MR. Markus Ganzera thank you for you answer.

My question is a sample tacking different wavelength reading. which wave length gives high/low value. why?

Dear Sivagnamam,

It also depends on your solvent. Polarity, pH and other solvent properties can interact with your analyte and modify the way it absorbs light. Thus, you detemine high/low absorbance values in exactly the same solvent. And you should report your values with this added information - the description of your solvent.

Respected Ademario junion,

I am extracted rice leaf pigments by 80% acetone method and different wavelength   (645-0.510, 652-0.755, 663-1.347, 480-0.884, 510-0.103) . first three wavelength 645, 652 & 663 nm Abs. value gradually increased. but wave length abs. from 480nm higher than 510 nm. What is the reason for increasing Abs at 480 nm than 510 nm.

Dear Sivagnamam,

The reason why a substance has its highest absorbance in a particular wavelength depends solely on its molecular structure. However, as I said, this molecular structure can be affected by molecular neighborhood - I mean polarity interactions, hydrogen bonding etc. - and this maximum can be dislocated when one uses different solvents. But absorbance maxima are almost always local maxima.

Even though there is a slight continuous decrease in absorbance along the whole wavelength spectrum from lower energy to higher energy - from microwave to gama rays, I mean - the oscillations in parts of the whole spectrum are due to quantum effects and you can find peaks of absorbance in all different regions. And the wavelength position of this peaks are determined by the 'real' molecular structure - the structure that takes into account 'neighbor' effects, when the molecule is analyzed in liquid or solid state.

I hope this theoretical explanation may help you.

Dear Sivagnanam, please have a look here:

http://omlc.org/spectra/PhotochemCAD/html/123.html

http://omlc.org/spectra/PhotochemCAD/html/125.html

http://omlc.org/spectra/PhotochemCAD/html/041.html

There you see the spectral shape for your three different compounds. This shapes are characteristic for the compounds. Dependent on concentration, the whole shape goes up or down in absorbance measurement (Lambert-Beer law). If there is a mixture (like in your extract, supposedly), the shapes are added up linearly. The reason for the spectrum's shape, the position of the maxima and so on can only be understood by quantumchemical explanations (read about Franck-Condon factors). the molecule as a whole reacts in a specific way to impinging light, this behaviour can be modified by the molecules surrounding (solvent polarity, viscosity, and so on)

Dear Peter,

I am a teacher - and a professor, too - and to explain concepts to other people is exactly what I do for a living. I guess Friedrich is also a teacher. And I really admire my students - and other people in general - when they are outspoken about their doubts. That's when they are able to learn.

Dear Peter,

If one does not have the live interaction, the measurement of what piece of knowledge is missing is somewhat difficult, don't you think? And the resources available today to complete the missing pieces are so much better than in the past. So, I try to nudge someone into knowledge so that he or she may be able to search for themselves.

Sometimes I am successful with this approach. Sometimes I have to add more explanations. Sometimes the student is not outspoken enough to say that he didn't understand and the doubt stays. I try hard not to silence their questions, but I am not always so lucky. And sometimes my explanation may not be good enough, so that I have to live with this possibility and try to improve my own ability to explain.