Basic considerations

The following are considerations in purchasing a basic pulse machine. The additional considerations for Fourier transform instruments are discussed in the next section. This section may be educational even if you are not in the market. 

Two important specifications are the maximum rf field intensity H and the duty cycle. The 90° pulse duration x (specified, for example, for protons) is related to H by YH t=7i/2. Hj is a measure of the power delivered to the 

Some modern (new in 1981) commercial NMR spectrometers are extremely versatile with computer controlled pulse sequence generators which are easily programmable even for complex sequences. Thus, even for a homemade machine, you should seriously consider purchasing the operating system software with a computer and a pulse sequence generator. 

As two phases in quadrature will allow Carr-Purcell Meiboom-Gill, T, and or other spin-locking experiments, we recommend this as a minimum. However, providing different rf phases for the transmitter pulses is not difficult and this capability should not increase the cost of the spectrometer very much so you might as well go for four orthogonal phases. As discussed elsewhere, the phase shifts can be performed in the IF stage rather than at the carrier frequency so that one phase shifting network can suffice for all carrier frequencies. 

In purchasing a new spectrometer, you should specify the desired frequency(ies) which will be dictated by the magnet available and the nuclei and experiments desired. If at all possible, a spectrometer which can operate over a large frequency range (a factor of 25 or more) should be chosen. This will enable you to study a variety of nuclei under conditions of optimum magnetic field. It also permits the study of relaxation times and lineshapes as a function of frequency. (This is an important method in identifying relaxation mechanisms and in separating different contributions to the line-shape. See III.C.2. and VI.D.l.) 

Prior to the development of the light diffraction methods and photon correlation spectroscopy to be described in chapters 6 and 8, sedimentation methods were the most widely used methods for characterizing the size distribution of a powder. In this chapter we will discuss the basic concepts involved in sedimentation techniques and review two or three methods still in relatively wide use. We will also describe briefly some of the methods which have been used extensively in past studies so that the reader may be able to assess the value of published data from the archives of powder technology. The reader interested in a detailed discussion of classical sedimentation size characterization techniques should refer to textbooks covering these earlier methods of size distribution in detail [1, 2]. 

In sedimentation methods for characterizing fineparticles the settling dynamics of a dispersion of the fineparticles under study in a gas or a fluid are studied. The sedimentation conditions are usually adjusted so that the fineparticles can be considered as falling freely at low Reynolds number in a viscous fluid. Under such conditions one can use Stokes law to interpret the size of the settling fineparticles. The Stokes diameter of a fineparticle is defined as the size of the dense smooth sphere of the same density material having the same falling speed as the fineparticle of interest in a viscous fluid at low Reynolds number. The reason that one has to specify low Reynolds number for sedimentation studies is that the falling conditions of the fineparticle must involve laminar flow without turbulence. The formulae for Stokes Law and Reynolds number are 

Readers unfamiliar with Stokes Law and Reynolds number will find a useful introduction to these concepts in Chapter 6 of Reference 3. 

Measured Velocity as a Fraction of Stokes Velocity for the Marker Sphere 

When choosing the concentration to be used in a given experimental study, scientists face a dilemma. If one chooses to operate at very low concentrations of solid then the analytical procedure for estimating changes in concentration of the fmeparticles in suspension becomes difficult. On the other hand, if one uses higher concentrations the interaction error can become significant. The consensus of opinion seems to be 

In today s discussion of the origin of life, the RNA World (Chapter 6) is seen as much more important, and is much better publicized, than the protein world . However, nucleic acids and proteins are of equal importance for the vital metabolic functions in today s life forms. Peptides and proteins are constructed from the same building blocks (monomers), the aminocarboxylic acids (generally known simply as amino acids). The way in which the monomers are linked, the peptide bond, is the same in peptides and proteins. While peptides consist of only a few amino acids (or to be more exact, amino acid residues), proteins can contain many hundreds. The term protein (after the Greek proteuein, to be the first) was coined by Berzelius in 1838. 

At first sight, equation 9.1 implies that the relationship between Q and AT is linear. Whereas this is approximately so over limited ranges of temperature difference for which U is nearly constant, in practice U may well be influenced both by the temperature difference and by the absolute value of the temperatures. 

If it is required to know the area needed for the transfer of heat at a specified rate, the temperature difference AT, and the value of the overall heat-transfer coefficient must be known. Thus the calculation of the value of U is a key requirement in any design problem in which heating or cooling is involved. A large part of the study of heat transfer is therefore devoted to the evaluation of this coefficient. 

