Showing papers in "Analytical Sciences in 1995"

Solvent Extraction of Trivalent Rare Earth Metal Ions with Carboxylate Derivatives of Calixarenes

Abstract: As novel calixarene or macrocyclic types of extractants, 37, 38, 39, 40, 41, 42-hexakis(carboxymethoxy)-5, 11, 17, 23, 29, 35-hexakis(1, 1, 3, 3-tetramethylbutyl)calix[6]arene and 25, 26, 27, 28-tetrakis(carboxymethoxy)-5, 11, 17, 23-tetrakis(1, 1, 3, 3- tetramethylbutyl)calix[4]arene as well as p-(1, 1, 3, 3-tetramethylbutyl)phenoxy acetic acid, as their monomeric analog, and 2, 6-bis[2-carboxymethoxy-5-(1, 1, 3, 3-tetramethylbutyl)benzyl]-4-(1, 1, 3, 3-tetramethylbutyl)phenoxyacetic acid as a linear trimer analog, have been synthesized in order to investigate their extraction abilities of rare earth metal ions, RE3+ (RE=Y, La, Pr, Nd, Sm, Eu, Gd, Ho, Er), from an aqueous nitrate solution. It was found that the calixarene derivatives provide much higher extractability and greater separation efficiency than do the monomeric analog and the other acidic carboxylate extractants. The selectivity for rare earth elements in this system is not affected by the ring size. In extraction from an aqueous mixture of nitric acid-glycine, the stoichiometry of the extracted species was determined and the extraction equilibrium constants as well as the separation factors were evaluated for each extractant. A stripping test was also performed and the stripping of rare earths was found to be successfully achieved with diluted hydrochloric acid.

High-Performance Liquid Chromatographic Determination of Ascorbic Acid in Soft Drinks and Apple Juice Using Tris(2, 2'-bipyridine)ruthenium(II) Electrochemiluminescence

Abstract: The electrochemiluminescence of tris(2, 2′-bipyridine)ruthenium(II) [Ru(bpy)32+] was applied to high-performance liquid chromatographic determination of ascorbic acid. Ascorbic acid was separated by a C18 reversed-phase column with a mobile phase containing 15mM NaH2PO4-K2HPO4 (pH 6.5). The eluted ascorbic acid was mixed with 0.5mM Ru(bpy)32+ within the flow tube. The solution then passed through a thin layer flow cell equipped with a glassy carbon electrode and both ascorbic acid and Ru(bpy)32+ were oxidized at +1.5V (vs. Ag/AgCl). The reaction of electrolytically formed Ru(bpy)33+ with oxidized ascorbic acid emitted light. The detection limit was 10pmol for ascorbic acid at an S/N ratio of 3, and the linear calibration range was 0.06-80nmol. The method was successfully applied to determination of ascorbic acid in soft drinks and apple juice.

Use of Surfactant Aggregates Formed on Alumina for the Preparation of Chelating Sorbents.

Abstract: To 1.5g of γ-alumina particles in40ml of water was added 10ml of ammoniacal solution containing 1.5mg of dithizone and 100mg of sodium dodecyl sulfate. The solution was acidified to pH 2 to form the surfactant aggregates on the alumina surfaces, into which dithizone was trapped. Traces of copper(II) and silver(I) in water were completely collected on the dithizone-coated alumina over a wide range of pH 1-7; cadmium(II), cobalt(II), nickel(II), lead(II) and zinc(II) were simultaneously recovered at pH 3-7. The chelating sorbent functioned well even in aqueous solutions containing a large amount of matrix element. The proposed sorption method has been successfully applied to the determination of traces of copper in lead metal.

Multi-Element Determination of Trace Elements in Seawater by Gallium Coprecipitation and Inductively Coupled Plasma Mass Spectrometry

Abstract: Multi-element determination of trace elements in seawater by inductively coupled plasma mass spectrometry (ICP-MS) has been investigated Coprecipitation with gallium hydroxide was used for preconcentration of trace elements and elimination of matrix elements Most trace elements were quantitatively collected at pH 10 with the concentration factor of 10 In the present experimental method, 17 elements (Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Y, Zr, Sn, La, Pb, Th, U) in a coastal seawater sample could be determined by ICP-MS In addition, 6 elements (Al, Mn, Fe, Ni, Cu, Zn) among them were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) The analytical results obtained by ICP-MS and ICP-AES were in fair agreement

