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Interaction of radiation & matter ä Electromagnetic radiation in different regions of spectrum can be used for qualitative and quantitative information.

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Presentation on theme: "Interaction of radiation & matter ä Electromagnetic radiation in different regions of spectrum can be used for qualitative and quantitative information."— Presentation transcript:


2 Interaction of radiation & matter ä Electromagnetic radiation in different regions of spectrum can be used for qualitative and quantitative information ä Different types of chemical information

3 Energy transfer from photon to molecule or atom At room temperature most molecules are at lowest electronic & vibrational state IR radiation can excite vibrational levels that then lose energy quickly in collisions with surroundings

4 UV Visible Spectrometry ä absorption - specific energy ä emission - excited molecule emits ä fluorescence ä phosphorescence

5 What happens to molecule after excitation ä collisions deactivate vibrational levels (heat) ä emission of photon (fluorescence) ä intersystem crossover (phosphorescence)

6 General optical spectrometer ä Wavelength separation ä Photodetectors Light source - hot objects produce black body radiation

7 Black body radiation ä Tungsten lamp, Globar, Nernst glower ä Intensity and peak emission wavelength are a function of Temperature ä As T increases the total intensity increases and there is shift to higher energies (toward visible and UV)

8 UV sources ä Arc discharge lamps with electrical discharge maintained in appropriate gases ä Low pressure hydrogen and deuterium lamps ä Lasers - narrow spectral widths, very high intensity, spatial beam, time resolution, problem with range of wavelengths ä Discrete spectroscopic- metal vapor & hollow cathode lamps

9 Why separate wavelengths? ä Each compound absorbs different colors (energies) with different probabilities (absorbtivity) ä Selectivity ä Quantitative adherence to Beers Law A = abc ä Improves sensitivity

10 Why are UV-Vis bands broad? ä Electronic energy states give band with no vibrational structure ä Solvent interactions (microenvironments) averaged ä Low temperature gas phase molecules give structure if instrumental resolution is adequate

11 Wavelength Dispersion ä prisms (nonlinear, range depends on refractive index) ä gratings (linear, Braggs Law, depends on spacing of scratches, overlapping orders interfere) ä interference filters (inexpensive)

12 Monochromator ä Entrance slit - provides narrow optical image ä Collimator - makes light hit dispersive element at same angle ä Dispersing element - directional ä Focusing element - image on slit ä Exit slit - isolates desired color to exit

13 Resolution The ability to distinguish different wavelengths of light - R= The ability to distinguish different wavelengths of light - R= ä Linear dispersion - range of wavelengths spread over unit distance at exit slit ä Spectral bandwidth - range of wavelengths included in output of exit slit (FWHM) ä Resolution depends on how widely light is dispersed & how narrow a slice chosen

14 Filters - inexpensive alternative ä Adsorption type - glass with dyes to adsorb chosen colors ä Interference filters - multiple reflections between 2 parallel reflective surfaces - only certain wavelengths have positive interferences - temperature effects spacing between surfaces

15 Wavelength dependence in spectrometer ä Source ä Monochromator ä Detector ä Sample - We hope so!

16 Photodetectors - photoelectric effect E(e)=h ä For sensitive detector we need a small work function - alkali metals are best ä Phototube - electrons attracted to anode giving a current flow proportional to light intensity ä Photomultiplier - amplification to improve sensitivity (10 million )

17 Spectral sensitivity is a function of photocathode material ä Ag-O-Cs mixture gives broader range but less efficiency ä Na2KSb(trace of Cs)has better response over narrow range ä Max. response is 10% of one per photon (quantum efficiency) 300nm Na2KSb AgOCs

18 Photomultiplier - dynodes of CuO.BeO.Cs or GaP.Cs

19 Cooled Photomultiplier Tube

20 Dynode array

21 Photodiodes - semiconductor that conducts in one direction only when light is present ä Rugged and small ä Photodiode arrays - allows observation of a number of different locations (wavelengths) simultaneously ä Somewhat less sensitive than PMT


23 T=I/Io A= - log T = -log (I/Io) Calibration curve


25 Deviations from Beers Law ä High concentrations (0.01M) distort each molecules electronic structure & spectra ä Chemical equilibrium ä Stray light ä Polychromatic light ä Interferences

26 Interpretation - quantitative ä Broad adsorption bands - considerable overlap ä Specral dependence upon solvents ä Resolving mixtures as linear combinations - need to measure as many wavelengths as components ä Beers Law.html

27 Resolving mixtures ä Measure at different wavelengths and solve mathematically ä Use standard additions (measure A and then add known amounts of standard) ä Chemical methods to separate or shift spectrum ä Use time resolution (fluorescence and phosphorescence)

