Analytical performance of a venturi-assisted array of micromachined ultrasonic electrosprays coupled to ion trap mass spectrometry for the analysis of peptides and proteins

Enhanced ultrasonic spray ionization for direct mass spectrometry analysis of aqueous solution and complex samples using a single-orifice piezoelectric atomizer

An efficient and superior soft ionization approach for direct mass spectrometry analysis of a variety of samples such as aqueous solution, raw biological sample and proteins, was developed based on commercially available piezoelectric atomizers. A single conical orifice (5 μm in diameter) was created on the atomizer, which resulted in generation of uniform fine droplets and long-duration of MS signal. The two electrodes of piezoelectric atomizer were connected to the two sides of ceramic ring which was insulated from the metallic substrate. The unique design allowed an additional high voltage input towards the spray reagents, which facilitated direct analysis of more complex samples without sample pre-treatment, such as biological samples (tomato tissue). The ionization was driven by an extremely low electrical power (3.5 V rechargeable battery) yet providing an efficient and superior soft ionization. The method displayed a better thermal and pH stability than nano electrospray ionization (nanoESI) and electrospray ionization (ESI) on direct analysis of Vitamin B and protein aqueous solutions. Quantitative analysis of Vitamin B and Rhodamine B aqueous solutions was also investigated, showing a good linearity (R² > 0.99). In addition, our results suggested that compared with ESI and nanoESI, the method not only could be used for direct analysis of intact protein, but also provide more information concerning the association between intact protein and the subunits.

Capillary Vibrating Sharp-Edge Spray Ionization Augments Field-Free Ionization Techniques to Promote Conformer Preservation in the Gas-Phase for Intractable Biomolecular Ions

Field-free capillary vibrating sharp-edge spray ionization (cVSSI) is evaluated for its ability to conduct native mass spectrometry (MS) experiments. The charge state distributions for nine globular proteins are compared using field-free cVSSI, field-enabled cVSSI, and electrospray ionization (ESI). In general, for both positive and negative ion mode, the average charge state (qavg) increases for field-free cVSSI with increasing molecular weight similar to ESI. A clear difference is that the qavg is significantly lower for field-free conditions in both analyses. Two proteins, leptin and thioredoxin, exhibit bimodal charge state distributions (CSDs) upon the application of voltage in positive ion mode; only a monomodal distribution is observed for field-free conditions. In negative ion mode, thioredoxin exhibits a multimodal CSD upon the addition of voltage to cVSSI. Extensive molecular dynamics (MD) simulations of myoglobin and leptin in nanodroplets suggest that the multimodal CSD for leptin may originate from increased conformational "breathing" (decreased packing) and association with the droplet surface. These properties along with increased droplet charge appear to play critical roles in shifting ionization processes for some proteins. Further exploration and development of field-free cVSSI as a new ionization source for native MS especially as applied to more flexible biomolecular species is warranted.

Mechanospray Ionization MS of Proteins Including in the Folded State and Polymers

Mechanospray ionization (MoSI) is a technique that produces ions directly from solution-like electrospray ionization (ESI) but without the need of a high voltage. In MoSI, mechanical vibrations aerosolize solution phase analytes, whereby the resulting microdroplets can be directed into the inlet orifice of a mass spectrometer. In this work, MoSI is applied to biomolecules up to 80 kDa in mass in both denatured and native conditions as well as polymers up to 12 kDa in mass. The various MoSI devices used in these analyses were all comprised of a piezoelectric annulus attached to a central metallic disk containing an array of 4 to 7 μm diameter holes. The devices vibrated in the 100-170 kHz range to generate a beam of microdroplets that ultimately resulted in ion formation. A linear quadrupole ion trap (LIT) and orbitrap mass spectrometer were used in the analysis to investigate higher mass proteins at both native (folded) and denatured (unfolded) conditions. MoSI native mass spectra of proteins acquired on the orbitrap and LIT instrument demonstrated that proteins could remain intact and in a folded state. In the case of native MS of holomyoglobin, the intact folded protein remained mostly bound noncovalently to the heme group, and typically, the spectra showed reduced loss of the heme group by MoSI as compared to ESI. In both non-native and native protein analyses examples, broader often multimodal distributions to lower charge states were observed. When using the LIT instrument, a significant increase in the relative abundance of dimers was observed by MoSI as compared to ESI. The softness of the MoSI technique was evidenced by the lack of fragmentation, the formation of dimers as also noted by others ( J. Mass Spectrom. 2016, 424-429) and under native conditions, the retention of proteins in one or more presumed folded structures and for holomyoglobin the high retention of the heme group. When analyzing polyethylene glycol (PEG) and polypropylene glycol (PPG), MoSI also generated a broader distribution to lower charge states than ESI. By using the improved separation of peaks at lower charge states and all the charge states available, MoSI data should provide an improved ionization method to obtain more accurate mass and dispersity values for some polymers.

