Electrospray Droplet Impact Research Articles

Relative secondary ion yields produced by vacuum-type electrospray droplet ion beams

The relatively new electrospray droplet impact method is based on electrospray at atmospheric pressure and has been successful in achieving efficient desorption/ionization of biological molecules, soft etching of polymers, and nonselective etching of metal oxides. However, the beam current and brightness of this method are not practical for surface analyses. To improve the performance of this method, the authors have developed a new droplet ion beam gun using an electrospray of aqueous solutions in a vacuum. This technique could be expected to be a high-intensity massive cluster ion beam and they evaluate the performance of the vacuum-type electrospray droplet ion (V-EDI) gun. In this study, V-EDI and gas cluster ion beam guns were installed in a time-of-flight analyzer to directly compare the relative secondary ion yields of several biomolecules produced by several cluster ion beams.

Analysis of fluorene and 9,9-dialkylfluorenes by electrospray droplet impact (EDI)/SIMS

Fluorene and 9,9-dialkyl fluorenes were examined by electrospray droplet impact/secondary ion mass spectrometry (EDI/SIMS). From the major peaks: M+, [M+H]+, [M−R]+, and [M−R−(R−H)]+detected by EDI/SIMS, information on the molecular masses (M) and the side chains (R) could be obtainedfor dioctyl-, dihexyltri-, dioctyltri- and dihexylheptafluorenes. Despite the different alkyl substituents, dihexyltrifluorene (C75H98) and dioctyltrifluorene (C87H122) gave similar fragment-ion distribution patterns with the common peak values at m/z 339 and 517 in the two envelopes. This suggests that the fragmentation paths are similar for these two compounds. The ions with m/z 339 and 517 were tentatively assigned asC27H15+ and C41H25+in which all the alkyl substituents (R) were eliminated. Fragmentation paths of protonated fluorenes were examined by DFT calculations.The reaction paths of the elimination of side two alkyl chains to give fluorenyl cations as the final products were postulated for 9,9-dimethylfluorene and 9,9-dioctylfluorene.

Desorption in Mass Spectrometry.

In mass spectrometry, analytes must be released in the gas phase. There are two representative methods for the gasification of the condensed samples, i.e., ablation and desorption. While ablation is based on the explosion induced by the energy accumulated in the condensed matrix, desorption is a single molecular process taking place on the surface. In this paper, desorption methods for mass spectrometry developed in our laboratory: flash heating/rapid cooling, Leidenfrost phenomenon-assisted thermal desorption (LPTD), solid/solid friction, liquid/solid friction, electrospray droplet impact (EDI) ionization/desorption, and probe electrospray ionization (PESI), will be described. All the methods are concerned with the surface and interface phenomena. The concept of how to desorb less-volatility compounds from the surface will be discussed.

真空エレクトロスプレーを用いるイオン銃の開発

Atmospheric pressure electrospray had been used previously in our laboratory to generate massive cluster ion beam for charged droplet ionization; a method we called electrospray droplet impact (EDI). Previous ion gun that employed atmospheric pressure electrospray lacks adequate beam current and density for surface and interface analysis. To improve the EDI beam performance, we have proposed a technique for producing a stable electrospray of aqueous solution under vacuum. It is well known that vacuum electrospray of volatile liquids is difficult because of the freezing of the liquids by evaporative cooling under vacuum. Vacuum electrospray of aqueous solution was achieved by irradiating the tip of the electrospray emitter with infrared or near-infrared laser to prevent the freezing under high vacuum (<0.05 Pa). A stable and large beam current was also obtained by optimizing the vacuum electrospray conditions. On the basis of these results, the vacuum electrospray can be expected to be a high-performance massive cluster ion gun.

