Magnetic Sector Field Research Articles

A novel ground-potential monochromator design

An electron monochromator design is presented as an instrumental development for electron energy loss spectroscopy (EELS) and imaging in (scanning) transmission electron microscopy ((S)TEM). The main purpose of this development is enhancing the energy resolving power in spectroscopy and filtering. In addition, it helps reducing the effect of the objective lens’ chromatic aberration Cc in imaging and therefore, enhancing the spatial resolving power of electron microscopes. General estimates for the performance of a monochromator in energy distribution and the resulting usable beam currents are given. The special monochromator design presented is a ground-potential monochromator based on magnetic sector fields. The monochromator generates a spatially and angular un-dispersed spot and has no mechanically actuated parts in the filter sections. The optics can be operated at electron acceleration voltages from 30kV to 300kV and shows an energy resolving power of better than 2⋅10−7 relative to the primary electron energy. The actual device is designed to be retro-fittable to microscopes from various manufacturers.

Microanalysis using secondary ion emission By Raimond Castaing and Georges Slodzian

Secondary ion mass spectrometry (SIMS) is an analytical technique used to probe the composition of solids through the sputtering of the sample with a focussed primary ion beam and analyzing the emitted secondary ions generated under impact. These secondary ions ranging from elementals to low molecular weight organics offer precious information on the molecular composition and spatial organization within samples. Today, the combination of modern SIMS methodologies and instrumentation offers the most sensitive surface analysis approach, with elemental detection limits ranging from parts per million to parts per billion. In this month’s special feature article, we retrace the steps taken by Raimond Castaing and Georges Slodzian who in the early 1960’s developed a SIMS instrument based on a magnetic double focusing sector field mass spectrometer and using argon for the primary ion beam. This seminal work was first published in 1962, in French, in the Journal de Microscopie, which limited access to the scientific community at large. This article is now available in English and is beautifully prefaced by Georges Slodzian who revisits his steps as a young graduate student in the Castaing laboratory and discusses the scientific legacy of this article in the development of modern SIMS instrumentation.

Investigation of mass-scale drift effects in the milli-mass range: Influence on high mass resolution mode multicollector-inductively coupled plasma mass spectrometer isotope ratio measurements.

The consequences, possible origins, and prevention of mass-scale drifts in the high mass resolution mode (HR, M/ΔM ≈ 8000) under constant conditions were investigated and simulated in case of a multicollector-inductively coupled plasma mass spectrometer (MC-ICP-MS) using silicon enriched in 28 Si as the main element in this survey. A drifting mass scale strongly impairs the precise and accurate determination of isotope ratios, depending especially on the peak/plateau width. For example, 29 Si+ in Si highly enriched in 28 Si has an extremely small mass plateau width of ΔM ≤ 4 × 10-3 u, compare ΔM(56 Fe+ ) ≈ 18 × 10-3 u, which is to our knowledge one of the smallest plateaus routinely investigated in isotope ratio measurements, thus requiring extreme stability. During warm-up of the double-focusing sector field mass spectrometer, a mass drift up to ΔM/Δt ≥ 0.006 u/hr has been observed. Long-term studies on mass scale stability and simulations concerning fluctuations of the magnetic field B, acceleration voltage Uacc and ESA voltage UESA are reported. A change of one of these quantities of 0.01% induces changes of the mass scale of 6 × 10-3 u, 3 × 10-3 u, and 1 × 10-3 u in the case of B, Uacc , and UESA , respectively. After identifying electrical charging/discharging effects in the mass spectrometer affecting the mass scale stability, the instrument was completely dismantled and carefully reinstalled. Additional stability tests using silicon, strontium, and lead finally yielded a mass drift of ΔM/Δt ≤ 0.001 u/8 h in the case of silicon. This enhanced stability guarantees measurements of isotope ratios on smallest plateaus with lowest uncertainty. The importance of a stable mass scale is pointed out and the relevant quantities of a typical magnetic sector field mass spectrometer are discussed.

Momentum dispersion and mass resolution of magnetic sector instruments for surface analysis

The momentum‐dependent dispersion of ion trajectories in a homogeneous magnetic sector field is derived by a trigonometric method. Contrary to the often used transfer matrix technique, the results are not restricted to small momentum differences. Based on the results of the trigonometric evaluation, an expression for the mass resolution of a magnetic sector for an entering ion beam is derived and extensively discussed. An example for high‐resolution magnetic mass spectrometry is presented.