Combining Field-Enabled Capillary Vibrating Sharp-Edge Spray Ionization with Microflow Liquid Chromatography and Mass Spectrometry to Enhance 'Omics Analyses

Field-enabled capillary vibrating sharp-edge spray ionization (cVSSI) has been combined with high-flow liquid chromatography (LC) and mass spectrometry (MS) to establish current ionization capabilities for metabolomics and proteomics investigations. Comparisons are made between experiments employing cVSSI and a heated electrospray ionization probe representing the state-of-the-art in microflow LC-MS methods for 'omics studies. For metabolomics standards, cVSSI is shown to provide an ionization enhancement by factors of 4 ± 2 for both negative and positive ion mode analyses. For chymotryptic peptides, cVSSI is shown to provide an ionization enhancement by factors of 5 ± 2 and 2 ± 1 for negative and positive ion mode analyses, respectively. Slightly broader high-performance liquid chromatography peaks are observed in the cVSSI datasets, and several studies suggest that this results from a slightly decreased post-split flow rate. This may result from partial obstruction of the pulled-tip emitter over time. Such a challenge can be remedied with the use of LC pumps that operate in the 10 to 100 μL·min-1 flow regime. At this early stage, the proof-of-principle studies already show ion signal advantages over state-of-the-art electrospray ionization (ESI) for a wide variety of analytes in both positive and negative ion mode. Overall, this represents a ∼20-50-fold improvement over the first demonstration of LC-MS analyses by voltage-free cVSSI. Separate comparisons of the ion abundances of compounds eluting under identical solvent conditions reveal ionization efficiency differences between cVSSI and ESI and may suggest varied contributions to ionization from different physicochemical properties of the compounds. Future investigations of parameters that could further increase ionization gains in negative and positive ion mode analyses with the use of cVSSI are briefly presented.

Inexpensive Ultrasonic Nebulization Coupled with Direct Current Corona Discharge Ionization Mass Spectrometry for Liquid Samples and Its Fundamental Investigations

The concept of direct mass-spectrometric analysis, especially exploited by ambient desorption/ionization (ADI) methods, provides numerous means for convenient sample analysis. While many simple and versatile ionization sources have been developed, challenges lay in achieving efficient sample introduction. In previous work, a sample introduction method employing direct current corona discharge (CD) coupled to a surface acoustic wave nebulization (SAWN) device enhanced sampling performance for both polar and nonpolar analytes by up to 4 orders of magnitude. In fact, the SAWN-CD method generated a multiply charged peptide ion signal comparable to that of conventional ESI. Unfortunately, the high cost of the SAWN devices themselves limits their accessibility. Herein, we report on an analogous implementation of CD with an inexpensive ultrasonic nebulizer (USN) on the basis of a commercial room humidifier demonstrating equivalent exemplary performance. We subsequently compare the two methods of SAWN-CD and USN-CD in a screening application of milk for the detection of two antibiotic drugs, ciprofloxacin and ampicillin. Finally, we further investigate the relative softness of these CD-coupled acoustic nebulization methods in comparison to that of ESI using a survival yield study of the thermometer ion nitrobenzylpyridinium.

Tuning the Coupled-Domain Response for Efficient Ultrasonic Droplet Generation

Acoustic microfluidic devices encompass mechanical, fluidic, and electromechanical domains. Complicated multidomain interactions require the consideration of each individual material domain, as well as coupled behaviors to achieve optimal performance. Herein, we report the co-optimization of components comprising an ultrasonic droplet generator to achieve the high-efficiency liquid atomization for operation in the 0.5-2.5-MHz frequency range. Due to the complexity of the real system, simplified 2-D representations of the device are investigated using an experimentally validated finite element analysis model. Ejection modes (i.e., frequencies at which droplet generation is predicted) are distinguished by maxima in the local pressure at the tips of an array of triangular nozzles. Resonance behaviors of the transducer assembly and fluid-filled chamber are examined to establish optimal geometric combinations concerning the chamber pressure field. The analysis identifies how domain geometries affect pressure field uniformity, broadband operation, and tip pressure amplitude. Lower frequency modes are found to focus the acoustic energy at the expense of field uniformity within the nozzle array. Resonance matching yields a nearly threefold increase in maximum attainable tip pressure amplitude. Significantly, we establish a set of design principles for these complex devices, which resembles a classical half-wave transducer, quarter-wave matching layer, and half-wave chamber layered system.

