Basic Principles and Apparatus

Scanning eiectron microscopy image of an etched STM tip. The scaie bar on the ieft of the image denotes a iength of 100 pm. From Ref. [5]. 

Aarhus STM head designed for high-speed scanning, (b) Schematic representation of a segmented tube scanner, illustrating the X, y, and z movements by applying voltages 

Low-temperature scanning tunneling microscopes (LT-STMs) are designed in a way that the entire STM and the sample are kept at a low temperature inside a cryostat [8, 9]. Such instruments use liquid helium (f He) as the cooling medium, allowing for operation at temperatures down to 4 K. Even ultralow temperatures below 1 K can be achieved with a mixture of He and He and specially designed mixing cryostats [10, 11]. 

Flow cryostats can be operated at various temperatures up to room temperature [9]. These cryostats allow for a faster cooling down than bath cryostats. However, the lowest temperature is usually a few degrees higher than that achieved with a bath cryostat and also the helium consumption rate is considerably higher, namely about 1.3 lh [12]. 

STM with tunneling electrode (TE), counter electrode (CE), working electrode (WE), and reference electrode (RE), (b) Commercial electrochemical STM with liquid cell, (a) From Ref [14]. (b) Courtesy of NT-MDT. 

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The result of the Back-to-Basics series is an accumulation of some 50 separate but interrelated expositions of mass spectrometric principles and apparatus. Some areas of mass spectrometry, such as ion cyclotron resonance and ion trap instruments, have not been covered except for passing references. This decision has not been due to any bias by the authors or Micromass but simply reflects the large amount of writing that had to be done and the needs of the greatest proportion of users. 

Despite some refinements in the methods, the basic principles and protocols of gel electrophoresis have not changed appreciably since their introduction. Proteins are introduced into a gel matrix and separated by the combined effects of an electrical field, buffer ions, and the gel itself, which acts as a protein sieve. At the completion of the electrophoresis run, separated proteins in the gel are stained to make them visible, then analyzed qualitatively or quantitatively. The topic has been covered in numerous texts, methods articles, and reviews.1-11 In addition, apparatus and reagents for analytical and preparative gel electrophoresis are available from several suppliers. 

The American Society of Lubrication Engineers has issued a compilation of friction and wear devices [2] which describes 234 different pieces of apparatus. However, the measurement of friction is governed by only a few basic principles, and consequently an appreciation of the practical techniques employed is not difficult to acquire. To quote Bowden and Tabor [3] Any method which will give at the same time a measure of the normal load between surfaces and of the tangential force necessary to cause sliding can be used to determine the coefficient of friction."... 

The basic principles to consider in establishing an experimental system for lung perfusion experiments are considered with regard to the apparatus and the mode of ventilation and perfusion. 

The apparatus and basic principles of SECM [28] were described in Section 5.6. 

Fractional solidification and its applications to obtaining ultrapure chemical substances, has been treated in detail in Fractional Solidification by M.Zief and W.R.Wilcox eds, Edward Arnold Inc, London 1967, and Purification of Inorganic and Organic Materials by M.Zief, Marcel Dekker Inc, New York 1969. These monographs should be consulted for discussion of the basic principles of solid-liquid processes such as zone melting, progressive freezing and column crystallisation, laboratory apparatus and industrial scale equipment, and examples of applications. These include the removal of cyclohexane from benzene, and the purification of aromatic amines, dienes and naphthalene. 

Before considering particular test methods, it is useful to survey the principles and terms used in dynamic testing. There are basically two classes of dynamic motion, free vibration in which the test piece is set into oscillation and the amplitude allowed to decay due to damping in the system, and forced vibration in which the oscillation is maintained by external means. These are illustrated in Figure 9.1 together with a subdivision of forced vibration in which the test piece is subjected to a series of half-cycles. The two classes could be sub-divided in a number of ways, for example forced vibration machines may operate at resonance or away from resonance. Wave propagation (e.g. ultrasonics) is a form of forced vibration method and rebound resilience is a simple unforced method consisting of one half-cycle. The most common type of free vibration apparatus is the torsion pendulum. 

The static laser light scattering apparatus used as an on-line GPC detector has been popular for a while. Here, we illustrate another but less known method of combining the results from (gel permeation chromatography) and DLS. The basic principle is as follows There is a similarity between these two tools in that the translational diffusion coefficient D obtained by DLS and the elution volume V in GPC are related to the hydrodynamic size of a given macromolecule. In a first approximation, if the hydrodynamic size is proportional to the molar mass, we have... 

Chaiken et al.24,37,38 proposed using Raman spectroscopy to measure glucose in vivo with a technique called tissue modulation, that is, continuously press/unpress the measurement site with a mechanical apparatus. The basic principle is that during the press phase, blood is expelled from the measuring site and thus the spectrum is considered as nearly devoid of blood. During the unpress phase, the spectrum is considered to be a sum of both blood and other tissue constituents. 

Owing to safety aspects, apparatus is much more restricted than mechanical machines in plants. Construction and manufacturing of machines are widely unrestricted if basic principles, fixed, for example, in national regulation collections such as the German VDMA-Einheitsblatter are considered. 

It is a fact of experience that pressurized apparatus should be fitted with a control and monitoring unit to ensure an appropriate purging procedure and maintain the protective gas flow and pressure differential which guarantee the safe operation of the apparatus. Purely hand-operated purging procedures are susceptible to errors and slips, and a permanently man-operated flow or pressure control for the protective gas is too remote from all basic principles of economic efficiency. Besides, the standards for pressurization contain requirements for safety devices for zone 1 apparatus (EN 50016) and for type px, py and pz-apparatus (IEC 60079-2, see Table 6.7). So, a safety device, or better a control and monitoring unit, is an essential part of a p-apparatus especially for Group I application and for zone 1. It is by no means an imperative that the control unit forms an integral part of the apparatus or has been made by its manufacturer pressurized apparatus without a control unit shall be marked X and the description documents shall contain all necessary information required by the user to ensure conformity with the requirements of the p-standards. 

MQDT is an evolving subject with a vast literature, covering both atomic and molecular physics. It finds extensive applications. Thus, the summary in the present chapter is of necessity incomplete. The reader should be warned that several different formulations of the theory exist, which can be transformed into each other but differ somewhat in apparent structure. It is unwise to use equations borrowed from different papers on MQDT without checking quite carefully which version the author uses and whether the formulations are consistent with each other. The full apparatus of MQDT is quite complex. We have emphasised only the basic principles for a complete discussion of the mathematical detail of MQDT, the reader is referred to a review paper by Seaton [126]. 

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