# The reason for selection of wavelengths in the spectrophotometry of Quinoline Yellow SS

The absorption spectrum for Quinoline Yellow SS has peaks at 224 nm and 414 nm, and a trough at 337 nm.

This experiment (see page 31), selects 224, 337 and 414 nm to be used in the spectrophotometric analysis.

As far as I have read, the wavelengths of maximum absorption (224 nm and 414 nm), give the best accuracy and least percentage error. However, why is 337 nm chosen?

• Troughs and such are often used to improve selectivity, if you use absorbance peak-trough difference. Similar techniques are used in satellite images in false colours when some of the basic colours is defined as difference of signals for 2 wavelengths. Nov 30, 2021 at 6:15
• If there is a peak-trough pair, it helps to deal with the spectrum baseline. If there is peak-trough-peak, it helps even with a slope. Nov 30, 2021 at 6:45
• Not the main issue usually, but at an absorption maximum the slope in the spectrum is zero, just like at a minimum. Thus, if you are a little inaccurate with the wavelength scale, error propagation is not so bad in those two cases compared to measuring at a wavelength with a large slope in the spectrum. Nov 30, 2021 at 16:24
• @Poutnik I don't quite understand what you mean by improving selectivity through the absorbance peak-trough difference and the spectrum baseline. Could you please explain or point me to a resource that explains it? Dec 1, 2021 at 1:04
• Higher peak absorbance is not caused by the dye if it is not followed by higher peak-trough absorbance difference. IF e.g a peak has absorbance A_0 + A_p and trough A_0 + A_t, that peak-trough difference is (A_0 + A_p) - (A_0 + A_t) = A_p - A_t. This ways can be compensated the sample matrix absorbance baseline. 2 peaks with trough could be used to estimate and compensate the slope of such baseline. Dec 1, 2021 at 11:40