A newly developed prototype magnetic sector mass spectrometer with high resolving power

In recent years, high-resolution mass spectrometers have been applied and made tunable to various fields. In this study, a prototype magnetic sector mass spectrometer with high resolving power has been developed. Its design is based on employing a 90̂ magnetic sector-field analyzer with double trajectory, a miniature Nier-type ion source and two channel-type electron multipliers for ion detection. The ion accelerating voltage is controllable from 200 to 2500 V and the mass range demonstrated herein is (1–73) amu. The resolution of the instrument for the lower mass channel helium and higher mass channel nitrogen is about 0.008 amu and 0.05 amu (full width at half maximum), respectively, or m/Δ m ≈550 in the whole mass range of (1–40) amu. The present sensitivity is measured to be approximately 8.4 × 10−5 A/Pa.

Design and aberration study of a new miniature energy analyzer with correctors in a scanning electron microscope

The authors design a new miniature energy analyzer with a magnetic sector and corrector systems that can be installed in a scanning electron microscope for measurement of electron energy loss spectra. In order to correct the geometrical aberration caused by a magnetic sector field, two new corrector systems are designed using ray tracing simulations. One corrector system, using a simple structure, fully corrects second-order aberration and partially corrects third-order aberration. Another corrector system fully corrects third-order aberration in the x-direction. By aberration correction, energy resolution of ∼10 meV at an initial angle of 5 mrad can be achieved with either of the two corrector systems. Additionally, we reveal an overall process of aberration correction in ray tracing simulations, which can be a useful guideline for aberration correction. Notably, we have established a new method of aberration correction using a simple matrix equation. Using this method, second-order aberration can be fully corrected both conveniently and rapidly. This study enables us to easily calculate and correct geometrical aberration without solving complicated equations.

Quantitative analysis of plant leaf elements using the LA-ICP-MS technique

The use of Laser Ablation coupled to Mass Spectrometry is a promising technique for analysis of plant tissue elements because it has advantages over conventional digestion techniques such as reduced levels of contamination and sample preparation time, possibility of spatial distribution analysis of elements, high number of repetitions, among others. This technique still has some limitations, especially with regard to calibration, which may result in inaccurate chemical analyses. The present study aims to compare two types of analyzers (quadrupole and magnetic sector field) regarding precision and accuracy. Certified material was examined, and also plant material analyzed by both solution in inductively coupled plasma optical emission spectrometry (ICP OES) and by laser ablation methods. It was observed that the magnetic sector field analyzer gave better results in relation to precision (relative standard deviation - RSD) and accuracy (recovery). An elemental composition pattern similar to that of the sample to be analyzed is relevant to validate the methodology for the analysis of leaves collected under different environmental conditions. Thus, pulverized Eremanthus erytropappus leaves were suggested as a second type of validation standard to be used in analyzes of plants belonging to the Atlantic and Cerrado Forest biomes.

Detrital zircon U–Pb ages of Paleo- to Neoproterozoic black shales of the Baikal-Patom Highlands in Siberia with implications to timing of metamorphism and gold mineralization

A U–Pb isotope system of detrital zircons was used to determine the provenance and maximum deposition age of the Paleoproterozoic Mikhailovka and the Ediacaran Khomolkho, Aunakit and Dogaldyn Formations in the Baikal-Patom Highlands, one of the most ancient components of the Central Asian Orogenic Belt (CAOB). The Baikal-Patom area hosts the world-class Lena orogenic gold province with total endowment of up to 6000 tons Au. The gold deposits are confined to the compressional syn- or post-metamorphic structures at various stratigraphic levels in Paleoproterozoic to Neoproterozoic black shale sequences deposited in the Bodaibo passive margin of the Siberian platform. Nine samples were collected from mineralized rocks to address possible detrital zircon recrystallization and transformation during superimposed metamorphic and hydrothermal events. Zircon was analyzed using laser ablation – single collector – magnetic sectorfield – inductively coupled plasma – mass spectrometry (LA-SF-ICP-MS).The Mikhailovka metasediments yielded three concordant isotopic age clusters: predominant Archean (2.45–3.25 Ga), Paleoproterozoic (1.96–2.10 Ga), and Cambrian-Ediacaran-Cryogenian (524–702 Ma), whereas most of the zircon grains yielded discordant isotopic ages. Multiple lines of evidence (i.e., grain morphologies, specific set of mineral inclusions, distinctly high U and Th in zircon, and several persistent Pb-loss arrays of discordant ages with lower intercepts of concordia line at closely the same point) suggest the youngest cluster combines formerly metamict zircon of different ancient ages recrystallized under the peak conditions of the regional metamorphic event synchronous with or slightly preceding mineralization. Therefore, the youngest detrital grain isotopic age of 1961 ± 20 Ma determines the maximum depositional age of the Mikhailovka sediments that predates the emplacement of the Paleoproterozoic collisional Chuya-Nechera granitoids.Zircon grains from these granitoids represent a major population in the overlying Ediacaran sequences that indicate the prevalence of supply from proximal local sources, such as the uplifts of the ancient Siberian basement within the Bodaibo paleobasin. The age span of the Neoproterozoic detrital zircon population (604–966 Ma) in the younger formations is in agreement with the events in the CAOB and Siberian platform.

