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

Abstract

266 nm 1+1 resonance-enhanced multiphoton ionisation has been used in conjunction with a liquid microjet time-of-flight mass (TOF) spectrometer to detect anisole, 4-methoxyphenol and hydroquinone in ethanol solution. We have demonstrated a detection limit of 1 ppm for the parent molecular ion of hydroquinone. A comparison of TOF mass spectra of samples introduced by way of the liquid beam with those entrained in a supersonic free-jet expansion shows that the solute molecules are ionised after evaporation from the surface of the liquid beam. Extensive fragmentation of each parent molecular ion is attributed to excess energy available to the molecules by way of additional photon absorption following the initial photoionisation event. Hydroquinone photoion fragmentation patterns are found to be independent of laser power in the range 2.4–7.1 GW/cm², suggesting that saturation of the multiphoton fragmentation channels has been achieved.