Enhanced aerodynamic reach of vapor and aerosol sampling for real-time mass spectrometric detection using Venturi-assisted entrainment and ionization

Venturi-assisted ENTrainment and Ionization (VENTI) was developed, demonstrating efficient entrainment, collection, and transport of remotely sampled vapors, aerosols, and dust particulate for real-time mass spectrometry (MS) detection. Integrating the Venturi and Coandă effects at multiple locations generated flow and analyte transport from non-proximate locations and more importantly enhanced the aerodynamic reach at the point of collection. Transport through remote sampling probes up to 2.5 m in length was achieved with residence times on the order of 10^-2 s to 10^-1 s and Reynolds numbers on the order of 10³ to 10⁴. The Venturi-assisted entrainment successfully enhanced vapor collection and detection by greater than an order of magnitude at 20 cm stand-off (limit of simple suction). This enhancement is imperative, as simple suction restricts sampling to the immediate vicinity, requiring close proximity to the vapor source. In addition, the overall aerodynamic reach distance was increased by approximately 3-fold over simple suction under the investigated conditions. Enhanced aerodynamic reach was corroborated and observed with laser-light sheet flow visualization and schlieren imaging. Coupled with atmospheric pressure chemical ionization (APCI), the detection of a range of volatile chemical vapors; explosive vapors; explosive, narcotic, and mustard gas surrogate (methyl salicylate) aerosols; and explosive dust particulate was demonstrated. Continuous real-time Venturi-assisted monitoring of a large room (approximately 90 m² area, 570 m³ volume) was demonstrated for a 60-min period without the remote sampling probe, exhibiting detection of chemical vapors and methyl salicylate at approximately 3 m stand-off distances within 2 min of exposure.

Rapid detection and isotopic measurement of discrete inorganic samples using acoustically actuated droplet ejection and extractive electrospray ionization mass spectrometry

RATIONALE

The rapid detection, screening, and isotopic signature analysis of inorganics provide invaluable information for a variety of applications including explosive device detection, nuclear forensics, and environmental monitoring. The coupling of ultrasonic nebulization and extractive electrospray ionization (EESI) enabled the mass spectrometric (MS) detection and analysis of inorganics from microliter sample solution aliquots.

METHODS

Ultrasonic nebulization and acoustic pressure wave focusing within an array of exponential horn structures were utilized for the efficient atomization of discrete liquid samples ranging in volume from 3 μL to 10 μL pipetted aliquots. In conjunction with an electro-flow focusing source for extractive electrospray ionization (EESI), in-source collision-induced dissociation (CID) was utilized to enhance inorganic detection through fragmentation of adducts and reduction in chemical noise from organic compounds.

RESULTS

The investigated system enhanced detection of the singly charged elemental cation species and provided accurate measurements of isotopic distributions for a number of metal ions. The extent of CID demonstrated the competition between ligand loss from hydrate clusters and charge reduction from the doubly charged to singly charged cations for the alkaline earth metal ions of strontium and barium. Inorganics were also detected from complex matrices, including synthetic fingerprint material and sediment, without detriment to device operation.

CONCLUSIONS

The described system provides a versatile tool for the rapid detection, speciation, and isotopic identification of inorganic compounds at nanogram to sub-nanogram levels from microliter aliquots. Published in 2014. This article is a U.S. Government work and is in the public domain in the USA.

Real time monitoring of accelerated chemical reactions by ultrasonication-assisted spray ionization mass spectrometry

