Subject: How does the acid-ninhydrin method for proline quantification work?

Why are plant extracts prepared in 3% sulphosalicylic acid (and not in some 
kind of buffer)? What happens when we add a mixture of acid-ninhydrin and 
acetic acid (what is the chemical function of these substances)? Why to boil 
the mixture for 1h? What does it mean "the reaction mixture was extracted with 
toluene"? Why is proline the only amino acid present in the toluene phase?

Re: How does the acid-ninhydrin method for proline quantification work?