Principle of Separation

The separation mechanism can be explained on the basis of a specific distribution of the separated particles between the eluent outside the porous particles of the column packing (mobile phase) and the solvent filling the pores (stationary phase). This distribution is due to the steric exclusion of the separated particles from a part of the pores according to the ratio of their size to the size of the pores. The particles whose sizes are larger than the size of the largest pores cannot permeate the pores, passing only through the interstitial volume, i.e., through the void volume between the particles of the column packing, whereas very small particles may permeate all the pores. Particles of intermediate size are, to a greater or lesser extent, excluded from the pores. Hence, the elution proceeds from the largest particles to the smallest ones. This mechanism is schematically demonstrated in Figure 10.

The total volume of a packed chromatographic column, Vt, is given by the sum of the total volume of the pores, Vp, the volume of the matrix proper of the porous particles, Vm, and the interstitial or void volume, Vo, between the porous particles:

The retention volumes, VR, of the separated particles lie within Vo and Vo + Vp. VR of a uniform particle size fraction of the sample is defined as a volume of the eluent that passes through the column from the moment of the sample injection to the moment when the given particles leave the separation system at their maximal concentration. The retention can alternatively be expressed in time units as the retention time tR. The particles permeating the pores are excluded from some of the pores and partially permeate the accessible pores. The retention volume of a given species can be written as:

Figure 10 Schematic representation of the chromatographic column for SEC. Column with the void volume between the spherical particles of the column packing, the structure of one porous particle with the pore and matrix volumes, and the imaginary shape of one pore allowing the total permeation of smallest separated particles, partial permeation of intermediate size particles, and exclusion of largest particles.

Figure 10 Schematic representation of the chromatographic column for SEC. Column with the void volume between the spherical particles of the column packing, the structure of one porous particle with the pore and matrix volumes, and the imaginary shape of one pore allowing the total permeation of smallest separated particles, partial permeation of intermediate size particles, and exclusion of largest particles.

where Ksec is the formal analogue of the distribution coefficient between the mobile and stationary phases.

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Solar Panel Basics

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