Vacuum microbalances and thermogravimetric apparatus Part I: Commercially available instruments

Most vacuum balances used today are electromagnetically compensating beam balances with sensitivities down to the nanogram range. Deflection sensors operate either according to the optoelectric or the electromagnetic method. One type of UHV balance is equipped with magnetic sample suspension. For special tasks, quartz spring balances and crystal oscillators are available. Beam balances in the microgram and milligram ranges are also used in thermogravimetric and sorption measuring apparatus. Instruments and manufacturers are compiled in tables.     

Greater analytical accuracy through gravimetric determination of quantity

Summary Scatter in analytical results caused by production tolerances in the volumetric measuring apparatus (pipettes, burettes, volumetric flasks), can be eliminated by weighing in the sample and standard solutions. For the techniques described, only conventional instruments and balances available in any analytical laboratory are required. In this way it is possible to improve the precision of the analytical result by a factor of 10.

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Größere Analysengenauigkeit durch gravimetrische Mengenbestimmung

     

The ancient Egyptian balance

The oldest known balances are equal-armed instruments that have been found in Egypt and are represented on Egyptian drawings and reliefs. In its simplest form the beam was made of wood or stone and provided with a central boring for the balance suspension and two borings at the ends of the beam for the pan suspensions. The existence of a balance standard in the Fifth Dynasty is testified. A plummet line was hung parallel to the balance suspension so that the horizontal position of the beam could be checked by comparing the right angles formed between beam and plummet line. From the time of the Middle Kingdom the pans were suspended by four cords. In the New Kingdom tubular beams were introduced whose ends were shaped in lotus flower or papyrus form. The strings of the pans came out together from inside the beam and diverged to the sides, the lower beam ends acting as knife edges. In most cases a pointer is either missing or so short that it is hardly helpful. This type of elaborate standard balance which was also distributed in Syria, Greece and Persia, is often depicted in the Books of Death of the New Kingdom in the scene showing the “Death Tribunal” with “Weighing of the Heart”.Unclear drawings led to misinterpretations as to the design and mode of operation of these balances. By style analysis and comparison with finds exhibited in the Cairo Egyptian Museum, it was possible, however, to clarify details.     

混合稀土对ZA27合金阻尼性能的影响

研究了ＺＡ２７合金的稀土变质机制及阻尼性能,利用悬臂梁测试了阻尼能力。结果表明,合金中加入Ａｌ－１０％ＲＥ中间合金可以细化组织和提高阻尼能力,稀土添加量为０．３％时有最佳的变质效果和最佳高的阻尼能力。     

Fabrication and vibration characterization of electrically triggered shape memory polymer beams

Shape memory polymers (SMP) exhibit temperature, frequency and strain rate dependent properties which may be manipulated by various types of external stimuli to achieve desirable response characteristics. In recent years, the emphasis has been on designing SMPs which do not require external stimuli (such as a heat source) and have a rapid response time with large homogenous and reversible deformation characteristics. In this research, the fabrication process and dynamic vibration testing of an electrically activated SMP are presented. It is shown that conductive SMP beams can be fabricated to achieve tunable stiffness and damping with a reasonable thermal gradient generated by electrical triggering. This can allow the tuning of a range of frequency bandwidth and damping properties of SMPs for vibration control applications. The experimentation yielded modal properties (natural frequencies and damping) of the SMP beams. These parameters were validated against values obtained from the estimated performance of these beams based on the complex modulus parameters obtained using dynamic mechanical analysis (DMA). For a modest 20 °C temperature range in an epoxy based SMP, a resulting shift of approximately 7% in the natural frequency and 100% change in the damping ratio of a rectangular beam was successfully attained. These results recommend SMPs as being tunable materials that can enhance vibrational performance and expand the operational envelope of structures.     

Damped and thermal motion of laser-aligned hydrated macromolecule beams for diffraction

We consider a monodispersed Rayleigh droplet beam of water droplets doped with proteins. An intense infrared laser is used to align these droplets. The arrangement has been proposed for electron- and x-ray-diffraction studies of proteins which are difficult to crystallize. This paper considers the effect of thermal fluctuations on the angular spread of alignment in thermal equilibrium, and relaxation phenomena, particularly the damping of oscillations excited as the molecules enter the field. The possibility of adiabatic alignment is also considered. We find that damping times in a high-pressure gas cell as used in x-ray-diffraction experiments are short compared with the time taken for molecules to traverse the beam and that a suitably shaped field might be used for electron-diffraction experiments in vacuum to provide adiabatic alignment, thus obviating the need for a damping gas cell.     

