Emitter Tip Research Articles

Supercritical Fluid Nanospray Mass Spectrometry: II. Effects on Ionization.

Nanospraying supercritical fluids coupled to a mass spectrometer (nSF-MS) using a 90% supercritical fluid CO2 carrier (sCO2) has shown an enhanced desolvation compared to traditional liquid eluents. Capillaries of 25, 50, and 75 μm internal diameter (i.d.) with pulled emitter tips provided high MS detection sensitivity. Presented here is an evaluation of the effect of proton affinity, hydrophobicity, and nanoemitter tip size on the nSF-MS signal. This was done using a set of primary, secondary, tertiary, and quaternary amines with butyl, hexyl, octyl, and decyl chains as analytes. Each amine class was analyzed individually to evaluate hydrophobicity and proton affinity effects on signal intensity. The system has shown a mass sensitive detection on a linear dynamic range of 0.1-100 μM. Results indicate that hydrophobicity has a larger effect on the signal response than proton affinity. Nanospraying a mixture of all amine classes using the 75 μm emitter has shown a quaternary amine signal not suppressed by competing analytes. Competing ionization was observed for primary, secondary, and tertiary amines. The 75 and 50 μm emitters demonstrated increased signal with increasing hydrophobicity. Surprisingly, the 25 μm i.d. emitter yielded a signal decrease as the alkyl chain length increased, contrary to conventional understanding. Nanospraying the evaporative fluid in a sub-500 nm emitter likely resulted in differences in the ionization mechanism. Results suggest that 90% sCO2 with 9.99% methanol and 0.01% formic acid yielded fast desolvation, high ionization efficiency, and low matrix effect, which could benefit complex biological matrix analysis.

Stable electrospray signal on a microfabricated glass chip with three-dimensional open edge and tiered depth geometries

This paper presents the microfabrication and performance of a three-dimensional electrospray ionization (ESI) emitter tip made from glass, which achieves stable current signals important for chemical analysis. Our novel microfabrication process and custom-built signal conditioning hardware provides the advantage of providing accurate features and steady signals. The fabrication process relies on standard microfabrication techniques (i.e., deposition, photolithography, and wet etching). This fabrication method involves the novel application of two layers of positive and negative photoresists in addition to Parafilm® wax tape. Open edge and tiered depth details were successfully created from a multilayer planar mask. This is a benefit for integrated miniaturized and microfluidic systems that often require micro features for their functionality but relatively large millimeter size features for their physical periphery. We demonstrate the fundamental performance of electrospray with this microfluidic chip. The emitter tip was fixed on a linear axis stage with high resolution (10 μm) to finely control the tip distance from a metal counter electrode plate. A custom printed circuit board system was built to safely control four voltages applied to the microchip ports from a single high voltage power supply. To readily form the electrospray, non-aqueous solvents were used for their low viscosity and a constant voltage of +2.7 kV was applied to the sheath electrospray microchannel. The liquid being sprayed was 80/20 (v/v) methanol/acetonitrile with 0.1% acetic acid in the sheath microchannel and with ammonium acetate (10–40 mM) in its remaining microchannels. The electrospray signal was measured in response to varying the distance (1.4 to 1.6 mm) between the electrospray emitter tip and a metal counter electrode plate in addition to the varying concentration of the background electrolyte, ammonium acetate. Stable and repeatable electrospray signal showed linear relationships with emitter tip distance and concentration (r2 ≥ 0.95).

Ultra-low current electrospray ionization of chloroform solution for the analysis of perfluorinated sulfonic acids.

