of
the absorbance scale at the 235 nm, 257 nm, 313
nm, 345 nm, and 350 nm wavelengths of absorption
spectrometers that can provide an effective
bandpass of 1.6 nm or less. Such verification is
accomplished by comparing the measured apparent
absorbances, Aa (meas.), to the calculated
absorbances Aa (calc.) from the certified
apparent specific absorbance 1 (?a ) values as
described under the Instructions for Use. SRM
935a consists of 15 g of crystalline potassium
dichromate of established purity. Solutions of
known concentrations of this SRM in 0.001 N
perchloric acid are certified for ?a at 23.5 C.
http//patapsco.nist.gov/srmcatalog/common/view_de
tail.cfm?srm935a