Notes on UV-Visible Spectroscopy and Beer-Lambert Law

Introduction

• Presented by Professor Davis from Chem Survival.
• Topic: Introduction to UV-Visible Spectroscopy and the Beer-Lambert Law.

UV-Visible Spectrophotometer Setup

• Typical components of a UV-Visible spectrophotometer:
  • Source Lamp: Can be a simple headlamp or complex lamps like deuterium or xenon arc lamps.
  • Monochromator: Contains two slits separated by a prism or diffraction grating to separate wavelengths of light.
  • Beam Splitter: Divides light into two equal beams.
  • Sample Compartment: Holds cells for reference and sample.
  • Detectors: Convert the impact of photons into electrical current monitored by a computer.

Operation of the Spectrophotometer

1. Ignite the source lamp, producing multiple wavelengths of light.
2. Light passes through the first slit of the monochromator to ensure parallel pathways.
3. Light is refracted into different colors by the prism.
4. Only one wavelength exits through the second slit, striking the beam splitter.
5. Two equal-intensity beams are created, one for the reference cell and one for the sample cell.
6. Detectors measure the intensity of light exiting both cells.
7. Example:
   • Zero Concentration: Both detectors show identical intensities (100% transmittance).
   • Adding Sample:
     • Reduces intensity in the sample cell (e.g., down to 50% transmittance).
     • Further additions of sample halve the intensity each time (e.g., 25%, then 12.5%).

Relationship Between Transmittance and Concentration

• Observation: The relationship is not linear but exponential.
• Preference for linear relationships for simpler discussion and predictions.

Beer-Lambert Law

• Converts percent transmittance to absorbance for a linear relationship.
• Conversion Method: Takes the negative logarithm of transmittance (logarithmic transformation).
• Result: Data plotted as absorbance is easier to interpret and predict.
• Utility: Facilitates extrapolation and interpolation within collected data sets.

Conclusion

• Importance of transforming transmittance to absorbance in UV-Visible spectroscopy.
• Professor Davis thanks the audience and invites them to check out his new organic chemistry course launching in October 2014.
• For more information, visit Chem Survival's website.

Note: Make sure to review the application of the Beer-Lambert Law in practical scenarios.