Iodine Determination in Natural and Tap Water Using Inductively Coupled Plasma Mass Spectrometry

Abstract: The measurement of iodine in natural-water samples by inductively coupled plasma mass spectrometry (ICP-MS) is described. The limit of detection is 10pg/ml. The element is easily determined by this method without separation or preconcentration. Because iodine is highly volatile, the obtained signal is somewhat unstable. Iodine vaporization, however, can be restrained by the addition of an organic alkali (for example tetramethyl ammonium hydroxide), making it possible to obtain a stable signal. Analytical results using the conventional calibration-curve method and the standard addition method agreed well. We determined the iodine concentration of 42 natural-water samples in the northern Kanto area. The concentrations of iodine ranged from 0.65 to 35.9ng/ml.

Supercritical Fluid Extraction of Trace Metals from Solid and Liquid Materials for Analytical Applications.

Abstract: Metal ions in solid and liquid materials can be extracted by supercritical CO 2 containing a suitable ligand. Bis(trifluoroethyl)dithiocarbamate is an effective ligand for the extraction of transition metals and non-metals in supercritical CO 2 . Fluorinated β-diketones and tributyl phosphate in supercritical CO 2 exhibit a positive synergistic extraction for the lanthanides and actinides from solid and liquid samples. Triazole containing crown ethers can also be used for selective extraction of heavy metals in supercritical CO 2

Fluorescence Characteristics of Methoxycoumarins as Novel Fluorophores.

Abstract: The fluorescence characteristics of various methoxycoumarin fluorophores for the development of fluorescence reagents were examined in relation to their structures. The fluorescence emission mechanisms were also considered from the viewpoint of the intramolecular charge-transfer (CT) between methoxyl substituents and the coumarin ring. The arrangement of 6-methoxyl and 3-acetyl group pairs on the coumarin ring significantly contributed to the fluorescence enhancement through the intramolecular CT. We found that the structural features of methoxycoumarins required for intense fluorescence are to hold both diether bonds at the 6- and 7-positions and an electron-withdrawing group at the 3-position, as shown in 3-acetyl-6, 7-dimethoxycoumarin with 0.52 in quantum yield.

Optimization of operating conditions in individual airborne particle analysis by inductively coupled plasma mass spectrometry

Abstract: Operating conditions of a particle analyzer using inductively coupled plasma mass spectrometry were investigated to optimize the analysis of individual aerosol particles for zinc in the picogram to femtogram range. The signal intensities and their fluctuations from individual particles were strongly affected by the carrier gas flow rate, radio frequency power and sampling depth. The optimum operating conditions were determined and are discussed based on the vaporization and ionization processes of particles and the ion sampling process. Under the optimum operating conditions, fluctuations of the signals from monodisperse aerosols were improved to ca. 5% of relative standard deviation under favorable conditions, and the detection limit of zinc in a particle could be lowered to ca. 3fg.

Comparison of Three Chromatographic Systems for Determination of Organic Acids in Wine

Abstract: Three chromatographic systems: ion exchange chromatography, ion exclusion chromatography and reversed phase chromatography, have been used for the simultaneous determination of organic acids in wine. The common organic acids were separated using all three chromatographic systems. When an ion exchange column (TSK gel IC Anion-PW) was used, organic acids and inorganic anions (Cl- and SO42-) in wine were determined simultaneously without interference. The sharp peaks were obtained when an ion exclusion column (TSK gel OApak-A) was employed. A rapid separation of organic acids has been achieved, within about 7min, when a reversed phase column (Zorbax ODS) was used. In ion exclusion and reversed phase systems combined with UV detection, however, other organic compounds which have ultraviolet absorption at 210nm interfered with the determination of organic acids when practical wine samples were analyzed. IC is an obvious alternative for determination of organic acids in wine.

In vivo 11B NMR Observation of Plant Tissue

Abstract: In vivo 11B NMR spectra of plant tissue, namely, radish roots, apple fruit, cabbage leaves and komatsuna plant leaves and roots, were successfully obtained. In the spectra, the signals of borate monoester and diester, together with boric acid, were observed, which shows that the in vivo 11B NMR technique provides direct evidence that the borate esters exist in plant tissue. The spectra of juice and residue prepared from radish roots and apple fruit revealed that the signals, especially those of borate esters, in the intact plant spectra were derived from the water-soluble fraction in the plant cell.

