Axial Geometries

The geometries commonly used for measurements in instruments generating a linear or axial motion are listed in Table 2-1. With the exception of the shear sandwich, all require a solid sample. The drawings and associated formula 

The compression mode has been used successfully for soft samples that can be accurately made with strictly parallel faces on the top and bottom of the 

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Figure 11.13 Comparison of crystal structures of BACE with 15 and 16. Addition of the C6 methyl group in 16 reduces the presumed energetic penalty for the aryl substituent to adopt the pseudo-axial geometry seen in the bound conformation and also makes van der Waals contact with the enzyme.

Figure 28-8 Viewing geometries for ICP sources, (a) Radial geometry used in ICP atomic emission spectrometers (b) axial geometry used in ICP mass spectrometers and in several ICP atomic emission spectrometers.

The quadrupole ion trap (QIT) mass spectrometer consists of three hyperbolic-shaped electrodes arranged in a cylindrical geometry (see Fig. 2.14). By considering its axial geometry, we can move from the classical Cartesian coordinates to the polar ones. In fact, each point of the space inside the trap can be defined by the value of its axial (z) and radial (r) coordinates. If a potential U + Vcos cot is applied at the intermediate electrode and the two end caps are grounded, a mathematical treatment analogous to that done for quadrupole mass filter can be employed. In this case, a and q values can be defined again as... 

Cr(V) species have been observed via their narrow EPR signal at g 1.98 and their maximum visible absorbance at wavelengths near 750 nm In addition, the existance of Cr(V) in reactions has been inferred from its ability to induce oxidation of I ion, which is accomplished only very slowly by Cr(VI) Salts of CrO " have been isolated from very basic solution and have been shown to contain a tetrahedral ion However, the EPR signals of some intermediates indicate an axial geometry and demonstrate that Cr(V) can have 5-fold coordination . ... 

Once the axial geometry in a polymeric chain is the main factor in determining the ability of a chain to form ciystallites, the ciystalline contribution is probably due to isotactic and syndiotactic structure sequences." " In fact, saturated PHAmcl> which are able to crystallise due to their isotactic configuration, are also seen to crystallise with allq l side chains in an extended conformation to form ordered sheets, but they still show a reduced degree of crystallinity when compared to P(3HB) or P(3HB-co-3HV), owing to low crystallization rates. ... 

The radial geometry provides better stability and precision while the axial geometry can achieve lower detection limits. Many ICP emission systems allow both geometries. 

Hieftje and co-workers pioneered the use of both the orthogonal and axial TOF-MS geometries for ICP-MS, resulting in the development of commercially available instruments by GBC Scientific Instruments (Melbourne, Victoria, Australia) (orthogonal geometry) and LECO Instmments (St Joseph, MI, USA) (axial geometry). 

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