21-08-2012, 05:15 PM
Spectroscopy
Spectroscopy(UV and IR).pptx (Size: 3.31 MB / Downloads: 55)
Introduction
The study of the interaction of electromagnetic radiation with matter as a function of wavelength (λ) or frequency (ν).
The collection of measurements signals of the matter as a function of electromagnetic radiation is called a spectrum.
In spectroscopy the emitted or absorbed radiation is measured by means of an instrument called a spectrometer.
Emission spectroscopy
When matter is energized (excited) by the application of thermal, electrical, nuclear or radiant energy, electromagnetic radiation is often emitted as the matter relaxes back to its original (ground) state. The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum and the science is appropriately called emission spectroscopy.
Absorption spectrpscopy
Another approach often used to study the interaction of electromagnetic radiation with matter is one where by a continuous range of radiation is allowed to fall on a substance; then the frequencies absorbed by the substance are examined. The resulting spectrum from the substance contains the original range of radiation with dark spaces that correspond to missing, or absorbed, frequencies. This type of spectrpscopy is called an absorption spectrpscopy.
UV-vis spectroscopy
All atoms absorb in the Ultraviolet (UV) region because these photons are energetic enough to excite outer electrons. If the frequency is high enough, photoionization takes place. The visible region of the spectrum comprises photon energies of 1.77 to 3.1 eV, and the near ultraviolet region, out to 200 nm, extends this energy range to 6.2 eV. Ultraviolet radiation having wavelengths less than 200 nm is difficult to handle, and is seldom used as a routine tool for structural analysis. Visible absorption spectroscopy is often combined with UV absorption spectroscopy in UV/Vis spectroscopy.
When sample molecules are exposed to light having an energy that matches a possible electronic transition within the molecule, some of the light energy will be absorbed as the electron is promoted to a higher energy orbital. An optical spectrometer records the wavelengths at which absorption occurs, together with the degree of absorption at each wavelength. The resulting spectrum is presented as a graph of absorbance (A) versus wavelength. Absorbance usually ranges from 0 (no absorption) to 2 (99% absorption), and is precisely defined in context with spectrometer operation. Because the absorbance of a sample will be proportional to the number of absorbing molecules in the spectrometer light beam (e.g. their molar concentration in the sample tube), it is necessary to correct the absorbance value for this and other operational factors if the spectra of different compounds are to be compared in a meaningful way. The corrected absorption value is called "molar absorptivity", and is particularly useful when comparing the spectra of different compounds and determining the relative strength of light absorbing functions (chromophores). Molar absorptivity (ε)