Ultrasonication has been used to accelerate chemical reactions. It would be ideal if ultrasonication-assisted chemical reactions could be monitored by suitable detection tools such as mass spectrometry in real time. It would be helpful to clarify reaction intermediates/products and to have a better understanding of reaction mechanism. In this work, we developed a system for ultrasonication-assisted spray ionization mass spectrometry (UASI-MS) with an ~1.7 MHz ultrasonic transducer to monitor chemical reactions in real time. We demonstrated that simply depositing a sample solution on the MHz-based ultrasonic transducer, which was placed in front of the orifice of a mass spectrometer, the analyte signals can be readily detected by the mass spectrometer. Singly and multiply charged ions from small and large molecules, respectively, can be observed in the UASI mass spectra. Furthermore, the ultrasonic transducer used in the UASI setup accelerates the chemical reactions while being monitored via UASI-MS. The feasibility of using this approach for real-time acceleration/monitoring of chemical reactions was demonstrated. The reactions of Girard T reagent and hydroxylamine with steroids were used as the model reactions. Upon the deposition of reactant solutions on the ultrasonic transducer, the intermediate/product ions are readily generated and instantaneously monitored using MS within 1 s. Additionally, we also showed the possibility of using this reactive UASI-MS approach to assist the confirmation of trace steroids from complex urine samples by monitoring the generation of the product ions.

Design, Modeling, Fabrication, and Evaluation of the Air Amplifier for Improved Detection of Biomolecules by Electrospray Ionization Mass Spectrometry

Through a multi-disciplinary approach, the air amplifier is being evolved as a highly engineered device to improve detection limits of biomolecules when using electrospray ionization. Several key aspects have driven the modifications to the device through experimentation and simulations. We have developed a computer simulation that accurately portrays actual conditions and the results from these simulations are corroborated by the experimental data. These computer simulations can be used to predict outcomes from future designs resulting in a design process that is efficient in terms of financial cost and time. We have fabricated a new device with annular gap control over a range of 50 to 70 μm using piezoelectric actuators. This has enabled us to obtain better aerodynamic performance when compared to the previous design (2× more vacuum) and also more reproducible results. This is allowing us to study a broader experimental space than the previous design which is critical in guiding future directions. This work also presents and explains the principles behind a fractional factorial design of experiments methodology for testing a large number of experimental parameters in an orderly and efficient manner to understand and optimize the critical parameters that lead to obtain improved detection limits while minimizing the number of experiments performed. Preliminary results showed that several folds of improvements could be obtained for certain condition of operations (up to 34 folds).

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

Distinct regimes of droplet charging, determined by the dominant charge transport process, are identified for an ultrasonic droplet ejector using electrohydrodynamic computational simulations, a fundamental scale analysis, and experimental measurements. The regimes of droplet charging are determined by the relative magnitudes of the dimensionless Strouhal and electric Reynolds numbers, which are a function of the process (pressure forcing), advection, and charge relaxation time scales for charge transport. Optimal (net maximum) droplet charging has been identified to exist for conditions in which the electric Reynolds number is of the order of the inverse Strouhal number, i.e., the charge relaxation time is on the order of the pressure forcing (droplet formation) time scale. The conditions necessary for optimal droplet charging have been identified as a function of the dimensionless Debye number (i.e., liquid conductivity), external electric field (magnitude and duration), and atomization drive signal (frequency and amplitude). The specific regime of droplet charging also determines the functional relationship between droplet charge and charging electric field strength. The commonly expected linear relationship between droplet charge and external electric field strength is only found when either the inverse of the Strouhal number is less than the electric Reynolds number, i.e., the charge relaxation is slower than both the advection and external pressure forcing, or in the electrostatic limit, i.e., when charge relaxation is much faster than all other processes. The analysis provides a basic understanding of the dominant physics of droplet charging with implications to many important applications, such as electrospray mass spectrometry, ink jet printing, and drop-on-demand manufacturing.

Regime transition in electromechanical fluid atomization and implications to analyte ionization for mass spectrometric analysis

The physical processes governing the transition from purely mechanical ejection to electromechanical ejection to electrospraying are investigated through complementary scaling analysis and optical visualization. Experimental characterization and visualization are performed with the ultrasonically-driven array of micromachined ultrasonic electrospray (AMUSE) ion source to decouple the electrical and mechanical fields. A new dimensionless parameter, the Fenn number, is introduced to define a transition between the spray regimes, in terms of its dependence on the characteristic Strouhal number for the ejection process. A fundamental relationship between the Fenn and Strouhal numbers is theoretically derived and confirmed experimentally in spraying liquid electrolytes of different ionic strength subjected to a varying magnitude electric field. This relationship and the basic understanding of the charged droplet generation physics have direct implications on the optimal ionization efficiency and mass spectrometric response for different types of analytes.

Ultrasound ionization of biomolecules

To date, mass spectrometric analysis of biomolecules has been primarily performed with either matrix-assisted laser desorption/ionization (MALDI) or electrospray ionization (ESI). In this work, ultrasound produced by a simple piezoelectric device is shown as an alternative method for soft ionization of biomolecules. Precursor ions of proteins, saccharides and fatty acids showed little fragmentation. Cavitation is considered as a primary mechanism for the ionization of biomolecules.