Sputtering properties for polyimide by vacuum electrospray droplet impact (V-EDI) using size-selected cluster ions

Sputtering properties for polyimide by vacuum electrospray droplet impact (V-EDI) using size-selected cluster ions

Secondary ions produced by electrospray droplet impact with m/z selection from 103 to 106

In the secondary ion mass spectrometry, cluster ion beams are used for efficient ionization and low-damage etching of organic materials. In earlier studies, the authors have suggested a new cluster ion beam method using electrospray droplet impacts in atmospheric pressure conditions. Recently, the authors have developed a technique for electrospraying aqueous solutions in vacuum, which improved the performance of this technique for practical use. In this study, the mass-to-charge (m/z) values of the electrospray droplets produced by vacuum electrospray were measured by the time-of-flight technique and were found to be between 103 and 106. In addition, the secondary ions produced by these droplets were measured on indium and arginine samples, and the effect that the m/z value of the primary droplets has upon the secondary ion emission was investigated.

Electrospray droplet impact secondary ion mass spectrometry using a vacuum electrospray source.

In electrospray droplet impact (EDI) developed in our laboratory, an atmospheric pressure electrospray source has been used. To increase the ion beam intensity and reduce the evacuation load, a vacuum electrospray cluster ion source using a silica capillary was developed. A silica capillary with a tip inner diameter of 8 µm was used for vacuum electrospray using aqueous 10% methanol. To stabilize the flow rate of the liquid for nano-electrospray, a home-made constant pressure liquid pump was also developed. By using the silica tip nano-electrospray emitter and a constant pressure pump, stable electrospray with flow rate of 22 nL/min was realized without using any heating system such as laser irradiation. Comparative study of mass spectra obtained by atmospheric pressure EDI (A-EDI) and vacuum EDI (V-EDI) was made for various samples such as thermometer molecule, peptide, polystyrene, Alq(3), NPD, C(60), indium, and SiO(2). V-EDI showed slightly milder ionization than A-EDI. Because V-EDI gave higher target current (5-10 nA) than A-EDI (a few nA at most), V-EDI secondary ion mass spectrometry (SIMS) would be a useful technique for the surface and interface analysis.

XPS depth analysis of metal/polymer multilayer by vacuum electrospray droplet impact

Recently, the vacuum electrospray droplet impact (V-EDI) was developed as a cluster ion beam source in our laboratory. In this work, V-EDI was applied to polymers [polyimide (PI) and polycarbonate (PC)] and metal/polymer multilayer samples (Au/PI and Au/PC). We compared the etching performance of V-EDI with that obtained by the conventional atmospheric-pressure EDI (A-EDI). The nonselective etching was observed for organic and also inorganic samples by V-EDI as by A-EDI. Etching rates for the metal and polymer analysis by V-EDI were almost the same as those observed by A-EDI. The interlayer components were clearly observed by V-EDI for multilayer samples of Au and synthetic polymers. Copyright © 2014 John Wiley & Sons, Ltd.

Evaluation of a diode laser‐assisted vacuum‐type charged droplet beam source

The electrospray droplet impact (EDI) method based on ambient electrospray technique was developed as a novel massive cluster beam gun, but in its current status, the EDI method lacks adequate beam focusing and brightness. To improve the performance of the current EDI method, we developed a technique for producing electrospray of aqueous solutions under vacuum. In a previous study, the tip of the electrospray emitter was irradiated with an infrared (IR) CO2 laser (wavelength: 10.6 µm) to prevent freezing of the aqueous solutions. In this paper, a near‐IR diode laser (wavelength: 808 nm) was adopted for heating the tip of the electrospray emitter as an alternative to the IR laser. A stable vacuum electrospray of aqueous solutions was obtained, whereas the 808 nm light was poorly absorbed by water compared with the 10.6‐µm IR laser. The characteristics of the vacuum electrospray assisted by a near‐IR laser are presented. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Development of a high‐performance electrospray droplet beam source

