Detection of N,N′-diphenyl-N,N′-dimethylurea (methyl centralite) in gunshot residues using MS-MS method

An MS-MS method has been developed to detect N,N′-diphenyl-N,N′-dimethylurea (methyl centralite, MC) in gunshot residues. Residues were recovered with cotton wool swabs soaked with methanol. MC solution was squeezed from the swabs using a medical syringe. After dilution, the extracted sample was injected into a tandem mass spectrometer (MS-MS) instrument directly. The conditions of MS were optimized in multiple reaction monitoring (MRM) mode, using ions of m/z 241 as precursor ions and ions of m/z 134 as product ions. High selectivity was obtained and interference was eliminated for the application of the MRM method. The proposed method is quite sensitive and the detection limit is 60 pg of MC per injection. The operations are very simple and rapid and the whole analytical procedure needs no more than five minutes if the instrument has been tuned to optimum conditions. The reliability of the method was confirmed by a test in which persons who had fired a pistol and persons who had not were discriminated successfully. In the test, twenty actual samples were analyzed. Ten of them were extracted from the hands of ten persons who had fired a pistol ten minutes earlier and the other ten were from the hands of persons who had never fired a pistol. There was a marked difference between the results of the shooters and that of the non-shooters and they could be easily and exactly distinguished. The effects of time and washing were also investigated. The results indicated that even if eight hours had passed after one person discharged a pistol for one round and he had washed his hand three times with water during this time, MC on his hand could still be detected reliably.