Rapid Field Scan L-Band Electron Spin Resonance Computed Tomography System Using an Air-Core Electromagnet.

Abstract: A rapid field scan L-band electron spin resonance computed tomography (ESR-CT) system has been developed in order to obtain three-dimensional images of the distribution of free radicals in a sample. The ESR system comprises an L-band (ca. 700MHz) ESR spectrometer, an air-core electromagnet equipped with a field-scanning coil, three field-gradient coils, and a microcomputer. The rapid field scan (15mT/s) causes a delay in the magnetic-field strength due to eddy-current losses. However, it was found that the magnetic field strength could be corrected by a simple mathematical equation related to a low-pass filter model. Using this system, the original ESR data used to obtain one three-dimensional ESR-CT image could be obtained in 150s. The time course of the ESR-CT images of the test sample containing the reaction of nitroxide radical with ascorbate ion has been shown as an example of a spatiotemporal measurement.

Continuous Monitoring of the Carbon Dioxide Concentration in the Urban Atmosphere of Nagoya, 1991-1993

Abstract: The present paper is a preliminary report concerning a continuous monitoring experiment for the atmospheric carbon dioxide (CO 2 ) concentration conducted since November, 1990, in Nagoya. Even though Nagoya is an urban area, the annual variation of the atmospheric CO 2 concentration shows a seasonal oscillation (lower in the summer and higher in the winter), as observed in the global atmosphere. In addition to the seasonal oscillation, a daily change in which the atmospheric CO 2 concentration was lower during the day and higher during the night was also observed. Furthermore, additional peak concentrations, which might arise from the exhaust gases of automobiles, were observed during the morning and evening rush hours. The annual mean C0 2 concentration was 381 ppmv in 1991, 382 ppmv in 1992 and 377 ppmv in 1993. These values were significantly higher than those of the global mean concentration (353 ppmv) in 1990. The annual rate of increase for three years (from January, 1991, to December, 1993) was -1.29 ppmv/year, which corresponds to -0.34%/year.

Total Reflection Method Applied to an X-Ray Absorption Fine Structure Study of Aqueous Solution Surface

Abstract: A new method has been developed for investigating the geometrical structure of chemical species present at a solution surface. The method is the X-ray absorption fine structure (XAFS) spectroscopy which is combined with X-ray introduction under a total reflection condition. It was applied to aqueous solutions of bromide and rubidium ions with and without surface-active counter ions. The solvation structure for Br- segregated at the surface by surface-active cation has been found to be different from that in the bulk. The coordination numbers for both the cation and anion at the surface have also been found to be different from those in the bulk. Due to the highly polarized X-ray radiation from a synchrotron used in this study, the coordination number determined by this technique must reflect the preferential orientation of the observing atom-atom bond with respect to the solution surface. Thus, the solvation structures and orientations for ions at the surface are discussed.

Chemiluminescent Determination of 6-Mercaptopurine in Pharmaceutical Preparation

Abstract: A new chemiluminescence method is described for the determination of 6-mercaptopurine. In the presence of tris(2,2'-bipyridine)ruthenium(II), 6-mercaptopurine (6MP) and hydrogen peroxide, upon the addition of hydroxide ion, resulted in intense light emission. The emission intensity is greatly enhanced by the presence of Tween-20. The linear range and detection limit of 6MP are 1.4x10 -5 -1.4x10 -8 g/ml and 3x10 -10 g/ml, respectively. The method was evaluated by carrying out an interference study with common excipients and other coexisting compounds, by a recovery study and by the analysis of commercial formulation. A satisfactory result was obtained.

X-Ray Absorption and Photoelectron Spectroscopies Using Total Reflection X-Rays

Abstract: X-Ray absorption and X-ray photoelectron spectroscopic measurements of specular surfaces using grazing incidence soft X-rays carried out by the present authors are summarized. The merits of using a sample current induced by totally reflected X-rays are discussed. The probing depth of the sample current has been found to be restricted to within a very shallow surface. The application of the sample current X-ray absorption experiment to the depth profile analysis of fly-ash powders is described. Very clear X-ray photoelectron spectra excited by totally reflected X-rays are for the first time reported.

