4-FMOC-piperazin-1-ylacetic acid was obtained from Fluka (Sigma Aldrich, Dorset, UK). The amino acid isotope 13 C 3 , 15 N-FMOC-L-alanine was obtained from Cambridge Isotope Laboratories, Inc (Andover, MA). The sequences of the TNT peptides used for the preparation of ESI-cleavable TNT-PNA probes are shown in Table 1 .