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Spectrophotometry

Beer's Law: The Effect of Cell Path Length

What are the factors that affect the amount of light a sample absorbs?

In this exercise, the role of the cell path length is examined. The cell path length is the length of sample that the light passes through. The cell path length is represented by the symbol l and is typically measured in centimeters.

In the experiment presented below, you will perform a series of simulations in which all experimental parameters are the same except for the cell path length. Each simulation will be performed using a different cell path length to isolate the effect of the cell path length on the absorption of light.

Your goal is to answer the following questions:

  1. How does the cell path length affect the intensity of light reaching the detector and why is this behavior observed?
  2. How does the transmittance vary with the cell path length? Is this plot linear?
  3. How does the absorbance vary with the cell path length? Is this plot linear?

Experiment

For each simulation:

  1. Select a cell path length.
  2. For each cell path length, perform the simulation to determine the intensity of light that reaches the detector.
  3. Calculate the transmittance.
  4. Calculate the absorbance.
  5. For each simulation, plot the l,T and l,A points on the graphs.
  6. After all the points are plotted, carefully examine the plots to see how T and A vary with l. Is either plot linear?

Notes:

Cell Path Length:
1.00 cm           2.00 cm           3.00 cm
4.00 cm           5.00 cm


Number of Photons Detected:

Elapsed Time: sec

Intensity of Detected Light: photons/sec


       
Light
Source
Cell Detector
Plot Point:       l = cm     T = %     A =        


Absorbance Spectrum                     Effect of Concentration

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© 2000-2014 David N. Blauch