Title:

Femtoamp and Picoamp Modes of Electrospray and Paper Spray Ionization​

Introduction: Ionization current is an important characteristic which yields insight about the performance and mechanism of the ionization processes. For the electrospray-based methods such as ESI, the measurement of ionization current has been typically reported for the nA to μA ranges. Since the detectors of mass spectrometers are extremely sensitive and work with ion currents in the pA range, and there is a continuing trend in improving ion transmission efficiency, it is necessary to investigate ionization behavior in the sub-nano ampere range. Decreasing the ion current produced by electrospray improves the ionization efficiency of the process. The impacts of this would be improving the overall efficiency for mass spectrometry, which is one of the most inefficient analytical instruments in use today. The other significance of evaluating lower current electrospray is the indication that decreasing ion current decreases the size of produced droplets. Decreasing the size of droplets provides a more uniform ionization for analytes, especially in mixtures where analytes frequently compete for charge and harder-to-ionize analytes may be suppressed. Methods: With the utilization of a faraday cage to decrease the level of background noise and customized ESI emitters, a region of current values before typical ESI onset currents have been observed and measured. Outside of our labs home-built faraday cage, the lowest current measurable is approximately 100pA. Within the faraday cage this limit of detection is lowered to 0.010 pA. This low current phenomenon has been investigated with a collection of several different sizes of electrospray emitters (100nm-10µm) including paperspray ESI. Several analytes and solvent systems have been evaluated to test the efficacy of this method and gain insight into the mechanisms at which ions may be generated with ultra-low currents. Preliminary Data: ESI emitters of 93 nm o.d. with long tapers (10mm) have been demonstrated to have an ESI onset of approximately 150V with currents on the order of 0.182pA. The nm sized emitters produce greater currents with increasing voltage and exceed 1 nA of current at an applied voltage of greater than 2500V. When conducting voltage vs. current measurements for 4.22 µm o.d. emitters, where the typical ESI onset is 1200 V with a current of 1.14 nA which can increase to 100s of nA at voltages as high as 4000V. A low current region below onset is also observed with a low current onset of 825V and 2-10? of pA. This technique is also applicable to paperspray emitters. For a dry paperspray emitter, a small current of 0.060 pA is produced when 500V is applied to the emitter. This current increases with increasing voltage to around 1.61pA at 3000V. Confirmation for the generation of analyte ions for these currents has been undertaken using mass spectrometry. Results show that the ion detection for the low current region is 2-3 orders of magnitude lower than for an emitter operating at typical ESI conditions. This agrees with the measured currents, where the low current region also produces currents 2-3 orders of magnitude lower.

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