The value of the coefficient will depend on the mechanism by which heat is transferred, on the fluid dynamics of both the heated and the cooled fluids, on the properties of the materials through which the heat must pass, and on the geometry of the fluid paths. In solids, heat is normally transferred by conduction some materials such as metals have a high thermal conductivity, whilst others such as ceramics have a low conductivity. Transparent solids like glass also transmit radiant energy particularly in the visible part of the spectrum. 

If heat is being transferred through three media, each of area A, and individual coefficients for each of the media are hu hi, and hi, and the corresponding temperature changes are AT, AT-, and ATj then, provided that there is no accumulation of heat in the media, the heat transfer rate Q will be the same through each. Three equations, analogous to equation 9.1 can therefore be written  

Noting that (AT + ATi + ATi) — total temperature difference AT then  

Component Combustion Wells/ Fermentation Hydrogen Phosphate Coal Ethylene Acid 

Note Examples of specific analytical methods are given in the ISBT document. Analytical Methods for C02 Analysis. 

The capillary pressure of interest in water-air-GDM systems is the difference between the pressures of the liquid and gas phases across static air-water interfaces within a GDM. This pressure difference is fundamentally related to the mean curvature H of the air-water interfaces through the well-known Young-Laplace equation 22 

Since water must be forced into a dry GDM regardless of PTFE-content, water is effectively the non-wetting phase. This process has the characteristics of primary drainage, for which invasion percolation30 is an adequate model.6,31 Invasion percolation describes 

In Section 5.4 we develop the condition of equilibrium for a chemical reaction for any chemical reaction written as Y.k VA = 0, the condition of equilibrium is vkfik = 0. In these equations each vk represents the stoichiometric coefficient of the substance Bk in the balanced chemical equation the sign is positive for products and negative for reagents. Expressions for the equilibrium constants are readily obtained from this condition. For a general relation we express the chemical potentials in terms of the activities, so 

With this general basis we consider the development of specific expressions for different types of chemical reactions. We first consider a homogenous reaction in the gas phase, and use the virial equation of state accurate to the second virial coefficient. The chemical potential of the fcth substance is given by Equation (7.72) 

The equilibrium constant in terms of mole fractions for the reaction is thus 

The right-hand side of Equation (11.9) is now a function of both the temperature and the pressure, and consequently so is Ky. The chemical potential of the fcth substance, when expressed in terms of concentrations, becomes 

Expressions for equilibrium constants for homogenous reactions taking place in solution are readily derived by use of the appropriate expressions of the chemical potentials. When mole fractions are used to express the compositions, no difficulty occurs, and the expression for the equilibrium constant for the generalized change of state vkBk = 0 is 

To an extent in accordance with the title and goal of this book, some fundamentals of the sampling process are discussed in more detail below. The reader should not, however, forget that the sampling process is very complex and, therefore, that environmental aspects, the analytical chemistry to be used, and mathematical statistics must be considered simultaneously. 

The most important norms dealing with the fundamentals of sampling and sampling design have been fixed by the International Organization of Standardization in Geneva (see for example [ISO 1213, Part 2, 1992 ISO/DIS 5667, Part 1-13, 1991 ISO 9359, 1989 ISO/DIS 10381, Part 1-6, 1995 ISO 11074, Part 2, in prep. ISO/DIS 11464, 1995 ISO/ DIS 14507,1995]. Some of these norms are in the process of revision and updating. 

A detailed overview on references for sampling by HANNAPPEL [1994] contains not only references for general, statistical, and detailed aspects, but also for norms of the International Organization of Standardization in Geneva. 

At the beginning of this passage some definitions concerning the term sample are necessary. According to the IUPAC Commission on Analytical Nomenclature [HORWITZ, 1990] the general definition is The sample is the actual material investigated, whether di- 

Each sample contains an uncertainty - the sampling error - arising from the heterogeneity of the parent material. The potential hazard resulting from extrapolation from the smaller portion (laboratory sample) to the larger portion (parent population) becomes obvious with the aid of the following theoretical example  

FIGURE 9.10 A schematic illustration of dishing on two individual vias (left) and an array of live vias (right). 

FIGURE 9.11 SKW tungsten line test wafer mask floor plan (from Ref. 13). 

