Multiphoton ionization of nitrotoluenes by means of ultrashort laser pulses

The potential of ultrashort laser pulses in the laser mass spectrometry of photounstable molecules is demonstrated for the case of nitrotoluenes. Nitro compounds tend to quickly dissociate after photoexcitation producing only unspecific fragments under conventional nanosecond multiphoton ionization conditions. The mass spectra of two isomers of mononitrotoluene, two isomers of dinitrotoluene and trinitrotoluene were recorded following multiphoton ionization with 170 fs laser pulses either with a wavelength of 412 or 206 nm. Although even these laser mass spectra are characterized by intense fragmentation they exhibit a clear molecular ion or OH loss signals depending on the substitution positions. Although the two mononitrotoluenes can be distinguished by their mass spectra at both wavelengths the two isomers of dinitrotoluene investigated show characteristic features which allow their clear differentiation only at 412 nm. (Int J Mass Spectrom 206 (2001) 245-250) (C) 2001 Elsevier Science B.V.