Column packing particles

Figure 15.2D shows the contribution of stagnant mobile-phase mass transfer to molecular spreading. With porous column-packing particles, the mobile phase contained within the pores of the particle is stagnant, i.e., it does not move (in Fig. 15.2D one such pore is shown for particle 6). Sample molecules move into and out of these pores by diffusion. Those molecules that happen to diffuse a short distance into the pore and then diffuse out, return to the mobile phase quickly, and move a certain distance down the column. Molecules that diffuse further into the pore, spend more time in the pore and less time in the external mobile phase. As a result, these molecules move to a shorter distance down the column. Again there is an increase in the molecular spreading. 

U. S. Sieves are identified by either micrometer (micron) designations or arbitrary numbers. Thus, a material referred to as 60/80 means particles which will pass a 60 mesh screen but not an 80 mesh screen. You may also see this written as -60+80 mesh. Particle size is much better expressed in micrometers (microns), therefore 60/80 mesh would correspond to 250-177 micrometers (micron) particle size range. Table 2.5 shows the conversion of column packing particle sizes. The table shows the relation... 

TABLE 2.5 conversion TABLE OF COLUMN PACKING PARTICLES ... 

As we discussed above, efficiency and selectivity are complementary descriptors dependent on the different sets of chromatographic parameters. Efficiency is more dependent on the quality of the column packing, particle size, flow rate, and instrumental optimization, while selectivity is more dependent on the stationary phase properties and the nature of the analytes themselves. However, efficiency is sometimes affected by nonideal interactions of the analyte with the stationary phase (i.e., peak tailing). 

The velocity-independent term A characterises the contribution of eddy (radial) diffusion to band broadening and is a function of the size and the distribution of interparticle channels and of possible non-uniformiiies in the packed bed (coefficient A.) it is directly proportional to the mean diameter of the column packing particles, dp. The term B describes the effect of the molecular (longitudinal) diffusion in the axial direction and is directly proportional to the solute diffusion coefficient in the mobile phase, D, . The obstruction factor y takes into account the hindrance to the rate of diffusion by the particle skeleton. 

CHROMATOGRAPHIC COLUMN AND COLUMN PACKING PARTICLES 1.2.1 HPLC column... 

Column manufacturers have developed a wide range of column bore size, length, and column packing particle size to accommodate the increased demand for smaller and more efficient columns. The column bore size will dictate the appropriate flow rate and can be tailored to meet the analyst s needs. Table 2 lists the general column bore size (inner diameter, or i.d.) and appropriate flow rates. 

If a chiral liquid is coated on the surface of the column packing particles, a chiral liquid-liquid partition system is obtained. The mobile phase must be saturated with stationary phase. A thermostat must be provided in order to keep the equilibrium of the solution constant. An example of this method is shown in Figure 22.3 with (+ )-dibutyl tartrate as the stationary phase, with which several /3-amino alcohols can be separated. 

The internal physical structure of column packing particles is responsible for their both porous structure and mechanical properties. 

Exclusion Column packing Particle Maximum Maximum Plate count Column length Separation Manufacturer ... 

The basic principle of the SEC can be best explained with the use of Figure 1. The separation is considered as a specific type of distribution of the separated species (solute) between the solvent filling the pores of the column packing particles (stationary phase) and the solvent outside the particles (mobile phase). The total volume of a packed column, Vt, is the sum of the total volume of all pores, Vp, of the volume of the particles matrix, Vm, and of the interstitial volume, Vo, outside the particles ... 

Large molecules diffuse more slowly, and this in turn requires lower flow rates and/or smaller column-packing particles if large values of /V are to be attained. However, with sufficiently small particles and low flow rates, quite respectable values of N are possible, e.g., 20,000 for various proteins [13]. 

The theory summarized in this chapter and elsewhere [l-3a,18] has enabled the development of computer programs that allow so-called computer simulation. In this approach, two to four experimental gradient runs are first carried out for the reversed-phase or ion-exchange separation of a biological macromolecular sample. After the results of these initial experimental runs are entered into the computer, separation times of analytes can be predicted based on initial and final %B, gradient time and shape, column dimensions, flow rate, column-packing particle size, and temperature [42-47]. 

TABLE 3.3 Conversion Table of Column Packing Particles of Chromatographic Significance... 

Gas chromatography is a physical separation method in which the components in a mixture are selectively distributed between the mobile phase, which is an inert carrier gas, and a stationary phase, which is present as a coating of either column packing particles or the inner column wall. The chromatographic process occurs as a result of repeated sorption/desorption steps during the movement of the analytes along the stationary phase by the carrier gas. The separation is due to the differences in distribution coefficients of the individual components in the mixture. Being a gas-phase separation method, GC requires the analytes to be volatilized prior to their separation. As such, the application of GC is limited to components with sufficient volatility and thermal stability. 

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