It certainly goes without saying that other general responsibilities, of management, of Study Directors, of personnel and of Quality Assurance, will also have to be applied with regard to computerised systems. Thus, personnel should be appropriately trained in the operation and maintenance of computerised systems, adequate facilities for the location of such systems (especially for the central, core hardware of the test facility s computer network) have to be provided by management in the same way as for the localisation of other equipment, and Quality Assurance has to ascertain routinely the GLP compliance in the utilisation of computerised systems. 

Suitability for the intended use is something else that holds for a number of apparatus, equipment, facilities etc., and is certainly not a specific aspect of computerised systems. Computerised systems, as any other apparatus or instruments, should be of appropriate design, adequate capacity and should be suitable for their intended purposes. In the same way as for other apparatus, procedures have to be developed and documented to control and maintain computerised systems, and these systems should be developed, validated and operated in a way which is in compliance with the GLP Principles. The key word in this sentence is validated , and this term returns in the list of responsibilities of the Study Director, who has to ensure that computerised systems used in the study have been validated . This translates into the requirement that only systems that have been proven to be GLP compliant should be used in GLP studies, and the Study Director is consequently held responsible for ensuring that these systems have indeed been validated. In order to be able to fulfil this obligation the Study Director has to be actually aware of aU computerised systems that are to be used in a study. 

The basic principle in the use of computerised systems within regulatory safety studies can be very concisely brought to the point  

All computerised systems used for the generation, measurement or assessment of data intended for regulatory submission should be developed, validated, operated and maintained in ways which are compliant with the GLP Principles. Appropriate controls for security and system integrity must also be adequately addressed during the whole life cycle of any computerised system. 

These requirements may not seem to be helpful suggestions, but they provide the basic understanding that data quality, reliability and integrity have to be ensured in ways that are not intrinsically different, whether they are acquired and handled by computerised systems or not. 

It is obvious that reiiable and reproducible results can only be expected if the subsample has a mean composition, similiar to the totality of the investigated material, as regards the analyte elements. 

The human biological materials may be solid (bone, teeth, hair, nails, tissue), semisolid (blood, faeces, viscera) or liquids (body fluids). Treatment of the solid samples usually demands some extra steps during sample manipulation, e.g. particle size reduction, homogenisation, sub-sampling etc. Heterogeneous liquid phases, e.g. blood and certain body fluids, additionally need stabilisation and homogenisation so as to avoid occurrence of any changes in their composition, prior to actual analysis (Anand et al., 1975). It is also advisable to keep the total number of transfers to a minimum, and use accessories made of non-wettable and inert materials in case of the liquids. 

The method to be employed for measurement of the analytical signal largely determines the form of the sub-sample and, consequently, the extent and type of sample preparation required. Nuclear activation methods, x-ray fluorescence techniques, graphite furnace atomic absorption, classical emission spectroscopy and many mass spectrome- 

A variety of other techniques eg. voltammetry, flame atomic absorption spectrometry, flame emission spectrometry. ICP-atomic emission, ICP-fluorescence and ICP-mass spectrometry are almost invariably applicable to aqueous solutions, though solid sampling using ETA technique with the ICP-MS, is well established by now (Baumann, 1992 Voell-kopf et al., 1992). If the analytical technique employed is amenable only to a sub sample in solution, or is sensitive to the sample matrix, additional steps for dissolution and removal (partial or complete) of the biological matrix become unavoidable. We shall be mainly concerned in this chapter with these steps. 

Earlier work was carried out with crystalline bulk InP samples prepared by the gradient freeze method [274]. As In could become scarce due to its present use in electronics [275], the use of bulk material appears too expensive for appHcations. Therefore, homoepitaxial thin films have been prepared on InP wafers which can be removed by estabhshed Hft-off techniques [276]. The preparation by MOVPE allows scaling up and fine tuning of the growth process, enabhng fabrication of films with high electronic quality [277]. Besides the hitherto used (111) A-face (see Section 2.4.2.1), the surprisingly stable In-rich (100) surface is considered here. 

Rgure 2.105 Energy scheme of the p-lnP-H2/H+ redox electrolyte contact. The larger electron affinity compared to Si (by about 0.3 eV) and the larger energy gap 

Raman scattering may be regarded as an inelastic collision of an incident photon fuo with a molecule in the initial energy level Ei (Fig. 3.1a). Following the collision, a photon hcos with lower energy is detected and the molecule is found in a higher-energy level Ef 

The energy difference AE = E — Ei may appear as vibrational, rotational, or electronic energy of the molecule. 

