Spectrophotometric measruments give us a little information about the comlpexity of the fractions that we obtain by solid phese extraction. To determine the identity and number of the compounds each fraction has , what should we do?
You haven't said if you know what group of compounds you might be looking at. You also have not said if this is Normal, Reverse or Ion exchange SPE.
A quick technique is to use TLC against known samples. This will show how many compounds in each fraction and may identify some. This approach can be further expanded by using 2D TLC.
If the compounds in the fractions are all unknowns then these will need to be separated further to enable identification.
Hope this helps.