Stability Of Ion Current Research Articles

Gas field ion source current stability for trimer and single atom terminated W(111) tips

Tungsten W(111) oriented trimer-terminated tips as well as single atom tips, fabricated by a gas and field assisted etching and evaporation process, were investigated with a view to scanning ion microscopy and ion beam writing applications. In particular, ion current stability was studied for helium and neon imaging gases. Large ion current fluctuations from individual atomic sites were observed when a trimer-terminated tip was used for the creation of neon ion beam. However, neon ion current was stable when a single atom tip was employed. No such current oscillations were observed for either a trimer or a single atom tip when imaged with helium.

Stability Measurement of Ion Beam Current from Argon Field Ion Source

We have been developing a next generation finely focused ion beam (FIB) system with noble gas field ion source (GFIS), which could reduce the contamination of irradiated ion species. Ion current stability of GFIS is one of the most fundamental factors for FIB applications. In this study, we report a preliminary measurement of current stability of field-ionized Ar+ ions from a tungsten emitter-tip which has a specific nanoprotrusion terminated by a trimer on the tip-apex. As a measure of long-term stability of Ar+ ion beam, current fluctuation of ∼5% for 1800 seconds was typically obtained at the beam current of 1.3 pA and the argon gas pressure of 5× 10-3 Pa. Furthermore it was found in argon-GFIS that the spike-like current fluctuation was correlated with the trimer-dimer transformation in a tungsten trimer-tip. [DOI: 10.1380/ejssnt.2011.371]

Silicon micropillar array electrospray chip for drug and biomolecule analysis

We have developed a lidless micropillar array electrospray ionization chip (microPESI) combined with mass spectrometry (MS) for analysis of drugs and biomolecules. The microPESI chip, made of silicon, contains a sample introduction spot for a liquid sample, an array of micropillars (diameter, height, and distance between pillars in the range of 15-200, 20-40, and 2-80 microm, respectively), and a sharpened tip for direct electrospray formation. The microchips were fabricated using deep reactive ion etching (DRIE) which results in accurate dimensional control. The chip, providing a reliable open-channel filling structure based on capillary forces and a electrospray emitter tip for ionization, allows an easy operation and reliable, non-clogging liquid transfer. The microPESI chip can be used for a fast analysis using single sampling or for continuous infusion measurements using a syringe pump for sample introduction. The microPESI-MS shows high sensitivity, with limit of detection 30 pmol/L (60 amol or 28 fg) for verapamil measured with tandem mass spectrometry (MS/MS) and using a sample volume of 2.5 microL. The system shows also good quantitative linearity (r2 > 0.99) with linear dynamic range of at least six orders of magnitude and good ion current stability (standard deviation <5%) in 1-h continuous flow measurement. The microPESI-MS is shown to be a very potential method for direct analysis of drugs and biomolecules.

Stable Gradient Nanoflow LC-MS

This paper demonstrates improved nanoflow LC-MS performance on a QqTOF instrument with the incorporation of a heated nanoflow interface (particle discriminator) and a nebulizer assisted sprayer. It is shown that the nebulizer broadens the usable range of electrospray potentials, simplifying the tuning procedure, particularly for negative mode nanoflow gradients. The improved desolvation capability with the particle discriminator results in signal/noise improvements of approximately 3.5x for negative ion mode samples prepared in predominantly acidified water as well as increased ion current stability. For nanoLC applications, the combined desolvation capabilities of a counter-current gas and heated laminar flow chamber provide reduced background, increased signal stability, reduced background drift, and improved protein sequence coverage when compared with data generated with only a counter-current gas for desolvation. This system is capable of subfemtomole nanoflow LC-MS sensitivity in both positive and negative ion mode across the solvent gradient.

Some aspects of the stability of ion current in a matrix-assisted laser desorption/ionization source.

The effect of laser exposure (the number of laser shots) on the stability of production of matrix (gentisic acid, sinapinic acid, nicotinic acid and benzoic acid) and biomolecule (insulin and cytochrome-c) ions is presented. Two methods of sample preparation, dried-droplet and pressure, were examined. In the case of pressure preparation, the dry powder sample was pressed mechanically with a pressure of 30 MPa. The best results were obtained for the pressure method and gentisic acid as matrix.