Femtoamp and picoamp electrospray ionization (ESI) characteristics of a nonpolar solvent were explored. The direct ESI mass spectrometry analysis of chloroform extract solution enabled rapid analysis of perfluorinated sulfonic acid analytes in drinking water. Neat chloroform solvent and extracts were directly used in a typical wire-in ESI setup using micrometer emitter tips. Ionization currents were measured with femtoamp sensitivity while ramping the spray voltage from 0 to -5000 V. Methanol was used as a comparison to illustrate thecharacteristics of electrospraying chloroform. The effects of spray voltage and inlet temperature were studied. A liquid-liquid extraction workflow was developed to analyze perfluorooctanoate sulfonate (PFOS) in drinking water using an ion-trap mass spectrometer. The ionization onset of chloroform solution was 41 ± 17 fA at 300 V. The ionization current gradually increased with voltage while remaining below 100 pA when using voltages up to -5000 V. The ion signal of PFOS was significantly enhanced to improve the limit of detection (LoD) to 25 ppt in chloroform. Coupled with a liquid-liquid extraction workflow, LoD of 0.38-5.1ppt and a quantitation range of 5-400 ppt were achieved for perfluorinated sulfonic compounds in 1-ml water samples. Femtoamp and picoamp modes expand the solvent compatibility range of ESI and can enable quantitative analysis in parts per trillion (ppt) concentrations.

Flexible Acid‐Responsive Polymeric Phosphorescent Microfibers for Tunable Optical Waveguide

AbstractPhosphorescence optical waveguides, flexible ones in particular, are fascinating on account of their fundamental and practical values in photonics and optoelectronics devices. However, the majority of optical waveguides reported so far are fluorescent waveguides, the phosphorescent waveguides, especially ones with both flexibility and stimulus‐response are rarely studied. Herein, one‐dimensional phosphorescent microfibers are developed by employing the flexible polymer as the matrix and the phosphorescent emitter as the transducer for out‐coupled light, and their phosphorescent waveguide attributes are studied in detail. Significantly, benefiting from the non‐covalent clipping of molecular tweezer for Pt(II) triplet emitter, the strong and robust tip emission could be obtained for phosphorescent waveguiding microfiber. Furthermore, the asymmetric propagation of phosphorescent waveguide could be achieved via partly acid treatment, since acid could trigger the guest release accompanied by the emission decline. More interestingly, the phosphorescent microfiber exhibits flexible optical waveguide feature and reversible switching of tip emission. Based on the above, Boolean logic operation is selected to demonstrate the application of tunable tip emission. This study establishes an effective and convenient strategy to realize flexible phosphorescent waveguide with tunable tip emission, which is of fundamental value for designing optical functional elements and manufacturing intelligent flexible photonic devices with promising application potential.

Structure Optimization of Planar Nanoscale Vacuum Channel Transistor

Due to its unique structure, discoveries in nanoscale vacuum channel transistors (NVCTs) have demonstrated novel vacuum nanoelectronics. In this paper, the structural parameters of planar-type NVCTs were simulated, which illustrated the influence of emitter tip morphology on emission performance. Based on simulations, we successfully fabricated back-gate and side-gate NVCTs, respectively. Furthermore, the electric properties of NVCTs were investigated, showing the potential to realize the high integration of vacuum transistors.

Effect of material composition of diamond field emission array cathodes on quality of transversely shaped beams

Diamond field emitter array field emission cathodes (DFEA FECs) are attractive for the next generation of injectors due to their ability to produce transversely shaped beams without the need for complex masking or laser schemes. However, reliability of this cathode technology remains a challenging issue as principal mechanisms guiding and allowing for output beam shaping remained poorly understood. This paper reports the results of testing two DFEA FECs with the same pattern and emitter tip geometry. Although both cathodes were able to sustain gradients of 44 MV/m and produce maximum output integral charge of 0.5 nC per radio frequency pulse, their emission patterns were different. One cathode did not produce a shaped beam, while the other one did. This difference was explained by the extent of the local variations of the diamond material across the arrays as discovered by spatially resolved Raman spectroscopy. The main practical takeaways were (i) tip sharpness was not a prerequisite for producing a shaped beam and instead (ii) material characteristics resulting in different cathode ballast resistance affected emission spatial uniformity across the array and hence the beam shaping.

