BIOL 1406
PreLab 7.4
How can a scanning spectrophotometer be used to analyze the
purified pigments that were separated using TLC?
 |
In Exercise 2, you learned that a spectrophotometer can be used to shine a
specific wavelength (color) of light through a solution and then measure how
much of the light is absorbed. This measurement is referred to as the absorbance
at that wavelength. In today’s lab you will use a special type of
spectrophotometer, called a scanning spectrophotometer, to automatically measure
the absorbance of a solution at many different wavelengths. The results are then
plotted to form a curve called an absorption spectrum, like the one for
beta-carotene shown to the left:
An absorption spectrum shows which wavelengths of light are most strongly absorbed by a solution (peaks on the curve) and which wavelengths are most weakly absorbed (low points on the curve). Wavelengths where peaks occur are called wavelength maxima or absorption maxima. |
| Your Turn | ||
| Based on the absorption spectrum shown above, identify the 2 strongest wavelength maximum for beta-carotene. Don't forget to include appropriate units in your answer! | ||
| Strongest wavelength maximum = | Hint | Check your answer. |
| Second strongest wavelength maximum = | Hint | Check your answer. |
Because each substance has a characteristic absorption spectrum, examining the
absorption spectrum of a solution can help determine what substances are present
in the solution and whether any contaminants are present. When examining an
absorption spectrum for purposes of identification, it is important to focus on
the location of the absorption peaks and valleys (i.e. at which wavelengths the
peaks and valleys occur) and NOT on the heights of the peaks. This is because a
given substance will always have peaks at the same locations (wavelengths), but
the heights of the peaks may vary depending on how concentrated the solution is.
Below you can see the absorption spectra for 2 different pigments, chlorophyll a
and chlorophyll b, plotted on the same axis. Note that each type of chlorophyll
has its own characteristic absorption spectrum:
| Your Turn | ||
| Identify the 3 strongest absorption maxima for chlorophyll a: | Hint | Check your answer. |
| Identify the 3 strongest absorption maxima for chlorophyll b: | Hint | Check your answer. |
| A student receives a green solution and is told it contains chlorophyll. The
student plots the absorption spectrum of the solution and finds 3 major peaks
located at 410 nm, 430 nm, and at 662 nm. Based on his analysis, the solution
most likely contains: A. pure chlorophyll a B. pure chlorophyll b C. a mixture of chlorophylls a and b
|
Hint | Check your answer. |
| Another student extracts some green pigment from a leaf and then plots the
absorption spectrum of the pigment. The spectrum shows major peaks at 410 nm,
430 nm, 463 nm, 642 nm, and 662 nm. Based on her analysis, the solution most
likely contains: A. pure chlorophyll a B. pure chlorophyll b C. a mixture of chlorophylls a and b
|
Hint | Check your answer. |
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