ANALYTICAL CHEMISTRY CHEM 3811

CHAPTER 19

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences

Clayton state university

CHAPTER 19

SPECTROPHOTOMETRY

COMPONENTS OF THE SPECTROPHOTOMETER

Po PLightsource

monochromator

(λ selector) sample readout detector

b

Absorption (UV-Vis)

COMPONENTS OF THE SPECTROPHOTOMETER

Lightsource

monochromator

(λ selector) sample readout detector

Absorption (IR)

COMPONENTS OF THE SPECTROPHOTOMETER

Source& sample

monochromator

(λ selector)

readout detector

Emission

- Sample is an integral portion of the source

- Used to produce the EM radiation that will be measured

COMPONENTS OF THE SPECTROPHOTOMETER

Sourceλ selector

sample

monochromator

(λ selector)

readout detector

Fluorescence

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 forUV

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)

ExamplesPhototube (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 cellb1 = b2 = b3 = b

AT = b(ε1c1 + ε2c2 + ε3c3 + ….)

LUMINESCENCE

- Includes any EM emission

Emission Intensity (I)

I = kPoc

k is a proportionality constantk is a proportionality constantPPoo is the incident radiant power is the incident radiant power

c is the concentration of emitting speciesc is the concentration of emitting species

- Only holds for low concentrations- 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

-2.5

-1.5

-0.5

0.5

1.5

2.5

-0.4-0.200.20.40.60.8Volts vs Ag/AgCl

Curre

nt (M

illia

mps

)

Epa

Epc

ipa

ipc

0

200

400

600

800

1000

0 100 200 300 400 500 600

Time (Seconds)In

ten

sity

Io

I

ipa = anodic peak current ipc = cathodic peak current

Modulated Absorbance

Am = -log(I/Io)

SPECTROELECTROCHEMISTRY