28 Improving resolution in mixtures ä Instrumental (resolution) ä Mathematical (derivatives) ä Use second parameter (fluorescence) ä Use third parameter (time for phosphorescence) ä Chemical separations (chromatography)

29 Fluorescence ä Emission at lower energy than absorption ä Greater selectivity but fluorescent yields vary for different molecules ä Detection at right angles to excitation ä S/N is improved so sensitivity is better ä Fluorescent tags

30 Spectrofluorometer Light source Monochromator to select excitation Sample compartment Monochromator to select fluorescence

31 Photoacoustic spectroscopy ä Edisons observations ä If light is pulsed then as gas is excited it can expand (sound)


33 Principles of IR ä Absorption of energy at various frequencies is detected by IR ä plots the amount of radiation transmitted through the sample as a function of frequency ä compounds have fingerprint region of identity

34 Infrared Spectrometry ä Is especially useful for qualitative analysis ä functional groups ä other structural features ä establishing purity ä monitoring rates ä measuring concentrations ä theoretical studies

35 How does it work? ä Continuous beam of radiation ä Frequencies display different absorbances ä Beam comes to focus at entrance slit ä molecule absorbs radiation of the energy to excite it to the vibrational state

36 How Does It Work? ä Monochromator disperses radiation into spectrum ä one frequency appears at exit slit ä radiation passed to detector ä detector converts energy to signal ä signal amplified and recorded

37 Instrumentation II ä Optical-null double-beam instruments ä Radiation is directed through both cells by mirrors ä sample beam and reference beam ä chopper ä diffraction grating

38 Double beam/ null detection

39 Instrumentation III ä Exit slit ä detector ä servo motor ä Resulting spectrum is a plot of the intensity of the transmitted radiation versus the wavelength

40 Detection of IR radiation ä Insufficient energy to excite electrons & hence photodetectors wont work ä Sense heat - not very sensitive and must be protected from sources of heat ä Thermocouple - dissimilar metals characterized by voltage across gap proportional to temperature

41 IR detectors ä Golay detector - gas expanded by heat causes flexible mirror to move - measure photocurrent of visible light source Detector IR beam Vis source GAS Flexible mirror

42 Carbon analyzer - simple IR ä Sample flushed of carbon dioxide (inorganic) ä Organic carbon oxidized by persulfate & UV ä Carbon dioxide measured in gas cell (water interferences)

43 Chopper SAMP REF Detector cell Filter CO2 CO2 Beam trimmer Press. sens. det. NDIR detector - no monochromator

44 Limitations Mechanical coupling Slow scanning / detectors slow

45 Limitations of Dispersive IR ä Mechanically complex ä Sensitivity limited ä Requires external calibration ä Tracking errors limit resolution (scanning fast broadens peak, decreases absorbance, shifts peak

46 Problems with IR ä c no quantitative ä H limited resolution ä D not reproducible ä A limited dynamic range ä I limited sensitivity ä E long analysis time ä B functional groups

47 Limitations ä Most equipment can measure one wavelength at a time ä Potentially time- consuming ä A solution?

48 Fourier-Transform Infrared Spectroscopy (FTIR) A Solution!

49 FTIR ä Analyze all wavelengths simultaneously ä signal decoded to generate complete spectrum ä can be done quickly ä better resolution ä more resolution ä However,...

50 FTIR ä A solution, yet an expensive one! ä FTIR uses sophisticated machinery more complex than generic GCIR

51 Fourier Transform IR ä Mechanically simple ä Fast, sensitive, accurate ä Internal calibration ä No tracking errors or stray light

52 IR Spectroscopy - qualitative Double beam required to correct for blank at each wavelength ä Scan time (sensitivity) Vs resolution ä Michelson interferometer & FTIR

53 Advantages of FTIR ä Multiplex--speed, sensitivity (Felgett) ä Throughput--greater energy, S/N (Jacquinot) ä Laser reference--accurate wavelength, reproducible (Connes) ä No stray light--quantitative accuracy ä No tracking errors--wavelength and photometric accuracy

54 New FTIR Applications ä Quality control--speed, accuracy ä Micro, trace analysis--nanogram levels, small samples ä Kinetic studies--milliseconds ä Internal reflection ä Telescopic

55 Attenuated Internal Reflection ä Surface analysis ä Limited by 75% energy loss

56 New FTIR Applications ä Quality control--speed, accuracy ä Micro, trace analysis--nanogram levels, small samples ä Kinetic studies--milliseconds ä Internal reflection ä Telescopic

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