Determination of carnitine and acylcarnitines in biological samples by capillary electrophoresis–mass spectrometry

Free carnitine and acylcarnitines (carnitine esters) play an important role in the metabolism of fatty acids. Metabolic disorders can be detected by abnormal levels of these compounds in biological fluids. Capillary electrophoresis–mass spectrometry has the advantage of combining an efficient separation technique with highly selective detection. Therefore, we have developed a method for the determination of carnitine and several of its esters implementing electrospray capillary electrophoresis–mass spectrometry in the positive ion selected reaction monitoring mode. A sheath-flow interface with a mixture of 2-propanol or methanol, water and acetic acid as sheath liquid and nitrogen as nebulizing gas was used. The zwitterionic analytes migrated as cations in the applied electric field using ammonium acetate–acetic acid or formic acid electrolytes. Separations were performed in aqueous, mixed organic–aqueous and non-aqueous media. The influence of the electrolyte composition on the separation efficiency was investigated. The electrospray conditions have been optimized regarding ion current stability and sensitivity. Ammonium acetate (10 mmol/l)–0.8% formic acid in water or 6.4% formic acid in acetonitrile–water (1:1) were used as running buffers for the determination of carnitine and acylcarnitines in human biological samples. Methanol extracts of dried blood spots were analyzed as well as urine and plasma following sample preparation via solid-phase or liquid–liquid extraction. Recoveries approaching 100% were achieved depending on the analytes and sample preparation procedures employed. Endogenous carnitine and acetylcarnitine were determined at concentrations between 2.7 and 108 nmol/ml in normal human urine and plasma. Other acylcarnitines were detected at levels of below the limit of detection to 12 nmol/ml. Good precision (0.8 to 14%) and accuracy (85 to 111%) were obtained; the achieved limits of quantitation (0.1 to 1 nmol/ml) are sufficient to characterize carnitine and acylcarnitine levels occurring as markers for metabolic disorders.

Transition and rare-earth metal ion sources

The design and operation of an original transition and rare-earth metal ion source are described. Ions were produced during collisions of evaporated metal atoms with fast electrons. The source delivered the following parameters: (1) an ion energy of (1–20) keV; (2) a total ion current of (1–25)× 10−6 A; (3) an ion current stability of 0.1%; (4) a beam cross-section diameter of (1–12) mm; and (5) a period of 30–60 h of uninterrupted operation on a charge of 1 g of metal under UHV conditions. The peculiarity of the source is the creation of multiple-charged ions Mq+(q=1–6), and some part of these ions are metastable-excited. The testing results and practical applications of a proposed source are demonstrated.

Improving the ion current stability of a thermospray source by improving the control of the vaporizer temperature and solvent flow-rate

The Original time-proportional vaporizer temperature control system of the thermospray source has been changed into a system based on phase-angele control, which enahces the vaporizer temperature stability, and hence the ion current stability of the source. When combined with a solvent pressure stabilization system, this new temperature control system stabilizes the vaporizer temperature to within 0.1°C of the set value.

Factors affecting high‐mass sensitivity and ion current stability in a thermospray source

Factors affecting high‐mass sensitivity and ion current stability in a thermospray source

Beam current stability from localized emission sites in a field ion source

The production of stable ion beam currents using localized, high brightness ion emission from a gas fed, field ion source depends upon the stability of the field emitter surface in the region of the ionization. Research has focused primarily on developing a means of generating a single microstructure on the emitter surface which locally enhances the electrostatic field and thereby serves as an ion beam source. The relative ease of processing and the reliability of such a technique to contour the emitter surface are also factors determining the usefulness of the ion source which is to be used in high resolution microprobe systems. A procedure involving field emission through hydrogen films has been found whereby such a single ion, or electron, emission structure can be formed in situ on 〈110〉 oriented tungsten field emitters. These sites are capable of producing at least 10 to 15 nA beams of H+2 in half angle apertures of 16 mrad. Because the basic physical process involved in the field ionization of H2 is known to yield energy spreads of 1 eV (FWHM), good ion-optical properties are inherently available. This technique is practical in that one may readily and reproducibly localize the emission, but it remains to address the question of whether or not the localized emission structures provide stable beam currents for extended periods of time. Field ion micrographs of these sites indicate that they are protruberances 200 Å in diameter with field enhancement factors of 1.5. The current from these relatively large structures is not affected by surface rearrangement on the atomic scale caused by field induced chemical reactions or molecular contamination. The ability of these sites to provide a stable ion current then depends solely on the structural integrity of the protruberance. As ion bombardment is a factor in microboss growth, the first few surface layers of the field emitter and the microboss itself are radiation damaged tungsten and hence subject to electromechanical detachment. Heating the embossed field emitter to 800–1100 K allows greater ion currents to be drawn from the microboss before detachment occurs. However, these investigations indicate that to provide long term ion current stability a complete anneal (1100 K) of the embossed field emitter, most likely in the presence of an electric field such that the emitter is the anode, thereby holding the microboss in place and simultaneously halting the ion bombardment, is required. The fundamental field ionization mechanism generates ion currents stable to within 0.1% indicating the stability achievable after recrystallization of the surface.