The electrospray droplet impact (EDI) method was developed as a new massive cluster source, but the current EDI method lacks adequate beam focusing and density to achieve imaging and depth profiling with sufficient spatial resolution and within a reasonable time. To improve EDI beam performance, we have developed a technique for stable electrospraying of volatile liquids under vacuum. However, as is well known, vacuum electrospray of volatile liquids is extremely difficult because the evaporative cooling causes freezing of the volatile liquids in vacuum, and to prevent the freezing of these liquids, we irradiated the tip of the electrospray emitter with an infrared (10.6 µm) laser. The tip of the emitter was directly observed with optical microscopy, and the droplet ion beam current was measured with a Faraday cup. A stable vacuum electrospray of pure water was achieved with continuous laser irradiation. Also, typical modes of electrospray were clearly observed with the microscope even under vacuum conditions. The addition of 0.01 M trifluoroacetic acid to water caused an increase in current by at least one order of magnitude compared with water only. From these results, the vacuum electrospray technique can be expected to be a high‐performance massive cluster ion beam source. Copyright © 2012 John Wiley &amp; Sons, Ltd.

XPS depth analysis of CuO by electrospray droplet impact

The electrospray droplet impact (EDI) is presented as a new etching technique that induces almost no surface damage left after irradiation. We have previously reported that selective etching was not observed after EDI irradiation for organic materials. The present work made a comparative XPS study on the CuO etched by Ar+ and EDI. Although marked reduction of CuO was observed with Ar+ etching, no chemical modification was occurred by EDI. The etching rate of a few nm/min with EDI was estimated for copper oxide under the present experimental conditions. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Surface characterization and depth profiling of biological molecules by electrospray droplet impact/SIMS

Electrospray droplet impact (EDI) was applied to the analysis of peptides. The etching rate of bradykinin was estimated to be ~2 nm/min. This value is about one order of magnitude greater than the etching rate for SiO2 (0.2 nm/min). Considering that the etching rate of argon cluster ions Ar700+ for organic compounds is more than two orders of magnitude larger than that for inorganic materials, the rather small difference in etching rates of EDI for organic and inorganic materials is unique. When water/ethanol (1/1, vol%) solution of gramicidin S and arginine was dried in air, [gramicidin S + H]+ was observed as a predominant signal with little [Arg + H]+ right after the EDI irradiation, indicating that EDI is capable of detecting the analytes enriched on the sample surface. Copyright © 2011 John Wiley &amp; Sons, Ltd.

X‐ray photoelectron spectroscopy depth analysis of metal oxides by electrospray droplet impact

The electrospray droplet impact (EDI) method is a newly developed etching method using extremely large charged water cluster ions with masses of several 106 u. This work presents a comparative XPS study of chemical states of the transition metal oxides, TiO2 and Ta2O5, etched by Ar+ and EDI. Selective sputtering of oxygen was observed by Ar+ etching for these samples, but no chemical modification took place by EDI. This finding provided further evidence that EDI has the capability of nonselective etching for both inorganic and organic materials. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Study of the desorption/ionization mechanism in electrospray droplet impact secondary ion mass spectrometry

Electrospray droplet impact (EDI) secondary ion mass spectrometry (SIMS) is a desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from an atmospheric-pressure electrospray are accelerated in vacuum by several kV and impact on the sample deposited on the metal substrate. The abundances of the secondary ions for C(60) and amino acids are measured as a function of the acceleration voltage of the primary charged water droplets. Two desorption/ionization mechanisms are suggested in the EDI ionization processes: low-energy and high-energy regimes. In the low-energy regime, the excess charges in the primary droplets play a role in the formation of secondary ions. In the high-energy regime, samples are ionized by the supersonic collision of the primary droplets with the sample. The yield of secondary ions increases by about three orders of magnitude with increase in the acceleration voltage of the primary droplets from 1.75 kV to 10 kV.

