ELECTROMAGNETIC SPECTRUM Wavenumber (cm -1 ) Frequency (Hz) Wavelength (m) LEARNING CHECK What experiment proved light behaves as a wave? What experiment proved light behaves as a par6cle? List 5 examples of phenomena in which light interacts with ma;er as a wave. 1
YOUNG EXPERIMENT (1800)- PROVED LIGHT IS A WAVE PHOTOELECTRIC EFFECT- LIGHT IS A PARTICLE 1. Light incident on the photocathode 2. Electrons liberated 3. Voltage at anode adjusted to stop current 4. Stopping voltage depends on cathode substrate and radiazon energy 2
LEARNING CHECK Suppose that the screen in the figure below is 3.51 m from the plan containing the slits and that the slits are 0.200 mm apart. What is the wavelength of the third band from the center, located at 13 mm from the central band? 3
LEARNING CHECK List 5 examples of phenomena in which light interacts with ma;er as a wave. LEARNING CHECK 4
INTERACTIONS BETWEEN LIGHT AND MATTER LIGHT AS A WAVE DiffracZon RefracZon Transmission ReflecZon Sca`ering PolarizaZon LIGHT AS A PARTICLE Photoelectric effect AbsorpZon Emission Sca`ering 5
ABSORPTION ABSORPTION Compare atomic and molecular absorpzon? Why does atomic absorpzon occur at discrete wavelengths while molecular absorpzon occur in broad peaks? 6
EMISSION 7
LUMINESCENCE JABLONSKI DIAGRAM 8
JABLONSKI DIAGRAM TRANSITIONS Electronic excita6on- promo6on of an electron to an excited state (electronic, vibra6onal, rota6onal). S 0 à S 1 Nonradia6ve decay (vibra6onal relaxa6on)- vibra6onal energy transferred to other molecules through collisions. Very fast. Excited state à S 1 ground vibra6onal state Fluorescence- emission of photon to return to S 0. S 1 às 0 +hν Internal conversion- radia6onless transi6on to an extremely vibra6onally excited state of S 0 without a change in energy. S 1 às 0 Intersystem crossing- radia6onless transi6on from S 1 to T 1 with no change in energy. Change of electron spin. S 1 àt 1 Phosphorescence- emission of photon to return to S 0. T 1 às 0 +hν SCATTERING 9
TIMESCALES OF THE FOLLOWING TRANSITIONS. BASED ON THESE OBSERVATIONS, CAN YOU PREDICT WHICH TRANSITIONS MIGHT BE MORE LIKELY TO OCCUR (AND THUS MORE COMMON)? Phenomena Transmission NonradiaZve decay Electronic transizons fluorescence phosphorescence Time scale 10-14 - 10-15 s 10-15 s 10-8 s 10-5 s 10-5 to 100 s s BEER S LAW CHEM 314 SKOOG N HOLLER CH 13 10
OBJECTIVES State and apply Beer s law Apply Beer s law to a variety of useful situazons IdenZfy condizons under which Beer s law is not linear Troubleshoot absorpzon experiments and suggest ways to regain linearity in Beer s law. Derive Beer s Law equazon A SIMPLE ABSORPTION EXPERIMENT Beer s Law T= transmission P 0 = incident power P= transmi`ed power A= absorbance ε= molar absorpzvity b= path length C= analyte concentrazon ConcentraZon relazve to mixing direczons 2.85 2.0 1.0 0.5 0.25 0.1 0.01 0.001 11
WHY DOES KOOL-AID APPEAR RED? What color does the Kool-aid absorb? Absorbance (arb.) 2.0 1.5 1.0 0.5 0.0 Red #40 495.2 nm 300 400 500 600 700 800 wavelength (nm) Energy E 2 e - e - 3 2 1 0 3 2 1 E 0 1 Jablonski Diagram h`p://img.photobucket.com/albums/v645/fadeout95/roygbiv.gif h`p://roygbivcolors7.files.wordpress.com/2013/01/complementry-color-wheel-copy2.jpg 12
