Displacement between adjacent peaks

Fig. 3-5. Displacement between adjacent peaks with non-linear isotherms.

There is one major problem associated with working at very high loading which is the determination of cut points. Ultraviolet detectors become overloaded at relatively low concentrations and, whilst refractive index detectors are better in this respect, they suffer from other disadvantages such as an inability to cope with solvent gradients. Split flow to the detector may be helpful, but can cause problems. Where displacement is taking place, the boundary between adjacent peaks can be very sharp, and if the materieils are closely related, may not show on a detector suffi-... 

Ultrasonic waves are a mechanical disturbance which passes thru the medium by the progressive displacement of particles. The particles do not travel in the direction of the source but vibrate about their mean fixed position. The amplitude of the wave is the distance from peak to peak and therefore is the maximum displacement of a particle in the medium. The period (T) is the time required to complete one cycle and the frequency (f) refers to the number of cycles per unit time. The unit of frequency is the Hertz (Hz, one cycle per second) and it is the reciprocal of the period. The rate at which sound travels thru the medium is the velocity (c, meters per second). The wavelength (X, meters), is the distance between adjacent cycles. Therefore, the relation,between wavelength, velocity and frequency is given by... 

Peaks occur in a difference map in positions in the unit cell where the model did not include enough electron density valleys appear in places where the model contained too much electron density. This information may be used to obtain more precise atomic positions, atomic displacement parameters, or atomic numbers. For example, in the last category, the identities of atoms (carbon or nitrogen) in a tricyclic molecule were established by setting all atoms to one type (carbon in this case) in the structure factor calculation. A difference map was calculated with the calculated phases and examined for excess electron density at atomic positions (Table 9.2). It was found to be possible to distinguish between nitrogen (seven electrons) and carbon (six electrons), even though these atoms are adjacent in the Periodic Table. 

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