Ultraviolet/Visible Spectroscopy (UV-Vis)

Learn how UV-Vis spectroscopy works by measuring electronic transitions in molecules. Understand that compounds absorb energy equal to the gap between energy levels, with visible-region absorbers appearing colored and UV-only absorbers remaining colorless.

Examine the two light sources in the UV-Vis spectrometer: a tungsten lamp for visible light and a deuterium lamp for ultraviolet light. Understand how the mirror directs light into the monochromator containing a diffraction grating.

Learn how the diffraction grating splits white light into constituent wavelengths and how the instrument scans through the spectrum. Understand the modulator's rotor that chops light into sample and reference beams for double-beam detection.

Place pure solvent in a sample cuvette as a blank reference. Select appropriate cuvette materials: glass or plastic for visible region only, or quartz cuvettes for UV work since glass and plastics absorb UV light.

Place the dye solution in a second cuvette and insert both blank and sample into the sample holders. Close the lid to prevent lab light interference and enter wavelength range and scanning speed parameters into the computer.

Zero the instrument at a wavelength where the sample does not absorb and start the scan. The spectrum appears on screen in real-time, is automatically saved to the computer, and can be printed.

Analyze the spectrum with wavelength on the horizontal axis and absorbance on the vertical axis. Identify peaks as wavelengths absorbed by the sample and troughs as wavelengths that pass through, interpreting the green dye's absorption of orange and blue light.