BEER S LAW: RELATING ANALYTE CONCENTRATION AND ABSORBANCE Absorbance (arb) 1.0 0.8 0.6 0.4 0.2 A=1.9 [Kool-aid] r 2 =0.97 ε molar absorbzvity 0.0 0.0 0.1 0.2 0.3 0.4 0.5 [Kool-aid] y=mx + b A=ε b [Kool-aid] b= path length APPLICATION OF BEER S LAW TO MIXTURES Spectra of mixtures can be assumed to generate a linear combinazon of the individual component spectra 13
ML 2 M ML SOURCES OF NONLINEARITY OF BEER S LAW 1. SoluZon factors 2. Non-monochromaZc light 3. Not analyzing at l max 4. Stray light 5. Mismatched cuve`es 6. Instrument noise Too much or too li`le absorpzon Absorbance (arb) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 [Kool-aid] 14
1. SOLUTION FACTORS High analyte concentrazons (>0.01M) Red Cl High electrolyte concentrazons - 40 Na Red + Cl Red - Na 40 + RefracZve index of medium Na + 40 Cl- Cl - Na+ Cl - Na + Cl- Red Na + Na + Cl- Cl - Na + Cl- Na 40 Red + Cl- Na Red ReacZons within the soluzon + 40 Na Cl- + 40 HIn ó H + + In - Na + Na + Cl- Red Na + Cl - Cl - Na 40 + Cl- Incident Light P 0 Emergent Light P 1. SOLUTION FACTORS ReacZons within the soluzon HIn ó H + + In - AbsorpZon primarily by In - Absorbance (arb.) 2.0 1.5 1.0 0.5 0.0 300 400 500 600 700 800 wavelength (nm) AbsorpZon primarily by HIn 15
2. NON MONOCHROMATIC SOURCE Absorbance (arb.) 2.0 1.5 1.0 0.5 0.0 ε ε 300 400 500 600 700 800 wavelength (nm) λ and λ are different wavelengths 3. NOT MEASURING AT LAMDA MAX 16
EFFECT OF SLIT WIDTHS Mono slit width determines spread of λ incident on sample (bandwidth) Image incident on mono exit plane Wide slits allow More light (higher throughput) More λ (larger bandwidth) No such thing as a free lunch EFFECT OF SLIT WIDTHS What slit width should you choose? 17
EFFECT OF SLIT WIDTHS 4. STRAY LIGHT 18
4. STRAY LIGHT 5. MISMATCHED CUVETTES Differences in: Path length OpZcal characteriszcs Most likely to affect calibrazon curve intercept SoluZon: Double beam: Use matched cuve`es Single beam; Use the same cuve`e 19
LEARNING CHECK List and describe 4 types of instrumental error A SIMPLE ABSORPTION EXPERIMENT Components of an absorp6on experiment: 1. 0% T measurement (adjustment) 2. 100% T measurement 3. Sample T measurement Uncertainty associated with measurement is the aggregate of uncertainty in each step Uncertainty associated with error measurement depends on T 20
6. INSTRUMENTAL NOISE IN TRANSMISSION MEASUREMENTS 21
6. INSTRUMENTAL NOISE IN TRANSMISSION MEASUREMENTS 22
LINEAR RANGE OF BEER S LAW Absorbance (arb) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 [Kool-aid] 23
DERIVATION OF BEER S LAW T= transmission P 0 = incident power P= transmi`ed power A= absorbance ε= molar absorpzvity b= path length C= analyte concentrazon LOOKING AHEAD Thursday (Jan 28)- Beer s Law (Ch 13, look through deriva6on on website) Project Overview (Due Feb 15) First day of Consumer Characteriza6on Project Experiment 1: Metals Analysis Due: Prelab 1, Experiment 1 Goals for lab today: disassemble your project, use handheld XRF Monday (Feb 1)- Instrument components (Ch 7) Standard Addi6on Due Tuesday (Feb 2)- Experiment 1 Metals Thursday (Feb 4)- Experiment 1 Metals Prelab 2, Experiment 1 Due 24