Uv-Visible Spectroscopy Flashcards
How is UV-Vis Spectroscopy different to Atomic Spectroscopy?
UV-Vis measure the electronic transitions undergone in a molecule, where as atomic spectroscopy only measures the electronic excitation in an element.
What compounds always absorb UV radiation and sometimes visible radiation?
Organic Molecules
How do we know what part of the molecule is affected by the electron transition?
We can assign the transition to a part of the molecule that would be expected to undergo this excitation. Such as lone pairs or pi-electrons. Valence electrons are also a good target. These points are called chromophore.
How do we report a UV-Vis spectroscopy?
We report it in a plot of energy, using wavelength (nm) versus absorbance.
Why do we see broad absorption bands in UV-Vis spec?
As molecules are constantly vibrating due to continuous movement between bonds, a broad band of absorption is reflected. It is essentially blurring due to vibration.
What are we measuring UV-Vis Spectroscopy?
We are measuring the difference in intensity of the light passing through the sample, versus the intensity of light passing through the reference.
What conflicting factors can decrease the intensity of light and how do we account for this?
The reflection of the cuvette.
The scattering of the light by particles in the solvent (non-dissolved).
The solution is to use a reference cell as a comparison.
What factors deviate an accurate result from beer-lamberts law?
When absorbance is great then 1.5
When concentration of analyte is greater then 1mole/L (creating shadowing)
Stray radiation or non-monochromatic light affecting the result
What practical considerations need to be considered with UV-vis spectroscopy?
We need to ensure that the analyte is homogenised to ensure that it is an accurate representation of the bulk
we need to ensure that the solution is clarified to ensure that there is minimal scattering of light due to particles.
Using a calibration curve of standards will help interpret results.