If the photon hcoi is scattered by a vibrationally excited molecule, it may gain energy and the scattered photon has a higher frequency as (Fig. 3.1c), where 

This superelastic photon scattering is called anti-Stokes radiation. 

In the energy level scheme (Fig. 3.1b), the intermediate state Ey = Ei - hco of the system during the scattering process is often formally described as a virtual level, which, however, is not necessarily a real stationary eigenstate of the molecule. If the virtual level coincides with one of the molecular eigenstates, one speaks of the resonance Raman ejfect. 

A polymer is a substance composed of macromolecules, built by covalently joining at least 50 molecular mers, or the constitutional repeating units or CRU. The longest sequence of CRU defines the main chain of a macromolecule. The main chain may be composed of a series of subchains, identified by some chemical of physical characteristic (e.g., tactic placement). The main chain may also contain long or short side chains or branches, attached to it at the branch points. A small region in a macromolecule from which at least four chains emanate constitutes a cross-linking point. A macromolecule that has only one cross-link is the star macromolecule. 

1 Structure Aliphatic, aromatic, heterocyclic, metallo-organic 

2 Joining similar CRU Homopolymers (linear, branched, dendritic, cross-linked, etc.) 

3 Joining different CRUs Copolymers, multipolymers, polyadducts, polycondensates... 

4 Joining polymer segments Block copolymers, graft copolymers, ladder polymers 

In the energy level scheme (Fig. 8.1b), the intermediate state Ey = E - hco of the system during the scattering process is often formally described as 

A classical description of the vibrational Raman effect (which was the main process studied before the introduction of lasers) has been developed by Placek [8.8]. It starts from the relation 

Some general rules that are useful in die design are  

In die design, problems often occur because the product designer has little or no appreciation for the implications of the product design details on the ease or difficulty of extrusion. In many cases, small design changes can drastically improve the extrudability of the product. Some basic guidelines in profile design to minimize extrusion problems are  

Let us consider a small alloying addition of Pt (about 5 at.%) to Ni and examine its influence on the result of a sohd-state reaction with Si, leading to the growth of intermediate phases in the difiusion zone [10,12-16]. Such silicides are widely used in microelectronics while manufacturing intercoimects. Silicides of different stoichiometry are formed as a result of sohd state-reactions at the interface of a metallic film and siUcon substrate. 

Due to the tendency of microelectronic devices to reduce their sizes it is necessary to take into account the peculiarities of sohd-state reactions between Ni nanofilms and siheon substrate. Industrial implementation of metaUic silicides requires the reduction of contact layers and resistance of source/sink transitions while producing high-speed multielement integrated microcircuits. This is the NiSi that is most frequently used in manufacture of CMOS (Complementary 

When considering a phase composition of the diffusion zone in the process of heating a binary Ni-Si couple, one can notice that Pt addition leads to the rise of temperature for NiSi2 phase formation. A possible explanation for this is the 

Data from BINARY (SGTE) alloy databases 

We will depend upon the equations of flux balance at interface boundaries, t] — P, fi-y  

Bioorganic chemistry is a new discipline which is essentially concerned with the application of the tools of chemistry to the understanding of biochemical processes. Such an understanding is often achieved with the aid of molecular models chemically synthesized in the laboratory. This allows a sorting out of the many variable parameters simultaneously operative within the biological system. 

For example, how does a biological membrane work One builds a simple model of known compositions and studies a single behavior, such as an ion transport property. How does the brain work This is by far a more complicated system than the previous example. Again one studies single synapses and single synaptic constituents and then uses the observations to construct a model. 

Organic chemists develop synthetic methodology to better understand organic mechanisms and create new compounds. On the other hand, biochemists study life processes by means of biochemical methodology (enzyme purification and assay, radioisotopic tracer studies in in vivo systems). The former possess the methodology to synthesize biological analogues but often 

All living processes require energy, which is obtained by performing chemical reactions inside cells. These biochemical processes are based on chemical dynamics and involve reductions and oxidations. Biological oxidations are thus the main source of energy to drive a number of endergonic biological transformations. 

These catalysts have to be highly efficient (a minimum of waste) and highly specific if precise patterns are to be produced. Structural components have a static role we are interested here in the dynamics. If bond-breaking and bond-forming reactions are to be performed on a specific starting material, then a suitable specific catalyst capable of recognizing the substrate must be constructed around that substrate. 

The second term describes the infrared spectrum, the third term the Rayleigh scattering, and the last term represents the Raman scattering. 

