Question

1. What is the parameter that makes the absorbance of a sample inside a UV-VIS cell to change as a function of wavelength? Why?

2. What are the benefits of using the wavelength of maximum intensity (max) for the analysis?

Answer 1

The Beer-Lambert law is the linear relationship between absorbance and concentration of an absorbing species. This is usually written as:

Absorbance A = ε • c • l
ε: Absorption coefficient
c: Concentration of sample
l: Optical path length

where A is the measured absorbance
1. E is a wavelength-dependent absorptivity coefficient.

For example, the table gives values for the molar absorptivity of a solution of ethanal in hexane. If you want units and assuming the length is in cm and the concentration is mol dm^-3, the units are mol^-1 dm³ cm^-1.

electron jump	wavelength of maximum absorption (nm)	molar absorptivity
lone pair to pi anti-bonding orbital	290	15
pi bonding to pi anti-bonding orbital	180	10000

So wavelength dependence of absorptivity coefficient depends on the molecule(its structure) and this changes the absorbance of a sample.

2. Wavelengths of maximum intensity must be used for the analysis because a large amount of intensity can be absorbed by the sample leaving out small traces of intensity which will harder for the detector to detect.
The formula of the absorbance in terms of transmitted intensity(I) and initial intensity(I_o).
A = log (I_o / I), hence it is just a ratio which determines the absorbance thereby allowing any intensities to be passed through sample.

I hope I have answered your question satisfactorily still if any doubt persists please let me know in the comment section and please rate the answer accordingly.