Electrochemical Ionization and Analyte Charging in the Array of Micromachined UltraSonic Electrospray (AMUSE) Ion Source

Electrochemistry and ion transport in a planar array of mechanically-driven, droplet-based ion sources are investigated using an approximate time scale analysis and in-depth computational simulations. The ion source is modeled as a controlled-current electrolytic cell, in which the piezoelectric transducer electrode, which mechanically drives the charged droplet generation using ultrasonic atomization, also acts as the oxidizing/corroding anode (positive mode). The interplay between advective and diffusive ion transport of electrochemically generated ions is analyzed as a function of the transducer duty cycle and electrode location. A time scale analysis of the relative importance of advective vs. diffusive ion transport provides valuable insight into optimality, from the ionization prospective, of alternative design and operation modes of the ion source operation. A computational model based on the solution of time-averaged, quasi-steady advection-diffusion equations for electroactive species transport is used to substantiate the conclusions of the time scale analysis. The results show that electrochemical ion generation at the piezoelectric transducer electrodes located at the back-side of the ion source reservoir results in poor ionization efficiency due to insufficient time for the charged analyte to diffuse away from the electrode surface to the ejection location, especially at near 100% duty cycle operation. Reducing the duty cycle of droplet/analyte ejection increases the analyte residence time and, in turn, improves ionization efficiency, but at an expense of the reduced device throughput. For applications where this is undesirable, i.e., multiplexed and disposable device configurations, an alternative electrode location is incorporated. By moving the charging electrode to the nozzle surface, the diffusion length scale is greatly reduced, drastically improving ionization efficiency. The ionization efficiency of all operating conditions considered is expressed as a function of the dimensionless Peclet number, which defines the relative effect of advection as compared to diffusion. This analysis is general enough to elucidate an important role of electrochemistry in ionization efficiency of any arrayed ion sources, be they mechanically-driven or electrosprays, and is vital for determining optimal design and operation conditions.

Electrohydrodynamics of charge separation in droplet-based ion sources with time-varying electrical and mechanical actuation

Charge transport and separation in mechanically-driven, droplet-based ion sources are investigated using computational analysis and supporting experiments. A first-principles model of electrohydrodynamics (EHD) and charge migration is formulated and implemented using FLUENT CFD software for jet/droplet formation. For validation, classical experiments of electrospraying from a thin capillary are simulated, specifically, the transient EHD cone-jet formation of a fluid with finite electrical conductivity, and the Taylor cone formation in a perfectly electrically-conducting fluid. The model is also used to investigate the microscopic physics of droplet charging in mechanically-driven droplet-based ion sources, such as array of micromachined ultrasonic electrospray (AMUSE). Here, AMUSE is subject to DC and AC electric fields of varying amplitude and phase, with respect to a time-varying mechanical force driving the droplet formation. For the DC-charging case, a linear relationship is demonstrated between the charge carried by each droplet and an applied electric field magnitude, in agreement with previously reported experiments. For the AC-charging case, a judiciously-chosen phase-shift in the time-varying mechanical (driving ejection) and electrical (driving charge transport) signals allows for a significantly increased amount of charge, of desired polarity, to be pumped into a droplet upon ejection. Complementary experimental measurements of electrospray electrical current and charge-per-droplet, produced by the AMUSE ion source, are performed and support theoretical predictions for both DC- and AC-charging cases. The theoretical model and simulation tools provide a versatile and general analytical framework for fundamental investigations of coupled electrohydrodynamics and charge transport. The model also allows for the exploration of different configurations and operating modes to optimize charge separation in atmospheric pressure electrohydrodynamic ion sources under static and dynamic electrical and mechanical fields.

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

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.

Generation of multiply charged peptides and proteins by radio frequency acoustic desorption and ionization for mass spectrometric detection

The design and implementation of a radio frequency acoustic desorption ionization (RADIO) source has been demonstrated for the analysis of multiply charged peptides and proteins. One muL aliquots of melittin, BNP-32, and ubiquitin ( approximately 1 mug of analyte) were deposited onto a quartz crystal microbalance (QCM) electrode before radio frequency actuation for desorption. Continuous electrospray parallel to/above the sampling surface enabled the ionization of desorbed species. Detection by a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer confirmed the intact and dissociated species observed during MS and MS/MS experiments, respectively.

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 (<or=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 microM 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.