Determination of sulfur and moisture in coal using capture gamma-rays

Samples of coal weighing approximately 200 g placed in a collimated beam of neutrons from the thermal column of the Ames Laboratory Research Reactor produce capture gamma-rays which can be used for the determination of sulfur in the presence of iron, a potential source of interference. Spectra from NaI(Tl) detectors were used with the help of Ge(Li) spectra to locate interferences. Corrections for iron interference was made by the use of a higher energy iron peak. A linear relation was found between the area of the hydrogen capture peak at 2.23 MeV and the amount of water added to coal.     

Parameter study of self-absorption effects in Total Reflection X-ray Fluorescence–X-ray Absorption Near Edge Structure analysis of arsenic

Total reflection X-ray Fluorescence (TXRF) analysis in combination with X-ray Absorption Near Edge Structure (XANES) analysis is a powerful method to perform chemical speciation studies at trace element levels. However, when measuring samples with higher concentrations and in particular standards, damping of the oscillations is observed. In this study the influence of self-absorption effects on TXRF–XANES measurements was investigated by comparing measurements with theoretical calculations. As(V) standard solutions were prepared at various concentrations and dried on flat substrates. The measurements showed a correlation between the damping of the oscillations and the As mass deposited. A Monte-Carlo simulation was developed using data of the samples shapes obtained from confocal white light microscopy. The results showed good agreement with the measurements; they confirmed that the key parameters are the density of the investigated atom in the dried residues and the shape of the residue, parameters that combined define the total mass crossed by a certain portion of the incident beam. The study presents a simple approach for an a priori evaluation of the self-absorption in TXRF X-ray absorption studies. The consequences for Extended X-ray Absorption Fine Structure (EXAFS) and XANES measurements under grazing incidence conditions are discussed, leading to the conclusion that the damping of the oscillations seems to make EXAFS of concentrated samples non feasible. For XANES “fingerprint” analysis samples should be prepared with a deposited mass and sample shape leading to an acceptable absorption for the actual investigation.     

The assessment of electronic balances for accuracy of mass measurements in the analytical laboratory

The accuracy of mass measurements when using electronic balances is considered within the frame of analytical assays in terms of the uncertainty budget according to the internal quality control routine, the calibration process, the balance specifications, and the weighing scenarios. Buoyancy corrections for both conventional and true mass are fully discussed. The procedure is illustrated with a worked example.     

Modern vacuumbalances

Different types of vacuum balances have been developed corresponding to the various applications [ 1 ]:

Special beam balances are applied for metrology and thermogravimetry and for the investigation of physical and chemical reactions of solid samples with the gas phase. Today, with all these balances the beam deflection is observed by an inductive or photoelectric sensor and electromagnetically restored.

In corrosive atmospheres or under very clean conditions e.g. for investigations in ultrahigh vacuum the magnetic suspension balance should be favoured. The sample tube is separated from the balance and can be hermetically sealed from the environment. It contains only the pan with the sample connected to a permanent magnet, suspended at a stable position in the field of a controlled electromagnet. The carrying balance is electrodynamically compensated.

The spring balance is an inexpensive alternative, if a minor relative resolution can be accepted. The simple design utilizing only few materials enables measurements under very clean conditions. A quartz spring allows for investigations in corrosive atmosphere. The extension of the spring is observed optically or by means of an inductive sensor.

Quartz resonators are used to control vacuum metallizing and other evaporating and sputtering processes. Resonator systems of any type need no gravitational field and can be applied, therefore, in space technology. The mass determination is restricted to samples which are strongly connected to the surface of the sensor.

     

Analysis of small amounts of solid samples by spark-source mass spectrometry

Various elements were determined in solid samples weighing < 2 mg by spark-source mass spectrometry. The samples were fixed on the top of a cylindrical graphite electrode using a conductive silver paint. After baking, the samples were sparked against a tantalum or gold wire. The method was used for the determination of impurities in steel, iron, molybdenum and CdSe. For samples weighing ca. 1 mg, detection limits of the order of 1 μg g^?1 were obtained. A relationship between the relative sensitivity factor and physical properties is proposed.     

