94. Solving a chemistry problem

During the fluorimetric determination of riboflavin (vitamin B2), an aliquot of the analyzed solution with a volume of Va was placed in a volumetric flask with a capacity of Vk, diluted with water to the mark, and the fluorescence intensity Ix of the resulting solution was measured on an EF-3MA fluorometer with primary and secondary light filters that transmit light in the wavelength range from 400 to 480 nm and 510 to 650 nm, respectively. Under the same conditions, we measured the fluorescence intensity Ist of standard solutions with different precisely known concentrations of cstriboflavin:
Cst, μg/ml 0.04 0.08 0.16 0.24 0.32 0.40
Ist* 10 19 40 61 79 100
* Fluorescence intensity is given in arbitrary units - according to the readings of the measuring device.
From the data below, use the calibration curve method1 to calculate the concentration (µg/mL) of riboflavin in the test solution.
What is the principle by which the primary and secondary light filters are chosen for fluorimetric measurements if an aqueous solution of riboflavin has a maximum in the absorption spectrum at a wavelength of 450 nm, and in the fluorescence spectrum at 535 nm?
Va, ml Vk, ml Ix*
5.00 50 70
* Fluorescence intensity is given in arbitrary units - according to the readings of the measuring device.