Comparison of the internal energy deposition of venturi-assisted electrospray ionization and a venturi-assisted array of micromachined UltraSonic electrosprays (AMUSE)

The internal energy deposition of a Venturi-assisted array of micromachined ultrasonic electrosprays (AMUSE), with and without the application of a DC charging potential, is compared with equivalent experiments for Venturi-assisted electrospray ionization (ESI) using the survival yield method on a series of para-substituted benzylpyridinium salts. Under conditions previously shown to provide maximum ion yields for standard compounds, the observed mean internal energies were nearly identical (1.93-2.01 eV). Operation of AMUSE without nitrogen flow to sustain the air amplifier focusing effect generated energetically colder ions with mean internal energies that were up to 39% lower than those for ESI. A balance between improved ion transfer, adequate desolvation, and favorable ion energetics was achieved by selection of optimum operational ranges for the parameters that most strongly influence the ion population: the air amplifier gas flow rate and API capillary temperature. Examination of the energy landscapes obtained for combinations of these parameters showed that a low internal energy region (<= 1.0 eV) was present at nitrogen flow rates between 2 and 4 L min(-1) and capillary temperatures up to 250 degrees C using ESI (9% of all parameter combinations tested). Using AMUSE, this region was present at nitrogen flow rates up to 2.5 L min(-1) and all capillary temperatures (13% of combinations tested). The signal-to-noise (S/N) ratio of the intact p-methylbenzylpyridinium ion obtained from a 5 mu M mixture of thermometer compounds using AMUSE at the extremes of the studied temperature range was at least fivefold higher than that of ESI, demonstrating the potential of AMUSE ionization as a soft method for the characterization of labile species by mass spectrometry. (J Am Soc Mass Spectrom 2008, 19, 1320-1329) (C) 2008 American Society for Mass Spectrometry.