Development of a dual-electrospray ionization source with in-line absorbance-based voltage control.

Emitter tip arrays for electrospray ionization have been used for a variety of MS sample introduction purposes, including detection of multiple sample eluent streams and improved accuracy through parallel infusion of an internal standard. User control is typically required for targeted application of high voltage to specific channels to maximize analyte signal and minimize other background signals. In this communication, an automated approach to applying electrospray voltage only when a detectable analyte is present is described. An in-line absorbance detector is used to identify the presence of an analyte in the fluidic path between the sample introduction valve and the mass spectrometer. A Raspberry Pi-controlled system is then used to apply high voltage to a downstream emitter tip at the MS inlet following a delay volume between the detectors. Demonstration of this technique on two parallel sample channels is reported, including a pulsed voltage application to maximize signal when analytes elute on each channel simultaneously.

Demonstration of a silicon gated field emitter array based low frequency Colpitts oscillator at 400 °C

Silicon gated field emitter arrays have been used as a vacuum transistor to demonstrate a 152 kHz Colpitts oscillator. The transfer and output characteristics of the 1000 × 1000 silicon arrays were measured using a collector placed ≈ 1 mm away with a gate voltage up to 40 V and a collector voltage up to 200 V. The data were used to establish an LTspice transistor model based on a field emission tip model and a collector current model that fit the characteristics. Then, the LTspice model was used to design a low frequency Colpitts oscillator. Furthermore, experiments were carried out to successfully demonstrate the oscillation. Oscillation frequency was 152 kHz with a peak to peak voltage of 25 V for a tip to ground series resistance value of 10 kΩ at 50 V on the gate and 210 V on the collector. Further, the oscillator was also tested at 50, 100, 200, 300, and 400 °C. It was observed that frequency shifts for each temperature which is due to the change in the overall capacitance of the test setup. This type of device could be used as a temperature sensor in harsh environments.

Field emission characterization of nanoFEAs on single crystal CeB6 surface fabricated by focused ion beam milling

The field emission tip arrays with sub-100 nm apices (nanoFEAs) on single crystal cerium hexaboride (CeB6) surface were fabricated by the focused ion beam (FIB) milling microtechnology. The surface morphologies and field emissions of the nanoFEAs are systematically characterized. FIB milling, similar to the physical stripping process, can fabricate the nanoFEAs single crystal CeB6 with uniform morphologies. The nanoFEAs with sharp tips of size about 50 nm demonstrate the lowest turn-on electric fields (2.0 V/[Formula: see text]m), as well as a high current ([Formula: see text]1 mA) at the field of 6.7 V/[Formula: see text]m and a high stable emission current. Such excellent performances make CeB6 nanoFEAs promising candidates for application in field emission electronics.

Processing of experimental current-voltage characteristics of single tip emitters taking into account the functional dependence of the emission area on the applied voltage

The work addresses the question of the field-dependence of the notional emission area for a field-emitter tip using computer simulation and experimental data. Based on finite-element method calculations, it was determined that for single field emitters having a given shape, there is a stable and characteristic shift of the degree of voltage in the pre-exponential voltage exponent of the field emission equation relative to the “planar” case. A power-law fit to the 3D-data was applied so as to use it in the analysis of the current-voltage data using a k-power plot (KP-plot) of semilogarithmic type. A comparison of effective emission parameters obtained from the model current-voltage characteristic with the classical Fowler–Nordheim plot, modern Murphy–Good plot, and new KP-plot is carried out. The KP-plot is subsequently used to analyze the experimental data of Tungsten emitters having a scanning emission microscope determined apex radius. The new semilogarithmic analysis reveals that the apex radius of curvature is within the observed range.