Rutile R632 – A New Natural Reference Material for U‐Pb and Zr Determination

A new natural rutile reference material is presented, suitable for U‐Pb dating and Zr‐in‐rutile thermometry by microbeam methods. U‐Pb dating of rutile R632 using laser ablation ICP‐MS with both magnetic sector field and quadrupole instruments as well as isotope dilution‐thermal ionisation mass spectrometry yielded a concordia age of 496 ± 2 Ma. The high U content (> 300 μg g−1) enabled measurement of high‐precision U‐Pb ages despite its young age. The sample was found to have a Zr content of 4294 ± 196 μg g−1, which makes it an excellent complementary reference material for Zr‐in‐rutile thermometry. Individual rutile grains have homogeneous compositions of a number of other trace elements including V, Cr, Fe, Nb, Mo, Sn, Sb, Hf, Ta and W. This newly characterised material significantly expands the range of available rutile reference materials relevant for age and temperature determinations.

Newly developed compact magnetic sector mass spectrometer

In this paper, a novel compact magnetic sector mass spectrometer has been developed and tested at the Lanzhou Institute of Physics. Its design is based on the mass separation of a 90° magnetic sector-field analyzer with a double trajectory, which has a magnetic induction of 0.7 T. The instrument can record mass range from 1 to 50 amu based on two special trajectories when the accelerating voltage of the ion source changes from 200 V to 2500 V. The dimensions of the device together with the electronic unit are approximately 220 × 165 × 165 mm3 (length × depth × height). The weight is 10.2 kg and the maximum power consumption was measured to be 20 W. The instrument successfully provided mass spectrum information on the vacuum test chamber, resolving power of about 58 at full width half maximum (FWHM), and demonstrated a sensitivity of 7.2 × 10−5 A Pa−1. The compact magnetic sector mass spectrometer is designed for space application and can be extended to other portable analytical fields on Earth.

Performance evaluation of a miniature magnetic sector mass spectrometer onboard a satellite in space.

With the rapid development of space technology in China, it is urgent to use mass spectrometer to detect the space environment. In this work, a space miniature magnetic sector mass spectrometer is evaluated, which consists of three subsystems: (1) physical unit, (2) electric control unit, (3) and high voltage power. It has 90° magnetic sector-field analyzer with double trajectory, in which a trajectory measurement range is from 1 to 12 amu, the other range is from 6 to 90 amu.The mass spectrometer has two work models, one is used to measure space neutral gas when the filament of mass spectrometer ion source turned on, the other is used to measure space charged ions when the filament turned off. The absolute resolution of this device is less than 1 amu, the minimum detectable ion current is about 10-13 A, and the sensitivity is 10-6 A/Pa (N2). Its overall size is 170 mm × 165 mm × 170 mm, its weight is 4.5 kg, and its power consumption is 18 W. A series of environmental adaptability tests, including high and low temperature cycle, shock, vibration, thermal vacuum cycle, were carried out on the ground before launching, and sensitivity and peak position were also calibrated on the ground. In November 2012, the mass spectrometer was carried by an experimental satellite to 499 km sun synchronization and is still working right now. It successfully detected the atmosphere compositions both in the satellite orbit and gas-emitted from satellite, including O, He, 12CO2, 13CO2, H2, N2, O2, H2O, and so on.

