1. ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 19 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 19 SPECTROPHOTOMETRY

3. COMPONENTS OF THE SPECTROPHOTOMETER PoPo P Light source monochromator (λ selector) samplereadout detector b Absorption (UV-Vis)

4. COMPONENTS OF THE SPECTROPHOTOMETER Light source monochromator (λ selector) sample readout detector Absorption (IR)

5. 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

6. COMPONENTS OF THE SPECTROPHOTOMETER Source λ selector sample monochromator (λ selector) readout detector Fluorescence

7. 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 (D 2 ) Arc Lamp - D 2 molecules are electrically dissociated - Produces radiation at wavelengths from 200 to 400 nm - UV region

8. 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

9. LIGHT SOURCE Also for IR Region - NiChrome wire (750 nm to 20000 nm) - ZrO 2 (400 nm to 20000 nm)

10. LIGHT SOURCE Laser - Produce specific spectral lines - Used when high intensity line source is required Can be used for UV Visible FTIR

11. 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

12. 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

13. MONOCHROMATOR

14. SAMPLE CELL Fused silica Cells (SiO 2 ) - Transmits visible and UV radiation Plastic and Glass Cells - Only good for visible wavelengths NaCl and KBr Crystals - IR wavelengths

15. 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)

16. PHOTODIODE ARRAY SPECTROPHOTOMETER - Records the entire spectrum (all wavelengths) at once - Makes use of a polychromator - The polychromator disperses light into component wavelengths

17. ANALYSIS OF A MIXTURE - Occurs when there is more than one absorbing species Absorbance at a given λ = sum of absorbances from all species A T = ε 1 b 1 c 1 + ε 2 b 2 c 2 + ε 3 b 3 c 3 + …. For the same sample cell b 1 = b 2 = b 3 = b A T = b(ε 1 c 1 + ε 2 c 2 + ε 3 c 3 + ….)

18. LUMINESCENCE - Includes any EM emission Emission Intensity (I) I = kP o c k is a proportionality constant P o is the incident radiant power c is the concentration of emitting species - Only holds for low concentrations

19. 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

20. 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

21. 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

22. i pa = anodic peak current i pc = cathodic peak current Modulated Absorbance A m = -log(I/I o ) SPECTROELECTROCHEMISTRY