the use of formulation of non-polar organic solvent solutions for ninhydrin [4] and 1,2-indanedione [5]; indeed, by using HFE-7100 solvent, it is possible to obtain minimum damage and
discoloration of the paper surface; the choice of solvent-free techniques, such as fuming methods
[6–8,2] or sandwich process [9]; the use of washing solutions to remove the active coat