Column packings physical properties

There are two fundamental aspects of HPLC packings (1) the surface dieinis-try of the packing which determines the interaction of the analytes with the packing and (2) the physical properties of the packing. Physic properties include the particle size and shape, the pore size and porosity, tte specific surface area, and last, but not l t, the particle strength. Th properties determine, among other factors, the column eflSciency, the retentivity, and whether the particle is suitable for HPLC at all. We will address these issues in this chapter. 

Determination of pressure drop through the column (for packed columns, correlations dependent of packing type, column-operating data, and physical properties of the constituents involved are available to estimate the pressure drop through the packing for plate columns, the pressure drop per plate is obtained and multiplied by the number of plates)... 

Equation (15) gives the variance per unit length of a GC column in terms of the outlet pressure (atmospheric) the outlet velocity and physical and physicochemical properties of the column, packing, and phases and is independent of the inlet pressure. However, equation (13) is the recommended form for HETP measurements as the inlet pressure of a column is usually known, and is the less complex form of the HETP expression. 

Unfortunately, any equation that does provide a good fit to a series of experimentally determined data sets, and meets the requirement that all constants were positive and real, would still not uniquely identify the correct expression for peak dispersion. After a satisfactory fit of the experimental data to a particular equation is obtained, the constants, (A), (B), (C) etc. must then be replaced by the explicit expressions derived from the respective theory. These expressions will contain constants that define certain physical properties of the solute, solvent and stationary phase. Consequently, if the pertinent physical properties of solute, solvent and stationary phase are varied in a systematic manner to change the magnitude of the constants (A), (B), (C) etc., the changes as predicted by the equation under examination must then be compared with those obtained experimentally. The equation that satisfies both requirements can then be considered to be the true equation that describes band dispersion in a packed column. 

The explicit form of those equations that satisfy the preliminary data criteria, must then be tested against a series of data sets that have been obtained from different chromatographic systems. As an example, such systems might involve columns packed with different size particles, employed mobile phases or solutes having different but known physical properties such as diffusivity or capacity ratios (k"). 

The nomographs can be used to make a quick, rough, estimate of the column height, but are an oversimplification, as they do not take into account all the physical properties and other factors that affect mass transfer in packed columns. 

As with distillation, the correlation for overall tray efficiency for absorbers, given in Equation 10.7, should only be used to derive a first estimate of the actual number of trays. More elaborate and reliable methods are available, but these require much more information on tray type and geometry and physical properties. If the column is to be packed, then the height of the packing is determined from Equation 9.64. As with distillation, the height equivalent of a theoretical plate (HETP) can vary... 

Supercritical fluids possess favorable physical properties that result in good behavior for mass transfer of solutes in a column. Some important physical properties of liquids, gases, and supercritical fluids are compared in Table 4.1 [49]. It can be seen that solute diffusion coefficients are greater in a supercritical fluid than in a liquid phase. When compared to HPLC, higher analyte diffusivity leads to lower mass transfer resistance, which results in sharper peaks. Higher diffusivity also results in higher optimum linear velocities, since the optimum linear velocity for a packed column is proportional to the diffusion coefficient of the mobile phase for liquid-like fluids [50, 51]. 

The physical properties of silica are determined by its specific surface area, pore volume, average pore diameter, porosity, and the particle diameter and shape [8]. The latter two are responsible for the efficiency, the physical stability and the pressure drop of the packed columns and do not contribute to retention and selectivity. 

Employing the conditions defined in the three data bases and the appropriate equations derived from the Plate and Rate Theories the physical properties of the column and column packing can be determined and the correct operating conditions identified. The precise column length and particle diameter that will achieve the necessary resolution and provide the analysis in the minimum time can be calculated. It should again be emphasized that, the specifications will be such, that for the specific separation carried out, on the phase system selected and the equipment available, the minimum analysis time will be absolute No other column is possible that will allow the analysis to be carried out in less time. 

The optimized operating conditions for each analytical method including the detector system of choice are reported in Table II. The reported columns and operating conditions yield satisfactory peak shapes and resolution of all the potential interferents evaluated for HCCP and HCBD. Two potential interferents—tetrachloro-l,2-difluoroethane and 1,2-dichloroethane— could not be separated from 1,2-DCP with conventional packed columns. Tetrachloro-l,2-difluoroethane, a compound with physical properties similar to 1,2-DCP, is not likely to be found with... 

Materials. Distilled water was used 2-propanol and trihydrous lithium perchlorate, of guaranteed reagent quality from Wako Pure Chemicals Co., were used without further purification. The purity of the 2-propanol was checked by gas chromatography, with Porapak-Q as the column packing, and found to be more than 99.9 mol %. The physical properties of pure solvents were compared with the literature values in a previous paper (2), and the agreement was satisfactory. 

A variety of physical parameters have been shown to correlate with chromatographic retention. Several physical properties, measured SFC capacity factors, as well as GLC derived retention indices for the PAHs studied are listed in Table II. The capacity factors, k, were calculated from an isoconfertic-isothermal SFC separation of a mixture of the PAHs on an octadecyl bonded packed column using CC>2 as the mobile phase (4500 psi, 100°C). 

Service-Oriented Rules of Thumb Strigle (Packed Tower Design and Applications, 2d ed., Gulf Publishing, Houston, Tex., 1994) proposed a multitude of rules of thumb as a function of the service, column pressure, and physical properties. These rules are based on the extensive experience of Strigle and the Norton Company (now merged with Koch-Glitsch LP). 

Packed height is determined from the relationships in Section III. Application of these relationships requires knowledge of the liquid and gas mass transfer coefficients. It is best to obtain these from experimental data on the system if available, but caution is required when extending such data to column design, because mass transfer coefficients depend on packing geometry, liquid and gas distribution, physical properties, and gas and liquid loads, and these may vary from one contactor to another. 

Thermodynamic non-idealities are taken into account while calculating necessary physical properties such as densities, viscosities, and diffusion coefficients. In addition, non-ideal phase equilibrium behavior is accounted for. In this respect, the Elec-trolyte-NRTL model (see Section 9.4.1) is used and supplied with the relevant parameters from Ref. [50]. The mass transport properties of the packing are described via the correlations from Refs. [59, 61]. This allows the mass transfer coefficients, specific contact area, hold-up and pressure drop as functions of physical properties and hydrodynamic conditions inside the column to be determined. 

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