Internal surface packings

Column-liquid chromatography (CLC) can be conveniently divided into those systems which use packed columns and those which use open tubes (Figure 3.1). Capillary tubes (<4 < 350 pm) are used in open-tubular chromatography and the stationary phase is coated on the internal surface. Packed-column systems can be sub-divided arbitrarily into capillary columns, microbore columns, analytical columns and preparative columns according to the internal diameter of the column (Figure 3.1). 

Impingement demister systems are designed to intercept liquid particles before the gas outlet. They are usually constructed from wire mesh or metal plates and liquid droplets impinge on the internal surfaces of the mist mats or plate labyrinth as the gas weaves through the system. The intercepted droplets coalesce and move downward under gravity into the liquid phase. The plate type devices or vane packs are used where the inlet stream is dirty as they are much less vulnerable to clogging than the mist mat. 

All commercially available precoated plates are manufactured with great care. But they are active layers which, on account of the numbers and structures of their pores, possess a very large internal surface area, on which water vapor and other volatile substances can condense, particularly once the packaging has been opened. In order to prevent this as far as possible the precoated plates are packed with the glass or foil side upwards. 

The packing itself may consist of spherical, cylindrical, or randomly shaped pellets, wire screens or gauzes, crushed particles, or a variety of other physical configurations. The particles usually are 0.25 to 1.0 cm in diameter. The structure of the catalyst pellets is such that the internal surface area far exceeds the superficial (external) surface area, so that the contact area is, in principle, independent of pellet size. To make effective use of the internal surface area, one must use a pellet size that minimizes diffusional resistance within the catalyst pellet but that also gives rise to an appropriate pressure drop across the catalyst bed. Some considerations which are important in the handling and use of catalysts for fixed bed operation in industrial situations are discussed in the Catalyst Handbook (1). 

Many other types of solid phase adsorbents, including those based on conventional and specialty materials like restricted access media (RAM), can increase analysis speed and improve assay performance. These types of materials, also known as internal reversed-phase packings, are especially useful for assaying target compounds in biological samples such as serum and plasma. They are chemically modified porous silicas that have hydrophilic external surfaces and restricted-access hydrophobic internal surfaces. The ratio of interior to external surface areas is large. Macromolecules such as proteins cannot enter the pores of the RAM (they are excluded from the hydrophobic internal surface) and they elute quickly through the column. However, the smaller analyte molecules that can enter the pores are retained via interactions with the hydrophobic bonded phase within... 

Besides the above differentiation, restricted-access media can be further subdivided on the basis of the topochemistry of the bonded phase. Packings with a uniform surface topochemistry show a homogenous ligand coverage, whereas packings with a dual topochemistry show a different chemical modification of the pore internal surface and the particle external surface (114). Restricted-access media of the former type are divided into mixed-mode and mixed-function phases, bonded-micellar phases, biomatrix, binary-layered phases, shielded hydrophobic phases, and polymer-coated mixed-function phases. Restricted-access media of the latter type include the Pinkerton s internal surface reversed-phase, Haginaka s internal surface reversed-phase diol, alkyl-diol silica, Kimata s restricted-access media, dual-zone phase, tris-modified Styrosorb, Svec s restricted-access media, diphil sorbents, Ultrabiosep phases. Bio Trap phases, and semipermeable surface phases. 

Despite their distinct advantages, on-line SPE and column-switching proce-dures do not always represent ideal separation techniques. In many cases, only a small number of samples can be analyzed before contamination of the precolumn by proteins occurs. Alternative techniques that prevent the adsorption of macromolecules onto column packings and allow direct injection of sample extracts are those based on use of specific LC columns. Shielded hydrophobic phase (27), small pore reversed-phase (28), and internal surface reversed-phase (29, 30) columns can be used to elute proteins in the excluded volumes, allowing small... 

These columns, less commonly used today, are made of stainless steel or glass. They have diameters of 1/8 or 1/4 in (3.18 or 6.35 mm) and range in length from 1 to 3m. The internal surface of the tube is treated to avoid catalytic interactions with the sample. These columns use a carrier gas flow rate of typically 10 to 40ml/min. Although they are still used in approximately 10% of cases for routine GC work, packed columns are not well adapted to trace analyses. 

Another requirement is the accessibility of the active surface. A pore size of 30 or 50 nm is adequate in most cases of interactive polymer chromatography. The pore diameter must not be too large because the internal surface of the packing decreases with increasing pore diameter. 

Protecting the can from the effect of corrosion is very important in order to achieve the very long shelf life that metal cans offer. Discussion with the can manufacturer will ensure the use of the appropriate grades of lacquers on the internal surfaces to prevent primary corrosion. The external surfaces must also be considered in order to prevent secondary corrosion, which will result in leakage. Handling on the filling line must ensure the smooth flow of cans and eliminate any sharp objects that may scratch or pierce them. A lot of moisture is present on the line because of the use of conveyor lubricants or from the pasteuriser, and it is important that cans are dried before being packed, particularly if they are to be shrink-wrapped. Cold cans, below the dew point, must be wanned to ambient temperature or else they will become wet when packed. 

As the ligand-protein interaction takes place at the internal surface of porous adsorbents, kinetics and equilibrium of the interaction should be independent of the interstitial voidage within an adsorbent bed. Therefore the equilibrium capacity of an adsorbent will not be influenced by different experimental configurations e.g. batch stirred tank, batch fluidized bed, frontal application to packed or fluidized beds. The major difference arises from the medium from which the protein is isolated. As fluidized beds are used for whole broth adsorption, the properties of the broth have to be considered regarding the possible influence of components which are removed in conventional primary recovery steps and therefore are not present during the initial chromatography operations in a standard downstream process. These are on one hand nucleic... 

Due to the fact that protein adsorption in fluidized beds is accomplished by binding of macromolecules to the internal surface of porous particles, the primary mass transport limitations found in packed beds of porous matrices remain valid. Protein transport takes place from the bulk fluid to the outer adsorbent surface commonly described by a film diffusion model, and within the pores to the internal surface known as pore diffusion. The diffusion coefficient D of proteins may be estimated by the semi-empirical correlation of Poison [65] from the absolute temperature T, the solution viscosity rj, and the molecular weight of the protein MA as denoted in Eq. (16). 

The threshold value derived from a plot of the residual amount of crystalline NaCl versus the total amount of NaCl in each sample, as shown in Fig. 15, is 0.39 g NaCl/g NaY. This value corresponds to a dispersion capacity of 10.5 NaCl molecules Vsodalite cage in the NaY zeolite. In view of the fact that a sodalite cage can accommodate only one NaCl molecule, most of the NaCl molecules are dispersed on the wall of the supercages. On the basis of the close-packed monolayer capacity of NaCl (0.085 g/100 m2) taken from Table II and the BET surface area of NaY zeolite (800 m2/g), we estimate the utmost monolayer capacity at 0.68 g/g NaY, which is reasonably higher than the experimental value of 0.39 g NaCl/g NaY, because in our calculation we have neglected heterogeneity of the internal surface of the zeolite. 

Xs.—The wetting efficiency is obtained by calculating the fraction of the pixels identifying the surface of the packing that are in contact with liquid during gas-liquid flow. Liquid-containing voxels adjacent to the wall of the column, and the internal surface of the porous packing elements are not considered in the analysis. 

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