Since an oscillating dipole moment is a source of new waves generated at each molecule, (9.5) shows that an elastically scattered wave at the frequency tx) is produced (Rayleigh scattering) but also inelastically scattered components with frequencies (to-w ) Stokes waves) and superelastically scattered waves with frequencies (w+co ) anti-Stokes components). The microscopic contributions from each molecule add up to macroscopic waves with intensities which depend on the population N(E. ) of molecules in the initial level E., on the intensity of the incident radiation and on the expression which describes the dependence of the polarizability components on the nuclear displacements. 

The soft segments of TPU can be designed as crystalline or amorphous depending on the type and length of the polyols being incorporated. Typically, ester and 

we first consider the shape memory effect, as well as basic structure-property relationships of TPU having crystalline soft segments, e.g., poly(tetra-methylene adipate) glycol (PTAd, M = 2000), and hard segments composed of 4,4 -diphenylmethane diisocyanate (MDI) and ethylene glycol (EG) or ethylene diamine (EDA) [25], 

Formulations to prepare the shape memory TPU are shown in Table 2. The OH-terminated polyols (OTP) in the table were obtained by extending the PTAd with hexamethylene diisocyanate (HDI). PH corresponds to a PU obtained from equimolar amounts of PTAd and HDI. The soft segment length was varied by changing the molar (n) ratio of PTAd (n + 1) to HDI (n) in OTP. 

Sample PTAd HDI OTP MDI moles) (moles) (moles) (moles) (moles) Soft segment content (wt%) rph ( C) (°C) (J/g(PTAd))  

With the rise in hard segment length and content, decreased, which is in line with the decreased glassy state modulus, and upon heating less residual strain was left [25]. That is, the shape is less fixable and more recoverable at high hard segment contents. 

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In the same section, we also see that the source of the appropriate analytic behavior of the wave function is outside its defining equation (the Schibdinger equation), and is in general the consequence of either some very basic consideration or of the way that experiments are conducted. The analytic behavior in question can be in the frequency or in the time domain and leads in either case to a Kramers-Kronig type of reciprocal relations. We propose that behind these relations there may be an equation of restriction, but while in the former case (where the variable is the frequency) the equation of resh iction expresses causality (no effect before cause), for the latter case (when the variable is the time), the restriction is in several instances the basic requirement of lower boundedness of energies in (no-relativistic) spectra [39,40]. In a previous work, it has been shown that analyticity plays further roles in these reciprocal relations, in that it ensures that time causality is not violated in the conjugate relations and that (ordinary) gauge invariance is observed [40]. 

H. C. Barnett and R. R. Hibbard, Basic Considerations in the Combustion of Hydrocarbon Fuels with Air, NASA Technical Report, 1959, p. 1300. 

The introdrrction to the code sets forth engineering reqrrirements deemed necessary for the safe design and constrrrction of piping systems. While safety is the basic consideration of the code, this factor alone will not necessarily govern final specifications for any pressrrre piping system. 

Another basic consideration is whether crystallization is best carried out on a batch basis or on a continuous basis. The present tendency in most processing plants is to use continuous equipment whenever possible. Continuous equipment permits adjusting of the operating variables to a relatively fine degree in order to achieve the best results in terms of energy usage and product characteristics. It allows the use of a smaller labor force and results in a continuous util-... 

The following subsections discuss the basic considerations involved in various unit operations of solid-solid separation and describe present industrial practice and equipment in general use. 

Basic consideration of stability-reactivity Some of these act as tnfluoro-methylatmg agents Te(CF3)2, (CH3)3SiCF-j... 

Section 6.2.1 offers literature data on the electrodeposition of metals and semiconductors from ionic liquids and briefly introduces basic considerations for electrochemical experiments. Section 6.2.2 describes new results from investigations of process at the electrode/ionic liquids interface. This part includes a short introduction to in situ Scanning Tunneling Microscopy. 

Refer to psychrometric chart. Figures 9-124A and B for basic considerations in establishing tower conditions. 

Equilibrium Basic Consideration, 1 Ideal Systems, 2 K-Factor Hydrocarbon Equilibrium Charts, 4 Non-Ideal Systems, 5 Example 8-1 Raoult s Law, 14 Binary System Material Balance Constant Molal Overflow Tray to Tray,... 

