Characterization of charge separation in the array of Micromachined UltraSonic Electrospray (AMUSE) ion source for mass spectrometry

Abstract

An initial investigation into the effects of charge separation in the Array of Micromachined UltraSonic Electrospray (AMUSE) ion source is reported to gain understanding of ionization mechanisms and to improve analyte ionization efficiency and operation stability. In RF-only mode, AMUSE ejects, on average, an equal number of slightly positive and slightly negative charged droplets due to random charge fluctuations, providing inefficient analyte ionization. Charge separation at the nozzle orifice is achieved by the application of an external electric field. By bringing the counter electrode close to the nozzle array, strong electric fields can be applied at relatively low DC potentials. It has been demonstrated, through a number of electrode/electrical potential configurations, that increasing charge separation leads to improvement in signal abundance, signal-to-noise ratio, and signal stability. (J Am Soc Mass

Droplet charging regimes for ultrasonic atomization of a liquid electrolyte in an external electric field

“…33 Characterization of the droplet formation and ejection physics, using stroboscopic visualization and scaling analysis has previously been conducted. 34 Finally, the AMUSE has recently been demonstrated as a "soft" ion source for bioanalytical mass spectrometry, 15,20,21 including investigations into charge separation 18 and droplet charging under static and dynamic electric fields. 17 We now consider droplet charging under static electric fields of varying magnitude for electrolytes of varying ionic strength.…”

“…Details of the experimental setup and equipment for the charge collection measurements are described in depth elsewhere. 17 As described in Forbes et al, 18 this specific configuration of the ultrasonic atomizer ͑AMUSE͒ is equipped for application of external electric fields. The local electric field for droplet charging is applied between the inner electrode of the piezoelectric transducer, V PZT , and an external counter electrode, V ext ͑Fig.…”

“…Another common technique, electrohydrodynamic atomization and tip streaming, relies on the application of a high electric field for fluid dispersion. [10][11][12][13][14] Still, there is a wide range of techniques for production of discretely charged droplets for applications including mass spectrometry, [15][16][17][18][19][20][21] drop-on-demand manufacturing of multilayer parts and circuits, 22,23 spray coatings, 24,25 biosensing and other lab-on-a-chip applications, [26][27][28][29] and wet scrubber systems for pollution control, 30 among others. The focus of this work is on mechanically driven atomization, which is coupled to an external electric field for droplet charging.…”

“…33,34 Again, an electric field in the region of droplet pinch-off is used to control the transport of charge into a forming droplet. 17,18 In these applications, precise control over droplet charging is imperative. For example, in mass spectrometry the charge-per-droplet level, as produced by the ion source, determines the level of available charge carriers for ionization of the analyte molecule.…”

“…For example, in mass spectrometry the charge-per-droplet level, as produced by the ion source, determines the level of available charge carriers for ionization of the analyte molecule. 16,18,20 This, in turn, determines the ion source's ionization efficiency. Forbes et al 17 recently demonstrated the ability to control droplet charging as a function of static ͑dc͒ and dynamic ͑ac͒ charging electric fields for an ultrasonic droplet-based ion source.…”