Because of measurement

limitations, it is usually best

to keep absorbances

between 0 and 1 to avoid

errors.

Absorbance Spectrum

The absorbance spectrum of

a solution is a measure of

how much light that solution

absorbs at each wavelength.

Figure 8.6 shows absorbance

spectra for an orange and

red forms of orange

carotenoid protein.

This absorbance spectrum is

taken by measuring the

absorbance of the solution at different wavelengths while keeping the path length

and concentration constant. If we look at the dotted line (red form) in the figure,

we see that the solution has zero absorbance between about 680 and 700 nm,

which means that all of the light at those wavelengths pass through the sample.

We will typically be interested in measuring the wavelength of maximum

absorbance, which is the wavelength where the absorbance has its largest value.

This is the location where molar absorptivity, ε, of the compound is the largest.

The wavelength of maximum absorbance for the dotted line in Figure 8.6 is about

525 nm.

If we change the concentration of our sample, the shape of the curve does not

change because the shape of the curve is determined by the identity of the

compound through its molar absorptivity, ε. When we change the concentration

all the points on the absorbance spectrum curve move up or down by the same

factor. If the solution was diluted so the concentration is half of the original, then

all the absorbance values would just be half as big. Using Figure 8.6 as an

example, if the concentration was half of the original, the wavelength of

maximum absorbance would still be 525 nm, but the absorbance would now be

about 0.2.

65

Figure 8.6: "File:Orange Carotenoid Protein spectra of orange vs

red form.svg" by Ryan Leverenz and Cheryl Kerfeld is licensed

under CC BY-SA 4.0.