Valence band spectra provide information about the electronic and chemical structure of the system, since many of the valence electrons participate directly in chemical bonding. One way to evaluate experimental UPS spectra is by using a fingerprint method, i.e., a comparison with known standards. Another important approach is to utilize comparison with the results of appropriate model quantum-chemical calculations 4. The combination with quantum-chcmica) calculations allow for an assignment of the different features in the electronic structure in terms of atomic or molecular orbitals or in terms of band structure. The experimental valence band spectra in some of the examples included in this chapter arc inteqneted with the help of quantum-chemical calculations. A brief outline and some basic considerations on theoretical approaches are outlined in the next section. 

The basic considerations regarding the cell safety and the test results are described briefly below. 

For one thing, the first part of the book presents a modest introduction to the basic considerations that applied ethics most often employs, namely, rights, justice, and utility. The concept of rights, which are an individual s entitlements to those liberties, choices, opportunities, and items having serious consequence for human life, is precisely what privacy depends on for protection. As well, the concept of rights significantly bears upon questions of ownership, that is, the right to property. 

The results of environmental monitoring exercises will be influenced by a variety of variables including the objectives of the study, the sampling regime, the technical methods adopted, the calibre of staff involved, etc. Detailed advice about sampling protocols (e.g. where and when to sample, the volume and number of samples to collect, the use of replicates, controls, statistical interpretation of data, etc.) and of individual analytical techniques are beyond the scope of this book. Some basic considerations include the following, with examples of application for employee exposure and incident investigation. 

Pharmaceutical packaging is the means of providing protection, presentation, identification and information, containment, and convenience to encourage compliance with a course of therapy. The period from product manufacture to ultimate use or administration lies within the product shelf life interval. Criteria for selecting a satisfactory packaging system for pharmaceutical products are established by addressing a checklist of basic considerations ... 

For a discussion of electronic rearrangements when binding 02 to the iron-porphyrin see Refs. [6b, 50]. The same basic considerations are valid here,... 

Lessons from Hypoxia Basic Considerations of HIF-laStability Regulation... 

The main focus of the following considerations is on catalysis using inorganic materials. Similar considerations come into play for catalysis with molecular compounds as catalytic components of course, issues related to diffusion in porous systems are not applicable there as molecular catalysts, unless bound or attached to a solid material or contained in a polymeric entity, lack a porous system which could restrict mass transport to the active center. It is evident that the basic considerations for mass transport-related phenomena are also valid for liquid and liquid-gas-phase catalysis with inorganic materials. 

The basic considerations for spectrophotometer maintenance are safely and cleanliness. Maintaining a safe and clean work environment ensures a longer life for the instruments and a lower likelihood for problems relating to contamination, broken or misaligned parts, or injury. Solutions spilled on sensitive electronic circuits can render them inoperative. Any spills should be cleaned up immediately according to local safety protocols. 

In actual practice, there are two basic considerations that prevail upon in gas chromatography, namely ... 

Special emphasis is placed on the carbohydrate-mediated cell - target system interaction by describing hints and pitfalls of assays for cytoadhesion, specificity, cytoinvasion, and cytoevasion. In addition, basic considerations are presented to discriminate between active and passive uptake as well as to detect lysosomal accumulation. Finally, the pros and cons of two useful analytical techniques, namely, flow cytometry and confocal laser scanning microscopy, are described in detail. 

It is readily seen that tha e are also two types of oxidizing molecular species. The reduction of oxygen according to (2) was included in the basic considerations. Reduction of hydrogen peroxide is of the same type... 

In the previous paragraph, the basic considerations of FEM modelling have been laid out. The outcome of a static thermal simulation based on this model is a 3-d temperature field T x,y,z). In this section it is discussed, how the characteristic figures, such as thermal resistance and thermal time constant of the membrane, can be deduced. 

In the following sections, some of the basic considerations and strategies used for SDM are discussed, as are the basic techniques that are currently in use in our laboratory and some of the caveats of which one must be aware. 

There are two basic considerations when attempting SDM. One is to determine the amino acids that should be mutated and the other is to decide what to replace them with. The first question is, of course, dependant upon information gathered from previous experimentation in order to target residues that are appropriate. Such information may be derived from biochemical techniques. For instance, in our binding site studies, we have specifically mutated amino acids that had previously shown to be covalently labeled by photoactive ligands. Additionally, we have used comparisons between the sequences of different receptor subunits that correlate with receptor function to identify domains of interest. Chimeragenesis, the technique described in the first half of this chapter, can provide important information in this regard. Obviously, those proteins for which a detailed structural model is available will lend themselves to more rational substitutions. 

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