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Introduction to Spectroscopy
Spectroscopy/Spectrophotometry✔✔The study of the interaction of electromagnetic radiation with a chemical species.
Interpretation of Spectroscopic Measurements✔✔Can provide a detailed picture of what atoms are present and how these atoms are connected in the molecule. A very easy technique to use for the analysis of exceedingly large numbers of samples, and the speed of modern instruments allows it to be used for in-line measurements as well.
Uses for Spectroscopy in Modern Industry✔✔Agricultural, clinical, environmental, pharmaceutical, and quality control.
Two Broad Areas of Spectroscopy✔✔Qualitative and Quantitative
Qualitative Spectroscopy✔✔Tries to determine what is in a sample by looking at overall line shapes, intensities, and the wavelengths at which they occur.
Quantitative Spectroscopy✔✔Tries to determine how much of a given component is present in a mixture.
Spectroscopic Measurements✔✔Presented in the form of a spectrum (spectra).
Spectrum✔✔A graph of the amount of radiation not absorbed (transmitted) or of the amount absorbed as the y-axis and some function of the light energy as the x- axis.
How to calculate transmittance✔✔I/I₀ (I = intensity of passage through the sample, I₀ = intensity of incident radiation)
How to calculate percent transmittance✔✔T% = 100 x (I/I₀)
How to measure absorbance✔✔A = log₁₀₀(1/T)
How to calculate frequency✔✔v = c/λ
How to calculate the energy of the photon✔✔E = hv
Wavenumber✔✔Directly proportional to the energy of the photons.
Common Types of Spectroscopy✔✔UV-vis (ultraviolet-visible), IR (infrared), NMR (nuclear magnetic resonance)
UV-vis Spectroscopy✔✔Used both quantitatively and qualitatively. Rarely used qualitatively by itself because the spectra for two extremely different molecules may be very similar to this. But if you know what molecules are in the sample, it can be used to determine how many of those molecules are present.
IR Spectroscopy✔✔Used mostly qualitatively to give some information about the structure of a molecule. Also used more rarely for quantitative purposes. Certain configurations confer specific functions (functional groups). Causes the molecule to vibrate, and specific functional groups tend to absorb the radiation at characteristic wavelengths. Using this data, information about parts of a molecule can be determined and thereby the molecule can be identified. Traditionally given as plots of the percent transmittance against wavenumber.
C-OH✔✔Gives a broad absorption band in the 3300 to 2500 cm⁻¹ region.
C=O✔✔Gives a strong band at about 1700 cm⁻¹
-OH✔✔Tend to absorb strongly in the 3500 to 3200 cm⁻¹ region.
NMR Spectroscopy✔✔Uses radio frequency light and is primarily used to deduce the structure of molecules (thus identifying the compound) and very rarely for the quantitative purposes. Especially useful in that it is essentially element specific - each element will have a normal range over which it will be resonant, and different frequencies within this range correspond to certain chemical environments. Also the basis for medical imaging. The technique depends on the interaction of the spin of the nuclei of the molecule with magnetic fields. The number of peaks in a group
Ultraviolet-visible absorption, emission, and fluorescence✔✔Bonding electrons transition
Infrared absorption and Ramon scattering✔✔Rotation/vibration of molecules transition
Microwave absorption✔✔Rotation of molecules transition
Electron spin resonance✔✔Spin of electrons in a magnetic field transition
Nuclear magnetic resonance✔✔Spin of nuclei in a magnetic field
Atomic Spectroscopy✔✔Appearance of spectra: Sharp peaks Quantum energy levels: well-defined peaks
Molecular Spectroscopy✔✔Appearance of spectra: unresolved group of lines Quantum energy levels: Broad bands
Assumptions of Beer's Law✔✔
- Electromagnetic radiation of monochromatic
- Each individual absorbing species acts independently
- If two or more species are present which absorb at a given wavelength, absorbances of the species are additive
Beer's Law Deviation: Real Limits✔✔What: At high analyte or electrolyte concentrations the species interact How: The electronics change Why: The interaction changes the molar absorptivity
Beer's Law Deviation: Chemical Deviation✔✔What: Result from chemical changes associated with concentration changes How: Positive or negative deviation Why: Analyte dissociates, associates, or reacts with solvent to produce a species with different absorption spectrum than analyte
Beer's Law Deviation: Instrumental Deviations✔✔What: Consequence of the manner in which absorbance measurements are made How: Polychromatic Radiation or Stray Radiation Why Polychromatic Radiation: More polychromatic Why Stray Radiation: the wavelength differs from the principal radiation
