Time-of-flight mass spectrometric detection of mono- and di-substituted benzenes at parts per million concentrations by way of liquid microjet injection and laser ionisation

266 nm 1+1 resonance-enhanced multiphoton ionisation has been used in conju nction with a liquid microjet time-of-flight mass (TOF) spectrometer to det ect anisole, 4-methoxyphenol and hydroquinone in ethanol solution. We have demonstrated a detection limit of 1 ppm for the parent molecular ion of hyd roquinone. A comparison of TOF mass spectra of samples introduced by way of the liquid beam with those entrained in a supersonic free-jet expansion sh ows that the solute molecules are ionised after evaporation from the surfac e of the liquid beam. Extensive fragmentation of each parent molecular ion is attributed to excess energy available to the molecules by way of additio nal photon absorption following the initial photoionisation event. Hydroqui none photoion fragmentation patterns are found to be independent of laser p ower in the range 2.4-7.1 GW/cm(2), suggesting that saturation of the multi photon fragmentation channels has been achieved. (C) 2001 Elsevier Science B.V.