Surface analysis of polyvinyl chloride bombarded by Ar+ and charged water droplets

AbstractA comparative study of 500 eV Ar+ ion impact and the electrospray droplet impact (EDI) applied to polyvinyl chloride (PVC) was made. In EDI, the charged electrospray water droplets are introduced in vacuum, accelerated and allowed to impact the sample. The samples, after etching by Ar+ and EDI, were examined by XPS. While chlorine is selectively etched by Ar+ ion impact, the XPS spectra did not change with prolonged charged water droplets irradiation, i.e. EDI is capable of surface etching with little damage of the sample after irradiation. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Molecular depth profiling of polystyrene by electrospray droplet impact

AbstractThe molecular depth profiling of polymer by the electrospray droplet impact (EDI) has been performed. The multiply charged water droplets with kinetic energy of ∼106 eV were irradiated on the polystyrene spin coated on silicon wafer. With EDI etching, the ablation of the target is suppressed to a minimal, i.e. the shallow surface etching with nonrecognizable damage on the surface is realized. It was found that EDI/SIMS spectra and EDI/XPS spectra for polystyrene (PS) has shown no damage accumulation on the surface. This indicates that EDI is a unique technique for surface etching of polymer materials without causing any damage to the etched surface. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Direct profiling of saccharides, organic acids and anthocyanins in fruits using electrospray droplet impact/secondary ion mass spectrometry

Electrospray droplet impact (EDI)/secondary ion mass spectrometry (SIMS) is a new desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from atmospheric-pressure electrospray are accelerated in vacuum by several kV and impact on the sample deposited on the metal substrate. In this study, we applied EDI/SIMS directly to fruits, such as bananas, strawberries, grapes and apples. The major components in the fruits--fructose, glucose, sucrose and organic acids--could be observed with strong signal intensities. EDI/SIMS was also applied to the analysis of different regions of strawberries and apples. Compared with matrix-assisted laser desorption/ionization (MALDI), ion signals with lower background signals could be obtained, particularly for the low molecular weight analytes.

Surface analysis of polyimide bombarded by charged water droplets

500 eV Ar+ ion impact and the electrospray droplet impact (EDI), in which the charged electrospray water droplets are introduced in vacuum, accelerated and allowed to impact the sample, are applied for etching of polyimide (PI). After the bombardment, x-ray photoelectron spectroscopy (XPS) was applied to the surface analysis. Although oxygen and nitrogen are selectively etched by Ar+ ion impact, the XPS spectra did not change with prolonged charged water droplets irradiation, i.e., EDI is capable of shallow surface etching for PI with little damage of the sample after irradiation.

XPS depth profiling of polystyrene etched by electrospray droplet impact

AbstractThe molecular‐level depth profiling of polystyrene (PS) has been performed by using a newly designed electrospray droplet impact (EDI) gun. The multiple charged water droplets with kinetic energy of ∼106 eV were irradiated to a bulk and a spin‐coated PS. When a PS target is etched by EDI, the ablation of the sample is suppressed to minimal, i.e. the shallow surface etching with nonrecognizable damage on the surface is realized. It was found that XPS spectra for PS were independent on the irradiation time by EDI. This indicates that EDI is a unique technique for the surface etching of synthetic polymer materials without resulting in any damage to the etched surface. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Direct analysis of lipids in mouse brain using electrospray droplet impact/SIMS

Electrospray droplet impact (EDI)/secondary ion mass spectrometry (SIMS) is a new desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from the atmospheric-pressure electrospray are accelerated in vacuum by 10 kV and impact the sample deposited on the metal substrate. EDI/SIMS was shown to enhance intact molecular ion formation dramatically compared to conventional SIMS. EDI/SIMS has been successfully applied to the analysis of mouse brain without any sample preparation. Five types of lipids, i.e. phosphatidylcholine (PC), phosphatidylserine, phosphatidylinositol (PI), galactocerebroside (GC) and sulfatide (ST), were readily detected from mouse brain section. In addition, by EDI/SIMS, six different regions of the mouse brain (cerebral cortex, corpus callosum, striatum, medulla oblongata, cerebellar cortex and cerebellar medulla) were examined. While GCs and STs were found to be rich in white matter, PIs were rich in gray matter.