Femtosecond laser ablation inductively coupled plasma mass spectrometry: Transport efficiencies of aerosols released under argon atmosphere and the importance of the focus position

Although the utilization of helium as aerosol carrier has been shown to improve both accuracy and sensitivity of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), occasionally, argon is being used due to practical and economic reasons. In order to provide more insight into the mechanisms underlying these performance differences, in this study, transport efficiencies of aerosols released by NIR- and UV-femtosecond laser ablation (LA) of brass applying laminar or turbulent in-cell flow conditions and argon as carrier gas were measured. Aerosol particles were collected by low-pressure impaction or filtered by fine porous membranes. On the basis of aerosol masses collected and mass differences derived from target weighing prior to and after LA, transport efficiencies approximately varied in between 75% and 95%. In addition, LA of a thin Cr layer was performed which allowed to release a well-defined amount of material and, thus, to correct mass balances for debris accumulating around the crater rim. The total aerosol mass released during LA was found to be strongly dependent on the relative focus position, i.e. surface area irradiated, even if the laser pulse energy delivered to the target was kept constant. Furthermore, a physical model only making use of input parameters such as laser spot size and pulse energy was implemented to qualitatively describe the correlation between aerosol mass and laser focus position.     

Effect of chemical exchange on radiation damping in aqueous solutions of the osmolyte glycine

Radiation damping is of central relevance in NMR spectroscopy especially with the advent of ultrahigh-field magnets and of supersensitive probes. Furthermore, the recent realization that the combined effect of the distant dipole field and of radiation damping causes the resurrection of undesired crushed water magnetization emphasizes the need for a thorough understanding of all the factors affecting radiation damping. While the effects of pulsed-field gradients and of active feedback have been extensively investigated, the consequences on radiation damping of chemical exchange between water and co-solutes is not as well understood. Here it is demonstrated that the rate of water radiation damping is significantly affected by free glycine (Gly), a representative of an important class of biocompatible osmolytes often used at molar concentrations as protein stabilizers. The pH and temperature dependencies of this effect were investigated and rationalized in terms of radiation damping attenuation caused by incoherent dephasing occurring in the intermediate exchange regime. For instance, at pH 6.0 and at a temperature of 313 K the Gly NH3+/water exchange has the same dramatic effect on radiation damping as a series of repeated weak PFGs, increasing the water inversion-recovery zero-crossing delay from approximately 30 ms to approximately 2.3 s. In addition, under these conditions, the Gly NH3+/water exchange suppresses the resurrection of unwanted crushed water magnetization. When used in combination with PFGs and water flip-back schemes, glycine is therefore expected to tame chaotic dynamics and improve the reproducibility of the NMR experiments affected by it.     

Advanced quality control for zinc speciation investigations in human breast milk

Summary Information is provided about the reliability of results obtained in Zn-speciation analyses of human milk. For such analyses, convenient techniques have already been given in an earlier paper [1], comprising mass balances of metal and ligand, purity of analytes, etc. Here, however, the influence of the eluent was checked in an extended manner. For this series of experiments, bi-distilled water was compared with different buffers as the eluent. Water proved to be a more suitable mobile phase than buffers with regard to contamination. Moreover, a method was developed to investigate the possibility of a transfer of Zn among the proteins during size exclusion chromatography. For these experiments, casein and metallothionein were chosen as competitive Zn-ligands, showing extremely different affinities to Zn. A possible Zn-transfer from one protein to the other was examined in combining a Zn-containing protein with a Zn-free one. No change of the Zn-status of the proteins was detected, indicating a stable protein-metal complex under the experimental conditions. Zn and protein mass balances (injected/eluted) were calculated and found to be 100%.     

Parametrical studies of electroosmotic transport characteristics in submicrometer channels

Spatially two-dimensional nonequilibrium mathematical model describing electroosmotic flow through a submicrometer channel with an electric charge fixed on the channel walls is presented. This system is governed by the hydrodynamic, electrostatic, and mass transport phenomena. The model is based on the coupled mass balances, Poisson, Navier-Stokes, and Nernst-Planck equations. Nonslip boundary conditions are employed. The effect of an imposed electric field on the system behavior is studied by means of a numerical analysis of the model equations. We have obtained the following findings. If the channel width is comparable to the thickness of the electric double layer, the system behaves as an ion-exchange membrane and the dependence of the electric current passing through the channel on the applied voltage is strongly nonlinear. In the case of negatively (positively) charged walls, a narrow region of very low conductivity (so-called ionic gate) is formed in the free electrolyte near the channel entry facing the anode (cathode) side. For a wide channel, the electric current is proportional to the applied voltage and the velocity of electrokinetic flow is linearly proportional to the electric field strength. Complex hydrodynamics (eddy formation and existence of ionic gates) is the most interesting characteristics of the studied system. Hence, current-voltage and velocity-voltage curves and the corresponding spatial distributions of the model variables at selected points are studied and described in detail.     

