Tandem nucleic acid mass tag-PNA oligonucleotide probe design and synthesis Two pairs of example TNT-PNA oligonucleotide probes are shown in part (i) of Figure 2a and b. The TNTs are peptides comprising two parts, the tag fragment portion, which carries a charge due to the presence of a tertiary amino-functionality and the mass normalization portion, which remains essentially uncharged. These two portions of the tag are linked by a cleavage enhancement group, a piperazine ring, which also carries the charge of the tag fragment on its tertiary amino-group. The two tag portions both comprise a mass modifier component, which are isotopes of alanine in this tag although a large number of different mass modifiers could be used. It can be seen from Figure 2 that the two tags shown employ the same mass modifier components but the order differs Beta Alanine -Aspartic Acid -Proline -13 C 3 , 15 N-Alanine -Piperazin-1-ylacetic acid -Alanine TNT2