Ion Chromatographic Determination of L-Ascorbic Acid, Sulfite, Sulfide and Thiosulfate Using a Cation-Exchanger of Low Crosslinking

Abstract: L-Ascorbic acid, sulfite, sulfide and thiosulfate have each been completely separated from their mixtures by a single run of ion chromatography using columns of a sulfonated styrenedivinylbenzene copolymeric cation-exchange resin of low crosslinking (1%) and an eluent of 2.3×10-3M (M=mol dm-3) sulfuric acid containing 4%(v/v) acetonitrile. L-Ascorbic acid and sulfite as sulfurous acid were eluted early by an ion-exclusion effect of the cation-exchanger in the hydrogen form, but both the sulfide as hydrogen sulfide and the thiosulfate ion were eluted late, owing to their adsorption onto the resin bed. The sample species in the effluent were monitored by photometric measurement of the excess of iodine (as triiodide) for its postcolumn reaction with each species separated. The chromatograms obtained gave negative peaks, based on the decrease in the absorbance from background. Calibration graphs for the four species, plotted as peak-heights vs. concentrations, were linear up to 3.00×10-5M for L-ascorbic acid, 1.25×10-4M for sulfite, 5.00×10-4M for sulfide and 1.20×10-4M for thiosulfate, respectively. Detection limits as S/N=3 were 1.00×10-7M for L-ascorbic acid, 1.15×10-6M for sulfite, 1.80×10-6M for sulfide and 6.80×10-6M for thiosulfate. The proposed method was successfully applied to the determination of L-ascorbic acid in soft drinks, sulfide in hot-spring waters and sulfite in wines.

Glutamate Sensors Carrying Glutamate Oxidase/Peroxidase Bienzyme System on Tin Oxide Electrode

Abstract: Amperometric L-glutamic acid sensors were fabricated by depositing L-glutamate oxidase (GLOD) and horseradish peroxidase (HRP) on the tin oxide surface by means of either encapsulation into electropolymerized pyrrole or sequential chemical modification via surface hydroxyl groups. After optimization for the mixing ratio of the two enzymes during polypyrrole (PPy) formation and for the thickness of the PPy film, the GLOD/HRP/PPy electrode gave a nearly linear response to L-glutamic acid in the concentration range of 10-7 to 10-4M, with a response time of about 1min. The GLOD/HRP bilayer-modified electrode showed a sensitivity comparable to that of the PPy-encapsulated bienzyme electrode. Both type of electrodes exhibited excellent substrate specificity. The pH dependence and lifetime of the sensor response are also given.

Separation and Recovery of Gold, Platinum and Palladium by a Trioctylamine Liquid Membrane

Abstract: The carrier-mediated transport of gold(III), platinum(IV) and palladium(II) through a supported liquid membrane (SLM) has been investigated by the use of trioctylamine (TOA) as a mobile carrier. The transport behavior of precious metals greatly depended on the carrier concentration. At a low TOA concentration, Au(III) was preferentially transported across an SLM from the HCl feed solution to the HClO 4 product solution. The transport of Pt(IV) and Pd(II) was enhanced with increasing carrier concentration. The separation factor of Au(III) over Pt(IV) and Pd(II) was improved with increasing concentration of HCl in the feed solution

Simultaneous Determination of 266 Chemicals in Water at ppt Levels by GC-Ion Trap MS

Abstract: The authors have developed an analytical method for determining trace amounts of 266 kinds of chemicals, such as pesticides, in natural water samples by GC-ion trap MS. The advantage of the method is not only that one can perform simultaneous determinations of a large number of chemicals within a short time, but also that experiments can be performed with high precision and accuracy, because the method has some quality-control systems. One liter of water was extracted by dichloromethane ; the extract was concentrated to 1 ml with a Kuderna-Danish concentrator. After adding internal standards, a qualitative and quantitative analysis by GC-ion trap MS was performed. The results concerning the overall recovery tests at 0.1 μg/l showed that the mean recovery was 92.1% and that the mean relative standard deviation was 10.8%. The mean detection limit of the method was 0.036 μg/l. To confirm the performance of the method, seawater and river water samples were analyzed. More than 100 chemicals were detected in water at below the 0.1 μg/l level. This method is useful for elucidating the concentration levels and the fate of chemicals in the aquatic environment.

