Analytical Performance of a Venturi-Assisted Array of Micromachined Ultrasonic Electrosprays Coupled to Ion Trap Mass Spectrometry for the Analysis of Peptides and Proteins

Abstract

The analytical characterization of a novel ion source for mass spectrometry named Array of Micromachined UltraSonic Electrosprays (AMUSE) is presented here. This is a fundamentally different type of ion generation device, consisting of three major components: 1) a piezoelectric transducer that creates ultrasonic waves at one of the resonant frequencies of the sample-filled device, 2) an array of pyramidally-shaped nozzles micromachined on a silicon wafer, and 3) a spacer which prevents contact between the array and transducer ensuring the transfer of acoustic energy to the sample. A high pressure gradient generated at the apices of the nozzle pyramids forces the periodic ejection of multiple droplet streams from the device. With this device, the processes of droplet formation and droplet charging are separated, hence, the limitations of conventional electrospraytype ion sources, including the need for high charging potentials and the addition of organic solvent to decrease surface tension can be avoided. In this work, a Venturi device is coupled with AMUSE in order to increase desolvation, droplet focusing, and signal stability. Results show that ionization of model peptides and small tuning molecules is possible with DC charging potentials of 100 V DC or less. Ionization in RF-only mode (without DC biasing) was also possible. It was observed that, when combined with AMUSE, the Venturi device provides a 10-fold gain in signal-to-noise ratio for 90% aqueous sample solutions. Further reduction in the diameter of the orifices of the micromachined arrays, led to an additional signal gain of at least 3 orders of magnitude, a 2-to 10-fold gain in the signal-to-noise ratio, and an improvement in signal stability from 47% to 8.5% RSD. The effectiveness of this device for the soft ionization of model proteins in aqueous media, such as cytochrome C was also examined, yielding spectra with an average charge state of 8.8 when analyzed with a 100 V DC charging potential. Ionization of model proteins was also possible in RF-only mode.

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

“…Fabrication of the 20 ϫ 20 array of 5-m nozzles in silicon [31] and assembly of the AMUSE ion source [17] have been described in detail elsewhere. Briefly, a gold-coated piezoelectric transducer (American Piezo Ceramics Inc., Mackeyville, PA, USA) was separated from the silicon ejector array by a series of silicone spacers (McMaster-Carr, Atlanta, GA, USA) that were used to define the volume of the analyte reservoir.…”

“…Comparison of the charge-state distributions of cytochrome C obtained using Venturi-assisted AMUSE ionization, conventional ESI, and nanoelectrospray ionization showed a slight shift toward lower charge states for AMUSE spectra [17]. Despite the analytical advantages observed for AMUSE coupled to an air amplifier, its intrinsic internal energy deposition, including the contributions from the air amplifier, were unknown to date.…”

“…There are several intrinsic advantages of this fundamentally different ionization method. First, unlike conventional ESI-which requires the use of high DC potentials (1-5 kV) to both form and charge droplets-the AMUSE provides sensitive ionization of biomolecules with only several hundred volts [17]. The inclusion of organic solvents to reduce the surface tension of the sprayed solution in ESI analysis is beneficial for facilitating Taylor cone formation but may perturb the native conformation of the solutes [18].…”

“…To mitigate this difficulty, several methods have been proposed including air amplifiers (also known as Venturi devices) [21,22], flared inlet capillaries [23,24], flared inlet capillaries coupled to air amplifiers [25], and ion funnels [26,27]. In previous work, we incorporated an air amplifier into the AMUSE setup to improve ion yields by increasing the linear velocity of the droplet streams ejected by the device [17]. Air amplifiers are devices that rely on the Venturi and Coanda effects to focus a charged droplet cloud toward the mass spectrometer inlet [28].…”

“…Air amplifiers are devices that rely on the Venturi and Coanda effects to focus a charged droplet cloud toward the mass spectrometer inlet [28]. Voltage-and heatassisted operation of these air amplifiers has been reported and it has been suggested that these devices provide enhanced droplet desolvation [17,29]. Previously, we reported that the coupling of an air amplifier to the AMUSE resulted in a tenfold gain in signal-tonoise (S/N) ratio of 10 M reserpine accompanied by a 77% reduction in relative standard deviation (%RSD) of the signal.…”