A process according to claim 1 wherein the marking material is a dye selected from the group consisting of dansyl-L-alanine; Dansyl-L-??-amino-n-butyric acid; ??-dansyl-L-arginine; dansyl-L-asparagine; dansyl-L-aspartic acid; dansyl-L-cysteic acid; N,N'-di-dansyl-L-cystine; dansyl-L-glutamic acid; dansyl-L-glutamine; N-dansyl-trans-4-hydroxy-L-proline; dansyl-L-isoleucine; dansyl-L-leucine; di-dan