ANALYTICAL CHEMISTRY CHEM 3811

CHAPTER 19

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciencesClayton state university

CHAPTER 19

SPECTROPHOTOMETRY

COMPONENTS OF THE SPECTROPHOTOMETERPoPLightsourcemonochromator( selector) samplereadout detectorbAbsorption (UV-Vis)

COMPONENTS OF THE SPECTROPHOTOMETERLightsource monochromator( selector) samplereadout detectorAbsorption (IR)

COMPONENTS OF THE SPECTROPHOTOMETERSource& sample monochromator( selector)readout detectorEmission - Sample is an integral portion of the source

- Used to produce the EM radiation that will be measured

COMPONENTS OF THE SPECTROPHOTOMETERSource selectorsample monochromator( selector)readout detectorFluorescence

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 SOURCEMercury 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 SOURCEAlso for IR Region

- NiChrome wire (750 nm to 20000 nm)

- ZrO2 (400 nm to 20000 nm)

LIGHT SOURCELaser

- 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 CELLFused 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 = sum of absorbances from all species

AT = 1b1c1 + 2b2c2 + 3b3c3 + .

For the same sample cellb1 = b2 = b3 = b

AT = b(1c1 + 2c2 + 3c3 + .)

LUMINESCENCE- Includes any EM emission

Emission Intensity (I)

I = kPoc

k is a proportionality constantPo is the incident radiant powerc is the concentration of emitting species

- Only holds for low concentrations

LUMINESCENCEFluorescence- 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

LUMINESCENCEPhotoluminescence (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

ipa = anodic peak current ipc = cathodic peak currentModulated AbsorbanceAm = -log(I/Io)SPECTROELECTROCHEMISTRY

Chart1

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0.333918

0.350978

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