Applications of a Glazing Incidence X-Ray Fluorescence Analysis to Forensic Samples

Abstract: A glazing incidence X-ray fluorescence analysis (GIXF) has been applied to forensic samples : a counterfeit 100-dollar bill, fragments of polyvinyl tapes, a trace of semen, illegal drugs, fingerprints and fake V.S.O.P brandy. Strontium could not be detected on the magnet-respondent letter of the counterfeit bill and Br was detected on the magnet-nonrespondent part of the counterfeit bill, while such phenomena could not be noticed on a true bill. Fragments of black vinyl tapes related to a sexual assault case could be discriminated from each other. Zinc as a characteristic ingredient could be detected in a trace of semen. Bromine was detected in each of what is called a pure methamphetamine crystal and K, Ca, Fe, Zn etc. were detected in heroin powders. Lead was sharply detected in gunshot residues attached to a finger after gun-firing. Sulfur as a contaminant was abundant in fake V.S.O.P brandy, while no S was detected in genuine V.S.O.P brandy.

Voltammetric Behaviors of Dopamine and Ascorbic Acid at a Glassy Carbon Electrode Anodized in 1,ω-Alkanediol

Abstract: The voltammetric behaviors of dopamine (DA) and ascorbic acid (AA) at a glassy carbon (GC) electrode anodized in 1, ω- alkanediol were examined. On cyclic voltammetry (CV) at a 1, 5-pentanediol-modified GC electrode, the electrode process of AA was totally depressed, while DA still showed an anodic peak. Although similar phenomena were observed at a GC electrode anodized in 1, 3-propanediol or 1, 4-butanediol, the 1, 5-pentanediol-modified electrode suppressed the anodic peak of AA most strongly. The voltammetric response of DA at the modified electrode was enhanced by the presence of AA, and the extent was controllable by a proper selection of the sweep rate. Thus, the electrochemical detection of DA with high sensitivity was achieved through CV with 0.1V s-1 at the modified electrode by adding excess AA, while electrochemical discrimination of DA from excess AA was realized at 20V s-1.

Syntheses of a Series of Fluorescent Carboxylic Acids with a 1, 3-Benzodioxole Skeleton and Their Evaluation as Chiral Derivatizing Reagents

Abstract: A series of fluorescent carboxylic acids with a common skeleton of 1, 3-benzodioxole-4- or 5-carboxylic acid were prepared as racemates or in an optically active form. Their potential as chiral derivatizing agents was evaluated in terms of the HPLC and 1H-NMR analyses of D, L-amino acids. Every acid with a carboxyl group at C-4 showed high separation ability of original enantiomers in both the HPLC and the NMR analyses, while the C-5 isomer showed little separation. As the difference in the bulkiness between the two substituents at C-2 became wider, the separation by HPLC increased. These studies allowed us to develop new chiral reagents useful both for HPLC and 1H-NMR analyses, along with our previous reagent, (S)-TBMB carboxylic acid.

A Standard Sample Preparation Method for the Determination of Metal Impurities on a Silicon Wafer by Total Reflection X-Ray Fluorescence Spectrometry

Abstract: Total reflection X-ray fluorescence spectrometry (TXRF) is widely used for the determination of surface metal contamination on a silicon wafer. In TXRF measurements, calibration standard samples are needed to quantify the metal concentration. We propose a new method for the preparation of calibration standard samples for TXRF. The method is called "Immersion in Alkaline Hydrogen Peroxide Solution (IAP)", in which the silicon wafers are immersed in an intentionally contaminated alkaline hydrogen peroxide solution. Samples made by the IAP method are suitable calibration standard samples for TXRF because of their good depth profile reproducibility and good uniformity on the surface as well as homogeneity in a given batch. They can also be applied for making a cross-check among plural TXRF instruments as well as among different analytical methods.

Laser Electrochemical Detection Technique in a Flow System

Abstract: A platinum electrode was illuminated with an intermittent Ar+ laser beam in a flowing solution from the front side of the electrode. The current induced by this laser illumination was recorded as a function of electrode potential with a lockin amplifier and was directly observed with an oscilloscope. Experimental results for the [Fe(CN)6]4-, [Fe(CN)6]3-, Fe2+ and Fe3+ solutions indicated that the laser heating of the electrode promotes or depresses the electron transfer process, depending on the standard entropy change of the electrode reaction, promotes the mass transfer process and induces adsorption or desorption of anions. Further, the induced current was proportional to the concentration and strongly depended on the chopping frequency and the flow rate. This laser electrochemical (LEC) detection can be used as a newelectrochemical detector in electroanalytical chemistry.