U-Pb detrital zircon dates and provenance data from the Beaufort Group (Karoo Supergroup) reflect sedimentary recycling and air-fall tuff deposition in the Permo-Triassic Karoo foreland basin

Detrital zircon U-Pb age dating was used for provenance determination and maximum age of deposition for the Upper Permian (upper Teekloof and Balfour formations) and Lower Triassic (Katberg Formation) lithostratigraphic subdivisions of the Beaufort Group of South Africa's Karoo Basin. Ten samples were analysed using laser ablation - single collector - magnetic sectorfield - inductively coupled plasma - mass spectrometry (LA-SF-ICP-MS). The results reveal a dominant Late Carboniferous-Late Permian population (250 ± 5 Ma – 339 ± 5 Ma), a secondary Cambrian-Neoproterozoic (489 ± 5 Ma to 878 ± 24 Ma) population, a minor Mesoproterozoic (908 ± 24 Ma to 1308 ± 23) population, and minor occurrences of Devonian, Ordovician, Proterozoic and Archean zircon grains. Multiple lines of evidence (e.g. roundness and fragmentary nature of zircons, palaeo-current directions, and previous work), suggest the older zircon populations are related to sedimentary recycling in the Gondwanide Orogeny. The youngest and dominant population contain elongate euhedral grains interpreted to be directly derived from their protolith. Since zircons form in felsic igneous rocks, and no igneous rocks of Late Permian age occur in the Karoo Basin, these findings suggest significant input of volcanic material by ash falls. These results support sedimentological and palaeontological data for a Lopingian (Late Permian) age for the upper Beaufort Group, but contradict previous workers who retrieved Early Triassic dates from zircons in ashes for the Beaufort and Ecca Groups. Pb-loss not revealed by resolvable discordance on the concordia diagram, and metamictization of natural zircons are not factored into the conclusions of earlier workers.

Near-road enhancement and solubility of fine and coarse particulate matter trace elements near a major interstate in Detroit, Michigan

Communities near major roadways are disproportionately affected by traffic-related air pollution which can contribute to adverse health outcomes. The specific role of particulate matter (PM) from traffic sources is not fully understood due to complex emissions processes and physical/chemical properties of PM in the near-road environment. To investigate the spatial profile and water solubility of elemental PM species near a major roadway, filter-based measurements of fine (PM2.5) and coarse (PM10-2.5) PM were simultaneously collected at multiple distances (10 m, 100 m, and 300 m) from Interstate I-96 in Detroit, Michigan during September–November 2010. Filters were extracted in water, followed by a hot acid extraction, and analyzed by magnetic sector field high resolution inductively coupled plasma mass spectrometry (HR-ICPMS) to quantify water-soluble and acid-soluble trace elements for each PM size fraction. PM2.5 and PM10-2.5 species measured in the near-road samples included elements associated with traffic activity, local industrial sources, and regional pollution. Metals indicative of brake wear (Ba, Cu) were dramatically enriched near the roadway during downwind conditions (factor of 5 concentration increase), with the largest increase within 100 m of the roadway. Moderate near-roadway increases were observed for crustal elements and other traffic-related PM (Fe, Ca), and the lowest increases observed for regional PM species (S). Water solubility varied by PM species and size, and for PM2.5 included highly (S, K, Ca, Mg, Zn, Ba), moderately (Cu, Mn, Sb, Pb), and minimally (Fe, Ti) water-soluble species, with lower water solubility for most species in PM10-2.5. Results from this study indicate that water-soluble PM2.5 and PM10-2.5 metals, particularly from brake/tire wear, were enhanced in the near-roadway environment which may have human health implications.

Mass Spectrometric Investigation of Silicon Extremely Enriched in (28)Si: From (28)SiF4 (Gas Phase IRMS) to (28)Si Crystals (MC-ICP-MS).

A new generation of silicon crystals even further enriched in (28)Si (x((28)Si) > 0.999 98 mol/mol), recently produced by companies and institutes in Russia within the framework of a project initiated by PTB, were investigated with respect to their isotopic composition and molar mass M(Si). A modified isotope dilution mass spectrometric (IDMS) method treating the silicon as the matrix containing a so-called virtual element (VE) existing of the isotopes (29)Si and (30)Si solely and high resolution multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) were applied in combination. This method succeeds also when examining the new materials holding merely trace amounts of (29)Si (x((29)Si) ≈ 5 × 10(-6) mol/mol) and (30)Si (x((30)Si) ≈ 7 × 10(-7) mol/mol) extremely difficult to detect with lowest uncertainty. However, there is a need for validating the enrichment in (28)Si already in the precursor material of the final crystals, silicon tetrafluoride (SiF4) gas prior to crystal production. For that purpose, the isotopic composition of selected SiF4 samples was determined using a multicollector magnetic sector field gas-phase isotope ratio mass spectrometer. Contaminations of SiF4 by natural silicon due to storing and during the isotope ratio mass spectrometry (IRMS) measurements were observed and quantified. The respective MC-ICP-MS measurements of the corresponding crystal samples show-in contrast-several advantages compared to gas phase IRMS. M(Si) of the new crystals were determined to some extent with uncertainties urel(M) < 1 × 10(-9). This study presents a clear dependence of the uncertainty urel(M(Si)) on the degree of enrichment in (28)Si. This leads to a reduction of urel(M(Si)) during the past decade by almost 3 orders of magnitude and thus further reduces the uncertainty of the Avogadro constant NA which is one of the preconditions for the redefinition of the SI unit kilogram.

