By looking at the absorbance

spectra of the pure dyes, we can see

the relationship between the

absorbance peak and the interfering

wavelength. Figure 9.2 shows the

absorbance spectra for FD&C Red

No. 40. We can see that the

absorbance at the peak of 505 nm is

about 1.04 while the absorbance at

425 nm is about 0.319. When

correcting for the interference

between two dyes, what we need is

the ratio of the absorbances at

those two wavelengths. The ratio of the absorbances is 0.319/1.04 = 0.31.

The red dye absorption value at 425 nm is 31% of what the absorption is at the

peak (505 nm). If we do a similar analysis, the yellow dye absorption at 505 nm

is about 3% of what the absorption is at the peak (425 nm).

This gives us these equations:

With some algebra, this allows us to determine the absorbances from each

individual dye at their peak wavelengths by measuring the absorbances at both

wavelengths. For the absorbance for yellow dye at the peak of 425 nm that

would give

For the absorbance for red dye at the peak of 505 nm that would give

These two values, A_Y5@425 and A_R40@505, are the absorbance values at those

wavelengths that come from the specific dye and would be used in determining

76

Figure 9.3: Absorbance spectra of Yellow Dye No. 5

highlighting important analysis locations. The raw data

is available as a supplementary resource.