Interaction of radiation matter Electromagnetic radiation in different

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

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

UV Visible Spectrometry äabsorption - specific energy äemission - excited molecule emits äfluorescence äphosphorescence

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

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

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)

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

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

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

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

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

Resolution ä The ability to distinguish different wavelengths of light - R=l/Dl 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

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

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

Photodetectors - photoelectric effect E(e)=hn - w 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)

Spectral sensitivity is a function of photocathode material ä ä ä Ag-O-Cs mixture gives broader range but less efficiency Na 2 KSb(trace of Cs)has better response over narrow range Max. response is 10% of one per photon (quantum efficiency) Na 2 KSb Ag. OCs 300 nm 500 700 900

Photomultiplier - dynodes of Cu. O. Be. O. Cs or Ga. P. Cs

Cooled Photomultiplier Tube

Dynode array

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

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

Deviations from Beer’s Law ä High concentrations (0. 01 M) distort each molecules electronic structure & spectra ä Chemical equilibrium ä Stray light ä Polychromatic light ä Interferences

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 Beer’s Law. html

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)

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)

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

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

Photoacoustic spectroscopy ä Edison’s observations ä If light is pulsed then as gas is excited it can expand (sound)

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

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

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

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

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

Double beam/ null detection

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

Detection of IR radiation ä Insufficient energy to excite electrons & hence photodetectors won’t 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

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

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)

NDIR detector - no monochromator SAMP REF Chopper Filter Detector cell CO 2 Beam trimmer Press. sens. det.

Limitations Mechanical coupling Slow scanning / detectors slow

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

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

Limitations ä ä ä Most equipment can measure one wavelength at a time Potentially timeconsuming A solution?

Fourier-Transform Infrared Spectroscopy (FTIR) A Solution!

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, . . .

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

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

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

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

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

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

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