Column permeability

An important measure concerning column characterization in LC is the column permeability, which represents the capacity of the support to transport the mobile phase as consequence of a pressure drop occurring over the column. In other words, the permeability of a column determines the required pressure to achieve the desired flow rate. The linear flow velocity (u) across an empty cylindrical column is given by... 

Small particles should be used in gas chromatographic columns since the HETP is directly proportional to particle diameter. However, column permeability is proportional (and pressure drop is inversely proportional) to the square of the particle diameter. Therefore, if particles are too small pressure requirements increase tremendously. 

Column gas velocity is dependent upon the column length, carrier gas viscosity, pressure drop through the column and the column permeability. Column permeability is expressed by the specific permeability coefficient, B0, which may be calculated by the following expression. 

With open tubular columns, permeability is given by Poiseuille s law, B = dll32, where dc is the column internal diameter. 

Product Characterization by GPC. A Waters gel permeation chromatograph, model 200, was used in this work. The instrument was operated at room temperature, and tetrahydrofuran was used as the elution solvent. Samples were analyzed on a four-column system column permeabilities were 2.5 X 105, 1.5 X 104, 103, and 102 A (Waters designation). Oligomer samples (0.5% by weight in tetrahydrofuran) were injected for 120 seconds. A solvent flow rate of 1 ml/minute was used. 

Selectivity Column efficiency Peak asymmetry Column permeability... 

The particle size of the packing material determines the number of theoretical plates per unit length that can be generated (see Section 1.3.4). Small particle sizes result in high efficiencies however, increased backpressure can occur as a result of the decreased column permeability. If the small particles are packed into shorter columns the speed of analysis will increase at the same efficiency. 

Measurement of column permeability is not clear-cut. The permeability of a column can be determined in several different ways. For example, it can be measured as the specific permeability, K° (EQ. 3), which corrects for mobile phase viscosity and column length (5) ... 

Electrical conductivity measurements in porous media have long been used for evaluation of porosity and permeability of various geological samples [33-36]. In CEC, it is important to characterize the column permeability in the appropriate manner. One way to characterize the permeability in CEC of the column packing is by the conductivity ratio, < >, which is defined as [36,37]... 

The introduction of monolithic columns in the 1990s was another and more successful attempt to increase column permeability while decreasing the gap in column dual porosity. Macropores in the monolith are between 4000 and 6000 A in diameter, and they occupy almost 80% of the column volume. Compared to the conventional packed column with 5- or even 3-pm particles, the silica skeleton in monolith is only approximately 1 pm thick, which facilitates accessibility of the adsorbent surface inside the mesopores of the skeleton (pores between 20 and 500 A in diameter are usually called mesopores). Comparison of the spherical packing material and monolithic silica is shown in Figure 3-1. 

The velocity we can obtain at a given pressure will also be limited by the resistance to flow presented by the column, known as the specific column permeability. In equation (17-16) the permeability is broken up into its two main components the flow resistance parameter, ( ), and the particle diameter squared, dl, and can be expressed as... 

The cohuim pressure drop as a function of the flow rate was measured on each cohimn packed with the particulate (Gl-G - G5-G) and monolithic silicas (PKlll - PK118) with n-heptane/ethylacetate (80/20, v/v). The flow rate was increased by 0.5 ml min intervals from 0.5 to 4.0 ml min and the corresponding pressure recorded. The same measurement was performed in the reversed order. The measured pressure was corrected by the pressure drop caused by the cotmecting tubings. This measurement was done without the column. The column permeability Kf was calculated according to equation (1) ... 

It is interesting to compare the flow characteristics of both types of silica columns with respect to the column pressure drop and column permeability. The column pressure drop / flow rate curves of both types indicate a much lower slope of the monolithic columns (not shown) than the particulate columns. In other words, the column pressure drop of the monolithic columns corresponds to that of a column packed with 15 pm silica particles [6]. The column permeability values are listed in Table 3. 

---