ANALYTICAL CHEMISTRY
CHEM 3811
CHAPTER 19
DR. AUGUSTINE OFORI AGYEMAN
Assistant professor of chemistry
Department of natural sciences
Clayton state university
CHAPTER 19
SPECTROPHOTOMETRY
COMPONENTS OF THE SPECTROPHOTOMETER
Absorption (UV-Vis)
b
Light
source
monochromator
(λ selector)
Po
sample
P
detector
readout
COMPONENTS OF THE SPECTROPHOTOMETER
Absorption (IR)
Light
source
sample
monochromator
(λ selector)
detector
readout
COMPONENTS OF THE SPECTROPHOTOMETER
Emission
Source
& sample
monochromator
detector
readout
(λ selector)
- Sample is an integral portion of the source
- Used to produce the EM radiation that will be measured
COMPONENTS OF THE SPECTROPHOTOMETER
Fluorescence
Source
λ selector
sample
monochromator
(λ selector)
detector
readout
LIGHT SOURCE
- A steady power supply is essential to provide constant signal
Tungsten Filament Lamp
- Glows at a temperature near 3000 K
- Produces radiation at wavelengths from 320 to 2500 nm
- Visible and near IR regions
Dueterium (D2) Arc Lamp
- D2 molecules are electrically dissociated
- Produces radiation at wavelengths from 200 to 400 nm
- UV region
LIGHT SOURCE
Mercury and Xenon Arc Lamps
- Electric discharge lamps
- Produce radiation at wavelengths from 200 to 800 nm
- UV and Visible regions
Silicon Carbide (SiC) Rod
- Also called globar
- Electrically heated to about 1500 K
- Produces radiation at wavelengths from 1200 to 40000 nm
- IR region
LIGHT SOURCE
Also for IR Region
- NiChrome wire (750 nm to 20000 nm)
- ZrO2 (400 nm to 20000 nm)
LIGHT SOURCE
Laser
- Produce specific spectral lines
- Used when high intensity line source is required
Can be used for
UV
Visible
FTIR
MONOCHROMATOR
- Optical device
- Disperses a beam of light into its component wavelengths
- Allows only a narrow band of wavelengths to pass
- Blocks all other wavelengths
Components
- Two slits (entrance and exit)
- Concave Mirrors
- Grating
MONOCHROMATOR
- Mirror collimates light (parallel rays)
- Gating disperses light
- Prisms were formerly used to disperse light
- Light coming through entrance slit is polychromatic
- Light out of exit slit is monochromatic
Dispersive Spectrophotometers
- Spectrophotometers that use monochromators
to select wavelengths
MONOCHROMATOR
SAMPLE CELL
Fused silica Cells (SiO2)
- Transmits visible and UV radiation
Plastic and Glass Cells
- Only good for visible wavelengths
NaCl and KBr Crystals
- IR wavelengths
DETECTOR
- Produces an electric signal proportional to the radiation intensity
- Signal is amplified and made available for direct display
- A sensitivity control amplifies the signal
- May be controlled manually or by a microprocessor
(the use of dynodes)
Examples
Phototube (UV)
Photomultiplier tube (UV-Vis)
Thermocouple (IR)
Thermister (IR)
PHOTODIODE ARRAY SPECTROPHOTOMETER
- Records the entire spectrum (all wavelengths) at once
- Makes use of a polychromator
- The polychromator disperses light into component wavelengths
ANALYSIS OF A MIXTURE
- Occurs when there is more than one absorbing species
Absorbance at a given λ = sum of absorbances from all species
AT = ε1b1c1 + ε2b2c2 + ε3b3c3 + ….
For the same sample cell
b1 = b2 = b3 = b
AT = b(ε1c1 + ε2c2 + ε3c3 + ….)
LUMINESCENCE
- Includes any EM emission
Emission Intensity (I)
I = kPoc
k is a proportionality constant
Po is the incident radiant power
c is the concentration of emitting species
- Only holds for low concentrations
LUMINESCENCE
Fluorescence
- Emission of light from a molecule in an excited state
- Initial absorbance followed by emission
Phosphorescence
- Similar to fluorescence
- There is a delay (from seconds to hours) before emission
LUMINESCENCE
Photoluminescence (PL)
- EM absorption and re-radiation
- Period between absorption and emission is usually very short
Chemiluminescence (CL)
- Emission of light as a result of a chemical reaction
Electrochemiluminescence (ECL)
- Emission produced during electrochemical reactions
- Luminescence as a result of electrochemically generated species
SPECTROELECTROCHEMISTRY
- Spectral and electrochemical signals are simultaneously obtained
- Change in absorbance is concurrent with the electrochemistry
Generally
- Positive scan (oxidation) - absorbance decreases
- Negative scan (reduction) - absorbance increases
SPECTROELECTROCHEMISTRY
-2.5
1000
Io
-1.5
800
ipc
-0.5
Intensity
Current (Milliamps)
Epc
0.5
600
I
400
ipa
1.5
200
Epa
2.5
0
0.8
0.6
0.4
0.2
Volts vs Ag/AgCl
0
-0.2
ipa = anodic peak current
ipc = cathodic peak current
-0.4
0
100
200
300
400
500
Time (Seconds)
Modulated Absorbance
Am = -log(I/Io)
600