587 08 wk08b mol spec uv_doas

587 08 wk08b mol spec uv_doas - Molecular Spectroscopy, UV,...

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1 Molecular Spectroscopy, UV, DOAS Chemistry 587, Weeks 8/9 Chapter 13: UV/Vis Molecular absorption spectroscopy Beer’s law: A = ε bc • application of Beer’s law to mixtures • limitations of Beer’s law Chapter 14: Applications of UV/Vis Mol. Abs. Spectrometry Molar absorptivities Magnitudes transition types vapor vs. solution and spectral characteristics effect of solvent derivative spectral deconvolution photoacoustic spectroscopy long path spectroscopy** Differential optical absorption spectroscopy (DOAS) If your solution was a mixture of two of these dyes, what would their visible spectrum look like? A = bc =- logI/I 0
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2 Addition of Bands • Overlapping bands/peaks add – For all linear optical spectroscopies • Infrared and Raman and UV/Vis – Beware: sometimes Raman spectra give spectral artifacts – Similarly, one can take the final spectrum and fit a series of peaks to the overall shape - Allowing quantification of different components – Problems can arise if one tries to use reference spectra from another solvent • DIFFERENT SOLVENTS AFFECT THE SPECTRAL PEAK WIDTHS AND SHAPE – Should therefore always use the same solvent as the unknown matrix for reference spectra Another example of bands adding - due to different environments - Infrared spectrum of the O-H stretching region of liquid water is fit to 3 component peaks. Liquids rarely give narrow peaks since molecules interact with adjacent molecules in the solution, causing broadening of the lines. This broadening is similar to a pressure effect, yet more complex. (The resolution of the instrument is ~ 4 cm -1 .) Note that if you add the 3 component peaks, the final spectrum fit (black line thru the green data points) is obtained.
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This note was uploaded on 07/17/2008 for the course CHEM 587 taught by Professor Allen during the Spring '08 term at Ohio State.

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587 08 wk08b mol spec uv_doas - Molecular Spectroscopy, UV,...

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