Close-packed silicon field emitter arrays with integrated anode fabricated by electron-beam lithography

Design, fabrication, and characterization of close-packed field emitter tips enclosed in an Si trench and stand-alone arrays are presented. The two types of field emitter arrays (FEAs) are fabricated using a combination of high-throughput electron-beam lithography, plasma etching, and anode bonding integration technology. The field emitter array inside the trench shows a higher turn-on voltage compared to the stand-alone array. Without any tip sharpening, a current of 7.5 μA was observed at 300 V from FEAs inside the trench, while a higher current of 12.5 μA was observed at the same voltage for the stand-alone array.

Laser Heating Nanoelectrospray Emitters for Fast Protein Melting Measurements with Mass Spectrometry.

Temperature-controlled nanoelectrospray ionization has been used to measure heat-induced conformational changes of biomolecules by mass spectrometry, but long thermal equilibration times associated with heating or cooling an entire emitter limit how fast these data can be acquired. Here, the tip of a borosilicate electrospray emitter is heated using 10.6 μm light from an unfocused CO2 laser. At 1.2 W, the solution inside the emitter tip can be heated from room temperature to a steady-state temperature of 78.2 ± 2.5 °C in less than 0.5 s and cools from 82.6 ± 0.6 °C back to room temperature within 4 s. The time required to establish a steady-state temperature is more than 100-fold faster than that required for a resistively heated emitter due to the low thermal mass. Protein unfolding curves measured as a function of laser power can be acquired in ∼40 s compared to a resistively heated apparatus that required ∼21 min to acquire similar data. Laser power is calibrated to temperature by comparisons of the average charge state of the protein cytochrome c measured with laser heating and with resistive heating. This laser heating method is applied to a three-component protein mixture to demonstrate the ability to rapidly acquire melting temperatures of proteins in mixtures. The ability to rapidly assess the thermal stabilities of multiple proteins simultaneously shows significant promise for coupling temperature-controlled electrospray ionization (ESI) to separation techniques, providing a high-throughput method for determining the effects of solution composition, drug binding, or sequence mutations on protein thermal stability.

Attomole-Level Multiplexed Detection of Neurochemicals in Picoliter Droplets by On-Chip Nanoelectrospray Ionization Coupled to Mass Spectrometry.

While droplet microfluidics is becoming an effective tool for biomedical research, sensitive detection of droplet content is still challenging, especially for multiplexed analytes compartmentalized within ultrasmall droplets down to picoliter volumes. To enable such measurements, we demonstrate a silicon-based integrated microfluidic platform for multiplexed analysis of neurochemicals in picoliter droplets via nanoelectrospray ionization (nESI)-mass spectrometry (MS). An integrated silicon microfluidic chip comprising downscaled 7 μm-radius channels, a compact T-junction for droplet generation, and an integrated nESI emitter tip is used for segmentation of analytes into picoliter compartments and their efficient delivery for subsequent MS detection. The developed system demonstrates effective detection of multiple neurochemicals encapsulated within oil-isolated plugs down to low picoliter volumes. Quantitative measurements for each neurochemical demonstrate limits of detection at the attomole level. Such results are promising for applications involving label-free and small-volume detection for monitoring a range of brain chemicals.

Supercritical Fluid Nanospray Mass Spectrometry.

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO2 with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO2, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.

Stacked conductive metal–organic framework nanorods for high-performance vacuum electronic devices

Metal-organic frameworks (MOFs) possessing many unique features have been utilized in several fields in recent years. However, their application in field emission (FE) vacuum electronic device is hindered by their poor electrical conductivity. Herein, a novel conductive MOF of Cu-catecholate (Cu-CAT) with the nanorod length of 200 nm and conductivity of 0.01 S/cm is grown on the graphite paper (GP). Under an applied electric field, a large number of electrons can be emitted from the nanoscale emitter tips of MOF surface to the anode. The great field emission performance of Cu-CAT@GP cold cathode film including a low turn-on field of 0.59 V/μm and ultra-high field enhancement factor of 29622, even comparable to most carbon-based materials that have been widely investigated in FE studies, is achieved in this work. Meanwhile, Cu-CAT@GP film has a good electrical stability with a current attenuation of 9.4% in 2 h. The findings reveal the cathode film fabricated by conductive MOF can be a promising candidate of cold electron source for vacuum electronic applications.

