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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?

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    $\begingroup$ 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. $\endgroup$
    – Poutnik
    Commented Nov 30, 2021 at 6:15
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    $\begingroup$ 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. $\endgroup$
    – Poutnik
    Commented Nov 30, 2021 at 6:45
  • $\begingroup$ 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. $\endgroup$
    – Snijderfrey
    Commented Nov 30, 2021 at 16:24
  • $\begingroup$ @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? $\endgroup$
    – StrugglingChemistryStudent
    Commented Dec 1, 2021 at 1:04
  • $\begingroup$ 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. $\endgroup$
    – Poutnik
    Commented Dec 1, 2021 at 11:40

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The experiment in your link pg 31 is "Path Length Dependence of Absorbance Values". As you stated, quinoline absorption spectrum has a dip near 337 nm. The reason for choosing the wavelength is not something fundamental. You already know that most sensitive analysis is carried out at the peak maximum rather than a wavelength corresponding to a valley. All Agilent wanted to show here is that Beer's law is valid for all wavelengths in the absorption spectrum, as long as A > 0.

If you are carrying out this experiment, check how the slopes vary with different path length and wavelengths. Are all the slopes the same?

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  • $\begingroup$ I graphed Absorbance vs Path length for the three wavelengths (224, 337 and 414nm). The slopes are similar for 224 and 414nm (the peaks), the slope is close to 0 for the trough, which doesn't seem useful. So just to clarify, the choice of the trough is simply to demonstrate that the linear relationship holds for all wavelengths? $\endgroup$
    – StrugglingChemistryStudent
    Commented Dec 1, 2021 at 1:10
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    $\begingroup$ @StrugglingChemistryStudent, Yes! You got it right. In analytical chemistry, sensitivity is defined as the slope. Higher the slope, the more sensitive the analysis is. $\endgroup$
    – ACR
    Commented Dec 1, 2021 at 1:11

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