High-resolving mass spectrographs and spectrometers

Discussed are different types of high resolving mass spectrographs and spectrometers. In detail outlined are (1) magnetic and electric sector field mass spectrographs, which are the oldest systems, (2) Penning Trap mass spectrographs and spectrometers, which have achieved very high mass-resolving powers, but are technically demanding (3) time-of-flight mass spectrographs using high energy ions passing through accelerator rings, which have also achieved very high mass-resolving powers and are equally technically demanding, (4) linear time-of-flight mass spectrographs, which have become the most versatile mass analyzers for low energy ions, while the even higher performing multi-pass systems have only started to be used, (5) orbitraps, which also have achieved remarkably high mass-resolving powers for low energy ions.

Lead Isotopic Composition in Biogenic Certified Reference Materials Determined by Different ICP‐based Mass Spectrometric Techniques

This work presents data for the radiogenic Pb isotopic ratios (206Pb/207Pb and 208Pb/206Pb) in nine biogenic certified reference materials (NIST SRM 1515, 1566b, 1570a, 1573a, 1575a; BCR 100, BCR 101, BCR 670 and IAEA 359), which are suitable for analytical quality control in environmental research. The results were obtained using three different types of ICP‐based mass spectrometer (quadrupole‐based/magnetic sector field single‐collector ICP‐MS instruments and a multi‐collector ICP‐MS) and applying different mass bias correction procedures (calibrator‐sample bracketing and external Tl normalisation) with and without Pb separation from the matrix using ion exchange chromatography. In the majority of the samples, the measurements from all three of the ICP‐MS instruments were in agreement within ± 0.1%, despite the lower analytical precision of the single‐collector ICP‐MS instruments. We demonstrate that the presence of the sample matrix did not significantly influence the Pb isotopic ratios measured by magnetic sector field ICP‐MS, whereas the use of the two different mass bias corrections resulted in a systematic difference of 0.09% for the 208Pb/206Pb ratio.

Determination of 234U/238U, 235U/238U and 236U/238U isotope ratios in urine using sector field inductively coupled plasma mass spectrometry

Quantification of the isotopic composition of uranium in urine at low levels of concentration is important for assessing both military and civilian populations' exposures to uranium. However, until now there has been no convenient, precise method established for rapid determination of multiple uranium isotope ratios. Here, the authors report a new method to measure (234)U/(238)U, (235)U/(238)U and (236)U/(238)U. It uses solid-phase chelation extraction (via TRU columns) of actinides from the urine matrix, followed by measurement using a magnetic sector field inductively coupled plasma mass spectrometer (SF-ICP-MS-Thermo Element XR) equipped with a high-efficiency nebulizer (Apex PFA microflow) and coupled with a membrane desolvating nebulizer system (Aridus II™). This method provides rapid and reliable results and has been used successfully to analyse Certified Reference Materials.

Quantification of trace element contents in frozen fluid inclusions by UV-fs-LA-ICP-MS analysis

We have developed a new analytical setup for the determination of trace element concentrations in fluid inclusions by UV-fs-LA-ICP-MS. Laser ablation was performed at a low temperature of −40 °C by using a modified heating–freezing stage as the ablation cell. With this method it was possible to successfully analyse 53 of 55 frozen synthetic NaCl–H2O fluid inclusions in quartz, covering a size range between 8 μm and 25 μm down to a depth of 50 μm. The high success rate could be achieved as the 194 nm UV-fs-laser allows excellent control over the opening procedure of frozen fluid inclusions. Trace element analyses were performed with a fast scanning magnetic sector field ICP-MS. The lower limits of detection for fluid inclusion analysis vary from 0.1 μg g−1 (for 209Bi) to 10 μg g−1 (for 39K). The typical analytical uncertainty, depending on the element and respective concentration level, ranges between 10% and 30% (1RSD), based on the reproducibility of experimentally synthesized fluid inclusions. All elements from a stock solution, which behaved inert during the HP/HT experiments (B, K, Cd, Te, Tl, Pb and Bi), could be recovered in the synthetic inclusions at concentrations that correspond within their specific analytical uncertainties to their original concentration of 53 μg g−1. The method represents a highly efficient tool for the determination of accurate trace element data on low concentration levels in small fluid inclusions with a high success rate of >90%. The latter is particularly advantageous considering the commonly time consuming characterization of fluid inclusions.

