Question

2) In your own words, explain why it is necessary to select only one wavelength to pass through the sample (i.e. why don’t we just shine white light into the sample and measure the amount of light that passes through?) (5 points)

Answer 1

In spectrophotometry we attempt to analyse the amount of light that passes through a sample and try to estimate the concentration of the sample based on this interaction. We also try to find unknown species in a sample by this method. The underlying priniciples of spectrophotometry are

1. The intensity of color is a measure of the amount of a material in solution.
2. That every substance absorbs or transmits certain wavelengths of radiant energy but not other wavelengths.

The light energy absorbed or transmitted must match exactly the energy required to cause an electronic transition from one quantum level to another. Only certain wavelength photons satisfy this energy condition. Thus, the absorption or transmission of specific wavelengths is characteristic for a substance, and a spectral analysis serves as a “fingerprint” of the compound.

If we use white light absoption will take place and the compound will be excited as white light contains the wavelength requied for the excitation but we will not get the information of the amount of light absorbed as the other wavelengths which are transmitted will overwhelm the measurement equipment and prevent any analysis.