Unsteady stretching of a glass tube with internal channel pressurization

Mathematical modeling is used to examine the unsteady problem of heating and pulling an axisymmetric cylindrical glass tube with an over-pressure applied within the tube to form tapers with a near uniform bore and small wall thickness at the tip. To allow for the dependence of viscosity on temperature, a prescribed axially varying viscosity is assumed. Our motivation is the manufacture of emitter tips for mass spectrometry which provide a continuous fluid flow and do not become blocked. We demonstrate, for the first time, the feasibility of producing such emitters by this process and examine the influence of the process parameters, in particular the pulling force and over-pressure, on the geometry. There is not a unique force and over-pressure combination to achieve the desired geometry at the tip but smaller over-pressure (hence force) yields a more uniform bore over the entire length of the emitter than does a larger over-pressure (and force). However, the sensitivity of the geometry to small fluctuations in the parameters increases as the over-pressure decreases. The best parameters depend on the accuracy of the puller used to manufacture the tapers and the permissible tolerances on the geometry. The model has wider application to the manufacture of other devices.

(Invited) Atom Probe Tomography: A New Tool For the Analysis of Solid Liquid Interfaces?

Atom probe tomography is an established method for the microsocopic analysis of hard materials, which stands out by combining single-atom sensitivity with a 3D volume reconstruction. The method is based on laser-assisted field evaporation from nanometric emitter tips. In solid state reactions it delivered decisive information to understand the detailed conditions of nucleation and growth of product phases. In recent efforts, we desire to break the materials limitations aiming for application to liquids, softmatter and polymeric materials. To this end, a new instrument has been constructed that joins cryogenic ion beam preparation with a flexible transfer into an atom probe chamber. Meanwhile, we are able to produce nanometric needles from frozen liquids, even from pure water [1]. Studies are performed with aqueous solutions [2], liquids of short alkanes [3], self-assembling monolayers [4]. The talk discusses the field desorption of complex materials and presents time of flight mass spectra and their dependence on field or laser intensity. The complex mechanisms of field desorption from frozen water surfaces are further elucidated by DFT and Monte-Carlo simulation. Finally, by evaluating evaporation probabilities and event correlations, even insight in bonding strength and the stability of molecules in strong electrical fields may be provided.[1] T. M. Schwarz, E. M. Weikum, K. Meng, E. Hadjixenophontos, C. A. Dietrich, J. Kästner, P. Stender, G. Schmitz, Field evaporation and atom probe tomography of pure water tips, Nature Scientific Reports (2020) 10:20271 | (DOI: 10.1038/s41598-020-77130-x[2] T. M. Schwarz, C. A. Dietrich, J. Ott, E. M. Weikum, R. Lawitzki, H. Solodenko, E. Hadjixenophontos, B. Gault, J. Kästner, G. Schmitz, P. Stender, 3D Sub-Nanometer Analysis of Glucose in an Aqueous Solution by Cryo-Atom Probe Tomography, Scientific reports 11 (2021) 11607 (DOI: 10.1038/s41598-021-90862-8)[3] Kuan Meng, Tim Schwarz, Eric Weikum, Patrick Stender, Guido Schmitz, Frozen n-Tetradecane investigated by cryo-Atom Probe Tomography, J. Microsc. Microanal. (2021), 1-11; (DOI:10.1017/S14319276210125)[4] Helena Solodenko, Patrick Stender, Guido Schmitz, “Atom Probe Study of 1-octadecanethiol self-assembled Monolayers on Platinum (111) and (200) surfaces”, J. Microsc. Microanal. (2021), 1–10. (DOI: 10.1017/S1431927621012654)