Comprehensive modelling of secondary-ion energy spectra measured with a magnetic sector field instrument: I. Concept and basic features

In order to correlate measured secondary-ion energy spectra with theoretically predicted energy distributions of sputtered atoms, one needs to know the properties of the employed mass spectrometer. Here the resolution function and the transmission of a frequently employed magnetic sector-field instrument (IMS-nf series) are described using a simple concept. Modelling of energy spectra is greatly facilitated by the recently provided evidence that the ionisation probability of secondary ions is independent of their emission velocity. Hence the ejected ions may be described by Thompson-type energy distributions which are fully defined by the surface binding energy, Es, of sputtered atoms. On passage through the mass spectrometer the energy distributions are modified in response to the energy dependent transmission of the ion optical system, the aberrations of the lenses and the non-negligible size of the resolution defining apertures, notably the circular contrast aperture (CA) on the entrance side of the spherical energy analyser (aperture diameter d) and the energy slit on the exit side (slit width s). The diameter d determines a characteristic energy Ec up to which secondary ions with emission angles up to 90° to the surface normal are fed through the CA. At higher energies the transmission theoretically decreases as 1/E. Resolution functions are derived by convolving a rectangular box of unit height and width ω with a Gaussian featuring a standard deviation σ. The shape, width and height of the modelled energy spectra depend in a characteristic manner on ω, σ, Ec and Es. Owing to the very low characteristic energies (Ec≤0.1eV) associated with small CAs (d≤50μm), the resulting energy spectra contain very little useful information on the physics of the emission process. The peak position and the width of energy distributions are reproduced best using contrast apertures large enough to achieve Ec>2Es, in combination with a slit width corresponding to ω<Es/4. Inspecting calculated spectra on absolute scales, variations of ω are seen to shift the low-energy edge towards lower (‘negative’) energies by ω/2. With Es as an input parameter, the true origin of the energy scale may be assessed with an accuracy of about ±0.05eV. In measured spectra, the full widths at half maximum of the original distribution and the resolution function add up roughly in quadrature. Evidence in favour of the described concept is presented in the companion paper.

Comprehensive modelling of secondary-ion energy spectra measured with a magnetic sector field instrument: II. Evaluation of experimental data

In the preceding paper an approach was described that allows measured secondary-ion energy spectra to be correlated with the original energy distribution. Here the model was applied to analyse energy spectra of Cs+ ions sputtered from a Cs bombarded Si sample. The aim was to show that relevant properties of the employed IMS-4f sector field mass spectrometers as well as the mean surface binding energy of the Cs+ ions can be derived in one effort. The novel results may be summarised as follows. (i) The width of the energy slit can be determined in situ by comparing integral intensities for the chosen and the fully opened slit, the latter of known size. (ii) Pronounced yield losses occur if the width-to-thickness ratio of the slit is reduced below about 0.1. (iii) The broadening of energy spectra is predominantly defined by the slit width. Unexpectedly, the size of the circular contrast aperture plays only a minor role. The reason needs to be clarified. (iv) Measured spectra can be reproduced with a remarkable accuracy, better than ±8% throughout the whole spectrum. The origin of the energy scale can be determined with an uncertainty of ±0.05eV. (v) Beyond a characteristic energy Ec the transmission decreases more rapidly with increasing emission energy E than the expected Ec/E fall-off. The difference is probably due to the detuning of the immersion lens, an action inevitably associated with increasing target-bias offset. (vi) The surface binding energy Es of Cs+ ions, derived from spectra measured with the largest contrast aperture, turned out to be surprisingly high, 3.9±0.2eV. Evidently, the Cs+ ions were emitted from sites featuring very strong ionic bonding. The derived value of Es presumably represents the centroid 〈Es〉 of a spectrum of binding energies which reflect different local configurations of damage around the Cs ions. Considering the much lower binding energy of adsorbed Cs atoms, we have to conclude that sputtered ions and neutrals do not exhibit the same energy spectrum. Hence the previous practise of using one global ionisation probability to correlate secondary ion emission with the sputtering of neutral atoms needs to be abandoned.