Six aliquots of 20 ml of stock solution of the first TNT-PNA probe was mixed with 20, 10, 4, 2, 1 and 0.5 ml of the second TNT-PNA, respectively. To these, water was added to make up the solution to a total of 40 ml. The aliquots were then made up to 80 ml with methanol and formic acid to give a final solution of the TNT-PNA probes in 50:50 water:methanol with 1% formic acid. These samples were then analysed by direct injection ESI-MS/MS.