The outline of the general approach for the detection of DNA sequences via ESI-cleavable TNT-PNA conjugates is presented in Figure 1 . The ESI-cleavable TNT-PNA conjugates consist of a PNA probe portion, which interacts with the immobilized target sequence (DNA or RNA) and a peptide tandem nucleic acid mass tag portion, which is ultimately detected. Note that the TNT shown is merely a representation of the tag and not a real structure ( Figure 2 ). An ESI-cleavable linker connects the PNA probe portion of the conjugate to the tandem nucleic acid mass tag peptide. The complete TNT 'Parent Tag' comprises the red 'Tag Fragment' portion and the blue 'Mass Normalizer' portion shown in Figure 1 . The TNT marker is designed to have a unique combination of parent tag mass and tag fragment mass, released during collision induced dissociation (CID), and it is this pair of masses that serves as the sequence identifier. In a typical scenario (Figure 1 ), a set of PNA hybridization probes labelled with different TNTs is first hybridized to the captured target nucleic acids of interest (step (1)). After stringent washes to remove the non-hybridized probes (step (2)), the probes are denatured from the target (step (3)) and injected into an ESI-MS/MS instrument for detection (step (4)). In the mass spectrometer, the TNTs cleave from the PNAs during electrospray ionization (step (5)). The TNT parent tag ions are then selected from the background, fragmented by CID and finally, daughter, tag fragment ions from the fragmentation are detected (step (6)), confirming that the signals are indeed due to the presence of tagged probes, thereby detecting the presence of the target sequences.