Scanning anode field emission microscopy of a single Si emitter

Emitter tip radius nonuniformity results in exponential variations in emission current and a relatively low array utilization. Here, we provide a method of mapping the current and field-factor from a single emitter over a small area using a scanning anode field emission microscope. A dull W probe is used as the anode, and an array of emitters is fabricated on silicon (Si) wafers. We use a relatively wide spaced (100 μm pitch) emitter array with each emitter having an integrated Si pillar. Current-voltage characteristics are used to extract the field-factor and to experimentally demonstrate the mapping of the currents and field-factor of a single emitter. From emission spot sizes, the emission half-angles are measured to be <14° at anode voltages 2.5 kV and a minimum resolvable feature of 2–3 μm at 1.8 kV. We also determine the field-factor dependence with the distance between the anode and the emitter, where limiting the current becomes essential to prevent early burn-out of the emitter that could reduce the current. We also simulated the maximum currents tolerated by the pillar to assess the thermal effects on the emitter. Finite element modeling confirms the experimental trend in the field-factor with the distance between the anode and the emitter tip, resulting in a value of approximately 105 cm−1 for an emitter tip radius of 5 nm and an emitter-anode distance of 50 μm.

Simulation of liquid meniscus formation in the ionic liquid electrospray process

Though tremendous efforts have been made to investigate electrospray, some aspects, such as the evolution of the menisci on the micropores of porous emitter tips and the transient response of the meniscus during the polarity alternation, need to be further understood. This paper presents a computation fluid dynamics (CFD) model to describe the meniscus formation in the ionic liquid electrospray process. The CFD model, based on the Taylor–Melcher leaky dielectric fluid theory and the volume of fluid (VOF) method, is validated by experiments. The evolution of the meniscus on the basis of a micropore is presented using two typical ionic liquids, EMI-BF4 and EMI-Im. The influences of the pore size, flow rate and applied voltage on the formation of the meniscus have been studied. Results show that a larger pore is more likely to start emission, and the time consumed for liquid meniscus formation decreases with increasing applied voltage and flow rate. Further, it is found that alternation of polarity does not destroy the structure of the meniscus but retards the formation process, and a faster polarity alteration leads to a shorter delay in meniscus formation time.

Fritted tip capillary column with negligible dead volume facilitated ultrasensitive and deep proteomics

Insufficient chromatographic performance results in reduced utilization of MS/MS scan capacity of advanced MS instruments. Improvement in peptide separation in liquid chromatography is critical for increasing the sensitivity and quantification performance of LC-MS-based proteomics. However, existing column fabrication methods suffer from slow packing, large dead volume, and band broadening. Herein, we reported that directly pulling emitter tips within short frits after fast packing (termed “filled tip”) can minimize the dead volume, improving ionization efficiency and reducing band broadening. Within 10 min, our method can pack over 10 cm for 50 μm I.D. capillary columns under 6–8 MPa and over 50 cm for 75 μm I.D. long capillary columns under 70 MPa. We can identify an average of 3043 protein groups and 33 309 peptide-spectrum matches (PSMs) from 1 ng of HeLa digest using a 50 μm I.D. x 20 cm “filled tip” column, with good reproducibility. The number of protein groups increased by 50% and 96% when compared with a 50 μm I.D. “void tip” column and a 100 μm I.D. column with a manually pulled tip, respectively. We identified an average of 5534 protein groups and 71 769 PSMs from 10 ng of HeLa digest. In addition, using 75 μm I.D. x 50 cm “filled tip” columns, we can identify on average 8829 protein groups and 170 751 PSMs in single-shot data-dependent acquisition analysis from 500 ng of 293T digested peptides. Importantly, good repeatability and reproducibility of “filled tip” method were verified by results from columns fabricated in three batches and by different persons. When compared with conventional columns with “void tips”, “filled tip” columns reduced median full peak widths by 19% and alleviated sampling redundancy by 10%. Collectively, we developed an easy-to-use, versatile and robust column fabrication method for both narrow-bore and long capillary columns, which achieved great sensitivity and depth in proteomic analysis.