Principles versus Techniques

All these technologies rigidly obey Newton s laws and the laws of thermodynamics. Students who learned cookbook techniques for solving problems in 1954 were not well prepared for the technologies that appeared during the next 10 years, but those who learned the basic, ideas and how to apply them could adapt to any one of them. There is little reason to believe that the pace of technological change will be slower in the future. If we 

---

There are many techniques available for measuring the particle-size distribution of powders. The wide size range covered, from nanometers to millimeters, cannot be analyzed using a single measurement principle. Added to this are the usual constraints of capital costs versus running costs, speed of operation, degree of skill required, and, most important, the end-use requirement. 

Composite materials have many distinctive characteristics reiative to isotropic materials that render application of linear elastic fracture mechanics difficult. The anisotropy and heterogeneity, both from the standpoint of the fibers versus the matrix, and from the standpoint of multiple laminae of different orientations, are the principal problems. The extension to homogeneous anisotropic materials should be straightfor-wrard because none of the basic principles used in fracture mechanics is then changed. Thus, the approximation of composite materials by homogeneous anisotropic materials is often made. Then, stress-intensity factors for anisotropic materials are calculated by use of complex variable mapping techniques. 

Ionisation processes in IMS occur in the gas phase through chemical reactions between sample molecules and a reservoir of reactive ions, i.e. the reactant ions. Formation of product ions in IMS bears resemblance to the chemistry in both APCI-MS and ECD technologies. Much yet needs to be learned about the kinetics of proton transfers and the structures of protonated gas-phase ions. Parallels have been drawn between IMS and CI-MS [277]. However, there are essential differences in ion identities between IMS, APCI-MS and CI-MS (see ref. [278]). The limited availability of IMS-MS (or IMMS) instruments during the last 35 years has impeded development of a comprehensive model for APCI. At the present time, the underlying basis of APCI and other ion-molecule events that occur in IMS remains vague. Rival techniques are MS and GC-MS. There are vast differences in the principles of ion separation in MS versus IMS. 

With the improvement of analytical techniques, it is now possible to measure oxygen isotopic profiles in minerals. By analogy to the concepts of bulk mineral closure temperature versus closure temperature at every point in the interior of a mineral for the case of thermochronology, future theoretical development will need to clarify the concepts and meanings of Tc and Tae based on interior isotopic or elemental compositions of two contacting minerals. Furthermore, the principles of the above discussion may also be applied to elemental exchange reactions, such as Fe-Mg exchange between minerals. 

Because of the dosing technique associated with the vacuum volumetric method there exists a potential source of error which in principle cannot be avoided on samples that are slow to equilibrate. Figure 14.2 represents a plot of the pressure in the sample cell versus time. The pressure up to time tj represents the equilibrium pressure in the sample cell before a new quantity of adsorbate is admitted. At time adsorbate is admitted into the sample cell and is accompanied by a rapid pressure rise. The pressure then gradually decreases to a new equilibrium value at time t2- If the decay to the new equilibrium pressure is slow, it is possible for open or accessible parts of the surface, such as the interior of wide pores, to contain more adsorbate before equilibrium is attained than after equilibrium is established. Less accessible regions, such as the interior of long narrow pores, will adsorb slowly and as the pressure falls because of adsorption in these pores, desorption must occur from the more accessible pores which tend to equilibrate more rapidly. If a porous sample is subjected to... 

In principle, the parameters can be evaluated from minimal experimental data. If vapor-liquid equilibrium data at a series of compositions are available, the parameters in a given excess-free-energy model can be found by numerical regression techniques. The goodness of fit in each case depends on the suitability of the form of the equation. If a plot of GE/X X2RT versus X is nearly linear, use the Margules equation (see Section 3). If a plot of Xi X2RT/GE is linear, then use the Van Laar equation. If neither plot approaches linearity, apply the Wilson equation or some other model with more than two parameters. 

Immediately it is clear that a plot of Pm versus Tutilizing measurements of k as a function of temperature will yield both oq and F- This technique is readily applicable to gases. In principle it is applicable to liquids and solutions also, but it is seldom convenient owing largely to the small temperature range accessible between the melting point and boiling point. 

Wang et al. (1992) developed a new technique for the measurement of electrical conductivity of fiuoride melts, which employs the principle of a Continuously Varying Cell Constant (CVCC) through a moving platinum disc electrode in a relatively large diameter capillary tube-type conductivity cell. At the same time, the real component of the circuit impedance, R a, at a fixed high-frequency current is measured. Since the Rm versus the cell constant plot is linear, the electrical conductivity of the electrolyte is given by the relation... 

The frequency dependence of the measured resistance of the CVCC conductivity cell was tested using molten KCl and three different compositions of cryolite melts. The statistical analysis of the results indicated that the electrical conductivity of each electrolyte is independent of the applied frequency. Figure 8.11 shows the conductivity results as a function of the applied frequency. No variation of the conductivity values was observed within dispersion of 1%. This verifies the principle on which the technique is based, i.e. that the slope of resistance versus the distance L in the tube-type conductivity cell is independent of the applied frequency. Conventional methods, on the other hand, have to take into account the applied frequency and many conductivity values were derived or extrapolated to the infinite frequency of the measuring current. 

In this introduction, we have presented an overview of the benefits of applying the technique of SFE to the area of food analysis. There are substantially reduced costs derived from use of SFE versus traditional extraction in the areas of solvent purchase costs, solvent disposal costs, reduced labour charges, and even less need to repeat experiments due to reduced human errors in the overall analytical scheme. Moreover, productivity can be improved and the use of environmentally-unfriendly solvents is greatly reduced. In the rest of this chapter we will explore the fundamental principles of SFE in more detail, discuss some of the aspects of current SFE instrumentation, present a number of examples of applying SFE to food samples, and briefly summarise some hints for methods development. 

---