Microanalysis in Forensic Science: Characterization of Single Textile Fibers by Total Reflection X-Ray Fluorescence

Abstract: Fibers of different textile materials, such as polyesters, viscose, and wool, were analyzed using TXRF, yielding a "fingerprint-type" trace element pattern. This pattern can be compared with quantitative results obtained from weighable amounts, e.g. 50μg of the material. It can be shown that, besides the omnipresent elements Ca, Fe and Zn, some of these materials contain specific elements, such as P, Mn or Sb, and that an identification is possible using these elements even when very small amounts of sample are involved (i.e. sub-microgram). Since many synthetics contain Ti as a matting additive, the Ti concentration can be used to further differentiate the sample material.

A Review of Standardization Issues for Total Reflection X-Ray Fluorescence and Vapor Phase Decomposition/Total Reflection X-Ray Fluorescence

Abstract: Total reflection X-ray fluorescence (TXRF) and vapor phase decomposition (VPD) followed by TXRF have become widespread methods for measuring surface metal contamination on silicon wafers. For quantification, TXRF and VPD/TXRF require reference samples. Since the approaches to making and using these reference samples have significantly varied among TXRF manufacturers and users worldwide, there exists some confusion about the quantification of TXRF and VPD/TXRF. This paper summarizes the basic issues behind TXRF and VPD/TXRF quantification using reference samples.

Determination of Nickel, Cobalt, Copper, Thorium and Uranium in High-Purity Zinc Metal by ICP-MS with On-Line Matrix Separation

Abstract: Traces of Co, Cu, Ni, Th and U in high-purity zinc metal were determined by inductively coupled plasma mass spectrometry (ICP-MS) in combination with flow-injection (FI) on-line matrix separation (FI-ICP-MS). The anion-exchange separation method of an HCl system was applied to the separation of Ni, Co, Cu, Th and U from zinc matrix. The matrix element, zinc, was adsorbed on an anion-exchange (BIO-RAD AGI-X8) mini-column (2.0 mm i.d.×300 mm bed length), while the analytes were directly introduced into the ICP-MS. The following detection limits (in ng g -1 ) were obtained: Ni, 3.1; Co, 1.2; Cu, 4.0; Th, 0.12 and U, 0.48. The reproducibility has proved to be satisfactory with a relative standard deviation of less than 5% (at the 10 ng ml -1 level, n=3). The method was successfully applied to determining trace impurities in three 6 and 7 nines grade of high-purity zinc metals and in three standard reference materials of high-purity zinc metal samples (from NIST)

Linear Polystyrene-Grafted Silica Gels for High-Performance Liquid Chromatography

Abstract: Linear polystyrene, having a trimethoxysilyl group at one side of the terminal group, was newly synthesized and readily grafted onto porous silica gels using the terminal reactive group. Its packed column showed good selectivity based on the π-π interaction for polyaromatics as well as the usual reversed-phase mode separation for alkyl compounds. The elution behavior (including the elution order) agreed with those that were observed in non-silica-supported poly(styrene- divinylbenzene) spherical particles. However, the asymmetric factor (As) of the elution peaks for polyaromatics was much better in silica-supported polystyrene than in non-supported polystyrene; for example, the values of As for triphenylene were 1.4 and 4.7, respectively. This difference is attributable to the flexibility of the polystyrene chain.

Probing Depth of Ultraviolet Photoelectron Yield Spectroscopy in Hydrocarbon Films

Abstract: In order to clarify the probing depth of ultraviolet photoelectron yield spectroscopy, the escape depth (L) of photo- electrons with kinetic energy (Ek) lower than 1.5eV in hydrocarbon films was determined by using self-assembled monolayers ofn-alkylmercaptane adsorbed on gold. The yield of photoelectrons from the gold substrate decreased exponentially as the chain length of n-alkylmercaptane increased. TheLwas 1.0±0.1nm for electrons with Ek lower than 0.3eV, and increased to 3.0±0.9nm for electrons with the maximumEk of 1.5eV.