Metastable effects on martensitic transformation in SMA

The use of Shape Memory Alloys (SMA) in technical applications as damping in civil engineering structures requires the characterization of the alloy for each specific application. This involves the evolution of the mechanical properties and damping capacity with the number of cycles, frequency, maximum deformation, applied stresses, and the evolution of the alloy with aging time and temperature. In particular, the temperature effects associated to self-heating need to be evaluated. In continuous cycling the effects of latent heat, the associated dissipation induced by the hysteresis, the heat flow to surroundings and the cycling frequency induce different states of temperature in the specimen, which in turn produces changes in the transformation-retransformation stresses. In this article, the temperature effects associated to cycling are outlined for different cycling frequencies. The results show that, for relatively faster frequency the temperature arrives at an oscillatory state superimposed to an exponential increase. For lower frequencies, some parts of the sample attain temperatures below room temperature. The experimental results are represented with an elementary model (the 1-body model or the Tian equation used in calorimetric representation) of heat transfer. For the higher fracture where life requirements are associated to damping in stayed cables for bridges, the results show (for the NiTi alloy) a reduction of the hysteresis width as the frequency increases for deformations up to 8%. For reduced deformation, under 2% appears an asymptotic behavior where the frictional area is practically independent of the cycling frequency (up to 20 Hz). In addition, it is shown that more than 4 million of working cycles can be attained if the maximum applied stress is kept below a threshold of about 200 MPa. Although under this condition the deformation must remain lower than 2% a reasonable damping capacity can still be obtained.     

Controlling the performance of silicalite-1 membranes

The structural and performance characteristics (for n- and i-butane separation) of self-supported silicalite-membranes, were optimised by fine-tuning their syntheses by screening a total of nine silica sources and many reaction conditions. The mass balances indicate that membrane thickness is a function of both the synthesis volume and the silica source used. The excellent properties of the final membrane are demonstrated by its high permselectivity of 31 for n-butane combined with a n-butane flux of 10 mmol m(-2)s(-1), indicating perfect performance. For 50/50 mixtures (of n and i) the selectivity for nbutane was 48 and its flux was 3.8 mmol m(-2)s(-1). For the given selectivities, in relation to the membrane thickness, the theoretical fluxes are the highest values ever reported, underlining the point that high structural integrity is essential to achieve superior functionality.     

聚合物二元体系动态力学性能的估算

动态热机械分析是多相聚合物体系的一个重要研究手段.分析动态力学性能可以研究共混高聚物的相容性、复合材料的界面特性以及高分子运动机理等.本文综述了聚合物二元体系,即填充、纤维增强、共混体系动态力学性能的估算方法.在填充体系中,分别概述有无界面作用两种情况,当存在界面作用时,界面作用越强,模量越大,阻尼越小.对纤维增强体系,讨论了玻璃纤维有无取向的情况下模量和阻尼的估算.特别对于聚合物二元共混体系,分"海-岛"结构和双连续相两种情况,分别讨论了模量与阻尼的估算.     

Determination of an amount of gas by weighing under vacuum conditions and uncertainty analysis

For the determination of the calorific value of gases, a mass determination system for the gas burnt in the calorimeter was developed. Contrary to classical methods, which weigh the gas bottle offline, the method applied here weighs the amount of gas online and continuously. For this purpose, a commercial analytical balance with a readability of 10 µg across the entire weighing range of 205 g was modified for use under vacuum conditions.Since the gas bottle has to be permanently connected to the calorimeter by a steel capillary, an additional force is exerted to the balance. The influence of this force is investigated and determined by a special weighing procedure.In addition, an automatic calibration technique must be developed to ensure the traceability of the results to the national standard of mass. This allows an uncertainty analysis according to the Guide to the Expression of Uncertainty in Measurement (GUM) to be performed.This revised version was published online in November 2005 with corrections to the Cover Date.