[ajax0604]

I am looking at a question which asks why a wavelength of 370nm is used instead of 250nm to analyze a fluorescent brightener in a laundry detergent (it absorbs strongly at both these wavelengths). Is it because the solvent (distilled water) absorbs at around 200nm and could interfere? Is there another reason?

[Big-Daddy]

Let's see the full question.

[DrCMS]

What part of the brightener absorbs at ~370nm and what part absorbs at ~250nm.  Which part is more unique to the OBA?

[ajax0604]

Here is the full question:
Fluorescent brighteners are added to washing powders and it is these agents that cause white clothes to appear blue under ultraviolet light. One particular brightener absorbs ultraviolet light strongly at 250 and 370nm. The absorbances for a series of standard solutions of the brightener were measured using a wavelength of 370nm. Suggest one reason why this wavelength might be selected in the analysis rather than a wavelength of 250nm.

[Corribus]

Maybe approach it generally. What are typical reasons you would select one wavelength over another?

[ajax0604]

Then is it sufficient to say that 370nm is more strongly absorbed than 250nm?

[DrCMS]

Then is it sufficient to say that 370nm is more strongly absorbed than 250nm?

No.

[ajax0604]

I don't understand so could you explain it?

[Babcock_Hall]

Based on the information you have provided so far, there is no reason to believe that the molar absorptivities at the two wavelengths are different.  I suspect the reason lies elsewhere.